Asian river ﬁshes in the Anthropocene: threats and ... river ﬁshes in the Anthropocene: threats and conservation challenges in an ... of the Mekong, ... that reduce its availability

Journal of Fish Biology (2011) 79, 1487–1524

doi:10.1111/j.1095-8649.2011.03086.x, available online at wileyonlinelibrary.com

Asian river fishes in the Anthropocene: threatsand conservation challenges in an era of rapid

environmental change

D. Dudgeon

School of Biological Sciences, The University of Hong Kong, Hong Kong SAR, China

This review compares and contrasts the environmental changes that have influenced, or will influ-ence, fishes and fisheries in the Yangtze and Mekong Rivers. These two rivers have been chosenbecause they differ markedly in the type and intensity of prevailing threats. The Mekong is relativelypristine, whereas the Three Gorges Dam on the Yangtze is the world’s largest dam representing theapotheosis of environmental alteration of Asian rivers thus far. Moreover, it is situated at the footof a planned cascade of at least 12 new dams on the upper Yangtze. Anthropogenic effects of damsand pollution of Yangtze fishes will be exacerbated by plans to divert water northwards along threetransfer routes, in part to supplement the flow of the Yellow River. Adaptation to climate changewill undoubtedly stimulate more dam construction and flow regulation, potentially causing perfectstorm conditions for fishes in the Yangtze. China has already built dams along the upper courseof the Mekong, and there are plans for as many as 11 mainstream dams in People’s DemocraticRepublic (Laos) and Cambodia in the lower Mekong Basin. If built, they could have profoundconsequences for biodiversity, fisheries and human livelihoods, and such concerns have stalled damconstruction. Potential effects of dams proposed for other rivers (such as Nujiang–Salween) arealso cause for concern. Conservation or restoration measures to sustain some semblance of the richfish biodiversity of Asian rivers can be identified, but their implementation may prove problematicin a context of increasing Anthropocene alteration of these ecosystems. © 2011 The Author

Journal of Fish Biology © 2011 The Fisheries Society of the British Isles

Key words: climate change; dams; flow alteration; Mekong; overfishing; Yangtze.

INTRODUCTION

Freshwater fishes face a global crisis, as is evident from the fact that almost 40% offishes in Europe and the U. S. A. are imperiled (Kottelat & Freyhof, 2007; Jelks et al.,2008). This illustrates the more general point that freshwater ecosystems tend to havea higher portion of species threatened with extinction than their marine or terrestrialcounterparts (Loh et al., 2005; Revenga et al., 2005; Dudgeon et al., 2006; Strayer& Dudgeon, 2010). There are four reasons for this (Dudgeon et al., 2006). First, only3% of the water on Earth is fresh (salinity < 0·5) and most of it comprises polar icecaps or is deep underground (Gleick, 1996); only c. 0·1% of global water reserves(i.e. 0·29% of the fresh water) is in liquid form habitable by fishes. Rivers contain amere 2% of surface fresh water, i.e. 0·006% of total freshwater reserves, although a

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1487© 2011 The AuthorJournal of Fish Biology © 2011 The Fisheries Society of the British Isles

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further 11% is in swamps of various types including floodplain water bodies (Gleick,1996). Second, fresh water is an irreplaceable resource that it is variously extractedand consumed or contaminated by humans, thereby reducing its quantity and qualityas a habitat. Meeting human water needs has direct implications for the availability ofwater to ecosystems; i.e. the wasted water remaining in rivers that flows into the sea.The absolute scarcity of river water can give rise to asymmetric competition betweenhumans and nature, resulting in cessation of downstream flows during part of the yearin some major rivers (e.g. Yellow, Ganges and Colorado Rivers). Third, the positionof freshwater bodies ensures that they serve as landscape receivers (Dudgeon et al.,2006), thus water and habitat quality are determined to a large extent by conditionswithin the catchment. Fourth, freshwater bodies or drainages are separated from eachother by land or sea barriers that are insurmountable for strictly freshwater species, sothat, in non-glaciated latitudes at least, they are equivalent to biogeographic islandswith separate evolutionary histories; consequently, beta diversity or spatial turnoverof fish species is high (Oberdorff et al., 1999; Leprieur et al., 2011). This localdiversification produces a lack of substitutability among freshwater habitat units thathave important consequences for conservation and choice of areas for protection andmanagement (Revenga et al., 2005; Dudgeon et al., 2006).

An additional feature of rivers that increases their vulnerability to human activitiesis attributable to their unidirectional water flow whereby pollutants and other mate-rial from the drainage basin are exported downstream; thus, rivers are transmittersas well as receivers. This contrasts with the relatively localized effects of humanactivity on terrestrial environments. Furthermore, because the effects are felt else-where, there is little impetus for humans to control the release or export of pollutantsdownstream from the site of origin. Movements of fishes and other animals, by con-trast, can transport material upstream: the contribution of marine-derived nutrients toheadwaters by Pacific salmon Oncorhynchus spp. (Gende et al., 2002) is an example.Oncorhynchus spp. and other migratory species are extremely vulnerable to the con-sequences of dams constructed variously for hydropower, water storage and irrigationor flood control (sometimes all three). Dams now fragment and regulate the flow ofmany major rivers worldwide (Nilsson et al., 2005), with consequences that includesequestration of transported sediments (Vorosmarty et al., 2003), blocked migrationroutes and transformation of free-flowing reaches into impoundments of standingwater where conditions are unsuitable for river fishes (Limburg & Waldman, 2009).

In addition to connectivity along the longitudinal axis, rivers link laterally withtheir flood plains during high-flow periods. Floodplain inundation is essential forfeeding and reproduction of many river fishes (Poulsen et al., 2002a), but channel-ization and levee construction reduces the extent and frequency of this inundation,degrading the flood plain and affecting fishes that depend on it (Lytle & Poff, 2004).The historical tendency for centres of human population to develop along flood plain(e.g. the Indus, Ganges and many others) exacerbates this trend. The inherent lateraland longitudinal connectivity of rivers means that threats to biodiversity that canoriginate or extend well beyond the river banks may be transmitted downstream and,under certain circumstances, upstream as well.

Stiassny (1999) co-opted Marshall McLuhan’s statement ‘the medium is the mes-sage’ to convey the notion that freshwater biodiversity is in peril precisely because itdepends on a resource subject to unprecedented and ever-increasing human demandsthat reduce its availability and compromise its value as a habitat. This catchphrase,

© 2011 The AuthorJournal of Fish Biology © 2011 The Fisheries Society of the British Isles, Journal of Fish Biology 2011, 79, 1487–1524

A S I A N R I V E R F I S H E S I N T H E A N T H RO P O C E N E 1489

however, fails to adequately capture the pressures on fishes due to overexploitation(Allan et al., 2005a), which are especially severe for larger species (Olden et al.,2007), or threats from introductions of exotic, non-native or alien species (Strayer,2010). Complex, often synergistic, interactions between ecosystem stressors or threatsto biodiversity (Ormerod et al., 2010) will be compounded by human-induced globalclimate change causing higher temperatures and shifts in runoff and precipitationpatterns (IPCC, 2007). The range and complexity of potential interactions amongstressors will make it difficult to predict probable outcomes (Moss, 2010) andconsequential extinction risks, but climate change considerations have begun todominate discussions about conservation planning for freshwater ecosystems (Heinoet al., 2009).

R I V E R S I N T H E A N T H RO P O C E N E

The phenomenon of human-induced climate change due to rising atmosphericcarbon dioxide concentrations (IPCC, 2007) demonstrates that the present is theAnthropocene epoch (Steffen et al., 2007) when humans are the dominant driverof environmental change (Zalasiewicz et al., 2008, 2011; Rockstrom et al., 2009;Steffen et al., 2011). The transition to the Anthropocene can, arguably, be dated at1980 when annual global nitrogen fixation by humans (primarily industrial productionof fertilizers) began to exceed that fixed by natural processes (Vitousek, 1994). Thenew epoch is evident with respect to a number of Earth-system processes, includingthe global water cycle (Meybeck, 2003) and land-to-ocean sediment fluxes mediatedby rivers (Syvitski & Kettner, 2011). Currently, humans appropriate more than halfof global surface runoff (Jackson et al., 2001; Vorosmarty et al., 2010) and, in someregions, the proportion is even higher (Rockstrom et al., 2009). A direct carbondioxide signal has been detected in continental river runoff records (Gedney et al.,2006). Human adaptation to climate change will certainly involve dam constructionfor flood control, water storage and hydropower generation, further altering river-flowpatterns and runoff (Palmer et al., 2008).

Nowhere has the Anthropocene transition become so apparent nor occurred sorapidly than in Asia. Much of the landscape is disturbed and degraded, the regionis very densely populated with vast urban conurbations, and there are striking con-trasts between the rural poor and growing, increasingly affluent urban populationswith relatively high per capita rates of resource consumption (Hannah et al., 1994;Corlett, 2009). It was surely for this reason that the Geophysical Society of Americaillustrated the cover of their flagship journal with a picture of Shanghai when theyhighlighted an article asking the question ‘Are we now living in the Anthropocene?’(Zalasiewicz et al., 2008). Furthermore, China has been the world’s highest emitterof greenhouse gases since 2007 (Boden et al., 2010), and the earliest sign of anthro-pogenic effects on land-to-water sediment fluxes originated within the Yellow Riversome 3000 years ago (Syvitski & Kettner, 2011). The history of flow alteration anddamming of Chinese rivers are even longer than this, extending over 4000 years(Dudgeon, 1995a).

One feature of the Anthropocene that illustrates the profound changes undergoneby rivers is recent delta shrinkage and reductions in the rate of aggradation reportedfrom the Yellow River, Zhujiang (Pearl River) and Yangtze in China as well as fromother east Asian rivers such as the Chao Phraya (Thailand) and Mekong (Syvitiski


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et al., 2009). This is due, in large part, to a 15% global average decline in annualsediment delivery to coastal zones during the 20th century (Syvitski & Kettner,2011). The role of dams in this sediment sequestration has been well established(Vorosmarty et al., 2003; Syvitski & Milliman, 2007) but must have been especiallyprofound in China where the number of large dams (>15 m tall) increased fromeight in 1950 to 18 600, more than half of the world’s total, by 1982 (Syvitski &Kettner, 2011).

In the human-dominated landscapes of Asia, the twin imperatives of economicdevelopment and livelihood improvement for the poor have led many nations to pri-oritize growth over environmental protection (Dudgeon, 1992; Dudgeon et al., 2000;Corlett, 2009). As a result, even where relevant laws and regulations are in place,deterioration in, for example, river water quality continues because enforcement isweak or standards are lax (Dudgeon et al., 2000; Xue et al., 2008). Trajectoriesof energy consumption, water use and consequential environmental alterations arerising steeply and can be projected to continue in the near future, which does notaugur well for freshwater biodiversity. In places such as China, where rivers arealready highly regulated and far from pristine, the outlook for some fishes is bleak(Dudgeon, 1995a, 2010), and there are good reasons for concern over biodiversityin other Asian rivers also (Dudgeon, 2000, 2005a). The implications for humansare also serious: a recent global analysis demonstrates that there are serious threatsto human water security and riverine biodiversity over much of Asia (Vorosmartyet al., 2010). These findings are of particular significance because the region accountsfor two-thirds of the world’s freshwater capture fisheries (FAO, 2010) contributingsignificantly to food security and livelihoods.

