Seasonal and Interannual Variations in the Abundance of Jellyfish in A

Plankton Benthos Res 8(3): 124–133, 2013 Plankton & Benthos Research © The Plankton Society of Japan Seasonal and interannual variations in the abundance of jellyfish in a southern coastal waters of Iyo-Nada, Japan: Influence of cyclonic gyre transport 1, 2 3 4 ATSUSHI KANEDA *, NAOKI FUJII , JUNICHI OHYAMA , DAISUKE TAKAHASHI , 5 3 FUSAICHI YAMAMOTO & HIDETAKA TAKEOKA 1 Faculty of Marine Bioscience, Fukui Prefectural University, 1–1, Gakuen, Obama, Fukui 917–0003, Japan 2 Institute of Lowland and Marine Research, Saga University, Honjo 1, Saga 840–8502, Japan 3 Center for Marine Environmental Studies, Ehime University 2–5, Bunkyo, Matsuyama, Ehime 790–8755, Japan 4 Graduate School of Agricultural Science, Tohoku University, 1–1 Amamiya-machi, Tsutsumidori, Aoba-ku, Sendai, Miyagi 981–8555, Japan 5 Yonden Consultants Co., Inc., 1007–3 Mure, Mure, Takamatsu, Kagawa 761–0121, Japan Received 15 November 2012; Accepted 21 May 2013 Abstract: The longterm data on daily abundance of jellyfish (mostly Aurelia aurita) trapped in the intake gates of the Ikata Nuclear Power Plant on the south coast of the central part of Iyo-Nada in the Seto Inland Sea, Japan was examined for a 14-year period (April 1998 to March 2012) in relation to environmental variables, i.e. temperature, salinity, and residual current, of which the last was measured in February, May, August and November. The jellyfish abundance, expressed as wet weight, generally increased in April, reached a maximum in September, markedly decreased from October to November, and was very low or null from December to March. The annual mean jellyfish abundance varied markedly. The interannual variations in the monthly mean jellyfish abundance were not significantly correlated with water temperature or salinity in any month, while correlations with eastward velocity of the residual current in May and August were generally significantly positive. These results indicate that the jellyfish population at the study site had been introduced from the Hayasui strait about 25 km westward, where jellyfish were more abundant. Since the eastward residual current is considered as a part of the cyclonic gyre in Iyo-Nada during the warm, stratification period, it is inferred that the strength of the cyclonic gyre is the main factor affecting seasonal and interannual variations in jellyfish abundance along the south coast of Iyo-Nada. Key words: Aurelia aurita, gyre, interannual variation, jellyfish, Seto Inland Sea for the management of industries and fisheries. Introduction The Seto Inland Sea consists of several interconnected In recent years, increases in the populations of jellyfish, wide areas called nada in Japanese and it is connected to such as Aurelia aurita Linnaeus, 1758, have been reported the open ocean primarily through two channels, the Bungo around the world (Arai 2001, Ishii 2001, Lynam et al. 2004, Channel and the Kii Channel (Fig. 1a). A marked increase Mills 2001, Purcell 2005). Dense aggregations of jellyfish in jellyfish, consisting mostly of A. aurita has been re- occasionally have had marked negative impacts on local ported in the Seto Inland Sea in recent years based on a ocean ecosystems, fisheries and industries (Möller 1980, poll of fishermen (Uye & Ueta 2004). In the survey, 70% Uye 2011). For example, aggregations of jellyfish at the of respondents believed that the numbers of A. aurita had intake gates of power plants and factories cause opera- increased in the last 10 years (1993–2002) and that the in- tional problems (Yasuda 2003). Thus, it is important to creases were most marked in the western Seto Inland Sea, understand the factors affecting the distribution of jellyfish especially along the coastal seas of the Bungo Channel and around the Hayasui Strait (Fig. 1b). Uye et al. (2003) * Corresponding author: Atsushi Kaneda; E-mail, [email protected] showed that A. aurita formed unusual, dense aggregations Jellyfish abundance in Iyo-Nada 125 in the coastal waters of the Bungo Channel in the summer of 2000 based on aerial photographs and sampling data. They reported that the unusual aggregations of jellyfish ap- peared as cloud-like features in the photographs. Since 2000, we have been investigating the spatial distribution of the jellyfish using aerial photographs around Iyo-Nada and the Bungo Channel several times per year, and have often detected cloud-like features, considered as jellyfish blooms, in the coastal waters of the Bungo Channel but never in the coastal waters of Iyo-Nada. In visual observations in 2008 from a research boat, one of us (N.F., unpub- lished) found that A. aurita was less abundant in Iyo-Nada than around the Hayasui Strait. These observations suggest that the jellyfish population density in Iyo-Nada is lower than in the Bungo Channel and the Hayasui Strait. The physical environment of Iyo-Nada