Effect of Fish Size on the Reactive Distance of Bluegill (Lepomis macrochirus) Sunfish

Abstract

Visual acuity in fishes and thus reactive distance should increase with fish size; visual acuity depends on eye lens diameter and cone density in the retina, and eye lens diameter increases with fish size. Though cone density declines in larger fish, this effect should be relatively small. This hypothesis was tested for a behavioral measure of visual acuity, the reactive distance of bluegill (L. macrochirus) sunfish to zooplankton prey, in aquaria (375 l) for fish from 27-162 mm standard length. Reactive distance increased nonlinearly with fish size; the rate of increase in reactive distance slows in larger fish. For fish of a given size, reactive distance was dependent on prey size, but visual angle measured from the fish eye was nearly constant. Lens diameter and visual acuity increase with fish size in bluegills, the acuity of larger fish is less than expected from eye lens diameter alone. This is probably a result of cone density decreasing with fish size, as has been found for other fishes. The observed fish-size-dependent differences in reactive distance imply very large differences in visual volumes and encounter rates with prey among size-classes of bluegills. Habitat segregation among bluegill size-classes may prevent the intense intraspecfic competition for prey that would be expected, in part, from the superior visual acuity of larger fish.