O B J E C T I V E S

The following sections will summarize, in general terms, the riverine fish bio-diversity of monsoonal east Asia, where the seasonal patterns of rainfall largelydetermine river ecology (Dudgeon, 1992, 1995b, 2000), and describe in more detailthe existing and potential threats to fishes in some major rivers focusing especiallyon the Yangtze and Mekong with particular reference to hydropower dams. Thesetwo rivers have been chosen because they differ markedly in the type and intensityof prevailing threats. The Yangtze is highly modified and degraded, and the Anthro-pocene’s influence on the river is manifested from the presence of the world’s largesthydropower project, the Three Gorges Dam (TGD). In contrast, the Mekong is rela-tively pristine, but is at a threshold and could be altered irreversibly by developmentplans that do not bode well for river ecology, biodiversity or fisheries. A concludingsection will outline the opportunities and challenges for conservation of river fishesin monsoonal east Asia.

CHINA AND THE YANGTZE

S TAT U S A N D T H R E AT S T O I C O N I C F I S H E S

Freshwater fishes throughout monsoonal Asia are incompletely known, with na-tional and local inventories of species bedeviled by misidentifications, taxonomic



inaccuracies and records that are out-of-date or otherwise suspect (Kottelat &Whitten, 1996). That said, the Chinese freshwater fish fauna numbers well over1000 species (Fishbase lists 1548; Froese & Pauly, 2011), and at least 717 species in33 families inhabit rivers (Li, 1981); a further 66 species spend part of their lives inrivers while others are mainly confined to estuarine reaches but occasionally swimupstream. Most of the Chinese river fishes are Oriental in distribution: 586 of the717 primary freshwater species occur in the Yangtze or farther south, of which 361are found in the Yangtze (Fu et al., 2003) making it fifth in the world in terms ofrichness, but 11th by basin area (Table I); 177 of them are Yangtze endemics (outof 265 species for China as a whole), but estimates of endemicity and richness varysomewhat among studies (Li, 1981; Chen et al., 2002, 2009; Fu et al., 2003; Parket al., 2003). Around 80% of the species in the Yangtze (including 124 endemics)occur in its upper course above the TGD (He et al., 2011). Elsewhere, the Zhujiang,China’s second largest river in the south of the country, supports 262 species (Liaoet al., 1989) and although Fu et al. (2003) claim 296 species, this may encompassmarine vagrants. Dudgeon (2002) gives a much lower total of 106 species, basedon raw data in Li (1981), but this is overly conservative given the river’s size andlatitude. Moreover, it is fewer than the 170 species in a Fishbase partial inventory forthe Xijiang (Froese & Pauly, 2011), which is a major tributary of the Zhujiang. Thespecies list for the Yellow River or Huang He in the north may once have comprised150 fishes (Fu et al., 2003), but the Chinese Ministry of Agriculture has declaredthat 30% of them are now extinct (Handwerk, 2007).

The presence of one of the world’s largest freshwater fish in the Yangtze is note-worthy. The Chinese paddlefish Psephurus gladius (Martens 1862) can grow to500 kg and ranks second among the world’s riverine megafishes (i.e. species over90 kg and 180 cm long; Hogan et al., 2004; Stone, 2007), but there are records sug-gesting P. gladius can reach 7 m long and weigh several thousand kg (Wang et al.,1998; Wei, 2009a). Psephurus gladius is categorized as critically endangered on boththe IUCN Red List (Wei, 2009a) and the China Red Data Book for fishes (Wanget al., 1998) and is endemic to the Yangtze, as is the more diminutive (16 kg; 130 cm)Yangtze sturgeon Acipenser dabryanus Dumeril 1869. It is likewise critically endan-gered (Wei, 2009b) with dramatic declines between 1958 and 1999 accompaniedby a significant loss of genetic variability (Wan et al., 2003). Recent evidence sug-gests that P. gladius are on the verge of extinction or possibly extinct (Wei, 2009a;Dudgeon, 2010): a 3 year hydroacoustic and fisheries survey of the upper Yangtzefailed to detect any P. gladius despite including the only known recent spawningsite of this species (Zhang et al., 2009b). Adults have not been captured since 2003and there does not appear to be any recruitment of juveniles (Zhang et al., 2009b).The situation of A. dabryanus seems little better: they are now extremely rare andseldom captured, and only a few small hybrid A. dabryanus were encountered duringthe P. gladius survey (Zhang et al., 2009b). This point has added significance giventhat, since 2007, more than 5000 artificially propagated juveniles have been releasedinto the upper Yangtze River, and this restocking may be all that is maintaining ofA. dabryanus in the wild (Wei, 2009b).

Also critically endangered is the Chinese sturgeon Acipenser sinensis Gray 1835:having disappeared from most its former range in China, it now breeds only inthe Yangtze where populations of adults declined to c. 2% of their former levelsbetween 1970 and 2007 (Wei et al., 1997; Wei, 2009c). This anadromous species is


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not among freshwater megafishes listed by Stone (2007) but given its dependence onthe Yangtze and potential maximum size (over 3 m long and perhaps up to 600 kg),it warrants inclusion among the top ranks of giant river fishes. Among other Yangtzeendemics, the Chinese sucker Myxocyprinus asiaticus (Bleeker 1864) (up to 1 mlong) is the only catostomid found in Asia. This migratory species breeds in theupper Yangtze but its range formerly included much of the mainstream. Although itis categorized as vulnerable in the China Red Data Book (Wang et al., 1998), theassessment pre-dates construction of the TGD which would prevent passage to andfrom the lower Yangtze.

R I V E R P O L L U T I O N

Declines in P. gladius and both Acipenser spp. are attributable to a combinationof overfishing, pollution, dam construction and heavy boat traffic (Wei et al., 1997;Fu et al., 2003; Hu et al., 2009; Wei, 2009a, b, c). They are classified as Grade 1Protected Species (Wang et al., 1998) and thus it is illegal to catch them, but thisdoes nothing to protect them from other threat factors. River pollution is epidemicin China and especially serious in the Yellow River, particularly during low-flowperiods, but water quality is also poor in parts of the Yangtze and Zhujiang (MEP,2008). By the beginning of the millennium, c. 80% of the 50 000 km of majorrivers in China had become too polluted to sustain commercial fisheries, and fisheshad been entirely eliminated from at least 5% of the total river length (Dudgeon,2005a). Given China’s rapid economic growth (c. 10% per annum) 2001–2010,further deterioration in river water quality has been inevitable.

Over 400 million people live in the 1·8 million km2 Yangtze Basin, and it isthe source of 40% of the country’s GDP (Pittock & Xu, 2010). For that reason, theYangtze receives around half of China’s waste-water discharge (Dudgeon, 2010), andpoint-source pollution by sewage and industrial waste is compounded by nitrogenrunoff (almost four times the global average) as well as phosphorus and pesticidesfrom agricultural land, and contaminants from vessels (Li & Zhang, 1999; Xueet al., 2008). Ambient levels of persistent organic pollutants and polycyclic aromatichydrocarbons from a variety of sources have increased along the river (Xue et al.,2008), and sublethal effects such as impairment of reproduction have been docu-mented for A. sinensis (Hu et al., 2009). The Chinese Ministry of EnvironmentalProtection (MEP) monitors river water quality at stations along the Yangtze gradingit according to six classes (Dudgeon, 2005a). Although the high-flow volume ofthe Yangtze (averaging 34 000 m3 s−1) has some potential to limit pollution-relateddegradation, MEP annual reports (MEP, 2008) show that pollution burdens in partsof the Yangtze have increased in recent years, especially in the lower course and insmaller tributaries (Xue et al., 2008). Only 31% of water samples, mainly from theupper Yangtze, are of first or second class quality, and much of the lower Yangtzewater is third class or poorer. There is, however, evidence of improvements in somelocalities due to enhanced treatment of industrial waste water (MEP, 2008).

The challenge of further enhancing water quality is very considerable in viewof the proximity of burgeoning urban conurbations to the Yangtze mainstream.Chongqing municipality, for example, has a population of around 32 million peo-ple, and is situated immediately upstream of the TGD Reservoir. This is likely tobe the reason why 20% of samples from the TGD Reservoir failed to meet third



class standard for water quality (i.e. <10 000 faecal coliforms l−1; <1·0 mg l−1

ammonia; MEP, 2008). Below the TGD, the lower Yangtze is navigable to the seaand connects the major cities of Yichang (4 million people) adjacent to the TDG,Wuhan (>9 million), Nanjing (almost 8 million) and Shanghai (>20 million). To alarge extent, these cities rely on the Yangtze for potable water and its polluted con-dition is thus a source of disquiet (Xue et al., 2008). It remains to be seen whetherefforts to safeguard drinking water will be implemented rapidly enough to preserveendangered Yangtze fishes.

OV E R F I S H I N G

The combined effects of pollution, habitat degradation and overexploitation havereduced fish catches from China’s rivers to a mere fraction of their former yield. TheYangtze fishery peaked in 1954 when it yielded 450 000 t, but catches from the riverfell by half between 1950 and 1970, and declined subsequently to c. 130 000 t year−1

in 2000 (Chen et al. 2004; Dudgeon, 2005a). Almost all (97%) of this yield camefrom the Yangtze below the TGD (Chen et al., 2004) and a considerable amountwas derived from floodplain lakes (Fang et al., 2006). The fishery for anadromousReeve’s shad Tenualosa reevesii (Richardson 1846), one of the most economicallyvaluable species in the Yangtze, collapsed in 1975 after a gradual decline in the meansize of individuals captured (Blaber et al., 2003; Wang, 2003; Chen et al., 2004;Fang et al., 2006). Another anadromous species, the Yangtze (or obscure) pufferfishTakifugu fasciatus (McClelland 1884), sustained a commercially important fisheryuntil the 1960s (Turvey et al., 2010). In addition to overexploitation, particularly byfine-meshed nets, the decline of T. reevesii can be attributed to dams on Yangtzetributaries, such that this fish is classified as endangered in the China Red Data Bookwith fishing of juveniles prohibited (but poorly enforced; Wang et al., 1998).

