Plankton Benthos Res 8(3): 124–133, 2013

Seasonal and interannual variations in the abundance of jellyfish in a southern coastal waters of Iyo-Nada, Japan: Influence of cyclonic gyre transport

ATSUSHI KANEDA1,*, NAOKI FUJII2, JUNICHI OHYAMA3, DAISUKE TAKAHASHI4, FUSAICHI YAMAMOTO5 & HIDETAKA TAKEOKA3

1 Faculty of Marine Bioscience, Fukui Prefectural University, 1–1, Gakuen, Obama, Fukui 917–0003, Japan2 Institute of Lowland and Marine Research, Saga University, Honjo 1, Saga 840–8502, Japan3 Center for Marine Environmental Studies, Ehime University 2–5, Bunkyo, Matsuyama, Ehime 790–8755, Japan4 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 signifi-cantly 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

Introduction

In recent years, increases in the populations of jellyfish, such as Aurelia aurita Linnaeus, 1758, have been reported around the world (Arai 2001, Ishii 2001, Lynam et al. 2004, Mills 2001, Purcell 2005). Dense aggregations of jellyfish occasionally have had marked negative impacts on local ocean ecosystems, fisheries and industries (Möller 1980, Uye 2011). For example, aggregations of jellyfish at the intake gates of power plants and factories cause opera-tional problems (Yasuda 2003). Thus, it is important to understand the factors affecting the distribution of jellyfish

for the management of industries and fisheries.The Seto Inland Sea consists of several interconnected

wide areas called nada in Japanese and it is connected to the open ocean primarily through two channels, the Bungo Channel and the Kii Channel (Fig. 1a). A marked increase in jellyfish, consisting mostly of A. aurita has been re-ported in the Seto Inland Sea in recent years based on a poll of fishermen (Uye & Ueta 2004). In the survey, 70% of respondents believed that the numbers of A. aurita had increased in the last 10 years (1993–2002) and that the in-creases were most marked in the western Seto Inland Sea, especially along the coastal seas of the Bungo Channel and around the Hayasui Strait (Fig. 1b). Uye et al. (2003) showed that A. aurita formed unusual, dense aggregations * Corresponding author: Atsushi Kaneda; E-mail, kaneda@fpu.ac.jp

Plankton & Benthos Research

© The Plankton Society of Japan

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 observa-tions 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 sea-sonal 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 cy-clonic gyre is similar to that observed in the western Irish Sea (Hill et al. 1994), which was shown to affect the trans-port 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 cool-ing 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 physi-cal environment, especially tide-induced eddies. In the present study, we focus on seasonal and interannual varia-tions in jellyfish abundance at the intake gate of the power plant. These variations are generally regulated by a complex interaction of factors. Considering the patchy distribution of jellyfish and the cyclonic gyre in this area, however, the transport of jellyfish from other coastal seas might be one of the major factors influencing the varia-tions. To test this hypothesis, we analyzed data collected in Iyo-Nada and in other seas adjacent to Iyo-Nada.

Materials and Methods

Field sampling and jellyfish records

Seawater for cooling the turbines is continuously

Fig. 1. Maps showing (a) locations of Iyo-Nada, the Bungo Channel and the Kii Channel in the Seto Inland Sea; (b) stations for monthly temperature and salinity data (single circles, IY1–IY7 and IY11), visual observations of jellyfish conducted by the Oita Institute of Marine and Fisheries Science (triangles), and Jellyfish sampling at the intake gates of the Ikata Nuclear Power Plant (double circle); and (c) station for current data collected by moored current meters (triangle, PC) located 1,800 m off the nu-clear power plant, and water temperature measured daily (filled square, PT), and locations of water gates of the plant (small circle and squares).

126 A. Kaneda et al.

pumped through two intake gates (17.7 m and 21.6 m in diameter) with three pumps (Fig. 1c). The amount of seawater pumped through the intake gates was not constant due to, for example, detailed safety inspections of the plant equipment several times a year. According to the data provided from the power plant, the daily amount of pumped sea water ranged between 3.3×106 and 1.2×107 m3, and the maximum daily volume (1.2×107 m3) was recorded for 48% of the study period.

Jellyfish were removed from the pumped seawater by a screen net (mesh size, approximately 1.0 cm) enclosing the intake gates about 50 m apart. Trapped jellyfish were re-moved from the net once a day except on Sunday, and their wet weight was measured by power-plant staff. The jelly-fish abundances are expressed as wet weight (mg) per unit volume (m3) of pumped seawater. Data collected for 14 years from 1 April 1998 to 31 March 2012 were used in the present study; we present yearly data according to the Japanese fiscal year, which starts on 1 April and ends on 31 March. According to the staff who measured the wet weight of jellyfish, captured jellyfish always consisted mostly of A. aurita. Two jellyfish, Chrysaora pacifica Goette, 1886 and Cyanea nozakii Kisinouye, 1891, were sometimes found but their numbers and volumes were negligible compared with A. aurita. Thus, the variation in the jellyfish abundance during the present study was primarily that of A. aurita.