has been studied by many researchers. Takeoka et al. (1993b) and Yama- moto et al. (2000) investigated the stratification and forma- tion of a cold layer of bottom water in summer. Guo et al. (2006) made intensive observations of the currents in the summer of 2005 and showed a cyclonic (anti-clockwise) gyre which accompanies the development of cold bottom water in Iyo-Nada. Based on a numerical study of the seasonal circulation in the Seto Inland Sea, Chang et al. (2009) showed that the cyclonic gyre in Iyo-Nada usually develops from April to October. The structure of the cyclonic gyre is similar to that observed in the western Irish Sea (Hill et al. 1994), which was shown to affect the transport of plankton and the distributions of pelagic juvenile fish (Dickey-Collas et al. 1997, Hill et al. 1996, White et al. 1988). In previous studies on jellyfish, changes in jellyfish distribution due to currents have been reported (Graham et al. 2001, Suchman & Brodeur 2005, Takahashi et al. 2010). At the Ikata Nuclear Power Plant, which is located on the south coast in the central part of Iyo-Nada (Fig. 1c), the Shikoku Electric Power Co., Inc. have recorded the wet weight of jellyfish collected at the intake gates of the cooling water system. Kaneda et al. (2007) showed the general relationships between short-term variations in jellyfish abundance based on these data and variation in the physical environment, especially tide-induced eddies. In the present study, we focus on seasonal and interannual variations in jellyfish abundance at the intake gate of the power Fig. 1. Maps showing (a) locations of Iyo-Nada, the Bungo plant. These variations are generally regulated by a Channel and the Kii Channel in the Seto Inland Sea; (b) stations complex interaction of factors. Considering the patchy for monthly temperature and salinity data (single circles, IY1– distribution of jellyfish and the cyclonic gyre in this area, IY7 and IY11), visual observations of jellyfish conducted by the however, the transport of jellyfish from other coastal seas Oita Institute of Marine and Fisheries Science (triangles), and might be one of the major factors influencing the varia- Jellyfish sampling at the intake gates of the Ikata Nuclear Power tions. To test this hypothesis, we analyzed data collected in Plant (double circle); and (c) station for current data collected by Iyo-Nada and in other seas adjacent to Iyo-Nada. moored current meters (triangle, PC) located 1,800 m off the nuclear power plant, and water temperature measured daily (filled square, PT), and locations of water gates of the plant (small circle Materials and Methods and squares). Field sampling and jellyfish records Seawater for cooling the turbines is continuously 126 A. KANEDA et al. pumped through two intake gates (17.7 m and 21.6 m in the present analysis; the data from 1998 to 2011 were diameter) with three pumps (Fig. 1c). The amount of provided by the Ehime Fisheries Research Center and seawater pumped through the intake gates was not those for 2012 were obtained from the website of the constant due to, for example, detailed safety inspections of Ehime Fisheries Research Center (2012). The frequency the plant equipment several times a year. According to the and locations for residual current and oceanographic con- data provided from the power plant, the daily amount of dition observations are summarized in Table 1. To test the pumped sea water ranged between 3.3×106 and relationships between variation in jellyfish abundance and 1.2×107 m3, and the maximum daily volume (1.2×107 m3) environmental factors, correlation analyses using both was recorded for 48% of the study period. Pearson’s and Spearman’s correlation coefficients were Jellyfish were removed from the pumped seawater by a conducted for monthly periods except for from January to screen net (mesh size, approximately 1.0 cm) enclosing the March, when jellyfish was absent or very rare. In this study intake gates about 50 m apart. Trapped jellyfish were re- it is hypothesized that the jellyfish population at the moved from the net once a day except on Sunday, and their present site Ikata is supplied from a denser population in wet weight was measured by power-plant staff. The jelly- the Hayasui Strait about 25 km west of Ikata by the fish abundances are expressed as wet weight (mg) per unit eastward current and therefore that the jellyfish population volume (m3) of pumped seawater. Data collected for 14 at Ikata would be correlated with the eastward component years from 1 April 1998 to 31 March 2012 were used in the of the residual current. To test this hypothesis, the correla- present study; we present yearly data according to the tion with the residual current was analyzed based on the Japanese fiscal year, which starts on 1 April and ends on 31 data when the eastward component of the current was >0.

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