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 appeared 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., unpublished) 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 formation 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. March. According to the staff who measured the wet weight of jellyfish, captured jellyfish always consisted Results mostly of A. aurita. Two jellyfish, Chrysaora pacifica Goette, 1886 and Cyanea nozakii Kisinouye, 1891, were Seasonal and interannual variability in environmental sometimes found but their numbers and volumes were factors negligible compared with A. aurita. Thus, the variation in the jellyfish abundance during the present study was The mean residual current for 14 years was eastward in primarily that of A. aurita. February, May and August with a mean velocity of 2.1– 5.1 cm sec－1, whereas in November its direction was west- Analysis of environmental factors ward (Fig. 2a). Mean monthly water temperatures at In the present study, the residual current data have been stations PT, IY5 and IY11 were at their minimum of 11.8– cited from the annual reports of Ehime Prefecture (1999– 12.7°C in March and their maximum of 23.9–24.3°C in 2012). The data in the reports were collected four times September (Fig. 2b). Mean salinity values at stations IY5 each year (February, May, August and November) by the and IY11 were high during the cold months (January– Shikoku Electric Power Co., Inc. at station PC (Fig. 1c) April) with a maximum of 33.7–34.1 in March and were using a current meter (RCM-10, AANDERAA Instru- low during the warm months with a minimum of 32.9–33.0 ments for 1998–2009 and INFINITY-EM, JFE Advantech in August (Fig. 2c). Co. Ltd. for 2010–2012), which was moored for 15 days The residual current, water temperature and salinity each month to estimate the harmonic constants of the four varied greatly interannually. These variations in May, principal tidal current components and velocities of the August and November are shown in Fig. 3. In most years, residual current. The daily water temperature data at 17 m the residual current was eastward in May and August, depth at station PT have been cited from the annual reports whereas the current was westward in November. of Ehime Prefecture (1999–2012). In addition, the monthly Fourteen-year mean sectional distributions of tempera- data for temperature and salinity at 0, 10, 25 and 50 m ture, salinity and density at stations IY1–IY7 for each depths at the eight stations (IY1–IY7 and IY11) in Iyo- month, which were made based on the data measured by Nada were measured by the Ehime Fisheries Research the Ehime Fisheries Research Center, indicated that the Center using the research vessel Yoshu, and were used for strength of the stratification in Iyo-Nada varied seasonally

Table 1. Summary of environmental parameters, stations, and intervals of the data used in the present study.

Parameter Station Sampling interval and frequency Organization

residual current PC Feb., May, Aug., Oct. Shikoku Electric Power Co., Inc. water temperature PT once a day Shikoku Electric Power Co., Inc. water temperature, IY 1–7 and 11 monthly Ehime Research Institute of Agriculture, Forestry and salinity Fisheries, Fisheries Research Center Jellyfish abundance in Iyo-Nada 127