Catches of major carp, i.e. black carp Mylopharyngodon piceus Richardson 1846,grass carp Ctenopharyngodon idellus (Valenciennes 1844), silver carp Hypo-phthalmichthys molitrix (Valenciennes 1844) and bighead carp Hypophthalmichthys(= Aristichthys) nobilis (Richardson 1845), from the Yangtze began to decline wellbefore the TGD was built, falling from c. 80 to 35% of the catch from the riverbetween the 1980s and 1990s in some localities, with even greater long-term reduc-tions from 90% in the 1960s to no more than 5% in the 1990s along parts of themainstream (Chen et al., 2004) and floodplain lakes (Fang et al., 2006) in down-stream provinces. Annual catches of major carp downstream of the TGD over theperiod 2003–2005 (1010–1680 t) amounted to only 30–50% of that prior to thestart of impoundment (2002:3360 t), with drifting eggs and larvae declining by upto 95% (Xie et al., 2007). Anecdotal accounts also suggest that economically impor-tant carp have been reduced by 90% since completion of the TGD (Watts, 2011). Aswith T. reevesii, Yangtze carp declines are unlikely to have been due solely to over-fishing. For example, cooler downstream temperatures caused by water drawn fromthe hypolimnion of the TGD Reservoir will tend to retard egg maturation. Whencombined with an altered flow regime (Fig. 1) involving release of large amountsof water prior to the flood season (in order to increase reservoir storage capacity),the outcome might be stimulation of upstream breeding migrations by carp that haveyet to ripen fully (Xie et al., 2007). Additional effects of altered flow regimes areattributable to a reduction in water release prior to the natural decrease in dry-season


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Fig. 1. Annual flow regime of the Yangtze River below the Three Gorges Dam. Natural flows ( ) arepre-2002 and regulated flows ( ) are 2003–2005. Redrawn from Xie et al. (2007).

flows (Fig. 1), a practice that allows the dam operator to store water for powergeneration during the winter (dry season) when demand peaks. This reduction maycause premature return migrations from floodplain feeding sites so that carp havenot built up sufficient energy reserves to sustain them over winter and into the nextbreeding season (Xie et al., 2007).

Other Yangtze fishes affected by overexploitation include the A. sinensis withlandings of up to 25 t each year and a smaller but significant yield of A. dabryanus(Wei, 2009b); these ceased to be viable fisheries over 30 years ago (Wang et al.,1998). The M. asiaticus formerly contributed >10% of the catch in sections of theYangtze above the TGD but have dwindled to virtually nothing, and this speciesvanished from the lower course following construction of the Gezhouba Dam onthe Yangtze mainstream during the early 1980s (Wang et al., 1998; Chen et al.,2004). Declines in the long-snouted catfish Leiocassis longirostris Gunther 1864 areprobably due to overfishing as the dried swimbladder is highly prized; landings havediminished to such an extent that it has almost disappeared from markets (Chenet al., 2004).

A good deal of the present-day Yangtze fish catch depends upon stocking withcultured fry of major carp species (Fu et al., 2003) which, in combination with anannual 3 month fishing moratorium on around two-thirds of Yangtze, including partof the upper course, since 2003, may go some way to maintain present-day yields.The possible benefits for threatened fishes, however, need to be balanced against theconsequences of annual releases of up to 100 million cultured fingerlings for geneticdiversity of wild populations of major carp (Dudgeon, 2005a). Artificial propagationand release of juvenile A. sinensis have been undertaken on a limited scale also (Weiet al., 2004; Wei, 2009c), but the remaining small wild population appears to be



sustained by natural recruitment (Ban et al., 2011) and thus the released individualsprobably do not pose any threat to the genetic variability of the wild population.

As in the Yangtze, fish catches from the Zhujiang peaked during the 1950s at c.

10 000 t year−1 but declined during the 1960s and were only c. 6500 t year−1 in theearly 1980s. Tenualosa reevesii declined by 80% over the same period (Liao et al.,1989) and has now virtually disappeared from the river, although it formerly sup-ported a highly lucrative fishery (Wang, 2003). Acipenser sinensis has been entirelyeliminated from the Zhujiang, in part because this large, long-lived species is espe-cially vulnerable to overfishing (Wei, 2009c). As in the Yangtze, reductions in fisheryyield have been attributed to a combination of overexploitation (plus the use of fishpoisons and electricity), pollution and habitat degradation. Monetary losses due tofish kills in the Zhujiang have been high (Jin, 2011) while, over the longer term, damconstruction on the eastern tributary of the Zhujiang (the Dongjiang) was primarilyresponsible for the eradication of migratory T. reevesii and Chinese gizzard shadClupanodon thrissa (L. 1758), as well as the destruction of an economically, viablefishery for mud carp Cirrhinus molitorella (Valenciennes 1844) (Liao et al., 1989).

In a recent development, the Chinese Ministry of Agriculture announced the impo-sition of an annual 2 month fishing ban on the entire 2400 km long Zhujiang andits tributaries, amounting to some 5365 km of river, starting in April 2011, com-bined with plans to stock 6 million major carp fry (Jin, 2011). This moratorium,like that imposed in the Yangtze, constitutes a welcome step towards reversingthe consequences of overfishing but, without other environmental mitigation, mayhave little effect on species limited by the presence of dams or affected by pollu-tion and flow alteration. The situation would be further improved by a year-roundfisheries moratorium in both rivers. This is feasible, given the political will and ade-quate compensation for affected fishers, because the current contribution of Zhujiangand Yangtze fisheries to China’s total annual marine and freshwater fish production[c. 15 million t (Mt) capture; 33 Mt aquaculture, c. 60% in fresh water (FAO, 2010)]is trivial (c. 1%), and could be easily compensated by the rapidly growing freshwateraquaculture sector.

CHINA: HYDROPOWER CHALLENGES AND THREATS

Among the strongest signals of the Anthropocene epoch is that of reduced sed-iment delivery to the sea due to construction of dams and reservoirs, which hasbeen particularly apparent in China (Syvitski & Kettner, 2011). Some aspects of theecological effects of these dams have been considered elsewhere (Dudgeon, 1995a,

2000, 2005a), but the effects on migratory Yangtze megafishes, that are all nowcritically endangered, can be considered representative. While the TGD is the mostobvious threat to such fishes, construction of the Gezhouba Dam first blocked theYangtze mainstream in 1981. It obstructed breeding migrations, fragmented popu-lations and degraded spawning sites of Acipenser spp. and P. gladius (Dudgeon,1995a). The former extensive ranges of P. gladius and A. dabryanus are now lim-ited to parts of the river above the TGD, while A. sinensis only occur downstreamof the Gezhouba Dam. In this section, the recent history and extent of projectedhydropower dams, some of equivalent scale to the TGD, will be described togetherwith their implications for river fishes in China and adjacent nations.


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T H E H Y D RO P OW E R I M P E R AT I V E

Most of the China’s electricity is generated by coal-fired power stations; in 2010,only 9% originated from non-fossil sources (Anon, 2011). The contribution from coalis projected to decrease to 55% by 2035 with the balance made up by non-fossilenergy. To meet this target, a massive expansion in hydropower will be needed, asthis shift from coal will occur in the context of a national economy growing at ratesof c. 10% annually so that, even with some efficiency gains, electricity consumptionis bound to increase. Furthermore, China has pledged to reduce carbon intensity(the amount of carbon dioxide produced per unit of economic growth), 40–45%from 2005 levels by 2020 (Qiu, 2011; Stanway, 2011). The 12th Five-Year Plan for2011–2015, adopted by the National People’s Congress of the People’s Republic ofChina in March 2011, states that 15% of energy will be generated from non-fossilsources by 2020; 9% will come from hydropower, requiring an addition of 230 GWto the present estimated gross installed capacity of 200 GW to give a total productionof 430 GW by 2020 (Anon, 2011). To achieve this, the 12th Five-Year Plan specifiesa target of 163 GW for 2015. By any standard, this objective is extremely ambitious.For example, the capacity of the TGD is 18 GW. Reaching the 2015 target willrequire construction of nine TGD equivalents, almost two a year, whereas the 2020target of 430 GW installed capacity will require construction of 13 TGD equivalents.To put this in a global perspective, China overtook the U.S.A. to become the worldleader in terms of installed hydropower capacity in 2004; the intention is to more thantriple that by 2020 (Stanway, 2011). Achieving this goal will involve construction ofa host of large-scale hydropower plants along the major rivers of south-west China;an indication of the extent of growth in generating capacity and reservoir storage isshown in Fig. 2. Note that this only includes the very largest dams that will eachgenerate >1000 GWh of electricity annually.

DA M S O N T H E N U J I A N G A N D S A LW E E N A N D U P P E RYA N G T Z E

The recent history of dam construction in China has been associated with a con-siderable controversy. Some was generated by the forced relocation of people fromareas submerged by rising waters (for instance, more than 1 million people weremoved as a result of the TGD), but that matter lies beyond the scope of this review.Of greater relevance here is that, in order to facilitate economic growth, the authorityfor infrastructural development has been decentralized from the state to provincial ormunicipal governments. The enticement of financial gain has permitted local officialsto prioritize economic returns above other considerations. Environmental legislationand pollution control laws are ignored or flouted, and corruption is rampant becausestate government, in the form of the MEP, lacks the capacity for oversight andenforcement at the local scale. The enticement of financial gain has led to unfettereddevelopment and environmental degradation. Although proposals to construct damsrequire environmental assessments and approval from the MEP, this requirement,in force since 2003, is routinely flouted, even by nationally owned companies. TheChina Three Gorges Corporation (CTGC), a state-authorized investment institutionresponsible for the construction of the TGD, began constructing the Xiluodu Dam onthe Jinsha River, the mainstream of the upper Yangtze, in 2004 without the necessarypermission. Work was halted temporarily in 2005 and CTGC was fined for breaches



100

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ervo

ir s

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(Gm

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d ge

nera

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city

(G

W)

2000 2010 20170

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Fig. 2. Increases in the number of very large dams (i.e. dams with annual electricity generation >1000 GWh)in China projected to 2017 as reflected in accumulated installed generating capacity ( ) and reservoirwater storage ( ). Projections are likely to be conservative due to incomplete data for some projectsin the planning stage. In 1980, China had only one dam generating 1050 GWh (installed capacity 0·22GW; reservoir 0·05 Gm3) at Bapanxia on the Yellow River.

of regulations prior, but MEP subsequently approved hastily compiled reports andstudies of potential effects on fishes (Dudgeon, 2010). Elsewhere in China, prepara-tions for some of a cascade on 13 dams along the Nujiang (Fig. 3), the upper courseof the Salween River, began illegally in 2003, and was suspended in 2004 only afterthe intervention of Premier Wen Jiabao (Dudgeon, 2005a).

The need for stricter control of dam building was affirmed by China’s State Councilin 2004 and 2007 with confirmation that large hydropower projects needed MEPapproval; furthermore, Premier Wen reiterated concerns over the effects of dammingthe Nujiang in 2009 (Beitarie, 2011). As a result, many dam projects were haltedor delayed, including some on the Jinsha River, and only around one-third of theprojects envisaged in the 11th Five-Year Plan had been completed by the end of2010. Thus far, however, there is no instance where an environmental assessmenthas permanently stopped a dam project in China, as project proponents must beaware. It is now clear that work on the stalled or delayed dams will be resumedand others initiated during the 12th Five-Year Plan. Among them will be the 13dam cascade (Fig. 3; total capacity c. 21 GW) along the Nujiang mainstream (Anon,2011; Beitarie, 2011). In addition to dams within China’s national boundaries, since2007, the CTGC and other Chinese hydropower companies have signed agreementsto construct dams along the Salween in eastern Myanmar (Burma) (Anon, 2010).Details are scarce, but at least five dams are planned for the mainstream includingthe massive 228 m tall Tasang Dam (7110 MW).


