5. COMPARISON OF SWORDFISH FISHERIESdata.daff.gov.au/.../data/12908_05-swordfishreport.pdf · swordfish and, in 1998, ICCAT began to incorporate sex-ratio at size information into

Comparison

Swordfish fisheries 153

5. COMPARISON OF SWORDFISH FISHERIESThe fisheries

Fishery developmentThe established swordfish fisheries1 started as nearshore subsistence activities thousands ofyears ago. They involved harpooning large, predominantly female swordfish as they werebasking or finning at the sea surface in subtropical regions. Anecdotal reports suggest thatswordfish were much more abundant in those early years than they are in many areas now.Australia and other developing swordfish fisheries do not have an early history of harpoon ordriftnet fishing.

In the 1950s Japan developed a distant-water longliner fleet. Swordfish have commercialmarkets in Japan, but it is less valuable than the tuna that they have targeted since the mid-1960s for sashimi markets. Apart from targeting in the north-western Pacific, swordfish areusually an incidental catch of distant-water longliners fishing for yellowfin and bigeye and, toa lesser extent, southern bluefin and northern bluefin tuna.

In the early 1960s a variety of longline fleets were established to target swordfish in theMediterranean Sea, north-eastern Atlantic and north-western Atlantic. Yet, it was not until themid-1980s that fishers realised the potential for small (10–30 m) longliners to make shortfishing trips, store tuna and swordfish on ice and airfreight the product to distant markets. Bydeploying shorter sets than distant-water longliners, the fresh-chill longliners were better ableto target regions where tuna and swordfish aggregate, such as at the boundaries of watermasses. They further improved their swordfish catch rates through refinements to fishinggear, such as the introduction of lightsticks and monofilament longlines. Fresh-chill longlinefleets were quickly established in many of the world’s ports during the late 1980s and early1990s. Swordfish are a high value catch and represent an increasingly important source ofrevenue to many coastal nations in the Indian Ocean and South Pacific, e.g., La Réunion,Seychelles and Fiji.

In the mid-1980s subsistence fishers in several areas (e.g., Chile) upgraded their fishing gearto monofilament driftnets, which had become available at attractive prices and weresubsidised by some governments. Pelagic driftnets were more efficient than harpoons atcatching swordfish. But, like harpoons, driftnets tended to take large adult swordfish ratherthan juveniles (<120 cm or <15 kg). Harpooning has declined as a result of increasing labourcosts and an apparent decline in the abundance of large, basking swordfish. In 1992 theUnited Nations placed a moratorium on the use of large-scale pelagic driftnets in internationalwaters. Consequently, longline has become the dominant fishing method for swordfish inmost areas.

CatchesCatch trendsThe established swordfish fisheries show the classic development pattern described byHilborn and Walters (1992, pp. 6–7) for uncontrolled fisheries: a period of rapid growthfollows the discovery of a valuable resource, with catches reaching a peak then decliningrapidly as sustainable catch levels are exceeded (Figure 32). A period of recovery may follow,as fishers adjust the scale of their operations to the catch levels that the stock can withstand. Akey hindrance to fishery management is the lag between fishery indicators showing aproblem, scientific advice demonstrating that problem and effective management response.

1 We refer to swordfish fisheries of the Mediterranean, North Atlantic, Chile, California, Hawaii and north-

western Pacific as ‘established fisheries’–fisheries that have passed an historical peak in catch levels. Theterm ‘developing fisheries’ refers to swordfish fisheries that were established in other areas (e.g., Australiaand the Seychelles) during the 1990s.

Bureau of Rural Sciences154

Our review highlights the ability of longline fisheries, unfettered by management regulations,to expand swordfish catches rapidly to high levels:

• the annual catch of swordfish in the North Atlantic expanded from 12 000 t to 21 000 twithin nine years;

• the South Atlantic catch expanded from less than 5000 t to 21 000 t within 15 years;

• the Mediterranean catch increased by 15 000 t over five years to peak at almost 20 000 t;

• in less than five years, Chile’s catch expanded from less than 500 t per year to peak atmore than 7000 t; and

• over three years Hawaii’s swordfish catch grew from less than 300 t to almost 6000 t.

Apart from Hawaii’s fishery, fishing effort continued to expand in each of those fisheries forseveral years after the peak in catches, highlighting the inability of fishery management toaddress growing overcapacity in the fishing fleets. Although swordfish catches peaked thendeclined, for only three of the five fisheries is there clear evidence that they have been fishedat unsustainable levels (see p. 166).

Figure 32. Phases of development of uncontrolled fisheries (Hilborn and Walters 1992, p. 7).

Comparison


Table 13. Comparison of levels of fishing effort, swordfish catch, longline catch rates and average size foreach fishery examined.

Fishery Longline effort (million hooks) Annual landed catch of swordfish (’000 t) Nominallongline catch rate

(swordfish /1000 hooks) Longline average size of swordfish (kg)

peak current peak current initial peak current initial peak current

North Atlantic ? ? ?20

north-west Pacific

Chile

Hawaii

eastern Australia (domestic)


Broad comparisons can be made between the established fisheries and developing fisheries,such as the south-west Pacific (Australia) and South Africa. Current catches in the south-westPacific (2000–4000 t per year) are smaller than the peak catches reached in the establishedfisheries (6000–20 000 t; Table 2). Nevertheless, developing fisheries show a similar trend ofrapid expansion in catches and effort.

After peaking, swordfish catches in each of the established fisheries declined by43–79 per cent (Table 2). Declines in catches after the peak have been gradual, rather thansudden, as observed in some collapsing fisheries, e.g., Peru’s anchovy fishery. The gradualdecline in swordfish fisheries might reflect the wide number of age classes exploited bylongliners and the ability of longliners to maintain catch levels by modifying their operations(e.g., by moving to other areas). For example, Spain’s fleet expanded during the 1980s to apoint where many longliners were forced to move to distant areas or to switch to targetingother species (e.g., shark and tuna). Fishery catch and effort data, aggregated over largegeographical areas, would not provide warning signals of such declines.

Within the bounds set by biological production, market demand has had an importantinfluence on expansions and contractions in swordfish catches. Two major markets forswordfish, Europe and the United States, determine global prices. Catch levels declined in the1970s in response to the imposition of mercury restrictions in the United States, thenincreased when the United States eased those restrictions. In 1998 swordfish prices fell inresponse to a restaurant boycott and oversupply of the United States market (Kronman 1999,p. 14).

