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

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.