1498 D . D U D G E O N

5

Lancang Jiang –Mekong River

Nujiang –Salween River

China(Yunnan Province)

Yangtze(Jinsha)River

China(Sichuan Province)

6

7

8

9

10

Myanmar

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12

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3

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N

400 km

Fig. 3. Map of the Nujiang–Salween in China, showing a 13 dam cascade planned for the river mainstream.Construction of a further six dams has been proposed for the Salween mainstream within Myanmar. Notethe proximity to the Lancang Jiang and Jinsha River, and UNESCO World Heritage Areas (indicated bycrosshatching). 1, Songta; 2, Bingzhongluo; 3, Maji; 4, Lumadeng; 5, Fugong; 6, Bijiang; 7, Yabiluo; 8,Lushui; 9, Liuku; 10, Shitouzhai; 11, Saige; 12, Yansangshu and 13, Gunagpo.

The potential effects of these dams cannot be predicted precisely because the fishfauna of the Nujiang–Salween is incompletely known. It is certainly diverse: at least143 species are recorded from the river (Table I) representing 77 genera (Dudgeon,2000); Fishbase lists 147 species (Froese & Pauly, 2011). An additional complicationis that there is uncertainty over which dams will be built, their configuration and theconstruction sequence. As this cascade of mainstream dams will obstruct migrationsand fragment populations, however, it is hardly conceivable that it will leave fishbiodiversity unaffected. A scenario of cumulative species loss and (at best) a gradualor more sudden reduction of capture fisheries is more plausible.

Considerably more is known about the plans for, and probable consequences of,dam building along upper Yangtze or Jinsha River, where the massive Xiluodu Dam(12·6 GW; 278 m tall) is now virtually complete (dam 3 in Fig. 4). It ranks secondin size to the TGD and is part of a 12 dam cascade along the Jinsha River (dams2–13 in Fig. 4) with a combined height of over 2000 m (Yao et al., 2006; Dudgeon,



Table I. Global ranking of Asian rivers (basins > 22 500 km2) according to fish speciesrichness. Ranks of drainage basin area for each river are given in parentheses. Table modi-fied and updated from Dudgeon (2002). Fish biodiversity estimates for river basins are notconsistent and this may affect relative rankings, e.g. totals for the Kapuas River (250 species)cited by Groombridge & Jenkins (1998) do not match those of Kottelat & Whitten (1996)

(320 species)

Approximatenumber of fish

species

Global rank:speciesrichness

Number of fishfamilies

Global rank:family richness

Mekong (25) 450–1700 3 37 10Ganges (16) 350 4 32 20Yangtze (11) 320–361 5 23 53Cauvery (98) 265 7 27 39Kapuas (88) 250–320 8 32 20Zhujiang (138) 262–292 9 21 62Chao Phraya (69) 222 11 36 11Brahmaputra (22) 200 12 32* 20*Sittang (132) 200 12 31 25Krishna (56) 187 14 29 35Song Hong (64) 180 16 24 51Mahakam (108) 174 17 33 20Indus (19) 147 22 24 52Salween (57) 143–147 23 34 16Fly (74) 101 34 13 94

*An estimate based on a figure for the combined Ganges–Brahmaputra drainage (Groombridge &Jenkins, 1998).

2010). The combined installed capacity will be 59 GW, i.e. three TGD equivalents oronly slightly less than the installed hydropower capacity of the U.S.A. The tailwatersof each dam will extend backward to the dam wall of its upstream counterpart sothat, upon completion of the cascade, the river will descend in a series of steppedimpoundments with few or no free-flowing sections. The ecology and topography ofthe Jinsha will thereby be completely transformed. Despite initial reservations dueto an absence of an environmental impact assessment by CTGC, the MEP allowedthe Xiluodu Dam to go ahead, and the company has been given permission forinitial work on two other dams. MEP also approved construction of two dams in theJinsha cascade (dams 7 and 8 in Fig. 4) by other power companies after issuing asuspension order in 2009 (Beitarie, 2011).

The Jinsha 12 dam cascade not only illustrates the lack of commitment to environ-mental regulations by national and provincial officials, but it also provides a dramaticexample of the threats posed to river fishes by China’s unfettered economic devel-opment. The CTGC is building one of its dams, the 161 m tall, 6·4 GW XiangjiabaDam, within the former boundaries of the upper Yangtze River Rare and EndemicFishes Reserve (dam 2 in Fig. 4). The 400 km long reserve was designated by theState Council of China in 1987 to protect P. gladius, A. dabryanus and 69 endemicor valuable fish species, 29 of which are listed as endangered in the China Red DataBook (Wang et al., 1998), and to offset potential adverse effects of the TGD. In 2005,


1500 D . D U D G E O N

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Upper Yangtze Basin boundary

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Three Gorges

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Guizhou0 100 km

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Upper Yangtze RiverRare and Endemic Fishes Reserve

Yunnan

2

Min Jiang

Jinsha River

1

10

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Fig. 4. The Jinsha (upper Yangtze) River in China showing the location of 13 dams planned or under con-struction above the Three Gorges Dam (TGD) and Gezhouba Dam (see inset). The original boundariesof the Upper Yangtze Rare and Endemic Fishes Reserve, adjacent to Chongqing municipality, are shown( ). The reserve contains the only known breeding site of Psephurus gladius. Redrawn from Dudgeon(2010) with permission from John Wiley & Sons. 1, Xiaonanhai; 2, Xiangjiaba; 3, Xiluodu; 4, Bai-hetan; 5, Wudongde; 6, Guanyinyan; 7, Ludila; 8, Longkaikou; 9, Jin’anqiao; 10, Ahia; 11, Liyuan; 12,Liangjiaren and 13, Hutiaoxia.

the CTGC successfully petitioned the State Council to amend the reserve boundariesto exclude the part of river where Xiangjiaba Dam is now being constructed. TheXiluodu is immediately upstream of the original reserve boundary. The CTGC alsoplans to build the 195 m tall Xiaonanhai Dam adjacent to, but within, its downstreamborder 30 km from Chongqing (dam 1 in Fig. 4). This project was subject to a 2008environmental assessment commissioned by the Chongqing municipal government(as part of China’s 11th Five-Year plan), although the details have yet to be released(Dudgeon, 2010). Subsequently, CTGC and municipal officials successfully peti-tioned the state council to further amend the reserve boundaries, and the MEP redrewthe downstream boundary of the fish reserve in January 2011 (Watts, 2011). Theboundary adjustment is not large, but when the Xiaonanhai Dam is completed its tail-waters will extend upstream and transform much of the reserve into an impoundment.

The reduction in reserve size, and the inevitability that upstream and downstreamdams will profoundly compromise conditions within the reserve, will reduce its via-bility and effectiveness. Given the apparent absence of mature P. gladius or breedingpopulations of A. dabryanus (Zhang et al., 2009b), however, it is doubtful that thereserve has been fulfilling its primary function. The most recent and comprehensivestudy of the 124 endemic fish species in the Jinsha River and upper Yangtze Basin(He et al., 2011) identified five assemblages differing in composition and richnesswith distributions that were highly correlated with the type of land cover in thebasin and river topography. Each assemblage comprised mainly species that sharedsimilar adaptations, and were characterized by different sets of indicator specieswith schizothoracine cyprinids tending to show altitudinal zonation along the upperYangtze (He et al., 2011). These distribution patterns demonstrate clearly that a



single reserve in one section of the river, however well it is protected, is unlikely toadequately protect endemic fish biodiversity. A network of sites is needed, includingthe full variety of habitats that support species of interest, combined with mainte-nance of flowing reaches that are essential for spawning, in all major tributaries (Heet al., 2011). Maintaining the widest possible range of habitats and flow conditionswill, hopefully, ensure that the requirements for recruitment can be met for the manyspecies whose biology and habits have yet to be studied adequately. A similar sug-gestion or reserve design was made in an earlier, but less inclusive study of the upperYangtze (Park et al., 2003), although no new reserves were established as a resultof this recommendation. Unfortunately, the richest assemblage of endemic Yangtzefishes tends to be centred around the lower part of the Jinsha River (He et al., 2011)overlapping with the locations of a number of dam sites. This leaves protecting thefishes that can persist in those tributaries as the only viable conservation option.

LARGE-SCALE WATER TRANSFERS IN CHINA

This review on threats to fishes in the Yangtze and other Chinese rivers presentedhere is far from comprehensive; for instance, it fails to include the effects arisingfrom an ambitious scheme, conceived by Mao Zedong in 1952, to transfer water fromthe Yangtze to the Yellow River and arid lands of northern China (Dudgeon, 1995a;Wong, 2007; Chang et al., 2011). The project has been stalled for decades, but initiat-ing construction was a key component of China’s 10th Five-Year plan (2001–2005).Work began in 2002 on an eastern route (Fig. 5) along the coast through the ancient,but now refurbished, Grand Canal that will transfer (polluted) water from the lowerYangtze. Upon completion in 2013, 14·8 Gm3 of water will be conveyed as far asTianjin, almost 1200 km north, each year. A central middle route (Fig. 5) transferringwater from the Danjiangkou Reservoir on a major Yangtze tributary (the Han River)has been under construction since 2003 and, by 2014, should be transferring 13 Gm3

of water annually 1300 km north to Beijing, Tianjin and other northern cities. Anadditional southern connection to the TGD has been suggested, so as to reduce theamount of water removed from the Han River. A western route (Fig. 5), intended todivert 8 Gm3 (and perhaps up to 20 Mm3) of water from the three Yangtze headwa-ter tributaries (over 3000 m above sea level) to the Yellow River, has stalled due toengineering difficulties and is unlikely to be completed in the foreseeable future (per-haps not until 2050), although targets for construction planning are envisaged underthe 12th Five-Year plan. The already ambitious western route could ultimately besupplemented by diversion of water from the upper Brahmaputra (Yarlung Zhangbo),Mekong and Salween but, thus far, this remains engineering fantasy.

The costs of transferring water north from the Yangtze cannot yet be ascertainedfully, but will certainly be far in excess of the TGD (Wong, 2007). The project willbring obvious benefits to the relatively arid north-east of China but, given that atleast 36 Gm3 of river water will be removed each year (c. 5% of annual flow), it willnot be without effect on the Yangtze, particularly during the dry season. Reduceddischarge, increased pollution burdens and saline intrusion in the estuary are amongthe likely outcomes relevant to fishes (Dudgeon, 1995a; Fu et al., 2003; Wong,2007). In addition, there are growing concerns about the quality and quantities ofwater to be transferred north (Chang et al., 2011), as some of the Yangtze source


1502 D . D U D G E O N

Westernroute

Jinsha River

India

Myanmar

LaosVietnam

GezhoubaDam

LakeTaihu

DanjiangkouReservoir

ThreeGorgesDam

China

N

Yangtze River

Han River

Yellow river

Centralroute

Easternroute Shanghai

BeijingTianjin

Fig. 5. The eastern, western and central routes intended to transfer water from the Yangtze River to the northof China. The section between the Three Gorges Dam, and Danjiangkou Reservoir on the Han River hasyet to begin, and the western route is still in the planning stage.

water is so polluted that water treatment plants along the transfer route are unableto bring it up to an acceptable standard (Simpson, 2010).

