Prey flux manipulation and the feeding rates of reef-dwelling planktivorous fish

Prey availability for site-attached planktivorous fish is delimited not only by the density of their prey, but also by the speed of the waters that carry the prey. The effects of both flow speed and prey density on the feeding rates of coral reef dwelling planktivorous fish were examined under controlled laboratory conditions. Fish placed inside a large recirculating flow chamber exhibited a limited enhancement of feeding rates following the increase of flow speed from 3 to 18 cm/s. Feeding rates peaked between 6 and 12 cm/s and appeared to decline upon subsequent increases in speed. Saturation of feeding rates did not correspond directly with prey flux, as higher rates were achieved at 'higher prey density-lower flow speed' treatments of equal flux. Video analysis of fish movements while foraging, together with experimentation involving fish feeding under restricted spatial conditions, indicated the narrowing of the fishes' reactive volume with increasing flow speed. The dependence of reactive volume size on flow speed was incorporated into the Holling disk equation and shown to reproduce much of the observed trends. Disparities between the observed and predicted dependence of feeding rates on flow speed suggest additional effects of flow speed on search efficiency parameters, such as prey detectability and capture success.