It is not surprising, then, that catch levels in the established fisheries do not provide a guide topotential catch levels for developing swordfish fisheries. Trends in annual catches in theestablished fisheries do, however, highlight the ability of commercial fisheries to rapidlyexpand swordfish catches and to create potential problems with fleet overcapacity.

Size and sex compositionSwordfish have complex geographical distributions, depending on size and sex. Analyses ofsex-ratio and size data indicate three types of behavioural patterns in the Atlantic: feeding,spawning and transitional (Dr J. Porter, 21 June 2000). Medium and large-sized swordfish(>25 kg) tend to dominate longline catches in temperate waters (south of 35°S and north of35°N). In warmer waters, smaller swordfish are more common, but large swordfish are takenthere too. The introduction of longline fishing gear and subsequent expansion of fisheries tolow latitudes resulted in an increased representation of juvenile swordfish in catches.

There is a marked difference in the growth rates of male and female swordfish (‘sexualdimorphism’). After about two or three years of age, females grow faster than males (p. 10).In most longline catches, there are equal proportions of male and female swordfish acrosssmall sizes, but female swordfish predominate amongst swordfish larger than about 120 cm(~15 kg). This implies that males either grow more slowly or have higher natural mortalityrates or are less vulnerable to fishing. There is no evidence of sex change in swordfish.

Except for the Hawaii-based fishery, the established swordfish fisheries initially involvedsurface fishing methods (harpoon and driftnet), followed by the introduction of longlinefishing gear and subsequent geographical expansion. Associated with those changes, wewould expect the sex composition of catches to shift from predominantly female swordfish tomore equal proportions of male and female swordfish. The shift from mature femaleswordfish is likely to have important implications for the effects of fishing on the stock.However, elucidating the effects of differential distributions based on sex and behaviour iscomplex. Long time series of sex composition data are rare, partly because of a lack ofobserver data spanning fishery development. Extensive data are available for Atlanticswordfish and, in 1998, ICCAT began to incorporate sex-ratio at size information into its age-structured population analysis (Dr J. Porter, 21 June 2000).

The size composition of swordfish caught by driftnet is different to that of longline catches. InChile, for example, driftnet-caught swordfish average 122 kg whereas those caught by

Comparison


longline in the same area are smaller (75 kg on average). The driftnets have a large mesh size(~50 cm stretched). Swordfish have rostrums that easily become entangled in driftnets and wewould therefore expect driftnets to take a wide size range of swordfish. One explanation forthe different selectivity of the two fishing gears is that the vertical distribution of swordfishmay depend on individual size; small swordfish might rarely move to near the sea surfacewhere they would be vulnerable to surface-set driftnets. Another hypothesis is that longlinebaits are less attractive to large swordfish.

Species associations The catch composition of swordfish fisheries suggests distinct associations between swordfishand a set of pelagic species. Blue shark are often the dominant species in the catches oflongliners targeting swordfish in subtropical and temperate waters, e.g., Hawaii and the NorthAtlantic. The established fisheries have a relatively small bycatch of tuna. Australia’s fisherymay be unique in having relatively low catch rates of swordfish with relatively high catchrates of bigeye.

Seabird, shark, killer whale and false killer whale interaction tends to be fishing gear and areaspecific and is not unique to swordfish fisheries. Shark and killer whale can be a problem tolongline operations when they damage or remove fish hooked on the line. False killer whaleare a problem to longliners targeting swordfish in tropical areas (e.g., the Seychelles),whereas killer whale are a problem in some temperate and subtropical areas, e.g, Tasmania(Australia). Seabird are also a concern at those higher latitudes.

Distribution Swordfish are broadly distributed, between about 50°N and 50°S in all the world’s oceans andmost seas. The waters they inhabit exhibit a wide range of oceanographic conditions,e.g., temperature ranges of 5–27°C and dissolved oxygen likely to be as low as 1.0 mg/l. Inaddition to a broad distribution, swordfish make daily migrations to great depths, from nearthe sea surface at night down to depths of more than 1000 m during the day. Swordfishprobably use the cover of darkness and their superior vision to locate and catch prey.Observations and preliminary results of sonic tracking suggest that depth distribution andbasking behaviour vary between areas (Anonymous 1977, p. 3).

Swordfish are caught by longline fishing gear within 120 m of the sea surface at night. Theyare less commonly caught by longliners that target tuna by setting fish baits deep (250–400 m)during the day. This might be because swordfish inhabit deeper depths during the day thanthose reached by tuna longlines or because they are inactive or not feeding during the day.

Swordfish catches show strong lunar patterns in most fisheries, with the highest longline catchrates usually reported during full moon periods. Yet, this observation would seem tocontradict the notion that swordfish are caught by shallow-set longlines at night when theymove near to the sea surface to use darkness to feed. We would expect catch rates to behighest during dark (no moon) periods. Swordfish might be too close to the surface to becaught by shallow-set longline gear during no moon periods. During full moons, they areslightly deeper and therefore vulnerable to shallow-set longlines. Indeed, anglers who fish awide depth range when targeting swordfish report that there is no clear lunar pattern to theircatches (Pepperell 2000, p. 56).

We distinguish two types of swordfish fisheries:

• convergence fisheries, which are broadly distributed and associated with currents andfronts in the open ocean; and

• topographic fisheries, which are associated with seabed features, such as continentalslopes, banks and seamounts.

Examples of topographic fisheries are the Florida and Hawaii-based fisheries when they wereinitially based on banks and Australia’s fishery when it was concentrated on seamounts. TheNorth Pacific and South Atlantic swordfish fisheries are convergence fisheries. Chile’slongline fishery is a convergence fishery, whereas its artisanal fishery is more a topographic


fishery because of its limited coastal focus. Similarly, the North Atlantic fishery involvesactivities both over shelving banks off north-eastern North America and broader convergencezones.

Chile and the Mediterranean Seapresent contradictory evidence of thesignificance of ocean productivity indetermining the size of swordfishfisheries. The relatively smallMediterranean Sea has supportedannual swordfish catches of 12 000 – 20 000 t since 1988. Some believethat extensive areas and prolongedseasons for spawning are responsiblefor the large catches there. TheMediterranean’s productivity mightalso be due to high oceanproductivity, driven by the upwellingof warm (12–13ºC) nutrient-rich,deep waters.

Yet, Chile contradicts thegeneralisation that swordfish will bemost abundant in areas of high oceanproductivity. Chile’s exclusiveeconomic zone would seem to be anideal habitat for swordfish. It has a narrow continental shelf with many bathymetric featuresof the kind with which swordfish often associate. Coastal upwelling and the nutrient-rich PeruCurrent create what is one of the world’s most productive pelagic fisheries. However, thosewaters supported an average catch of only 886 t per year and declines in Chile’s longlinecatch and the demise of Chile’s artisanal fishery followed a peak catch of 7225 t of swordfish.There are similarities with California too, where ocean productivity is high but swordfishcatch levels are relatively low.