Smaller-scale, but nonetheless substantial, water transfers from the Yangtze havealso been deployed for other purposes. Eutrophication due to increased nutrient load-ing has led to algal blooms and the disappearance of as many as 41 fish species fromLake Taihu (Fig. 5) since the late 1980s (Guan et al., 2011). Annual transfers of upto 1·2 Gm3 of Yangtze water, over periods lasting up to 130 days, were initiated in2002 in an effort to flush out the 2338 km2 lake and improve its water quality (Zhaiet al., 2010). While a reduction in algal blooms within the lake was noted, the effectsof eutrophic effluent water flushed into the Yangtze were not evaluated, presumablybecause of the potential diluting effect of the mainstream flow.

THE MEKONG RIVER

I C O N I C R I V E R F I S H E S

The tropical Mekong is the largest river in Asia and, unsurprisingly, supports a richfish fauna (Table I). Estimates of total richness range from 450 species (Kottelat &Whitten, 1996) to around 1000 or perhaps as many as 1200 species (Rainboth, 1996),although one assessment extrapolates this total to 1700 (Sverdrup-Jensen, 2002).Fishbase (Froese & Pauly, 2011) lists only 773 species, while Hortle (2009) statesthat the Mekong hosts c. 850 freshwater species with the total rising to around 1100



if estuarine species and marine vagrants are included. This most recent inventorycomprises 781 species, although a complete list might ultimately reach 1300 (MRCS,2011a). On the basis of these figures, the Mekong has been ranked among the topthree rivers in the world in terms of fish species richness (Dudgeon, 2000), after theAmazon (South America) and the Congo (central West Africa), yet it is relativelysmall, ranking 15th globally in terms of discharge, 16th in terms of length and25th in terms of drainage basin area (Table I; Dudgeon, 1995b). If the estimateof 850 (or 1300) species is correct, then the Mekong may actually rank second,after the Amazon, in terms of species richness. In terms of species per unit area,the Mekong would rank top. Interestingly, the family-level diversity of rivers inmonsoonal east Asia is not exceptional: the Mekong ranks 10th only, with otherrivers in the region below it (Table I). This presumably reflects the overwhelmingdominance of Cyprinidae in the region, since genus-level diversity in these riversfollows a similar trend to species richness (Dudgeon, 2000).

The Mekong does not want for charismatic or iconic species (Hogan et al., 2004),including the giant freshwater stingray Himantura chaophraya Monkolprasit &Roberts 1990, which reaches c. 600 kg and may be the world’s largest freshwa-ter fish. The endemic Mekong giant catfish Pangasianodon gigas Chevey 1931 (c.350 kg) ranks third among the megafishes (Stone, 2007), although historical recordsof maximum size of this catfish (Smith, 1945) suggest that an even higher rankingmay be warranted. Three other megafishes are found in the Mekong: both the giantbarb Catlocarpio siamensis (Boulenger 1898) and dog-eating catfish Pangasius san-itwongsei Smith 1931 are reported to attain 300 kg, while the silurid Wallago attu(Bloch & Schneider 1801), also known as freshwater shark, can reach 2·4 m in totallength (LT) (Hogan et al., 2004; Stone, 2007).

T H E I M P O RTA N C E O F M E KO N G F I S H E R I E S

The richness of the Mekong fauna, and its charismatic and endemic species, com-bine to present a strong case for conservation. The case is strengthened by the factthat the Chao Phraya River in Thailand, the other major river of the Greater MekongRegion, has been degraded by pollution, dam building and a complex of other factors(Dudgeon et al., 2000) such that as few as 30 of the 190 native species can repro-duce in the river mainstream (Compagno & Cook, 2005). The importance of Mekongfishes in terms of global biodiversity is paralleled by its significance for humans:the annual yield from the lower Mekong Basin (LMB; i.e. the Mekong downstreamof China) is the world’s largest freshwater capture fishery at an estimated 2·2 Mt(Hortle, 2009). It may amount to 2·5 Mt if freshwater shrimps, crabs, snails andfrogs are included (MRCS, 2011a), i.e. one quarter of the global total of c. 10 Mt(FAO, 2010).

The Mekong fishery has three distinctive characteristics: first, it is based upon alarge number of species and second, much of the catch (40–70%) is constituted byc. 50 species that migrate within the river (Barlow et al., 2008). A third feature isinvolvement of c. 40 million people (more than half of them women) in fishing ona small-scale, subsistence or ad hoc basis, with the catch contributing to family orlocal welfare and food security (MRCS, 2011a). In land-locked People’s DemocraticRepublic (Laos), for example, 83% of households engage in capture fishery some ofthe time, with 90% of the catch derived from rivers and streams, and fishes provide


1504 D . D U D G E O N

20% of animal protein consumed (FAO, 2010). Elsewhere in the LMB (especiallyCambodia), fish constitutes 47–80% of animal protein intake or 29–39 kg per capita(Hortle, 2007).

OV E R F I S H I N G A N D T H R E AT T O I C O N I C F I S H E S

Unlike the Yangtze, which has serious pollution problems along much of its course,urban and industrial discharges do not yet present a significant threat to fish biodi-versity in the Mekong mainstream, although some local degradation of water qualitydue to saline intrusion, as well as acidification and eutrophication (mainly from aqua-culture), are evident in the delta (MRC, 2008). Biomonitoring at >50 sites in theLMB between 2004 and 2008 uncovered signs of impairment at scattered locations,but most sites maintained excellent or good ecological health and a few (in thedelta) even improved (Dao et al., 2010). In contrast, overfishing presently representsa more substantial risk to the Mekong fishes. While there are serious limitationson the reliability of freshwater capture fishery statistics (FAO, 2010), the aggregatecatch from the Mekong appears to be increasing, although catch per fisher may bedeclining (FAO, 2010). For example, the catch from the Tonle Sap floodplain lakein Cambodia doubled between 1940 and 1995; the number of fishers tripled overthe same period. Stability or increases in total catches, however, can conceal thefact that individual species in a multispecies fishery are being overexploited, withdeclines in the contribution of large, long-lived species to the overall catch beingoffset by increased capture of small, short-lived species with rapid life cycles as thecommunity is fished down. This is quite evident in the Tonle Sap, where the 120 000t annual catch in 1940 consisted mainly of large fishes while the 235 000 t caughtduring 1995 was almost exclusively small fishes (FAO, 2010). The dai (stationarytrawl) fishery of the Tonle Sap River has been monitored since 1997, and initiallyshowed a strong correlation with the height of the annual flood peak but this rela-tionship broke down after 2003. Catches in 2010 were unusually low despite highwater levels, perhaps due to scarcity of fry and poor recruitment success caused byoverfishing adults in dry-season refuges (Sopha et al., 2010).

The fate of P. gigas typifies trend towards overexploitation of large species. Pan-gasianodon gigas formerly supported a commercial fishery in Cambodia for the oilthat could be extracted from its fatty flesh, but stock decline attributable to overfishingbegan decades ago (Smith, 1945) and the fishery collapsed. Stocks are estimated tohave declined by 80% during the last two decades alone, and P. gigas is categorizedas critically endangered by the IUCN (Hogan, 2003). It has virtually disappearedfrom the Mekong in Thailand, although some licensed fishing of the species isstill allowed, but it is protected by law in Cambodia and Laos (Kottelat & Whitten,1996). Populations of C. siamensis are also in decline in the Mekong, and it is legallyprotected from fishing in Cambodia. This species, and the other two non-endemicMekong megafishes, are also known from the Chao Phraya River in Thailand, whereH. chaophraya have diminished greatly in numbers (Compagno & Cook, 2005)and P. sanitwongsei, which are critically endangered throughout their range (Jenkinset al., 2007), are apparently nearing extinction (Stone, 2007). Large migratory fishesare susceptible to overfishing and other human activities because they are long-livedand reach sexual maturity relatively late. The need to use different parts of the riverat different stages of the life cycle increases their vulnerability, even in the absence of



dams, because it increase their chances of encountering fishers, pollutants or degradedconditions. The further they migrate, the greater this risk, as exemplified by P. gigaswhich migrates throughout the LMB. It must be stressed, however, that c. 70% of theLMB catch (i.e. 1·8 Mt) is based upon whitefishes, which undertake long-distancemigrations up and downstream (Dugan, 2008), and which are not yet endangered.

In addition to the exceptional megafishes, several other Mekong fishes are cat-egorized as endangered by the IUCN: the endemic Mekong freshwater stingrayDasyatis laosensis Roberts & Karnasuta 1987 and Laotian (or Mekong) shad Ten-ualosa thibaudeaui (Durand 1940), as well as non-endemic Jullien’s golden carpProbarbus jullieni Sauvage 1880; they are either large, or migratory, or both. Tenu-alosa thibaudeaui is thought to be close to extinction (Blaber et al., 2003). AnotherMekong endemic, the small-scale croaker Boesemania microlepis (Bleeker 1858) hasbeen reduced to no more than 20% of previous stock levels in Laos, despite a lawprohibiting its capture during the breeding season or sale at any time of the year(Baird et al., 2001). Further evidence of fishing down the Mekong assemblage isevident from the Khone Falls in Laos where declines in the catches of long-distancemigrants such as P. jullieni and the thicklip barb Probarbus labeamajor Roberts1992, which both reach c. 70 kg and 1·5 m long, are accompanied by a shift tosmaller species (Baird, 2006). Wallago attu, another long-distance migrant, has alsodeclined due to overfishing but is more widely distributed across the region thanother Mekong species (Hossain et al., 2007).

Fisheries laws have been presented for some time in some LMB countries, butenforcement has often been weak (Sverdrup-Jensen, 2002), while relevant legislationwas only endorsed by the Laos National Assembly in 2009, and effort is needed toensure that it is adequately implemented (Phouthavongs, 2010a). Nonetheless, localfisher communities have responded to depletion of fishing stocks in parts of the LMBby establishing fishing control zones that protect spawning habitat (Baird et al., 2001)or dry-season refuges such as deep pools in the Lao section of the Mekong that sustainP. gigas (Poulsen et al., 2002b). Such measures may be more effective in protectingdepleted stocks of large fishes than legal restrictions on methods or gear type, whichare largely unenforceable (Baird et al., 2001; Sverdrup-Jensen, 2002). Communityfisheries are especially well developed in Cambodia: >450 have been establishedof which 236 have been formally registered with the government (Phouthavongs,2010b). While such local management schemes are potentially very beneficial, andhave been successful for some fisheries (Hilborn, 2007), by no means all community-based fisheries result in conservation gains. Some are directed towards maximizingfish yields (van Zalinge et al., 2004) and this tends to promote overexploitation oflarger species.

