Predation by a visual planktivore perch (Perca fluviatilis) in a turbulent and turbid environment

Turbulence and turbidity are thought to independently affect the foraging success of fish, but little is known about their interactive effects on the feeding of fish larger than a few centimetres. We experimentally tested for this interaction on the feeding of planktivorous perch (Perca fluviatilis). There was an interactive effect of root mean square (RMS) velocity (0, 1.3, 2.7, 5.5, and 18.3 cm.s(-1)) and turbidity (0, 30, and 60 nephelometric turbidity units; NTU) on perch feeding on phantom midge larvae (Chaoborus flavicans). In the 0 and 60 NTU conditions, there was no significant change in the feeding efficiency of perch. However, at 30 NTU, increasing turbulence enhanced perch feeding by increasing encounter rates and disabling the prey escape response. The proportion of encountered Chaoborus larvae that were consumed showed a linear decline with increasing turbulence under clear and 30 NTU conditions and a dome-shaped response under 60 NTU. The results indicate that turbulence has a strong effect on the post-encounter stages of the foraging cycle.