2012) are shown in Fig. 5. The 14-year mean value of the monthly-mean jellyfish abundance showed the following seasonal fluctuations (Fig. 6): it varied between 2.4 and 5.7 mg m－3 from April to August, suddenly peaked in Sep- tember (16.1 mg m－3), then markedly decreased in Novem- ber, almost disappearing from January to March. The annual mean jellyfish abundance ranged from 1.1– 9.3 mg m－3 (Fig. 7). The abundance was high (>5.0 mg m－3) in 1998–2000, 2002, 2006 and 2011, whereas in 2001, 2003–2005 and 2009 it was low (<2.0 mg m－3). The year- to-year change was sometimes drastic such as a sudden increase from 2001 to 2002 by about nine–fold, but no notable trends nor periodicity were detected. The jellyfish abundance in May, August and November also showed significant interannual variations (Fig. 8), but their patterns were different from that of the mean abundance in Fig. 7. Relationships between jellyfish abundance and environmental factors The correlation analyses of the interannual variation of mean jellyfish abundance in each month from April to December with those of environmental factors revealed that it was never significantly correlated with water temperature or salinity for both parametric and nonparametric correlation coefficients. The correlation with the eastward component of the residual current at station PC was calculated only for May and August based on 13 and 11 data points where the eastward component was >0, because the number of the data points where the eastward component was >0 was only three in November and jellyfish were absent in February. Since the mean eastward velocity of the residual current in May and August was 5.1 and 3.0 cm sec–1, respectively, the time necessary for the transportation of jellyfish from the Hayasui Strait to the present site about 25 km downsteam would be about 6–10 days. Thus, investigating the correla- Fig. 2. Seasonal variations in 14-year mean values of (a) resid- tion between the jellyfish abundance and the eastward ual current (lines indicate current direction and velocity) with current in the same month is a reasonable test of their dotted ellipses representing SE; (b) monthly water temperatures relationship. The correlation coefficients for the parametric at stations PT (at 17 m depth), IY11 (at 10 m depth) and IY5 (at analyses were significantly positive in May (r=0.69, 10 m depth) [SE was not shown because the values were very p<0.01, Fig. 9a). The correlation in August was not signifi- small (<0.37)]; and (c) salinity at stations IY11 and IY5 (vertical cant (r=0.49, p=0.12) due to one data points with high lines represent SE). abundance (25.4 mg m–3) at a velocity of about 5.5 cm sec－1, though a positive trend between jellyfish abundance and as follows. Seawater was well mixed in February and strat- the eastward component of the residual current seemed to ification developed in May, when a cold (<14°C) and dense be apparent (Fig. 9b). The positive correlation for the non- dome structure appeared around station IY5 (Fig. 4). In parametric analysis was significant in August (r=0.71, August, the stratification became stronger, and the cold, p<0.03). However, the correlation in May was not signifi- dense structure was more clearly developed around the cant, only being nearly significant (r=0.49, p=0.08) due to same station. The dome structure disappeared in Novem- several data points with low abundances (<5 mg m–3) at ber when strong vertical mixing occurred due to cooling. high velocities (>5 cm sec–1) (Fig. 9a). Seasonal and interannual variability in jellyfish abundance Discussion Jellyfish abundance at the intake gates of the Ikata Nu- The present study revealed that the velocity of the east- clear Power Plant over the 14 years (April 1998–March ward component of the residual current at station PC was 128 A. Kaneda et al.

Fig. 3. Year-to-year variations in (a) eastward velocity of the residual current at station PC, and monthly mean values of (b) water temperature and (c) salinity at station PT in May, August and November. significantly positively correlated with jellyfish abundance of Marine and Fisheries Science 2012). According to their in May according to the parametric correlation. The corre- results from 2004 to 2011, jellyfish, of which the dominant lation coefficient in August, using the nonparametric species was Aurelia aurita, were observed in the west part Spearman’s correlation, was also significant, though the of Iyo-Nada and around the Hayasui Strait in almost all correlation in May was nearly significant. Considering the months from April to September. patchy distribution of jellyfish, for which the abundance is In recent years, numerous aggregations of A. aurita have generally hard to estimate at a single sampling point, the appeared in the coastal seas of the Bungo Channel (Taka- present result most probably indicated a positive relation- hashi et al. 2010, Takeoka et al. 2009, Uye et al. 2003). Two ship between the jellyfish abundance and the eastward cur- important factors causing the high jellyfish population in rent at the present site during the stratification period. This the Bungo Channel were proposed by Takeoka et al. implies that the transport of jellyfish from the Hayasui (2009): (1) medusae reproduce along the coastal seas of the Strait, which is inhabited by a denser jellyfish population, Bungo Channel and (2) dense aggregations are induced by is an important factor regulating the jellyfish population the Kyucho, a warm water intrusion from the Pacific around the Ikata power plant about 25 km downstream (Takeoka et al. 1993a). Studies on water exchange and from the strait. In fact, one of us (N.F., unpublished) material transport in the Seto Inland Sea have identified visually observed that the jellyfish population around the exchanges between the Bungo Channel and Iyo-Nada Hayasui Strait was generally denser than in the central and through the Hayasui Strait (Kawamura 1975, Takeoka east parts of Iyo-Nada. 1984). Therefore, it is considered likely that jellyfish repro- Since 2004, the Oita Institute of Marine and Fisheries ducing in the Bungo Channel are introduced to the west Science has been conducting monthly visual observations part of Iyo-Nada through the Hayasui Strait by this water on the distribution of jellyfish in Suo-Nada, the west part exchange. This introduction of jellyfish from the Bungo of Iyo-Nada and the Bungo Channel from April to Novem- Channel is probably one of the main reasons for the higher ber and provides the results on their website (Oita Institute density of jellyfish around the Hayasui Strait. Jellyfish abundance in Iyo-Nada 129