MEKONG MAINSTREAM DAMS

C U R R E N T S I T UAT I O N , G OV E R NA N C E A N D P L A N N E DD E V E L O P M E N T S

China has already dammed the mainstream of the upper Mekong or Lancang Jiangwith three dams completed (in 1995, 2003 and 2008), another under construction anda further four planned (Dudgeon, 2005a; Barlow et al., 2008; dams 1–8 in Fig. 6).


1506 D . D U D G E O N

In addition, there is a host of dams planned for the lower Mekong Basin (LMB; thesection of the Mekong downstream of China) with as many as 71 tributary damsoperating by 2030 (MRCS, 2011a). Eleven dams proposed for the mainstream in theLMB, 10 in Laos and one in Cambodia (dams 9–19 in Fig. 6), are the primary focusof attention in this review. These dams will have implications for the livelihoods of>60 million people that live in the LMB, including the inhabitants of countrieswith high levels of malnutrition (Cambodia) and food insecurity (Laos). Ten of the

1

3

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67

8

9 10

11

12

13 14

1516

1718

19

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400 km

Cambodia

Thailand

Laos

Vietnam

China

Myanmar

Fig. 6. Map of the Mekong–Lancang Jiang showing the location of existing or planned dams. 1, Gongguoqiao;2, Xiaowan; 3, Manwan; 4, Dachaoshan; 5, Nuozhadu; 6, Jinghong; 7, Ganlaba; 8, Mansong; 9, PakBeng; 10, Luang Prabang; 11, Xayaburi; 12, Pak Lay; 13, Sanakham; 14, Pak Chim; 15, Ban Koum;16, Lat Sua; 17, Don Sahong (at Khone Falls); 18, Stung Treng and 19, Sambor. Dams 1–8 are situatedon the Lancang Jiang. Estimates of total annual catch aggregated across all fish species in the threemigration systems in the lower Mekong Basin, based on Halls & Kshatriya (2009), are 60 000 t ( ),0·9–1·2 Mt ( ) and 1·0–1·3 Mt ( ). Note that other estimates of catch (Barlow et al., 2008; MRCS,2011a) differ slightly from these figures.



11 dams will span the entire mainstream, with the other at Don Sahong in Laosdamming one of several branches of the mainstream at the Khone Falls. Two of theLaotian dams will be joint ventures between Laos and Thailand, with the latter beingthe primary recipient of the electricity generated.

Plans for dams on the Mekong mainstream date back to the 1950s (Dudgeon,1992), but were stalled by regional conflicts and other constraints on development.It was not until four decades later that they appeared to be reaching fruition witha plan to construct 12 dams along the lower Mekong (Dudgeon, 2000). A range ofenvironmental concerns, especially those relating to the effects on migratory fishesand fisheries led to suspension of the projects in 2002, largely due to concernsraised by the Mekong River Commission (MRC), an intergovernmental organiza-tion established by the four riparian states (Cambodia, Laos, Thailand and Vietnam)under the Mekong Agreement on the Cooperation for Sustainable Development ofthe Mekong River Basin in 1995. The MRC represented an evolution of the MekongRiver Committee, itself a modified version of an organization established in 1957intended to co-ordinate water resource development among LMB states (Dudgeon,2003). The 1995 agreement mandated international cooperation ‘. . . in all fields ofsustainable development, utilization, management and conservation of the water andrelated resources of the Mekong River Basin’ (MRC, 2002). National representationfrom each country provides a basis for prior consultation over water resource devel-opments in the LMB, ensuring mutual notification about intended projects, followedby a process of review by MRC experts, offering the potential to achieve consensuson whether or not they would be beneficial for the region.

Rather than adopting a utilitarian approach to maximizing economic opportunitiespresented by hydropower, the MRC philosophy favours the broader perspective ofintegrated management. This was reflected in an overall Basin Development Plan pro-mulgated in 2002, intended ‘. . . to identify, categorise and prioritise the projects andprogrammes to be implemented at the basin level . . .’ (MRC, 2002). In accordancewith the stated objectives of the Basin Development Plan, in 2002 the MRC decidednot to pursue the long-standing proposal to build a 12 dam cascade on the Mekongmainstream (Dudgeon, 2003, 2005a). To further formalize the 1995 Mekong Agree-ment and enhance cooperation, the MRC introduced the Procedures for Notification,Prior Consultation and Agreement (PNPCA) in 2003, under which consequencesor transboundary effects arising from proposed projects can be evaluated by theMRC Joint Committee comprising representatives of the four riparian states (MRCS,2011a). This process must be completed before approval of a project can be givenby national regulating authorities. In addition, refinement of the Basin DevelopmentPlan led to the MRC Council approving a new integrated water resources manage-ment (IWRM)-based development strategy in December 2010 that constitutes theframework for co-ordinated implementation of sustainable development of the LMBas set out in the 2011–2015 MRC Strategic Plan (MRCS, 2011b).

The latest 11 dam scheme includes eight of the sites intended for the previous12 dam scheme that was abandoned in 2002. Of these, the 1260 MW Xayaburi (orSayaboury) Dam in Xayaboury Province, Laos, is closest to construction (dam 11 inFig. 6). It will be in the third position in a six dam cascade envisioned for northernLaos (MRCS 2011a; dams 9–14 in Fig. 6). The MRC Secretariat received the rele-vant consultation documents on the Xayaburi Dam from the Lao National MekongCommittee in September 2010, and the PNPCA report was published in March 2011


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(MRCS, 2011a). It was intended to inform the consultation process scheduled forcompletion by 22 April 2011, after which a decision on dam construction was to bemade by the Laotian authorities. The PNPCA considered the Xayaburi Dam withinthe context of the planned six dam cascade, a process that was facilitated by aStrategic Environmental Assessment (SEA) of all 11 mainstream dams undertakenby independent consultants commissioned by the MRC (ICEM, 2010). The primaryrecommendation of the SEA consultant team was a deferral of up to 10 years ofany decisions to proceed with mainstream dams, because of the scale and serious-ness of potential risks of hydropower development, including potential effects on thelivelihood of c. 2·1 million people living in the LMB (ICEM, 2010).

After consideration of the PNPCA report, national representatives on the MRCJoint Committee were unable to reach agreement. On 19 April 2011, they deferredany decision on the Xayaburi Dam by referring it to ministerial level, citing as reasonsthe potential for transboundary effects and Cambodian concerns over knowledge gapsthat needed further study (MRC, 2011). While the reprieve set an important precedentfor the development of other mainstream dams in the LMB, the Laos position is thatthe consultation process has been completed and dam construction should beginwithout delay. The national representatives also disagreed on the duration of thedeferral; Vietnam supported the 10 year time frame advocated in the SEA, whileLaos remains unwilling to extend consultation beyond 6 months (MRC, 2011).

One conclusion to draw from the resurgence of plans to build mainstream dams inthe LMB is that the MRC may have little ability to influence events in the riparianstates, presumably because prior consultation provides no guarantee of cooperationwhen national interests conflict with (and override) international relations. This mayyet prove to be the case with the Xayaburi Dam given the preference of Laos that theproject proceeds. Moreover, the fact that China, which has already constructed main-stream dams, is not a member of the MRC strengthens the impression that nationalinterests trump transboundary concerns (Dudgeon, 2005a). Dugan et al. (2010) reacha similar conclusion, finding no evidence that dam construction would not proceedas planned, their prognosis being that a large portion of Mekong fish production andits associated economic and social benefits would be lost. This accords with virtuallyall evaluations of the probable consequences of mainstream dam construction (Dud-geon, 2000; Barlow et al., 2008; Dugan, 2008; ICEM, 2010; MCRS, 2011a), withRoberts (2001a) designating such developments as ‘fluvicidal’. Nonetheless, the factremains that the MRC offers the best available framework to ensure conservationof migratory fishes in the Mekong (van Zalinge et al., 2004; Dudgeon, 2005a) and,thus far, has had sufficient influence to prevent construction of mainstream dams inthe LMB.

E F F E C T S O F DA M S O N F I S H E S A N D F I S H E R I E S

Adequate understanding of the potential effects of dams in the LMB depends onthe knowledge of patterns of fish migrations to spawning areas in the river. There arethree main upstream migration systems in the lower Mekong Basin (Fig. 6): a lowerzone below the Khone Falls, a zone upstream from the Khone Falls to Vientianeand a zone upstream of Vientiane where the six-dam cascade is planned (Poulsenet al., 2002a). As mentioned above, a substantial number of commercially valuablewhitefish species migrate longer distances, as do all five of the globally endangered



Mekong fishes. There is a considerable degree of interspecific variation in the timingof up and downstream migration (or larval drift), but individual species appear to becued by particular components of the annual flood cycle, such as rising water levels,with much of the upstream migration in the early wet season and least activityin the middle of the dry season (Poulsen et al., 2002a; MRCS, 2011a). Clearly,maintenance of the natural flood cycle and connectivity that allow unobstructedpassage along the river is essential for fish reproduction and hence a productivefishery (Barlow et al., 2008; Dugan, 2008).

At least 23 and probably >100 migratory fish species could be affected by thesix Laotian dams. Much of the effect would be associated with construction ofthe first dam at Xayaburi, and the associated transition from prevailing fast andseasonally diverse flow regimes to the limited water movement in a large reservoir.The six-dam cascade would convert c. 40% of the mainstream riverine habitat inthe LMB into a series of lacustrine water bodies, representing a loss of 90% of theupper migration system (MRCS, 2011a). The predicted fisheries loss to the basin-wide capture fishery due to the reduction in the area accessible to fishes migratingupstream would be c. 66 000 t or an overall, basin-wide reduction of c. 6% of theannual 2·5 Mt fishery yield (MRCS, 2011a). This is significant, but not as large asmight have been feared because the effects would be largely confined to those speciesin the upper Mekong migration system (Fig. 6). The middle and lower migrationsystems have much greater biomass (Poulsen et al., 2002a); although estimates varysomewhat (Barlow et al., 2008; Halls & Kshatriya, 2009; MRCS, 2011a), migratorybiomass in the upper system may be as little as 36 000 t compared to 950 000 tin the lower system. The loss of 66 000 t capture capacity, however, is equivalentto 73% of the estimated river–floodplain fishery yield for Laos, and would not becompensated by reservoir-based fisheries (Dugan, 2008), placing the livelihoods of anestimated 450 000 people at risk. To put this another way, if the effects of the reducedcapture fishery were fully absorbed by the 2 million people living in and around thedam cascade in northern Laos, the reduction in food security due to capture fishlosses could amount to 33 kg−1 individual−1 year−1 (MRCS, 2011a). The effectson biodiversity would also be considerable: all of the IUCN-listed Mekong fishesare long-distance migrants, and their movement among the three migration systemsalong the river would be blocked by the Laotian dams. Pangasianodon gigas wouldprobably become extinct since its only confirmed spawning site at Luang Prabang isupstream of all of the Laotian dam sites (MRCS, 2011a). The dams will also leadto loss of critical habitat in the river between Luang Prabang Xayaburi (dams 10and 11 in Fig. 6), i.e. the potential Xayaburi Reservoir, which contains a numberof deep pools that are key dry-season habitat for megafishes and many whitefishes;some species also rely on the pools for spawning (Poulsen et al., 2002b).

