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Interdisciplinary Studies on Environmental Chemistry—Marine Environmental Modeling & Analysis,Eds., K. Omori, X. Guo, N. Yoshie, N. Fujii, I. C. Handoh, A. Isobe and S. Tanabe, pp. 65–71.© by TERRAPUB, 2011.

Establishing a Conceptual Design for Jellyfish Bloomsin the Seto Inland Sea

Naoki FUJII1, Atsushi KANEDA2, Shinya MAGOME3 and Hidetaka TAKEOKA1

1Center for Marine Environmental Studies, Ehime University,2-5, Bunkyo-cho, Matsuyama 790-8577, Japan

2Department of Marine Bioscience, Fukui Prefectural University,1-1, Gakuen-cho, Obama 917-0003, Japan

3Sanyo Techno Marine Inc.,1-3-17, Nihonbashi Horidome, Chuo-ku, Tokyo 103-0012, Japan

(Received 8 January 2011; accepted 13 January 2011)

Abstract—The increase in moon jellyfish (Aurelia aurita) populations in theSeto Inland Sea of Japan have caused damage in fisheries and many significantsocial and economic problems. To fully understand the population dynamics(e.g., abundance) of moon jellyfish in this region, analysis of patterns injellyfish temporal abundance in relation to environmental fluctuation is essential.However, an effective monitoring method of jellyfish abundance appears to belacking and there remains no quantitative data on jellyfish abundance in thepast. Some attempts to identify patterns in temporal abundance of jellyfishhave recently been carried out. Uye and Ueta (Bull. Jpn. Soc. Fish. Oceanogr.,68, 9–19) reported that A. aurita populations have shown apparent increasesince the 1980s, most dramatically in the last 10 years. Through a questionnairetargeting commercial fishers in the Seto Inland Sea, it was found that 65%believed that jellyfish populations have increased since 1982. This reportsuggests the long-term increasing trend of jellyfish population in the region. Inanother study, Nagai (Mar. Pollut. Bull., 47, 126–131) reported similar resultsby using other survey data. In this paper, we discuss the factors that contributeto jellyfish biomass increase in the Seto Inland Sea and present the ecologicalpoint of view of “jellyfish bloom dynamics.”

Keywords: Aurelia, jellyfish, blooms, Seto Inland Sea

INTRODUCTION

In recent years, the expanding influence of anthropogenic activities has causedmany changes in coastal sea ecosystems, such as harmful algal blooms (red tide),hypoxia, and loss of biodiversity. In addition, it is currently argued that jellyfish(Cnidaria and Ctenophora) populations are increasing in a variety of coastalregions worldwide. Previous reports have shown scientific evidence of anincrease in jellyfish populations in some water bodies such as the Black Sea(Shiganova and Bulgakova, 2000), Bering Sea (Brodeur et al., 1999, 2002), and

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northern Gulf of Mexico (Graham, 2001). Although scientific evidence isunavailable in many other coastal seas, some scientists and fisherpersons havenoticed changes in the occurrence of jellyfish in their neighboring waters. Forexample, in the Seto Inland Sea of Japan, fisherpersons have perceived theincrease in population biomass of the common jellyfish Aurelia aurita, thescyphozoan Chrysaora melanaster, and the ctenophore Bolinopsis mikado. Theyalso have observed a consistent decrease in fish catch. In recent decades, theecological importance of jellyfish has been increasingly recognized in biologicaloceanography (reviewed by Schneider and Behrends, 1998; Arai, 1988, 2001;Purcell et al., 2001, 2007; Parsons and Lalli, 2002).

Some scientists have provided meaningful discussions on why jellyfish

Fig. 1. Schematic representation of the jellyfish spiral theory.

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blooms occur. The most popular explanation is the “jellyfish spiral” theory, themechanism of which was proposed by Uye and Ueta (2004). However, to predictthe variations in jellyfish populations, the concept of jellyfish blooms should bemore easily understood. Therefore, in this study, we reexamined the mechanismof the jellyfish spiral theory. Here, we describe recent jellyfish blooms, mainlyA. aurita, in the Seto Inland Sea and propose the new “jellyfish bloom dynamics”theory.

THE JELLYFISH SPIRAL THEORY

Uye and Ueta (2004) proposed the mechanism of the jellyfish spiral theoryas shown in Fig. 1. Although the causes for the recent increase of A. auritapopulation in many coastal seas are still speculative, they may be associated with

1) Overfishing of planktivorous fish;2) Increase in overwintering populations due to warming of seawater

temperature;3) Increase in polyp attachment area due to waterfront constructions;4) Increase in jellyfish food due to eutrophication or modification of

nutrient composition. Once an ecosystem falls into this spiral, it wouldhave more jellyfish and less finfish.

Fig. 2. Circumstantial diagram of the jellyfish spiral theory.

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NEW THEORY: JELLYFISH BLOOM DYNAMICS

To predict the variations in jellyfish populations, the concept of jellyfishblooms should be more easily understood. In this paper, we reexamine themechanism of the jellyfish spiral theory.

Figure 2 shows a diagram of the modified jellyfish spiral theory. This figureoffers a detailed description of the “jellyfish spiral.” Figure 3 shows a diagramused to represent words, ideas, tasks, or other items linked to and arranged arounda central key factor of the previous slide. We explain some part, because thisfigure is too complicated.

1) The top left part indicates a causal relationship between the variation injellyfish population and nutrient dynamics.

2) The top right part indicates a causal relationship between the variation injellyfish population and changes in waterfront constructions. Becausewaterfront constructions provide increased polyp attachment area, A.aurita polyps can increase in number, which may eventually lead toincrease in the medusa population.