Fig. 4. Mean vertical proﬁles of temperature (left), salinity (center) and density (right) along the line from IY-1 to IY7 in February, May, August and November.

The present results concerning the residual current at sta- 2004 a current with a two-layer structure was formed tion PC off the coast of Ikata in May and August indicated while in July 2005 cyclonic circulation accompanied cold that eastward flow occurs in most years. Dominance of the bottom water. They considered the former as part of a eastward flow in the southern part of Iyo-Nada during the basin-scale estuarine circulation in the Seto Inland Sea, stratification period was also revealed by Chang et al. and explained that the different current patterns result (2009). Since the eastward current observed at station PC is from competition between the basin-scale estuarine circu- considered as a part of the cyclonic gyre developed in Iyo- lation and the cyclonic gyre. Murakami et al. (1985) Nada during the stratification period (Guo et al. 2005), it is showed that differences in density between the inner part deduced that the strength of the cyclonic gyre determines of the Seto Inland Sea and the open ocean induced a basin- the transport of jellyfish in this coastal area. scale estuarine circulation pattern. They concluded that the Guo et al. (2006) found different circulation patterns in year 2004 was an abnormal year because large fresh water Iyo-Nada between August 2004 and July 2005; in August inputs, due to several typhoons, intensified the basin-scale 130 A. Kaneda et al.

Fig. 5. Temporal variation in daily abundance of jellyfish trapped at the intake gates of the Ikata power plant from April 1998 to March 2012. The data are smoothed by 3-day running means. circulation, while the current in 2005 was representative of between Guo et al.’s (2006) data and our own data confirm the usual one in this area. Our data at station PC also indi- that the eastward flow along the south coast of the central cate that westward flow occurred in August 2004, while part of Iyo-Nada is part of the cyclonic gyre in Iyo-Nada, eastward flow developed in August 2005. The coincidence and therefore that the year-to-year fluctuation in the Jellyfish abundance in Iyo-Nada 131

Fig. 6. Seasonal variations in the 14-year mean values of mean monthly jellyﬁsh abundance. Vertical lines represent SE.

Fig. 7. Year-to-year variation in annual mean jellyfish abundance at the intake gates of the Ikata power plant from 1998 to 2011. strength of the gyre can be regarded as a main factor causing the interannual variation in jellyfish abundance in the present study field. Chang et al. (2009) showed the cyclonic gyre develops from April to October but not in November. The present results concerning the residual current corresponds with their results, that is, the direction of the residual current was westward in November in many years, in contrast to the dominance of eastward flows in May and August. Accordingly the extinction of the cyclonic gyre was probably one of the reasons for the marked decrease in jellyfish Fig. 8. Year-to-year variation in monthly mean jellyfish abundance at the intake gates of the Ikata power plant in May (a), in November. On the other hand, Uye & Shimauchi (2005) August (b) and November (c). suggested that A. aurita disappeared by November in the Seto Inland Sea due to death. In the present study, the population decrease after November is considered to result from the extinction of the cyclonic gyre and death of jelly- consisting mostly of A. aurita, in Iyo-Nada is certainly fish introduced from the west part of Iyo-Nada in October. regulated by a complex interaction of multiple factors. It has been suggested that environmental variables, such However the present study indicates that the strength of the as temperature, salinity and eutrophication, are factors cyclonic gyre is probably the main factor regulating the causing variations in A. aurita populations (Lucas 2001, abundance of jellyfish in the central part of Iyo-Nada. Miyake et al. 2002, Omori et al. 1995, Purcell et al. 2009, Watanabe & Ishii 2001). The abundance of jellyfish, 132 A. Kaneda et al.

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