Do small riverine fish move inshore at night?

Abstract

We tested the hypothesis that numbers of small fish increase along river banks with decreasing light levels. Fish were sampled every 3h over a 24‐h period (30–31 August 1992) along two adjacent stretches of bank (one shallow sand, one steep boulder) on the R. Morava near Breelav, Czech Republic. The size of fish was significantly greater along the boulder bank than along the sand bank. Numbers of fish along the steep boulder bank did not increase with decreasing light levels, except in Rhodeus sericeus and perch Perca fluviatilis, which were significantly more abundant at night. Along the shallow sand bank, all species increased in number at night, most significantly so. As light levels decreased, numbers of whitefin gudgeon Gobio albipinnatus and roach Rutilus rutilus decreased along the boulder bank, as their numbers increased along the sand bank, suggesting a dusk migration to the sand bank to avoid predation. This assumption was corroborated by the significantly higher number of potentially piscivorous fishes (P. ftuviatilis and chub Leuciscus cephalus≥ 80 mm) along the boulder bank during the night.