Short-term changes in the population structure of hydromedusa Aglantha digitale during the spring phytoplankton bloom in the Oyashio region

To evaluate the effects of water mass changes and the spring phytoplankton bloom, we analysed short-term changes in the population structure of the dominant hydromedusa Aglantha digitale in the Oyashio region during March-April 2007. Samples were collected with Bongo nets at night via oblique towing from a depth of 200 m to the surface on eight occasions in March and April of 2007. The abundance, biomass and mature individual compositions of A. digitale rapidly increased from April onward. During April, the reproduction and growth of A. digitale is achieved due to the initiation of the spring phytoplankton bloom and the increase in small copepods. Based on dry mass, the growth rate of A. digitale (1.3% day super(-1)) was lower than the reported maximum value (4.9% day super(-1)) for this species. The rapid exchange of the dominant water masses was also observed during spring in the Oyashio region. Significant effects of water mass exchange were detected in the abundance, biomass and body sizes of A. digitale. Thus, the abundance, biomass and population structure of A. digitale in the Oyashio region may be altered during spring by two factors: internal growth and external water mass exchange.