The Xayaburi Dam design provides for a fish pass, but its effectiveness forupstream migrants, and especially large species (>150 cm LT) such as the P. gigas, islikely to be low (MRCS, 2011a). There are significant knowledge gaps over the abil-ity of different migratory species to traverse a fish pass and travel upstream throughthe reservoir. The design of fish passes and ladders has mainly been intended fortemperate-zone salmonids, but Mekong fishes such as large pangasiids and cyprinidslack their jumping ability (Roberts, 2001a; MCRS, 2011b). Furthermore, the rangeof species, body sizes and biomass of migrants in the Mekong is much greater than intemperate rivers, with biomass of the order 100 times higher (Dugan, 2008; MRCS,


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2011a). Downstream migration of adults (which is not typical of Oncorhynchus spp.)includes the same diversity of species and sizes, plus eggs and larvae. Downstreampassage of large fishes through dam turbines would be especially problematic and are‘. . . predicted to be terminal . . .’ for exploited populations of P. gigas and P. jullieni‘. . . even if upstream migrations were completely unhindered’ (Halls & Kshatriya,2009). Reduced water velocities in the reservoir will also compromise the drift oflarvae (MCRS, 2011a), and may provide inadequate cues for adult migrants. Sucheffects cannot be mitigated and would result in cumulative species loss or a moredramatic decline, as documented for Pak Mun Dam on the Mekong’s largest tributaryin Thailand where the fishery collapsed following dam completion in 1994 (Roberts,2001b).

S E D I M E N T S E Q U E S T R AT I O N A N D OT H E R E F F E C T S

The area of the drainage basin upstream of the Manwan Dam in China (dam 3in Fig. 6) provides 45% of the total Mekong sediment load of c. 160 Mt annually(MRCS, 2011a), and the three Chinese mainstream dams already completed (dams3, 4 and 6 in Fig. 6) have reduced sediment loads in the LMB by 35–40% relativeto pre-dam conditions prior to 1993 (MRCS, 2011a), although other studies suggesta smaller reduction (Wang et al., 2011). This could rise to 45% by 2015 when theeight Lancang Jiang dams have been completed (MRCS, 2011a). A broadly similarconclusion is reached by Kummu et al. (2010) who estimated that these dams wouldtrap >50% of the total sediment load of the basin. Such projections are sensitive toassumptions about trapping efficiency, and they do not include the effects of tributarydams, nor climate and land-use changes. Factoring in the effects of the Laotian six-dam cascade and planned tributary dams (n = 71) results in a prediction that atleast 75% of the baseline sediment load would be trapped (MRCS, 2011a), with aconsiderable proportion of this settling out in the 100 km long Xayaburi Reservoir.The trapping of sediments and the nutrients bound to them has already led to anestimated 15–35% reduction in nutrient supply relative to the pre-dam condition;this is predicted to rise to 15–40% by 2015 and thence to 20–45% if the six Laotiandams are completed, with a worst-case scenario (all mainstream dams plus plannedtributary dams) envisaging a decline of up to 70% (MRCS, 2011a). Although thesecumulative effects are likely to be significant, the reductions in sediment flux andnutrient balance downstream of the Xayaburi Dam appear small compared to thoseattributable to the Lancang Jiang dams.

A related issue of major importance is that flow alterations in the lower Mekongdue to the Lancang Jiang dams are projected to result in dry-season increases0·2–0·6 m in the level of Tonle Sap floodplain lake while decreasing the extent ofwet-season inundation: flood duration would be reduced by 14 days, while the flood-plain area, total flood volume and amplitude would be reduced by 7–16% (Kummu& Sarrkula, 2008). A similar analysis by Campbell et al. (2006) envisaged a 10%decrease in the area inundated, and consequential reductions in gallery swamp for-est, lake productivity and fishery yields, with implications for over 1 million peoplewho depend on the lake’s resources (Sverdrup-Jensen, 2002). Although the addi-tional effects of the Xayaburi Dam on quantity and timing of river flows into TonleSap are expected to be very minor (MCRS, 2011a), none of these projections takeaccount of reduced sediment and nutrient loads on lake productivity even though



>70% of the Tonle Sap sediments (and the nutrients bound to them) are derivedfrom the river (Kummu et al., 2008).

There is also uncertainty about how sediment sequestration within mainstreamdams and consequent nutrient reduction will interact with the effects of dams asobstacles to migration and thereby influence fish biodiversity and fishery yieldsacross the basin (MRCS, 2011a). Their effects seem likely to be synergistic ratherthan merely additive. Other outstanding concerns relate to the effect of dams ondownstream water quality and the need to establish environmental flow regimes.While the barrier effects of dams are a major focus of the PNPCA report (MRCS,2011a), the downstream effects of altered flow or thermal regimes below dams willcertainly be significant. Flow variability is known to influence lotic biodiversity viaa number of interacting processes (Bunn & Arthington, 2002; Lytle & Poff, 2004),and regulation of flows may result in a loss of hydrographic cues for fish migrationor reproduction (Lytle & Poff, 2004) and reduce the intensity of annual fluvial dis-turbance needed to rejuvenate habitat (Bunn & Arthington, 2002; Poff et al., 2007).Thus, environmental flow regimes that mimic the variability inherent in the naturalflow regime will be essential for sustaining fishes and fisheries (Poff et al., 2007;Arthington et al., 2010), and a recent approach for establishing environmental flowcriteria shows considerable promise (Poff et al., 2010). While implementation ofsuch environmental flows, especially on large scales, can be problematic, adjustmentof dam operations can result in environmental gains in downstream reaches (Olden& Naiman, 2010).

C L I M AT E C H A N G E A N D E A S T A S I A N R I V E R SI N T H E A N T H RO P O C E N E

Signs of global climate change on rivers are evident from runoff records (Gedneyet al., 2006) and warmer water temperatures leading to shifts in geographic rangesor phenology of freshwater animals (Allan et al., 2005b; Heino et al., 2009). Therehas been almost no research, however, on the incidence or consequences of thesechanges in monsoonal east Asia, with the notable exception of a preliminary studyon Yangtze alligators Alligator sinensis (Zhang et al., 2009a). Fishes with narrowthermal tolerances are likely to be especially at risk (Poff et al., 2001; Allan et al.,2005b), but such data are not available for most Asian river fishes, perhaps becauseecophysiological research has been directed towards understanding their responses topollution and eutrophication. Although climate change scenarios predict that temper-ature increases in the tropics will be less than those further from the equator (IPCC,2007), the effects of any rises could be considerable because tropical ectotherms maybe closer to their upper tolerance limits (Deutsch et al., 2008).

Climate change has begun to affect rivers such as the Mekong and the Yangtze(He & Zhang, 2005; Yang et al., 2005; Xu et al., 2009). Between 1960 and 2000, forexample, mean annual temperatures rose at a rate of 0·01–0·04◦C at 12 stations alongthe Lancang River in the Yunnan Province. Significant changes in precipitation of3–7 mm year−1 were also detected, with some sites increasing and others decreasing,but there was a tendency for the most downstream sites (580–1300 m elevation) toexhibit the greatest temperature rises and declines in rainfall, and thus a highertendency to develop dry-season droughts (He & Zhang, 2005). In the LMB, greaterprecipitation during the early monsoon and an overall increase in runoff with a higher


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frequency of floods has been projected (Xu et al., 2009; Bezuijen, 2011), in generalagreement with earlier predictions that extreme flow events in the LMB will becomemore common (Dudgeon, 2000). Other projections for the LMB include higher meanannual temperatures and greater duration of warm periods, as well as an overallincrease in annual precipitation (and greater river flows), although the magnitude ofthis change will show marked spatial variation (Bezuijen, 2011). Monsoonal flowsin the Nujiang and Salween also are expected to increase, although annual dischargewill fall initially (until c. 2040) before exceeding present levels over the longer term(2070–2099; Xu et al., 2009).

Some ambiguity over the climatic future of rivers that originate in the Himalayashas arisen due to the Intergovernmental Panel on Climate Change (IPCC) projectionthat melt rates of Himalayan glaciers are so fast that there is high likelihood they willdisappear by 2035 (Cruz et al., 2007). While this particular forecast was incorrect(Khadka, 2009; Black, 2010), rapid retreats of glaciers in Tibet and Kashmir appearnonetheless to be underway (Khadka, 2009). Thus, Xu et al. (2009) predict that thearea of glaciers in upper Yangtze will have decreased 12% by 2050 when runoffattributable to glaciers and thawing permafrost in Yangtze headwaters will increaseby c. 29% (Anon, 2009). While melt water is less important than rainfall as a supplyfor the Yangtze, such projections suggest an immediate future of increasing flows,followed by a longer-term decline (Cruz et al., 2007). Flows have, in fact, beendeclining for some time, with an 8% decline in annual discharge over the period1865–2004, although this seems to have been caused by reservoir construction andincreased water consumption within the Yangtze Basin (Yang et al., 2005). Changesin rainfall have the potential to further alter Yangtze flows: Xu et al. (2009) envisagean increase in precipitation, extreme rainfall events and frequent floods but suggestthat, when combined with the effects of glacial melt, total runoff will not alter greatly.The projected frequency of heavy rain events along the Yangtze, however, couldincrease by 31–57% at the end of the century, and will have greater intensity than1980–1999 (Chen et al., 2010). Lower dry-season water levels are also anticipated(Anon, 2009). Heavy rainfall events will increase at faster rates than increases intotal precipitation, indicating that the Yangtze, already a flood-prone region, willexperience greater flooding, and this prediction is consistent among the various IPCCscenarios (Chen et al., 2010). Data from 147 monitoring stations across the YangtzeBasin indicate that their mean annual temperature rose by 0·33◦C during the 1990s,and the rate of annual increase reached 0·71◦C between 2001 and 2005 (Anon,2009). Such changes may be affecting fish migration pathways and leading to earlierinitiation of spawning (Anon, 2009).

While there is much uncertainty over the precise effects of climate change onAsian river fishes, increasing deviations from the natural flow regime to which theseanimals are adapted, and upon which they depend, will add to any thermal stressesimposed by gradual warming. Extreme rainfall events, whether they give rise tofloods or droughts, will inevitably lead to new water-engineering projects and con-structions of dams for water storage and flood control intended to enhance humanwater security. A greater or lesser degree of future transformation of rivers thusseems inevitable. Engineering solutions associated with climate change adaptation(Palmer et al., 2008) will be especially rapid and extensive in the densely populatedYangtze Basin, where both the pressure to protect cities and associated infrastructureand the availability of resources that can be deployed to those ends will be far greater



than in the LMB. Greater societal and governmental awareness of the importance ofriver fishes may help to ensure that they continue to be afforded some considerationwhen decisions about Mekong development or climate change adaptation are made.This will assuredly not be the case, however, for Yangtze fishes where the combi-nation of overexploitation, pollution and habitat degradation, as well as dams andwater-engineering projects built for hydropower production, protection from extremeflow events and to facilitate navigation and shipping, are leading to perfect stormconditions (i.e. where the combination of threat factors create great hazard) that willirreversibly compromise riverine biodiversity. It may already be too late to save theYangtze megafishes.

