Population dynamics and production of Jesogammarus annandalei, an endemic amphipod, in Lake Biwa, Japan

1. Population dynamics and production of Jesogammarus annandalei, an endemic amphipod in Lake Biwa, were examined from April 1997 to June 1998. The life cycle of this species was 1 year with the new generation beginning 7 in early autumn. They preferred low temperature (<12 °C) and their spatial distribution varied seasonally and accordingly. 2. In deep water, the abundance of J. annandalei ranged from 200 to 63 000 m^-2 and decreased towards summer and the biomass (0.01 - 3.6 g C m^-2) was on average comparable that of zooplankton. The density was much higher than that recorded by a study conducted 35 years ago. 3. Individual growth rate of this amphipod was high in winter and spring but decreased in summer. Annual production of J. annandalei (6.2 g C m^-2 year^-1) was only 2% of primary production but was at the higher end of the range reported for amphipods in oligo- and mesotrophic lakes. 4. These results are consistent with the view that Lake Biwa is becoming more eutrophic, with a consequent decrease in the abundance of predatory fish in the profundal zone.