The reason for the decline in Chile’s swordfish catches is unclear. It might have been theresult of local depletion, stock-wide overfishing, long-term climatic shifts (the El Niño eventof the early 1990s) or massive increases in Chile’s jack mackerel catch.

Pelagic production In the open ocean, upwelling and the divergence ofcurrents bring nutrient-rich water near to the seasurface. Depending on sunlight, temperature and theavailability of trace elements, nutrient-rich water cansupport the rapid growth of microscopic plant life(‘phytoplankton’). The phytoplankton in turn supportmicroscopic animals (‘zooplankton’) and largeranimals, such as squid and small pelagic fish. In turn,squid and small pelagic fish attract higher predators,like tuna and swordfish. Zooplankton and small animals are not strongswimmers for prolonged periods; they are at themercy of ocean currents. Converging currents andeddies are critical in concentrating plankton for higherpredators to efficiently find and harvest their prey.Oceanic fronts represent boundaries between watermasses. Such fronts may be indicated by sharpgradients in water temperature, ocean colour orsalinity.

Comparison


Australia’s waters are far less productive than those supporting other swordfish fisheries. Yet,our assessment of the established swordfish fisheries suggests caution on drawing conclusionson how gross productivity might influence swordfish catch levels in Australia.

Deep-sea bathymetry and oceanographic conditions suggest that there may be lightly fished orunexploited resources of swordfish in several locations of the Southern Hemisphere:

• the south-eastern Indian Ocean;

• the southern Tasman Sea;

• across the subtropical convergence zone of the South Pacific; and

• associated with seamounts and banks south of French Polynesia, Fiji and Tonga (20–35ºS).

A warm ‘western boundary current’ (the Kuroshio) extends eastwards across the NorthPacific interacting with westward flowing currents to create zones of divergence andconvergence (Pickard 1979, p. 184). New Zealand’s North Island blocks the eastwardsextension of the South Pacific’s western boundary current (the East Australian Current).Consequently, the subtropical convergence zone is not as productive there, e.g., about40 000 t of albacore are caught in the South Pacific each year compared to about 72 000 t peryear in the North Pacific (SCTB 1999, p. 73). Bathymetry, oceanography and productivityprovide only a very general guide to potential swordfish fisheries. Commercial feasibilityfishing, involving fishers experienced in catching swordfish, would be required to evaluatethe resources of those unfished areas.

Gear and targetingFleets

Around the world, a wide variety of boat types target swordfish, with many small (<15 m)boats fishing opportunistically or seasonally. The average size of boats has increased, with20–25 m purpose-built, steel longliners becoming more common in several swordfishfisheries, e.g., Australia and Hawaii. Those purpose-built longliners are larger than fresh-chilllongliners designed to target sashimi tuna. Swordfish fishing can involve longer trips becauseswordfish have good storage characteristics and their prices are less sensitive to productquality. Unit prices of swordfish, however, are generally lower than sashimi tuna prices.

The development of longline fisheries for swordfish exhibits a pattern where the risk-takers inthe fleet move further offshore, taking initially high catch rates. Other operators, attracted byreports of high catch rates, follow the risk-takers offshore. Swordfish fisheries consequentlyexpand, with longliners ranging further offshore for longer trips under more extreme weatherconditions. Hawaii-based and many North America-based longliners, for example, regularlyundertake fishing trips of more than 30 days’ duration, venturing 1000–2000 nm from theirhome port. They average 246 sea days per year. This shows the potential for increases ineffort in developing fisheries, such as Australia’s.

Another feature of swordfish fisheries is the ability of longliners to switch to target otherspecies (e.g., sashimi tuna) or to relocate to distant ports in response to declining prices, poorcatch rates or the establishment of management regulations. Many of Spain’s longliners, forexample, started to target tuna or shark or relocated to the South Atlantic when swordfishcatch rates declined in the North Atlantic.

Fish-finding In topographic fisheries, oceanographic conditions seem to be less critical for fisherssearching for high concentrations of swordfish. Australia’s longliners, for example, operateover a wide range of sea surface temperature, from about 20 to 27°C. In those fisheries,fishers focus on a few local areas and do not move seasonally as oceanographic conditionschange. As a result, fishing activity and catches vary seasonally in topographic fisheries.Convergence fisheries usually involve medium-sized longliners tracking swordfish over long


distances as oceanographic conditions vary with latitude, month and the position and strengthof currents.

To improve the efficiency of fishing operations, many fleets have installed sophisticatedelectronic equipment. Fishing power (and presumably catch rate) is expected to increase indeveloping fisheries as longliners continue to:

• gain experience in swordfish fishing;

• install new technology (e.g., access to real-time maps of remotely sensed ocean colour);and

• improve searching efficiency by forming communication links between boats.

Fishing gear and practices During the late 1980s many fishing fleets adopted monofilament nylon longline gear to targetswordfish. Monofilament seems to produce higher catch rates of most target species,including swordfish (and it does not reduce bycatch; Dr J. Porter, 21 June 2000).Furthermore, monofilament has several advantages over rope gear in terms of ease ofdeployment and retrieval (the monofilament mainline is stored on a hydraulic drum). Most ofJapan’s distant-water longliners in the North Atlantic use monofilament gear (Dr J. Porter,21 June 2000). However, many operating in the Pacific reverted to traditional rope gear,reporting problems with durability and in handling monofilament gear.

Longlines take a much wider size range of swordfish than taken by harpoon and driftnetfishing gear. Longliners can operate under more extreme weather conditions and have alonger range than the subsistence operations.

The longline fishing gear used for swordfish is identical to that used by fresh-chill longlinersto target sashimi tuna. The maindifferences are the attachment oflightsticks to branchlines andfishing practices (bait, depth andtime of set, described below).

Most longliners use squid baitswith lightsticks, set shallow(depth range of 20–90 m) at nightto target swordfish. Longliningfor swordfish is a moretechnologically advanced andcostly fishing operation thanharpoon or driftnet fishing and itis more expensive than longliningfor tuna. Most longlinerstargeting swordfish deployhundreds or thousands ofexpensive squid baits andlightsticks each night. Thosepractices, which were widelyadopted in the late 1980s,probably resulted in significantimprovements in swordfish catch rates. Campbell (1998, p. 4), for instance, reports thatAustralia’s longliners had about seven or eight times the catch rates of swordfish as Japan’sdistant-water longliners fishing the same one-degree squares during June–August 1997.

