Establishing a Conceptual Design for Jellyfish Blooms in the Seto Inland Sea

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1 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 by TERRAPUB, Establishing a Conceptual Design for Jellyfish Blooms in the Seto Inland Sea Naoki FUJII 1, Atsushi KANEDA 2, Shinya MAGOME 3 and Hidetaka TAKEOKA 1 1 Center for Marine Environmental Studies, Ehime University, 2-5, Bunkyo-cho, Matsuyama , Japan 2 Department of Marine Bioscience, Fukui Prefectural University, 1-1, Gakuen-cho, Obama , Japan 3 Sanyo Techno Marine Inc., , Nihonbashi Horidome, Chuo-ku, Tokyo , Japan (Received 8 January 2011; accepted 13 January 2011) Abstract The increase in moon jellyfish (Aurelia aurita) populations in the Seto Inland Sea of Japan have caused damage in fisheries and many significant social and economic problems. To fully understand the population dynamics (e.g., abundance) of moon jellyfish in this region, analysis of patterns in jellyfish temporal abundance in relation to environmental fluctuation is essential. However, an effective monitoring method of jellyfish abundance appears to be lacking and there remains no quantitative data on jellyfish abundance in the past. Some attempts to identify patterns in temporal abundance of jellyfish have recently been carried out. Uye and Ueta (Bull. Jpn. Soc. Fish. Oceanogr., 68, 9 19) reported that A. aurita populations have shown apparent increase since the 1980s, most dramatically in the last 10 years. Through a questionnaire targeting commercial fishers in the Seto Inland Sea, it was found that 65% believed that jellyfish populations have increased since This report suggests the long-term increasing trend of jellyfish population in the region. In another study, Nagai (Mar. Pollut. Bull., 47, ) reported similar results by using other survey data. In this paper, we discuss the factors that contribute to jellyfish biomass increase in the Seto Inland Sea and present the ecological point of view of jellyfish bloom dynamics. Keywords: Aurelia, jellyfish, blooms, Seto Inland Sea INTRODUCTION In recent years, the expanding influence of anthropogenic activities has caused many 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 coastal regions worldwide. Previous reports have shown scientific evidence of an increase in jellyfish populations in some water bodies such as the Black Sea (Shiganova and Bulgakova, 2000), Bering Sea (Brodeur et al., 1999, 2002), and 65

2 66 N. FUJII et al. Fig. 1. Schematic representation of the jellyfish spiral theory. northern Gulf of Mexico (Graham, 2001). Although scientific evidence is unavailable in many other coastal seas, some scientists and fisherpersons have noticed changes in the occurrence of jellyfish in their neighboring waters. For example, in the Seto Inland Sea of Japan, fisherpersons have perceived the increase in population biomass of the common jellyfish Aurelia aurita, the scyphozoan Chrysaora melanaster, and the ctenophore Bolinopsis mikado. They also have observed a consistent decrease in fish catch. In recent decades, the ecological importance of jellyfish has been increasingly recognized in biological oceanography (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

3 Establishing a Conceptual Design for Jellyfish Blooms 67 Fig. 2. Circumstantial diagram of the jellyfish spiral theory. blooms occur. The most popular explanation is the jellyfish spiral theory, the mechanism of which was proposed by Uye and Ueta (2004). However, to predict the variations in jellyfish populations, the concept of jellyfish blooms should be more easily understood. Therefore, in this study, we reexamined the mechanism of the jellyfish spiral theory. Here, we describe recent jellyfish blooms, mainly A. 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 theory as shown in Fig. 1. Although the causes for the recent increase of A. aurita population 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 would have more jellyfish and less finfish.

4 68 N. FUJII et al. Fig. 3. Diagram showing the effects of jellyfish blooms. NEW THEORY: JELLYFISH BLOOM DYNAMICS To predict the variations in jellyfish populations, the concept of jellyfish blooms should be more easily understood. In this paper, we reexamine the mechanism of the jellyfish spiral theory. Figure 2 shows a diagram of the modified jellyfish spiral theory. This figure offers a detailed description of the jellyfish spiral. Figure 3 shows a diagram used to represent words, ideas, tasks, or other items linked to and arranged around a central key factor of the previous slide. We explain some part, because this figure is too complicated. 1) The top left part indicates a causal relationship between the variation in jellyfish population and nutrient dynamics. 2) The top right part indicates a causal relationship between the variation in jellyfish population and changes in waterfront constructions. Because waterfront constructions provide increased polyp attachment area, A. aurita polyps can increase in number, which may eventually lead to increase 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 on mesozooplanktons such as copepods, cladocerans, 1arvaceans, and larvae of various benthic animals and fish eggs, it is a competitor as well as a predator of zooplanktivorous fish. If there are significant changes in the secondary production processes of zooplanktons, there would be a change in the relationship between jellyfish and zooplanktivorous fish

