Indirect impact of bluegill Lepomis macrochirus on the development of cyanobacterial blooms in Lake Biwa, Japan

Abstract

Seasonal variations in the abundance of cyanobacteria and catch per unit effort (CPUE) of bluegill (Lepomis macrochirus) were investigated in Lake Biwa of Japan. High CPUE occurred from late spring to autumn when cyanobacteria dominated the community of photosynthetic plankton. However, the CPUE decreased substantially when the population density of cyanobacteria increased dramatically within an hour. Seasonal variation in the body weight of the fish with a fixed length was estimated from the length-weight relationships, revealing that body weight increased rapidly in early spring and reached its first peak in late spring, eventually decreasing in autumn after reaching the second peak. In May, bluegill fed on extremely large numbers of Bosmina longirostris when it dominated the zooplankton community. From June, the main diet of the bluegill became epiphytic filamentous green algae, which are the competitors for cyanobacteria. The biomass of submersed macrophytes in Lake Biwa has increased recently, potentially affecting cyanobacteria adversely since submersed macrophytes release allelochemicals and provide a habitat for cladocerans. However, propagation of submersed macrophytes also benefits bluegill, possibly contributing to the dominance of cyanobacteria by reducing the amounts of cladocerans and epiphytic algae. © 2011 Taylor & Francis.