Longliners in the Mediterranean and Japan’s longliners in the North Pacific differ from otherfleets in the use of lightsticks and type of bait. They have opted for lower-cost operations bynot using lightsticks and, in the case of several Mediterranean fleets, using cheaper baits, suchas mackerel.

Attached to branchlines, luminescent lightsticks revolutionisedlongline fishing for swordfish.

Comparison


Killer whale and shark often damage or remove fish hooked on longlines. Such damagelowers the value of the swordfish and shark catches reduce the number of baited hooksavailable for target species. There is at present no effective deterrent to killer whale and shark.Longliners will often move large distances from areas where killer whale or shark damage ishigh in relation to swordfish catch rates. In some fisheries (e.g., Japan’s longline), wireleaders are occasionally used on shallow hooks to reduce gear loss to shark. The sharp teethor abrasive skin of shark sometimes cut through monofilament leaders, whereas wire leadersallow shark to be landed so that hooks and leaders are not lost.

AssessmentData collection

Current approaches to fish stock assessment require a detailed breakdown of catches bylocation and size or, preferably, age. For swordfish, assessments also need information on sexbecause female swordfish grow faster than males and the size and sex composition ofswordfish catches varies considerably between areas and seasonally. Consequently, swordfishstock assessments should use size and sex composition data that take into account area andseasonal effects (DeMartini 1999).

Our review revealed a wide range in the quality of fishery data available for swordfishfisheries. Most rely on programs that sample catches when they are landed at ports. Port-based sampling was the sole source of fishery data for several Mediterranean fisheries,e.g., Morocco’s fishery. However, port-sampling cannot collect reliable data on the levels offishing effort or the location and timing of fishing activities. In particular, it is inadequate forswordfish fisheries because swordfish are processed at sea and the viscera discarded.Consequently, sex cannot be determined when the swordfish is landed. However, there ispotential for biochemical tests for certain hormones to be developed for sexing swordfishlandings (DeMartini 1999, pp. 165–67).

There is also uncertainty about the processing of catch data by distant-water fishing nations,such as Japan and Taiwan. Those fleets fillet many of the swordfish that they catch, yet it isunclear how fillet weights are raised to whole weights. Consequently, global catch statisticsmight substantially underestimate true catch levels of swordfish.

Small (<20 kg) and badly damaged swordfish fetch low prices on United States markets.Longliners that supply those markets often discard unmarketable swordfish, creating furtherobstacles for estimating catch levels and size composition through port sampling. In contrast,European markets do not strictly differentiate between small and large swordfish.Consequently, the decline in the size of Mediterranean swordfish has not created any greatproblems to size sampling.


In addition to port-sampling, many swordfish fisheries have logbook programs. In some caseslogbooks are voluntary, e.g., Spain’s North Atlantic fishery. In other fisheries (e.g., Canada)compliance is mandatory. Canada has a unique arrangement where boats cannot leave port fortheir next fishing trip until data requirements are fulfilled.

Logbook programs can provide useful information on catch levels and the distribution offishing effort and catches. However, the specialised nature of swordfish targeting meanslogbooks are of limited value without independent verification of the data. Ideally, researchand management of swordfish fisheries require integrated logbook, vessel monitoringsystems, port sampling and at-sea observer programs to collect fine-scale data on fishingpractices, catch levels, discarding (e.g., condition of small swordfish released where minimumsize restrictions apply) and the size and sex composition of catches. During the 1990s theUnited States and Spain established significant observer programs for the North Atlanticfishery.

For many fisheries, scientists are able to clearly separate swordfish activities from operationstargeting tuna or other species. In other fisheries, swordfish targeting is more difficult todistinguish. In Hawaii, scientists distinguish between ‘swordfish’ trips and ‘tuna’ trips and‘mixed’ trips through analyses of catch rates, gear and operational data gathered fromskippers or crew members at dockside interviews. The use of lightsticks and set time wouldbe useful criteria for separating swordfish targeting in many longline fisheries. However, fewlogbooks record information on those fishing practices. Until 1993, for example, Japan’slogbooks did not collect the data needed to distinguish swordfish targeting amongst theirlongliners. Data on the use of lightsticks and set time are available for Australia’s fishery butcollection did not commence until mid-1997.

Research Whereas data collection programs are in place for most swordfish fisheries, few nations havecommitted resources to undertake research on the biology of swordfish. Biological researchon large pelagic fish like swordfish is difficult and expensive. High priority research areas forswordfish assessment are similar to those for other pelagic fish species: stock structure andmixing, age and growth, natural mortality and reproduction.

For many pelagic fish species (e.g., skipjack) growth rates vary with size, areas or stocks, andover time. Swordfish are likely to show similar variation in growth but, until recently,inconsistencies between ageing methods concealed those variations. Specialists areconverging on fin rays as the preferred hard part for ageing adult swordfish. Counts of dailyrings of otoliths are used to age juvenile swordfish. Independent verification of age estimates(e.g., by marking bony parts with OTC)2 are required, especially to verify the apparently highgrowth rates of juvenile swordfish.

Scientists generally accept that swordfish form separate stocks in the Mediterranean, NorthAtlantic and South Atlantic, although the precise boundaries between stock are not known.The situation in the Pacific and Indian oceans is less clear. Genetics studies have not beenparticularly successful in delineating stock structure for highly migratory species like tuna.This is because some individuals are long-lived and only a few need to undertake longmigrations each generation to maintain genetic mixing. Results of the latest research suggestthat different north-western and south-western stocks exist in the Pacific and severaloverlapping swordfish stocks occur in the eastern Pacific.

For many years ICCAT used tag–recapture data to determine growth curves for Atlanticswordfish. Tag–recapture experiments could also provide useful information on stockstructure and estimates of mixing rates between stocks and within stocks. They might alsoprovide a point estimate of population size and population parameters, such as natural

2 Oxy-tetracycline (OTC) is an antibiotic injected in a fish to leave a mark on calcified structures, such asotoliths. When the fish is later recaptured, the mark left by the OTC can be used to validate estimates of agederived by other methods.

Comparison


mortality, fishing mortality and growth. However, prospects of a successful large-scaletagging experiment using conventional tags are poor because swordfish are notoriouslydifficult to tag (see p. 123). ‘Archival’ (data-logging) and pop-up (satellite communicating,data-logging) tags utilising small longline-caught swordfish or swordfish in harpoon fisheriesmight provide information on movement and behaviour.