Analysis of environmental factors

In the present study, the residual current data have been cited from the annual reports of Ehime Prefecture (1999–2012). The data in the reports were collected four times each year (February, May, August and November) by the Shikoku Electric Power Co., Inc. at station PC (Fig. 1c) using a current meter (RCM-10, AANDERAA Instru-ments for 1998–2009 and INFINITY-EM, JFE Advantech Co. Ltd. for 2010–2012), which was moored for 15 days each month to estimate the harmonic constants of the four principal tidal current components and velocities of the residual current. The daily water temperature data at 17 m depth at station PT have been cited from the annual reports of Ehime Prefecture (1999–2012). In addition, the monthly data for temperature and salinity at 0, 10, 25 and 50 m depths at the eight stations (IY1–IY7 and IY11) in Iyo-Nada were measured by the Ehime Fisheries Research Center using the research vessel Yoshu, and were used for

the present analysis; the data from 1998 to 2011 were provided by the Ehime Fisheries Research Center and those for 2012 were obtained from the website of the Ehime Fisheries Research Center (2012). The frequency and locations for residual current and oceanographic con-dition observations are summarized in Table 1. To test the relationships between variation in jellyfish abundance and environmental factors, correlation analyses using both Pearson’s and Spearman’s correlation coefficients were conducted for monthly periods except for from January to March, when jellyfish was absent or very rare. In this study it is hypothesized that the jellyfish population at the present site Ikata is supplied from a denser population in the Hayasui Strait about 25 km west of Ikata by the eastward current and therefore that the jellyfish population at Ikata would be correlated with the eastward component of the residual current. To test this hypothesis, the correla-tion with the residual current was analyzed based on the data when the eastward component of the current was >0.

Results

Seasonal and interannual variability in environmental factors

The mean residual current for 14 years was eastward in February, May and August with a mean velocity of 2.1–5.1 cm sec－1, whereas in November its direction was west-ward (Fig. 2a). Mean monthly water temperatures at stations PT, IY5 and IY11 were at their minimum of 11.8–12.7°C in March and their maximum of 23.9–24.3°C in September (Fig. 2b). Mean salinity values at stations IY5 and IY11 were high during the cold months (January–April) with a maximum of 33.7–34.1 in March and were low during the warm months with a minimum of 32.9–33.0 in August (Fig. 2c).

The residual current, water temperature and salinity varied greatly interannually. These variations in May, August and November are shown in Fig. 3. In most years, the residual current was eastward in May and August, whereas the current was westward in November.

Fourteen-year mean sectional distributions of tempera-ture, salinity and density at stations IY1–IY7 for each month, which were made based on the data measured by the Ehime Fisheries Research Center, indicated that the 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, salinity

IY 1–7 and 11 monthly Ehime Research Institute of Agriculture, Forestry and Fisheries, Fisheries Research Center

Jellyfish abundance in Iyo-Nada 127

as follows. Seawater was well mixed in February and strat-ification developed in May, when a cold (<14°C) and dense dome structure appeared around station IY5 (Fig. 4). In August, the stratification became stronger, and the cold, dense structure was more clearly developed around the same station. The dome structure disappeared in Novem-ber when strong vertical mixing occurred due to cooling.

Seasonal and interannual variability in jellyfish abundance

Jellyfish abundance at the intake gates of the Ikata Nu-clear Power Plant over the 14 years (April 1998–March

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 tem-perature 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 east-ward 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-tion between the jellyfish abundance and the eastward current in the same month is a reasonable test of their relationship. The correlation coefficients for the parametric analyses were significantly positive in May (r=0.69, p<0.01, Fig. 9a). The correlation in August was not signifi-cant (r=0.49, p=0.12) due to one data points with high abundance (25.4 mg m–3) at a velocity of about 5.5 cm sec－1, though a positive trend between jellyfish abundance and the eastward component of the residual current seemed to be apparent (Fig. 9b). The positive correlation for the non-parametric analysis was significant in August (r=0.71, p<0.03). However, the correlation in May was not signifi-cant, only being nearly significant (r=0.49, p=0.08) due to several data points with low abundances (<5 mg m–3) at high velocities (>5 cm sec–1) (Fig. 9a).

Discussion

The present study revealed that the velocity of the east-ward component of the residual current at station PC was

Fig. 2. Seasonal variations in 14-year mean values of (a) resid-ual current (lines indicate current direction and velocity) with dotted ellipses representing SE; (b) monthly water temperatures at stations PT (at 17 m depth), IY11 (at 10 m depth) and IY5 (at 10 m depth) [SE was not shown because the values were very small (<0.37)]; and (c) salinity at stations IY11 and IY5 (vertical lines represent SE).

128 A. Kaneda et al.

significantly positively correlated with jellyfish abundance in May according to the parametric correlation. The corre-lation coefficient in August, using the nonparametric Spearman’s correlation, was also significant, though the correlation in May was nearly significant. Considering the patchy distribution of jellyfish, for which the abundance is generally hard to estimate at a single sampling point, the present result most probably indicated a positive relation-ship between the jellyfish abundance and the eastward cur-rent at the present site during the stratification period. This implies that the transport of jellyfish from the Hayasui Strait, which is inhabited by a denser jellyfish population, is an important factor regulating the jellyfish population around the Ikata power plant about 25 km downstream from the strait. In fact, one of us (N.F., unpublished) visually observed that the jellyfish population around the Hayasui Strait was generally denser than in the central and east parts of Iyo-Nada.

