Effects of light intensity on activity and pelagic dispersion of fish: studies with a seabed-mounted echosounder

A seabed-mounted, upwards-pinging echosounder was used to study fish activity and pelagic dispersion in relation to fish size, light, and temperature. Four phases (day, dusk, night, dawn) in fish dispersion were distinguished over the diel cycle, and the swimming speed of fish varied among these phases. Notably, average swimming speed by day was twice as high as by night. For all phases combined, fish size, light intensity, and temperature explained 52% of the variability in swimming speed. When different phases were analysed separately, fish size was the most important variable by day, whereas light had the strongest effects on swimming speed in the evening. During the mornings, variability in swimming speed was best correlated with temperature, but by night all factors (fish size, light intensity, temperature) had similar effects on activity. These results have implications for fish bioenergetics models. Such models should account for seasonal, light-driven cycles in activity-induced respiration estimates, in particular when modelling populations at high latitudes.