Swordfish distribution and abundance are closely linked to ocean temperature andproductivity. Investigations of inter-annual variability in swordfish abundance, the effects ofbroad-scale oceanographic events like El Niño – Southern Oscillation and decade-scalechanges in ocean productivity may be particularly useful to the development and managementof several swordfish fisheries, such as Chile and Australia. Such environmental studiescombined with information on fishing gear (e.g., depth of hooks) are required for interpretingcatch rate trends.

Stock assessmentFishery indicators Comprehensive data on swordfish catch and effort were rarely gathered from swordfishfisheries as they first developed. However, anecdotal records suggest that swordfish weremuch more abundant before commercial fishers began targeting them. Several tuna andbillfish fisheries (e.g., blue marlin in the eastern Pacific) also exhibited high catch rates whenfirst exploited (Joseph et al. 1974, pp. 326–27).The decline from high initial catch ratesprobably reflects the fishing down of an accumulated biomass and resident components of thepopulations. In particular, components of swordfish populations are associated withtopographic features, which were often the early focus of developing fisheries. Modernlongliners often remove those resident components at a greater rate than can bed replaced bynatural growth and immigration. Swordfish may thus be prone to local depletion aroundtopographic features.

Nominal catch rates of swordfish generally increased to a peak, then declined in most of thefisheries examined. Exceptions to the declining trend were Hawaii and Spanish longliners inthe Mediterranean (in both fisheries catch rates declined then eventually returned almost topeak levels). In some fisheries (e.g., Japan’s distant-water longliners in the Pacific) thedeclines were attributed to changes in targeting, from shallow-sets for yellowfin to deep setsfor bigeye. Below we highlight the pitfalls in drawing conclusions from trends in nominalcatch rates. However, it is noteworthy that very long time series of catch rate data areavailable for several fisheries and the declines are witnessed in almost all fisheries.Furthermore, those declines occurred despite likely improvements in fishing gear andpractices and increased experience.

Changes in fishing gear and practices complicate the interpretation of catch rates in manyfisheries. To maintain the economic feasibility of their operations in the face of falling catchrates and swordfish prices, fishers have purchased larger boats and ranged progressivelyfurther offshore on longer fishing trips (e.g., Hawaii-based longliners and United Stateslongliners fishing across the Atlantic). Such geographical expansion may mask decliningcatch rates close to ports.

There have been several attempts at standardising swordfish catch rates in the North Atlantic,South Atlantic, Mediterranean and the Pacific. However, most of those analyses have usedbroad factors, such as area, quarter and year effects. Few have incorporated gear effects ortaken targeting into account. Nakano’s (1998) standardisation of Pacific-wide catch ratesincludes gear effects. ICCAT’s SCRS has developed separate analyses for each of nine agecohorts in the North Atlantic and major North Atlantic fishing nations have taken a similarapproach to their swordfish fisheries. In 1999 SCRS produced sex-structured and age-structured virtual population analyses as the basis for their scientific advice on North Atlanticswordfish. Indices used by SCRS now take into account targeting, gear configuration and, insome cases, environmental effects (Dr J. Porter, 21 June 2000).


Analyses can often benefit from the use of finer area and time scales and detailed informationon fishing gear and practices, e.g., investigating geographical changes that may indicate localdepletion or standardising catch rates for improvements in fishing gear. It is rarely possible toreconstruct historical data to obtain the necessary detail, yet assessments require reasonablylong time series of catch, effort and size data.

Further complicating the interpretation of fishery indicators is the strong influence ofoceanographic conditions on the distribution, abundance and catchability of swordfish. TheInter-American Tropical Tuna Commission has begun to include habitat parameters inanalyses that standardise swordfish catch rates in the eastern Pacific. Similar ‘habitat models’are providing new insights on trends in the Pacific longline fishery for bigeye (Bigelowet al. 1999a) and, more recently, yellowfin (Bigelow et al. 1999b). The bigeye model adjustscatch rates for basic parameters, such as area and quarter of fishing activity and the depthrange of longline gear. It is innovative in also adjusting for variations in bigeye densitypredicted from changes in the depth and geographical expanse of habitat defined by ocean-scale models of temperature and dissolved oxygen. SCRS uses models for some fleets thatincorporate limited environmental information, such as sea surface temperature (Dr J. Porter,21 June 2000).

We have highlighted the effects of fishing gear and geographical location on the size and sexcomposition of swordfish catches (p. 156). Increased exploitation often results in a decreasein the representation of large swordfish in catches and a resulting increase in therepresentation of juveniles. The limited information on size composition suggest that verylarge swordfish (200–500 kg) might have been more abundant when fisheries firstdeveloped.3 In the Mediterranean the size composition of catches has clearly declined so thatthe fishery now relies on only two or three mostly immature age classes. The size at maturityis smaller in the Mediterranean than in other fisheries (it might have declined there inresponse to increased exploitation or it might be a characteristic of the Mediterranean stock).For most swordfish fisheries, however, changes in fishing gear (surface methods to longline)and geographical expansion into higher latitudes confound interpretation of size trends.

Simulations, undertaken by CSIRO, suggest that size, particularly the frequency of the verylargest swordfish in catches, may be a more sensitive indicator of stock status than catch rates(Punt et al. 1999, pp. 14–16). A single index like average weight may, however, concealsignificant changes in size composition. As indicators of stock status, size trends also sufferfrom many of the problems highlighted above for catch rates. Size composition is closelyrelated to geographical distribution, season, fishing gear and, potentially, exploitation levelsand stock status. Increasingly, size composition data is becoming dependent on informationfrom markets and minimum size regulations. For size data there are particular concerns overthe effects of at-sea discarding on the fidelity of size as an indicator of stock status.

For swordfish the available fishery indicators are all fishery dependent. We did not identifyany specific program collecting fishery independent data on swordfish, e.g., research vesselsurveys and aerial surveys. Neither did we find any study that used the fishery’s geographicalextent as an indicator of status.

Stock assessment models4

Stock assessment of swordfish has received less attention than that of tuna (e.g., southernbluefin and northern bluefin tuna) because most fisheries that target swordfish are smaller andless valuable than the fisheries that supply tuna to sashimi markets. Commercial tuna fisherieshave existed for a long time, providing a longer history of experience in assessment andmanagement.

3 See, for example, Tibbo et al. (1961, p. 10).

4 The discussion of stock assessment models combines conclusions of our case studies with Kleiber’s (1999)discussion of the strengths and weaknesses of various stock assessment approaches for swordfish.

