Activity patterns in northern rock sole are mediated by temperature and feeding history

Studies of activity patterns of fishes rarely consider the impact of environmental conditions or organism state. In this paper we demonstrate the influence of temperature and feeding history on the diel activity patterns of juvenile (age 1+) northern rock sole Lepidopsetta polyxystra (Orr and Matarese). Activity type (benthic vs. water column) and level were determined from hourly video recordings of fish in laboratory tanks with the aid of infrared illumination. Groups of rock sole (n = 4 per group) were observed at 2, 5, 9 and 13 °C without food and at 9 °C with food offered once in the morning, at dusk or at night. In all experiments, rock sole exhibited peak activity levels during the dusk and dawn transition periods and a temporal segregation of activity mode. Daytime and crepuscular activity was predominantly benthic, occurring within a few centimeters of the tank bottom, whereas a significant fraction of nighttime activity (up to 90%) occurred with rock sole swimming in the water column or at the water surface. The primary effect of water temperature on rock sole behavior was a reduction in daytime and crepuscular activity at low temperatures. Conversely, nighttime activity levels were independent of water temperature, resulting in a thermally induced shift in the primary activity period. Morning and dusk feeding produced a short-term (1 h) elevation in activity followed by activity suppression until the subsequent crepuscular phase. Nighttime feeding resulted in a longer period of elevated activity (2 h) and did not suppress further benthic or water column activity. These observations suggest a temporal segregation of activity between daytime foraging and nighttime movement that balances energetic demands with predation risk.