Reaction distance of Bythotrephes longimanus, encounter rate and index of prey risk for Harp Lake, Ontario

1. Some predatory zooplankton, such as the large cladoceran Bythotrephes longimanus ( Leydig), have a large compound eye, hence vision may play an important role in prey detection and encounter. Our objectives were to quantify, under different light regimes, the reaction distance of Bythotrephes to an assortment of zooplankton prey and to model encounter rate with prey from Harp Lake, Ontario. Reaction distance to prey increased at higher light intensity. 2. Results from the encounter model show that small, slow-moving prey faced the greatest risk from Bythotrephes and most encounters occurred in the upper 10 m of the water column throughout the 24-h period. The model was highly sensitive to ambient light. Encounter rate and prey risk were two to three orders of magnitude more sensitive to swimming velocity of the predator than to that of the prey.