Comparison


In the 1970s and 1980s scientists applied surplus-production models to aggregated swordfishcatch and effort data. Those assessments were not particularly useful because they aggregatedall age classes and assumed equilibrium population sizes and constant selectivity. In mostcases the data provided insufficient contrast in the range of fishing effort and stock size toallow accurate estimation of maximum sustainable yield. Several fisheries (e.g., Pacific)subsequently sustained swordfish catches that were larger than the maximum sustainableyields predicted by the early production models.

During the late 1980s and 1990s production models developed to allow the relaxation ofvarious constraints, such as the equilibrium assumption (Prager 1999, p. 103). However,problems remain with having sufficient contrast in the data, the need to have catch rates as areliable index of abundance and age-related or size-related effects, e.g., variations indistribution, natural mortality, fishing selectivity and reproductive output that depend on age(Kleiber 1999, p. 202).

A lack of age (and size) data has hampered the application of age-structured models toswordfish. Virtual population analyses of Mediterranean swordfish have encounteredproblems with age and size data, as well as uncertainties in biological parameters, inaccuratecatch data and unreliable standardised catch rates used to tune the model.

Modelling of the dynamics of swordfish populations has developed to a sophisticated level inthe North Atlantic, where ICCAT has fostered regular stock assessments using virtualpopulation analyses. The virtual population analyses are calibrated to a suite of standardisedcatch rates using the ADAPT modelling framework (ICCAT 1997a, p. 176). SCRS hasincluded sensitivity analyses to determine the effects on model results of certain assumptionsor input data, e.g., plus group size, fishing mortality ratio (‘F-ratio’), length-based and sex-based analyses (Porter 1999, p. 82). SCRS’s models do not have catch-at-age data. Insteadthey use ‘cohort slicing’ where ages are derived from applying an age–size curve to size data.The most recent analyses lump all age 9 and older female swordfish into one ‘plus group’ andage 5+ males into another ‘plus group’ because of an inability to age older swordfish (ICCAT1997, p. 14).


In 1991 SCRS began using non age-structured production models in conjunction with age-structured virtual population analyses. Since 1996 SCRS has also considered age-structuredproduction models (Porter 1999, p. 82). Although the production models and virtualpopulation analyses are based on quite different assumptions, they give similar results. Thesimilarities are not completely unexpected because the models and analyses are tuned withabundance indices (catch rates) derived from the same data (Dr J. Porter, 21 June 2000).

Status Stock assessments suggest that three of the six established fisheries (the Mediterranean, SouthAtlantic and North Atlantic) are, or have been, fished at unsustainable levels. There is noquantitative assessment of swordfish in Chile’s fishery and we therefore classify its status as‘uncertain’. While there are problems and uncertainties with the assessments of swordfish inthe North Pacific (Japan’s fishery and the Hawaii-based fishery), there are no clear signs ofstock decline. The production models applied to the North Pacific and wider Pacific, suggestthat the stock has not been exploited heavily enough to cause a noticeable decline in catchrates.

North Atlantic swordfish have been fished at levels beyond the estimated maximumsustainable yield for 13 of the past 16 years. The 1999 assessment suggests, however, that thedecline in swordfish biomass has slowed. It showed strong recruitments of swordfish in 1997–98 that should eventually contribute to the parent stock. Yield-per-recruit analyses suggestedthat increases in the minimum size offered the best opportunity for increasing long-term yieldsin the North Atlantic. But, overall reductions in fishing mortality are required to increasebiomass per recruit and, ultimately, the size of the parent stock (Porter 1999, p. 84).

In the Mediterranean, GFCM–SCRS also used yield per recruit and spawning-biomass perrecruit analyses to examine how changes in fishing mortality would affect annual recruitment.Growth rates are particularly high in the first year, yet the assessments suggested thatreducing the catch of 70–120 cm swordfish would increase yield per recruit. The spawningpotential of the stock in 1994 was estimated to be 10–20 per cent of that for the virgin stock.The size composition of catches has declined so that the fishery now relies on 1–3 year-oldswordfish. Continued reliance on juvenile swordfish may reflect significant reductions in thesize of the parent stock which might cause wide fluctuations in recruitment.

Alternatively, catches of small swordfish in the Mediterranean might suggest that recruitmentcan be maintained at very low levels of parent stock. One scenario is that intense exploitationcauses depletion in local areas, but leaves scattered adults that are able to maintainrecruitment. Swordfish in the Mediterranean have a relatively young age at first maturity andhigh juvenile growth rates. They are therefore less likely to suffer recruitment overfishingthan slow-growing, late-maturing species like southern bluefin. Being long-lived and slow-growing after maturity, adult swordfish are more likely to be prone to growth overfishing.

Fishing for swordfish in the pelagic environment is simultaneously taking predators,competitors and prey. There is little understanding of what effects (if any) fishing has on thewider ecosystem that will, in turn, affect swordfish population dynamics (Kleiber 1999,p. 205). In addition to robust assessments of swordfish status, scientists need to conductmultispecies assessments and ecosystem approaches to facilitate informed managementdecisions.

Comparison


Reliability of assessments A key uncertainty, particularly for the Mediterranean fishery, is the maintenance ofrecruitment at low levels of the parent stock. For this (and other) swordfish fisheries there isno experience to show that recruitment is related to parent biomass (a ‘stock–recruitmentrelationship’).

Virtual population analyses must deal with uncertainties in biological parameters (particularlyage, growth, natural mortality and sex ratio), inaccurate catch data and unreliable standardisedcatch rates. In particular, the assessments of Mediterranean swordfish have required extensivesubstitution of size and sex composition to data across fishery components that are known tobe quite different. Those uncertainties result in wide confidence limits for parameterestimates.

The accuracy of assessments and uncertainty surrounding parameters are tied to data qualityand coverage. SCRS, for example, attributed part of the improved outlook for North Atlanticswordfish in 1999 to improvements in data quality and processing. Retrospective analysesrevealed that, had the revisions to data and processing techniques used in 1999 been availablein 1996, a 1996 assessment based on trends up to 1995 would have been similarly optimistic.

Production models and virtual population analyses are tuned with abundance indices derivedfrom catch and fishing effort data. Expansion of areas fished, the type of fishing gear used,technological advances and size regulations complicate interpretation of changes in fishingeffort, catch rates and size composition.

Total catch for several fisheries (e.g., the Mediterranean) are suspected to be underestimated.The lack of information on catches of small swordfish and swordfish discards is a commonsource of uncertainty in assessments of the fisheries examined. What may also be significantis the unknown quantity of swordfish that escape hooking on the longline, only to die frominjuries sustained by the hook tearing their mouths.

