Changes in depth distribution and activity in small benthic riverine fishes under gradually changing light intensities

Abstract

Three experiments were performed to determine how changes in light conditions affect the activity patterns of three small benthic fishes (Barbatula barbatula, Gobio gobio and Cottus gobio) and if they had any influence on fish microhabitat selection. First, the depth used by three individuals of each species was monitored when light conditions changed at dusk in an aquarium with three depth levels. During daylight most fish stayed in the deep level, but at dusk fish began to move from deeper zones to the shallow one. Second, to test the effect of gradually changing light intensity on bullhead depth use and activity, the aquarium was set up with controlled artificial light in a completely dark room. As light intensity increased, the mean number of fish in the deep level significantly increased and, reciprocally, the mean number of fish in the shallow and medium levels significantly decreased. Third, the variation in bullhead activity under different light intensities was measured by counting the number of pecks and jumps of focal individuals per five minutes. Fish activity diminished as light intensity increased, being highest when light intensity was near zero. Thus, small benthic fish such as bullhead use different microhabitats during the day than at night and display activity peaks during night.