Reaction field, capture field, and search volume of 0+ nase (<i>Chondrostoma nasus</i>): effects of body size and water velocity

Abstract

The reaction field, capture field, and search volume of four size-classes of 0+ nase (Chondrostoma nasus) were investigated at two water velocity regimes using flow-tank experiments. Reactive and capture distances, measured in three dimensions, increased linearly with fish size and were larger in flowing water than in calm water. The 0+ nase reacted almost exclusively to prey that were situated in their forward-directed hemisphere. In flowing water, 0+ nase systematically drifted and (or) swam downstream with the current in pursuit of prey that had drifted past their stations. Captures thus occurred in their backward-directed hemisphere. Based on the fish's swimming speed and its reaction field, we propose a modified method for estimating search volume of 0+ nase and other rheophilic cyprinid species with a similar feeding mode. Search volume also increased linearly with fish size and was larger in flowing water than in calm water. We argue why the foraging mode of 0+ nase in flowing water is the effect of a constraint imposed by water current rather than the result of an economic strategy.