Uncertainties over stock structure, particularly in the Pacific, have created further problems inthe interpretation of the results of stock assessment models. Many of the assessments assumea distinct stock and that mixing of swordfish through the stock is instantaneous.

ManagementObjectives

Development goals were the prime objective for many swordfish fisheries when theycommenced. As fisheries developed, fishery managers addressed conservation issues througha variety of approaches, including, catch quotas, resource allocation between competinggroups by limiting entry to fisheries (e.g., Hawaii) and restricting access to geographicalareas. To protect artisanal fishers in Chile, for example, commercial longliners are excludedfrom waters within 120 nm of the coastline.

The 1966 convention that created ICCAT introduced an objective of maximum sustainableyield (termed ‘maximum sustainable catch’). During the 1980s many nations andorganisations adopted the optimum utilisation as an objective of conservation. Under theMagnuson–Stevens Act, the United States established fishery management plans for each ofits fisheries during the 1990s. The plans have a wide range of objectives, such as optimumyield, securing food production, recreational access, preserving traditional fisheries andprotecting marine ecosystems.


During the 1990s fishery managers began to focus on environmental issues in severalswordfish fisheries, e.g., bycatch of seabird in Hawaii. The 1995 United NationsImplementing Agreement (UNIA) directed regional fishery managers to consider non-target,associated or dependent species and articulated the application of the precautionary approachto fishery management, including guidelines for the development of precautionary referencepoints. A 1998 workshop (SCTB 1998) onprecautionary limit reference points suggestedthat the avoidance of recruitment overfishingshould be the major objective of fisheriesmanagement. The workshop recommended thatfishery managers define at least two limitreference points to avoid recruitment overfishing:

• a percentage of the maximum observedspawning biomass beyond which the stockmust not be allowed to fall; and

• a level of fishing mortality that should not beexceeded in any year (SCTB 1998, p. i).

Where stock assessments cannot be undertaken,the workshop proposed that a suite of fisheryindicators be used as provisional referencepoints. Scientific advice on stock status andreference points need to be embedded in decisionrules specifying the implementation of fisherycontrol measures and monitoring of harvest(SCTB 1998, p. 2).

ICCAT takes a different approach toprecautionary fishery management. It bases totalallowable catch recommendations on the objective of a 50 per cent chance of rebuilding theNorth Atlantic swordfish stock to the level at maximum sustainable yield (BMSY) by 2010.Few other regional or national agencies have applied the precautionary approach orestablished provisional reference points for their swordfish fisheries.

Management measuresNations fishing for swordfish in the Mediterranean have introduced a variety of managementregulations to govern the activities of their national fleets, but there is virtually nocoordination of regulations over the entire fishery and there is no overall limit on fishingeffort or catch levels. Greece, for example, has a seasonal closure on fishing to protect smallswordfish. Yet, the closure has been ineffective because it does not prevent fishing in adjacentwaters or the bycatch of swordfish by other fishing operations. Some nations enforce aminimum size limit of 120 cm for Mediterranean swordfish and a limit of 2.5 km for pelagicdriftnets. However, several nations have inadequate enforcement arrangements and manyfleets do not to comply with the regulations. The regulations may also have contaminatedfishery data, with suspicions of under-reporting of the length of driftnets so that theconsequently inflated catch rates derived from those data are unreliable.

In the North Atlantic, ICCAT has no regulatory authority and member nations are not legallybound to accept ICCAT recommendations. Since 1991 ICCAT has recommended minimumsize regulations and limits on catch for swordfish in the North Atlantic. Several members haveignored the national quotas allocated by ICCAT and catches have regularly exceeded the totalallowable catch. Failure to accept and enforce quotas had contributed to the continued declineof the North Atlantic swordfish stock during the 1990s.

Minimum size limits have been ineffective in Chile, with longliners not moving away fromareas where small swordfish are abundant. Consequently small swordfish continue to becaught but most are discarded dead. Dead discards are not counted against national quotas,

Overfishing Recruitment overfishing involves thereduction of adult fish in a stock to apoint where the production of youngfish (‘recruitment’) declines or fails. Growth overfishing is a reduction inoverall yields resulting from theremoval of young fish before they cangrow to the optimum size forharvesting. The terms ‘fishery collapse’ and ‘stockcollapse’ refer to problems arising fromoverfishing. We define stock collapseas a situation where overfishing resultsin recruitment failure. The fishing fleetand industry that depends on the stockmight subsequently decline. Such adecline might be an acute fisherycollapse. More commonly, however,evidence of collapse is difficult toidentify because the fleet responds toearly signs of declining catch rates bydiverting to other species or moving toother areas.

Comparison


thus contributing to the breaching of the total allowable catch. Similar problems haveemerged with the size limits recommended by ICCAT. Area and season closures might bemore effective, but they also create complexities for monitoring and enforcement.

For swordfish fisheries, effective ‘output controls’ (e.g., total allowable catches) need to becombined with ‘input controls’, such as limits on fishing effort. The established fisheries allhave limited entry arrangements, but those limits have often been established after the fisheryhas expanded. Furthermore, inadequate support measures for limited entry have often allowedlatent effort to build up in the fishery and have not restrained fishing effort. Owners purchasedlarger fishing boats. Longliners have increased fishing effort by setting more gear or spendingmore days at sea. An important lesson from this review is the need to put in place acomprehensive suite of mechanisms to control fishing effort before geographical expansionand overcapitalisation commence. However, precise limits are extremely difficult to define fordeveloping fisheries. At a very early stage in the fishery’s development, they would involvepreliminary assessment of the distribution and abundance of swordfish, their productivity,seasonal fluctuations and the returns determining commercial feasibility.

Nations have not controlled the relocation of their fleets. When swordfish catch rates declinedin the North Atlantic, for instance, many of Spain’s longliners relocated to the South Atlantic.Others moved to the Indian Ocean and some (United States) longliners moved to the Pacific.

World Conservation Union (IUCN) Red Lists provide governments and internationalorganisations with a guide to the conservation status of species threatened with extinction. In1996 the World Conservation Union listed North Atlantic swordfish as a species facing a veryhigh risk of extinction in the near future. Worldwide, it considers that there is inadequateinformation to assess the risk of swordfish becoming extinct (IUCN 1999, pp. 1–11). Note,however, that those lists are largely based on criteria developed for terrestrial species withlimited distributions; they might not be directly applicable to widely distributed fish speciesthat support commercial fishing industries.

BycatchLongliners targeting swordfish take a wide variety of other species. Compared with longliningfor tuna, longlining for swordfish is more likely to interact with vulnerable species, such asseabird, seal and turtle. This is because swordfish longlining involves shallow-sets usuallybeginning in the late afternoon and early morning hauls at subtropical and temperate latitudeswhere those species are often active.