WHAT ARE THE PROSPECTS?

Ongoing construction of mainstream dams along the Jinsha River limits con-servation opportunities to protection of those fishes that can persist in tributarieswithin the upper Yangtze Basin. Under these circumstances, mitigation of potentialeffects arising from tributary dams could involve some or all of the following options(Richter et al., 2010): avoid siting dams at the downstream end of tributaries becausethis blocks upstream migration, locate dams upstream of major fish spawning sites,restrict dam construction to those tributaries that are dammed already, ensure that atleast one branch within a tributary network remains undammed so that fishes haveaccess to headwaters, and establish and operate environmental flows regimes belowdams. Even these options may be highly constrained: >1000 hydropower dams ofvarious sizes have been built or are under construction along the upper Yangtze andits tributaries, and more are planned, c. 100 of them exceeding 1 GW generatingcapacity (Beitarie, 2011; He et al., 2011; see also Fig. 2).

Elsewhere along the Yangtze, some limited conservation opportunities may stillbe available. Extensive wetland restoration has been undertaken in parts of the lowerYangtze to increase the river’s flood mitigation capacity (Pittock & Xu, 2010), andthis has benefits for fish populations as it increases their access to floodplain spawningand feeding sites. This soft-path approach, however, is unlikely to replace hard-pathengineering solutions to flood control in densely populated areas, especially in thecontext of the extremes of flow envisaged under IPCC scenarios (Chen et al., 2010).Furthermore, the costs of floods in terms of human misery provide the impetus forbuilding protective dykes and barriers; for instance, the 1998 Yangtze flood killedover 4000 people and inflicted economic losses (Pittock & Xu, 2010).

Establishment of captive breeding populations of Acipenser spp. and P. gladiusfrom the Yangtze as an insurance policy of ex situ conservation has been advocatedrepeatedly (Wang et al., 1998; Wei et al., 2004; Zhang et al., 2009b), and was alsoproposed for the Yangtze dolphin or baiji Lipotes vexillifer (Dudgeon, 2005b). Giventhe failure to implement those ex situ conservation measures, it might be concludedthat P. gladius and A. dabryanus will follow L. vexillifer into extinction. There is stillsome scope, albeit limited, for in situ conservation of A. sinensis, but it cannot relysolely on artificial propagation and will necessitate protection of spawning sites in theriver. Ban et al. (2011) have employed the widely used in-stream flow incrementalmethod (Tharme, 2003) to develop environmental flow recommendations to sustainthe very limited area of spawning habitat of the A. sinensis that remains downstream


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of the Gezhouba Dam. Completion of that dam blocked upstream migrations and thesituation was exacerbated in 2003 when the TGD began to cause downstream reduc-tions (up to 40%) in flow at Gezhouba, thereby degrading the quality of spawninggrounds. Applying the environmental flow recommendations, even on a trial basis,would require adjustment of water releases from both dams during the A. sinensisbreeding season followed by further modification based on the outcome of monitor-ing spawning success (Ban et al., 2011). Implementation of fish-friendly controlledwater releases from the world’s largest operating hydropower scheme, even on atrial basis, would be a major conservation achievement. In the unlikely event thatan environmental flow regime could be brought about there is no guarantee that itwould be successful, as the scale and complexity of the Yangtze, combined withthe long life span and small remnant population of A. sinensis, limit the odds ofreversing its decline.

As for the Mekong, current best evidence suggests that there will be a signifi-cant loss of fishes and fisheries if hydropower development on the LMB mainstreamproceeds. While the basin institutions and communities could, in theory, adapt suc-cessfully to the loss of the benefits currently derived from the productive Mekongfisheries, this would require diversification of national economies in Cambodia andLaos; in particular for the Laotian economy, which is being driven by the export ofelectricity to Thailand. Such adaptation will be especially difficult in the Mekonggiven the limited capability of the MRC and national institutions to implement inte-grated development within the basin as a whole, notwithstanding the lofty aspirationsof the Basin Development Plan (MRCS, 2011b). Furthermore, the adaptive potentialof impoverished fisher communities seems low without prior investment in alterna-tive livelihoods (Dugan et al., 2010; MRCS, 2011a). Stake-holder involvement in,or influence on, decision-making is also likely to be rather limited, and is likely togive rise to a situation where the effects of hydropower development such as col-lapsing fish stocks, habitat degradation and inundation of farmland and forest arefelt locally (Roberts, 2001b; Richter et al., 2010). By contrast, most of the benefitsaccruing from dams, such as power generation and associated industrial or economicdevelopment, as well as enhanced navigation and flood protection for people liv-ing in low-lying areas, are felt some distance away, especially in towns and cities.The disparity between local adverse effects and regional benefits (which is certainlynot confined to the LMB) creates conflicts between rural and urban dwellers thatare usually settled in favour of the latter because they live closer to centres ofpolitical power and are able to become more involved in whatever public consulta-tion may take place (Dudgeon, 2005a). Obviously, conflicts between parties withinnational boundaries have parallels with the international transboundary disputes thatcan arise when upstream states (such as China) take unilateral action to build damswith consequences felt downstream by stakeholders who were not involved in thedecision-making process.

WHAT CAN BE DONE?

If no further dams are built on the Yangtze or Mekong, then river fishes will stillhave to cope with global climate change as the Anthropocene unfolds. Scope foradaptation may appear limited, but fishes could, conceivably, adjust to rising water



temperatures by making compensatory movements upstream to higher elevations ornorthwards (Dudgeon, 2007; Heino et al., 2009). They may be constrained, however,by river topography (e.g. an east-west direction of flow, as in the Yangtze, rather thana north-south), the presence of dams or other in-stream barriers, dispersal through aterrestrial landscape, availability of suitable habitats or some combination of these(Dudgeon, 2007). Translocation or assisted migration of species at risk is one possible(Dudgeon, 2007; Olden et al., 2010), albeit controversial (Ricciardi & Simberloff,2009; Schwartz et al., 2009), solution that warrants further consideration, although itis less likely to be applicable to megafishes than to smaller, non-migratory species.The option of doing nothing in case the translocated species causes harm at thenew site may, in this instance, be unwise. Doing nothing equates with neither firstdo no harm nor the precautionary principle in a world where temperature risesare unavoidable (Schwartz et al., 2009), as existing habitats will become unsuitableunless thermal adaptation by fishes proceeds quickly enough to compensate for fitnesslosses associated with warmer conditions.

The primary rationale for dams in China, the Mekong and elsewhere is hydropowergeneration to fuel economic development and, more incidentally, reduce dependenceon fossil carbon. Dams within China or dams that export power to China fromneighbouring countries could help to reduce carbon emissions which would other-wise result from burning China’s huge reserves of sulphur-rich coal. While thesehydropower projects will help to meet non-fossil energy targets and increase carbonefficiency (Stanway, 2011), total carbon emissions will not decline because coal usewill need to increase if China’s economic growth is to be maintained at or near itscurrent pace (probably c. 7%), a stated goal of the 12th Five-Year Plan. Further-more, any environmental benefit will also need to be offset against the effect onfishes which seems likely to include an extinction debt that will grow rapidly. Chinaseems certain to lose some of its iconic freshwater species in the near future, as wellas the remaining ecological functionality to be derived from viable stocks of riverfishes. One such iconic species was L. vexillifer, confirmed as the first human-causedcetacean extinction in 2007 (Turvey et al., 2007), and it might be reasonably con-cluded that many of the factors leading to the demise of this animal might be equallydeleterious to river megafishes. There is little or no evidence that conservation offishes is being factored into decisions about dam construction, indeed the example ofthe Xiaonanhai Dam suggests that such considerations are being set aside or overrid-den. The contrary situation prevails in the LMB, at least for now, as the postponementof the Xayaburi Dam demonstrates, but this collaborative decision throws a harshlight on unilateral Chinese action in building dams on the Lancang–Mekong despitetheir potential implications for downstream nations. Interestingly, the multinationalnature of the MRC, which might have been anticipated to result in conflicts thathamper sustainable development, has achieved the opposite outcome of a decisionthat, for now at least, protects LMB fisheries. And the benefit of a centrally plannedeconomy and a government that speaks with a single voice, as in China, has failedto bring about integrated development of the resources of the Yangtze, resulting in aparlous ecological situation (Dudgeon, 2010). The political circumstances along theNujiang are somewhat different from the Mekong, given that only one downstreamstate (which also intends to build several dams) will be affected, but the ecologicalconsequences of a dam cascade on that river, in qualitative terms, will be much thesame as on the Mekong, Jinsha or any other large river in the region.


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There are many possible explanations as to why the Chinese government has failedto adequately consider the consequences of dam building on river fishes, whereas thisconsideration carries more weight in the LMB. One reason for this is that Chineseriver fisheries have been in decline for several decades, in contrast to the relativelyhealthy and very important LMB fishery, and that citizens and decision makers havefallen victim to the shifting baseline syndrome (Humphries & Winemiller, 2009),i.e. the current or recent condition of the river is assumed to be its natural state,and the formerly more bountiful, healthier river has been forgotten or was neverdocumented. In other words, a false impression arises if conditions in the immediatepast are thought to reflect the intermediate and distant past. Interviews with fishercommunities along the lower Yangtze have revealed a marked decrease in localknowledge about megafishes, with P. gladius unknown to >70% of all informantsbelow the age of 40 years and to those who began fishing after 1995 (Turvey et al.,2010). Less charismatic species that had formerly been economically important, suchas T. reevesii, were also subject to rapid baseline shift, but even large species werequickly forgotten by local communities when they were no longer encountered on a‘ . . . fairly regular basis’ (Turvey et al., 2010). It remains to be seen whether fishercommunities in the LMB have likewise experienced a baseline shift following thecollapse of the fishery for P. gigas decades ago, and the virtual disappearance ofthis megafish from the river.

An overriding and urgent priority for scientists is to convey citizens and their gov-ernments that holistic river management and ecosystem restoration or rehabilitationwill benefit humans through the provision of fisheries resources, as well as the cleanwater upon which fishes and humans depend. This message has gained some tractionin the LMB, and is far more likely to lead to a consensus on policies and associ-ated legislation that will produce conservation gains than an approach that stressesthe intrinsic values inherent in genes, species and natural communities. A furtherobjective must be fostering the work of organizations such as the MRC that arecharged with implementation of management and conservation of rivers and fishesas, to a large extent, it is they, not the general public nor government ministers andpolicy-makers, who determine what beneficial changes are made and the applicationand enforcement of legislation. In other words, as well as communicating the resultsof their science (of discovery) in the usual way to inform conservation, scientistsmust attempt to build community consensus on what should be done, and providerelevant information (that can be used) to ensure that it will be done. It is hard tothink of anywhere else in the world where such efforts to conserve river fishes areneeded more urgently than in Anthropocene Asia.

I am grateful to R. Gozlan for prompting me to write this review, M. Bezuijen for dis-cussions on climate change, K. Sloman, D. Allan and an anonymous reviewer for helpfulcomments on the manuscript and L. C. Y. Ng for assistance with map production and graphics.

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