3) The bottom-left part indicates a causal relationship between jellyfish(medusa stage) and other organisms. Since A. aurita preys primarily onmesozooplanktons such as copepods, cladocerans, 1arvaceans, and larvaeof various benthic animals and fish eggs, it is a competitor as well as apredator of zooplanktivorous fish. If there are significant changes in thesecondary production processes of zooplanktons, there would be achange in the relationship between jellyfish and zooplanktivorous fish

Fig. 3. Diagram showing the effects of jellyfish blooms.

Establishing a Conceptual Design for Jellyfish Blooms 69

production (in this study, we consider only jellyfish and finfish).4) The bottom-right part indicates a causal relationship between the variation

in jellyfish abundance and water temperature. For example, an increasein water temperature during winter may lead to an increase in theoverwintering population of A. aurita.

Although Fig. 3 is too complicated, this figure encompasses numerousinfluences of variation in jellyfish population.

THE SETO INLAND SEA

There is an absence of scientific monitoring data on long-term variation inthe biomass of A. aurita population in the Seto Inland Sea. Therefore, 2investigations (Nagai, 2003; Uye and Ueta, 2004) were carried out to obtain suchinformation.

Uye and Ueta (2004) conducted a poll to survey the recent increase in A.aurita population and its negative effects on fisheries in the Seto Inland Sea ofJapan. A total of 1152 respondents, with >20 years experience as a net fisherperson(85%), angling fisherperson (7%), and others (8%), were included in the study.Their results show variations in A. aurita populations in the Seto Inland Sea over2 decades (1982–2002). On average, 65% of total respondents indicate that A.aurita population has increased every 4 years from 1982 to 2002. In addition,there is a clear regional difference in the increasing pattern of A. aurita abundance.For example, the increase is most remarkable, particularly during the 10 yearsfrom 1992 to 2002, in the western Seto Inland Sea. On the other hand, the A. auritabiomass level is largely the same as before 1982 in the central Seto Inland Sea,where the seasonal occurrence pattern has also been more or less constant. Anincrease in the time spent in the medusa stage was obvious in jellyfish in both theeastern and western Seto Inland Sea, which may be due to the recent increase inannual minimum water temperature.

Nagai (2003) compiled the monthly fishing reports of the prefectural fisheriesexperimental stations in the Seto Inland Sea and counted the frequency ofjellyfish blooms. Their study showed that jellyfish (Cyanea nozakii, A. aurita,Pelagia panopyra, Dactylometra pacifica, Beroe cucumis, and others) occurred,up until the first half of the 1990s, only in and near the entrance areas of the SetoInland Sea, such as the Kii Channel, Bungo Channel, Suo-Nada, and Iyo-Nada.In the latter half of the 1990s, however, jellyfish became more common over theentire area of the Seto Inland Sea. The number of cases of jellyfish bloomsreached a maximum in 1997.

We considered factors of jellyfish blooms in the Seto Inland Sea on the basisof the jellyfish bloom dynamics theory. In the Seto Inland Sea, eutrophicationsignificantly progressed during the 1960s. However, Uye and Ueta (2004)reported that A. aurita populations have shown apparent increase only since the1980s. Thus, eutrophication was not a major factor, but a change in the low-trophic food web due to eutrophication may be a principal factor in the jellyfishblooms. Several districts around the Seto Inland Sea have been reclaimed, and thecoastal area has dramatically increased to about 225 km2 from 1950 to 1973

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(EMECS, 1997). Although, Uye and Ueta (2004) reported that A. aurita populationshave shown apparent increased since the 1980s, the change in waterfrontconstructions was not a major factor. The annual catch of zooplanktivorous fishfrom the Seto Inland Sea of Japan was highest (>350,000 metric tons) in the mid-1980s, and then declined to ca. 130,000 metric tons in the late 1990s. Under suchconditions, an increase in jellyfish population leads to a decrease in the finfishpopulation; in other words, the ecosystem shifts from a finfish-dominatedcommunity to jellyfish-dominated community. The connection between thepopulations of jellyfish and other organisms requires further research. Theseawater temperature around the Seto Inland Sea is increasing, particularly thewinter water temperature in the entire area of the Seto Inland Sea (Bungo-Channel). Considering that Uye and Ueta (2004) and Nagai (2003) reported thatA. aurita populations have increased in the entrance areas of the Seto Inland Sea,there may be a relationship between jellyfish abundance and water temperaturein winter. However, we do not yet understand the nature of this relationship.

To more fully understand the population dynamics of moon jellyfish in thisregion, analysis of patterns in jellyfish temporal abundance in relation toenvironmental fluctuation is essential. However, it is important to obtain scientificmonitoring data on long-term variations in A. aurita population. Thus, in a futurestudy, we will attempt to create a dataset of jellyfish abundance by using thefollowing novel methods:

1) Jellyfish Aggregation Monitoring System in Uwa Sea (JAMSUS; 2003–2010);

2) Jellyfish landings from the Ikata Nuclear Power Plant (1998–2010).Using the JAMSUS method, we had set up a video monitoring system on a

hill with a full view of Hokezu Bay (part of the Bungo-Channel) during summerand autumn from 2003 to 2010. We observed temporal shifts in dense aggregationsof moon jellyfish, the frequency of occurrence of which varied widely in HokezuBay. From jellyfish landings in the Ikata Nuclear Power Plant, which takes incooling water from the Iyo-Nada (part of the Seto Inland Sea), we maintainedrecords of the jellyfish clogging biomass cleaned daily from the intake screens,providing a decade-long record of jellyfish abundance. Using data from these 2methods, we will attempt to understand the factors influencing jellyfish populationsin the Seto Inland Sea.

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N. Fujii (e-mail: medusae@sci.ehime-u.ac.jp)