Diel activity and home range size in relation to food supply in a drift-feeding stream fish

Individuals within a species often compete for resources in both space and time. In dominance hierarchies individuals with the greatest competitive ability will occupy prime locations during optimal periods. We compared spatial and temporal use of habitat, aggression, and feeding success among giant kokopu (Galaxias argenteus) in dominance hierarchies under normal and reduced food supply. Under normal food supply conditions dominant fish were predominantly nocturnal and maintained large home ranges at night. Conversely, subdominant fish were diurnal and occupied large home ranges by day but were generally not observed at night. When the food supply was limited, the utilization of pools was determined by social rank. Dominant fish from each pool increased diurnal activity, home range size, aggressiveness, and the capture of food items offered by day while simultaneously reducing the spatial and temporal activity and habitat use of subdominant fish. These results indicate that behavioral changes in large dominant fish influence and reduce the amount of resources available to subdominant fish. Copyright 2005.