The effects of turbidity and light intensity on the consumption of mysids by juvenile perch (Perca fluviatilis L.)

Nursery areas of perch, Perca fluviatilis L., in the shallow coastal areas of the Baltic Sea are affected by increased water turbidity due to the ongoing eutrophication and other physical disturbances. Visually feeding fish, such as perch, which depend on clear water and good light conditions for effective foraging are often negatively affected by turbidity. We tested the following hypothesis: in laboratory experiments increased turbidity combined with decreased light intensity impairs the foraging efficiency of juvenile perch. Consumption of the mysid shrimp, Neomysis integer Leach, by 0+ perch (4-5.2 em) and 1+ perch (6.1-7.7 cm) was tested at four levels of turbidity (1, 10, 20 and 30 NTU = Nephelometric Turbidity Units) combined with three light intensities imitating daylight (64-71 mumol m(-2) s(-1)), twilight (1.0-1.2 mumol m(-2) s(-1)) and night (complete darkness). There were no significant reductions in the consumption of mysids by 0+ and 1+ perch with increased turbidity and decreased light intensity in 3 h trials. The consumption of mysids by 0+ perch decreased slightly, although not significantly, with increasing turbidity in 1.5 h trial. This indicates that there are compensatory factors (e.g. increased activity of perch, increased prey encounter, reduced anti-predator behaviour of prey, altered contrast of prey) acting on reduced visual ability of juvenile perch due to increased turbidity and low illumination.