5 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 increase in water temperature during winter may lead to an increase in the overwintering population of A. aurita. Although Fig. 3 is too complicated, this figure encompasses numerous influences of variation in jellyfish population. THE SETO INLAND SEA There is an absence of scientific monitoring data on long-term variation in the biomass of A. aurita population in the Seto Inland Sea. Therefore, 2 investigations (Nagai, 2003; Uye and Ueta, 2004) were carried out to obtain such information. 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 of Japan. 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 over 2 decades ( ). On average, 65% of total respondents indicate that A. aurita population has increased every 4 years from 1982 to 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 years from 1992 to 2002, in the western Seto Inland Sea. On the other hand, the A. aurita biomass 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. An increase in the time spent in the medusa stage was obvious in jellyfish in both the eastern and western Seto Inland Sea, which may be due to the recent increase in annual minimum water temperature. Nagai (2003) compiled the monthly fishing reports of the prefectural fisheries experimental stations in the Seto Inland Sea and counted the frequency of jellyfish 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 Seto Inland 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 the entire area of the Seto Inland Sea. The number of cases of jellyfish blooms reached a maximum in We considered factors of jellyfish blooms in the Seto Inland Sea on the basis of the jellyfish bloom dynamics theory. In the Seto Inland Sea, eutrophication significantly progressed during the 1960s. However, Uye and Ueta (2004) reported that A. aurita populations have shown apparent increase only since the 1980s. Thus, eutrophication was not a major factor, but a change in the lowtrophic food web due to eutrophication may be a principal factor in the jellyfish blooms. Several districts around the Seto Inland Sea have been reclaimed, and the coastal area has dramatically increased to about 225 km 2 from 1950 to 1973

6 70 N. FUJII et al. (EMECS, 1997). Although, Uye and Ueta (2004) reported that A. aurita populations have shown apparent increased since the 1980s, the change in waterfront constructions was not a major factor. The annual catch of zooplanktivorous fish from 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 such conditions, an increase in jellyfish population leads to a decrease in the finfish population; in other words, the ecosystem shifts from a finfish-dominated community to jellyfish-dominated community. The connection between the populations of jellyfish and other organisms requires further research. The seawater temperature around the Seto Inland Sea is increasing, particularly the winter water temperature in the entire area of the Seto Inland Sea (Bungo- Channel). Considering that Uye and Ueta (2004) and Nagai (2003) reported that A. aurita populations have increased in the entrance areas of the Seto Inland Sea, there may be a relationship between jellyfish abundance and water temperature in winter. However, we do not yet understand the nature of this relationship. To more fully understand the population dynamics of moon jellyfish in this region, analysis of patterns in jellyfish temporal abundance in relation to environmental fluctuation is essential. However, it is important to obtain scientific monitoring data on long-term variations in A. aurita population. Thus, in a future study, we will attempt to create a dataset of jellyfish abundance by using the following novel methods: 1) Jellyfish Aggregation Monitoring System in Uwa Sea (JAMSUS; ); 2) Jellyfish landings from the Ikata Nuclear Power Plant ( ). 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 summer and autumn from 2003 to We observed temporal shifts in dense aggregations of moon jellyfish, the frequency of occurrence of which varied widely in Hokezu Bay. From jellyfish landings in the Ikata Nuclear Power Plant, which takes in cooling water from the Iyo-Nada (part of the Seto Inland Sea), we maintained records of the jellyfish clogging biomass cleaned daily from the intake screens, providing a decade-long record of jellyfish abundance. Using data from these 2 methods, we will attempt to understand the factors influencing jellyfish populations in the Seto Inland Sea. REFERENCES Arai, M. N. (1988): Interactions of fish and pelagic coelenterates. Can. J. Zool., 166, Arai, M. N. (2001): Pelagic coelenterates and eutrophication: a review. Hydrobiologia, 451, Brodeur, R. D., C. D. Mills, J. E. Overland, G. E. Walters and J. D. Schumacher (1999): Evidence for a substantial increase in gelatinous zooplankton in the Bering Sea, with possible links to climate change. Fish. Oceanogr., 8, Brodeur, R. D., H. Sugisaki and G. J. Hunt, Jr. (2002): Increase in jellyfish biomass in the Bering Sea: implications for the ecosystem. Mar. Ecol. Prog. Ser., 233, EMECS (1997): Environmental Conservation the Seto Inland Sea. 40 pp. Graham, W. M. (2001): Numerical increases and distribution shift of Chrysaora quinquecirrha (Desor) and Aurelia aurita (Linne) (Cnidaria: Scyphozoa) in the northern Gulf of Mexico. Hydrobiologia, 155,

7 Establishing a Conceptual Design for Jellyfish Blooms 71 Nagai, T. (2003): Recovery of fish stocks in the Seto Inland Sea. Mar. Pollut. Bull., 47, Parsons, T. R. and C. M. Lalli (2002): Jellyfish population explosions: revisiting a hypothesis of possible causes. Lar mer, 40, Purcell, J. E. (1997): Pelagic cnidarians and ctenophores as predators: selective predation, feeding rates and effects on prey populations. Ann. Inst. Oceanogr., 73, Purcell, J. E., W. M. Graham and H. J. Dumont (2001): Jellyfish Blooms: Ecological and Societal Importance. Kluwer, Dordrecht. Purcell, J. E., S. Uye and W. Lo (2007): Anthropogenic causes of jellyfish blooms and their direct consequences for humans: a review. Mar. Ecol. Prog. Ser., 350, Schneider, G. and G. Behrends (1998): Top-down control in a neritic plankton system by Aurelia aurita medusae a summary. Ophelia, 48, Shiganova, T. A. and Y. V. Bulgakova (2000): Effects of gelatinous plankton Black Sea and Sea of Azov fish and their food resource. ICES J. Mar. Sci., 57, Uye, S. and Y. Ueta (2004): Recent increase of jellyfish populations and their nuisance to fisheries in the Inland Sea of Japan. Bull. Jpn. Soc. Fish. Oceanogr., 68, 9 19 (in Japanese with English abstract). N. Fujii ( medusae@sci.ehime-u.ac.jp)