Predator-prey relationship between the landlocked dwarf ayu and planktonic Crustacea in Lake Biwa, Japan

The predator-prey relationship was investigated between the landlocked dwarf ayu (koayu; Plecoglossus altivelis) and three planktonic crustaceans, Daphnia galeata, Eodiaptomus japonicus, and Mesocyclops dissimilis, in Lake Biwa. The abundance of each prey species and daphniid size composition in the lake was compared with those in the stomachs of koayu collected at the same time and place. D. galeata was the main item of diet of koayu in spite of their low density, and the larger individuals were selectively preyed upon by the predator. In the stepwise multiple regression of the mean number of each prey species in the stomachs on prey densities in the lake and predator mass, the density of the prey itself was always selected as the predictor variable. The density of E. japonicus was also selected as the regressor of daphniid number in the stomachs with a negative slope. Multiple regression was also performed each for two size-classes of D. galeata, and a negative effect of large daphniids was detected for the consumption of small ones. The present results and additional information enable estimation of the collective predation rate by the koayu population on the prey populations.