Many fishing nations have agreed to the Food and Agriculture Organization InternationalPlan of Action to reduce the incidental take of seabird on longline (FAO 1998). Fisherymanagers are also responding to concern over the bycatch of other species in swordfishfisheries. The potential for incidental catches of seal and turtle, for instance, resulted insignificant area closures around the Hawaiian Islands. Some swordfish fisheries are institutingother seabird mitigation measures, such as tori lines (Australia) and dyed baits (Hawaii).However, there is scant information about the catch levels of bycatch species becauseobservers are rarely placed on longliners targeting swordfish.

Longliners targeting swordfish often take large incidental catches of shark, particularly blueshark. Crew members sometimes release large shark by cutting the leader close to the hook.However, most shark are usually hauled aboard. The carcasses of most species (e.g., mako)are often retained for sale. Finning—the practice of remove the shark’s fins and discarding thecarcass—is common in many swordfish fisheries, especially for the most abundant species,blue shark. Bigelow et al. (1999, p. 195) found no evidence of decline in standardised catchrates of blue shark in the Hawaii-based fishery. Nevertheless, the waste of finning and thebroader effects of longlining on shark populations are a growing concern to fishery managersand the wider public. There is a need to broaden research and management to address themultispecies nature of swordfish fisheries. Shark and killer whale represent a cost to fishersthrough gear loss, bait loss and damage to target species. It is therefore in the interests of


fishers to develop mitigation measures to reduce interaction with those species as well asother bycatch species.

Fisheries interactionMajor recreational fisheries occur in the Caribbean, Mexico, California and, in the past, Chile.Recreational swordfish fisheries are also developing off the western coastline of Africa(Squire and Muhlia-Melo 1993, p. 8; Mr J. Findlay, 28 January 2000). Apart from those areas,recreational–commercial interaction is not a concern in swordfish fisheries. Of more concernto anglers is the bycatch of marlins by swordfish fisheries. However, the use of night sets forswordfish seems to result in low catch rates of marlin compared with marlin catch rates in daysets for tuna.

Longliners targeting tuna take significant amounts of swordfish as a bycatch, creatingproblems for the implementation of fishery management measures. Those catches may alsocreate problems for assessment because they might not be correctly reported and cannot becovered by data collection programs that are focused on swordfish.

Concluding remarksFishing for swordfish in the pelagic environment is simultaneously taking predators,competitors and prey. There is little understanding of the effects of fishing on the widerecosystem that will, in turn, affect swordfish population dynamics. In addition to robustassessments of swordfish status, fishery managers need multispecies assessments and adviceon the wider, ecosystem effects of fishing. The effects of longlining on non-target species arethe focus of fishery management in many areas and may result in more severe limitations andregulations than will concern over the status of target stocks.

Globally, swordfish catches are increasing. In particular, the introduction of longline hasincreased swordfish catch levels, broadened the size range taken and expanded thegeographical extent of fisheries. Swordfish fisheries often show a development pattern ofrapid growth, with catches reaching a peak then declining as fishing effort overshootssustainable levels. The pattern highlights the ability of commercial fisheries to rapidly expandswordfish catches and to create problems with overcapacity.

Like tuna fisheries, swordfish fisheries exhibit very high catch rates when first exploited. Theinitial high catch rates probably reflect a fishing down of an accumulated biomass andresident components of the population. Components of swordfish populations are associatedwith topographic features, which are often the initial focus of developing fisheries. Modernlongliners are capable of removing those resident components at a greater rate than they canbe replaced. Swordfish may thus be prone to local depletion. In response, longline fisheriesexpand geographically, masking declining abundance in local areas.

There is a pattern for longliners in developing swordfish fisheries to undertake progressivelylonger trips to obtain the catch levels necessary to maintain economically viable operations.Such expansion may attract overcapitalisation, with larger longliners needing to fish in newareas more often. To ensure economic efficiency, fishers and fishery managers will require anunderstanding of the geographical and boat size constraints to commercial feasibility inswordfish fisheries. In response to declining catch rates, longliners may also switch to targetother species, further complicating research and management.

Our review revealed no clear evidence that swordfish stocks or their fisheries had collapsedfrom overfishing. But, assessments suggest that three fisheries (the Mediterranean, SouthAtlantic and North Atlantic) were or are fished at levels that are not sustainable. Inadequateresearch and management are putting those stocks at risk and failing to achieve considerableeconomic benefits that would be derived from optimum use of the swordfish resources. Thereare some indications that swordfish may be resilient to intensive harvesting. With just twoyears of management action involving strict quotas, there are signs of improved catch rates inthe North Atlantic.

Comparison


In most swordfish fisheries, fishing effort continued to expand for several years after the peakin catches, highlighting the inability of fishery management to address growing overcapacityin the fleets. Mechanisms to control effort need to be established before the fishery expands.In some cases, fishery managers have not implemented restraints recommended by scientificadvisers. This is partly due to communication problems and uncertainties in assessments,which stem from inadequate data and biological information. It is also due to political, socialand economic imperatives. In isolation, fishery management measures, such as limited entry,size limits and catch limits, have been inadequate for swordfish fisheries. A combination ofinput controls and output controls should be considered for swordfish fisheries.

Small swordfish have high growth rates and, apparently in some areas (e.g., theMediterranean), are able to withstand relatively heavy fishing pressure. The broad distributionof swordfish, both in the water column and across the oceans, combined with year-roundspawning in a broad tropical–subtropical band might contribute to the resilience of swordfishstocks to intensive harvesting. In the North Atlantic, swordfish have apparently recoveredrapidly in response to firm management action (Dr J. Porter, 21 June 2000). This is not to saythat yield per recruit or biomass per recruit can be maximised by heavily fishing smallswordfish. Nor is it clear how far a parent stock can be reduced before recruitment isdepressed.

Swordfish are a highly migratory species. But, apart from those cooperating in ICCAT, mostnations have taken unilateral approaches, attempting to independently research and manageswordfish within their exclusive economic zones (Squire and Muhlia-Melo 1993, p. 19). Toavoid the problems experienced in areas like the Mediterranean, nations must work throughregional bodies to ensure proper monitoring and implementation of management measures.


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Comparison


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Documents

5. COMPARISON OF SWORDFISH FISHERIESdata.daff.gov.au/.../data/12908_05-swordfishreport.pdf · swordfish and, in 1998, ICCAT began to incorporate sex-ratio at size information into