Impact of fish predation on a small-bodied cladoceran: Limitation or stimulation?

Abstract

In seven enclosure experiments the short-term effects of exposing Bosmina longirostris, a small bodied cladoceran, to high levels of fish predation were analyzed. The density of Bosmina did not change significantly in the experimental enclosures, suggesting no direct fish impact on Bosmina demography. However, detailed studies revealed strong fish effects on some demographic and life history parameters of Bosmina. While the mean size of individual Bosmina decreased, reproduction enhanced (due to larger clutch sizes and a higher proportion of gravid females) in the enclosures with fishes. There is life history shift towards lower females sizes at a first reproduction in the fish enclosures. The maintenance of Bosmina density in the fish enclosures may be attributed to: (1) higher reproductive rate due to elevated food level; (2) a life history shift towards smaller size reducing vulnerability to fish predation; (3) release from copepod predation caused by the presence of fish.