Visual Predation by Planktivores

Abstract

The distance at which planktivorous fish strike at prey was measured under a variety of conditions. Several lake trout, Salvelinus namaycush (Walbaum), had statistically indistinguishable reaction distances, or RD, for the same size prey. Lake trout and brook trout, Salvelinus fontinalis (Mitchill), increased their RD linearly as prey size increased. The RD by lake trout for Daphnia with high concentrations of haemoglobin were greater than the RD for more translucent Daphnia of the same size. Daphnia with their intestine darkened from ingesting India ink were not detected at a greater distance than normal Daphnia. Light intensities between 10 and 50 lux reduce RD for 3 species of fish while RD remained constant between 50 and 1400 lux. Lake trout RD in narrow aquaria were about 30% greater than RD in large aquaria. The probable usefulness of several models of feeding by planktivores is discussed. Laboratory results suggest models will need to incorporate choice of prey by the fish at high prey densities but not at low densities.