Since 2004, the Oita Institute of Marine and Fisheries Science has been conducting monthly visual observations on the distribution of jellyfish in Suo-Nada, the west part of Iyo-Nada and the Bungo Channel from April to Novem-ber and provides the results on their website (Oita Institute

of Marine and Fisheries Science 2012). According to their results from 2004 to 2011, jellyfish, of which the dominant species was Aurelia aurita, were observed in the west part of Iyo-Nada and around the Hayasui Strait in almost all months from April to September.

In recent years, numerous aggregations of A. aurita have appeared in the coastal seas of the Bungo Channel (Taka-hashi et al. 2010, Takeoka et al. 2009, Uye et al. 2003). Two important factors causing the high jellyfish population in the Bungo Channel were proposed by Takeoka et al. (2009): (1) medusae reproduce along the coastal seas of the Bungo Channel and (2) dense aggregations are induced by the Kyucho, a warm water intrusion from the Pacific (Takeoka et al. 1993a). Studies on water exchange and material transport in the Seto Inland Sea have identified exchanges between the Bungo Channel and Iyo-Nada through the Hayasui Strait (Kawamura 1975, Takeoka 1984). Therefore, it is considered likely that jellyfish repro-ducing in the Bungo Channel are introduced to the west part of Iyo-Nada through the Hayasui Strait by this water exchange. This introduction of jellyfish from the Bungo Channel is probably one of the main reasons for the higher density of jellyfish around the Hayasui Strait.

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.

Jellyfish abundance in Iyo-Nada 129

The present results concerning the residual current at sta-tion PC off the coast of Ikata in May and August indicated that eastward flow occurs in most years. Dominance of the eastward flow in the southern part of Iyo-Nada during the stratification period was also revealed by Chang et al. (2009). Since the eastward current observed at station PC is considered as a part of the cyclonic gyre developed in Iyo-Nada during the stratification period (Guo et al. 2005), it is deduced that the strength of the cyclonic gyre determines the transport of jellyfish in this coastal area.

Guo et al. (2006) found different circulation patterns in Iyo-Nada between August 2004 and July 2005; in August

2004 a current with a two-layer structure was formed while in July 2005 cyclonic circulation accompanied cold bottom water. They considered the former as part of a basin-scale estuarine circulation in the Seto Inland Sea, and explained that the different current patterns result from competition between the basin-scale estuarine circu-lation and the cyclonic gyre. Murakami et al. (1985) showed that differences in density between the inner part of the Seto Inland Sea and the open ocean induced a basin-scale estuarine circulation pattern. They concluded that the year 2004 was an abnormal year because large fresh water inputs, due to several typhoons, intensified the basin-scale

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.

130 A. Kaneda et al.

circulation, while the current in 2005 was representative of the usual one in this area. Our data at station PC also indi-cate that westward flow occurred in August 2004, while eastward flow developed in August 2005. The coincidence

between Guo et al.’s (2006) data and our own data confirm that the eastward flow along the south coast of the central part of Iyo-Nada is part of the cyclonic gyre in Iyo-Nada, and therefore that the year-to-year fluctuation in the

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.

Jellyfish abundance in Iyo-Nada 131

strength of the gyre can be regarded as a main factor caus-ing 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 proba-bly one of the reasons for the marked decrease in jellyfish in November. On the other hand, Uye & Shimauchi (2005) suggested that A. aurita disappeared by November in the Seto Inland Sea due to death. In the present study, the pop-ulation decrease after November is considered to result from the extinction of the cyclonic gyre and death of jelly-fish introduced from the west part of Iyo-Nada in October.

It has been suggested that environmental variables, such as temperature, salinity and eutrophication, are factors causing variations in A. aurita populations (Lucas 2001, Miyake et al. 2002, Omori et al. 1995, Purcell et al. 2009, Watanabe & Ishii 2001). The abundance of jellyfish,

consisting mostly of A. aurita, in Iyo-Nada is certainly regulated by a complex interaction of multiple factors. However the present study indicates that the strength of the cyclonic gyre is probably the main factor regulating the abundance of jellyfish in the central part of Iyo-Nada.

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 abun-dance at the intake gates of the Ikata power plant from 1998 to 2011.

Fig. 8. Year-to-year variation in monthly mean jellyfish abun-dance at the intake gates of the Ikata power plant in May (a), August (b) and November (c).

132 A. Kaneda et al.

Acknowledgements

We thank the Ehime Fisheries Research Center and the Oita Institute of Marine and Fisheries Science for providing the data on environmental variables and jellyfish abundance, respectively. We thank the laboratory group at the Center for Marine Environmental Studies for thoughtful discussions leading to this paper. We also thank the editor and anony-mous reviewers for comments, which improved this paper.

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