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 inboth space and time. In dominance hierarchies individuals withthe greatest competitive ability will occupy prime locationsduring optimal periods. We compared spatial and temporal useof habitat, aggression, and feeding success among giant kokopu(Galaxias argenteus) in dominance hierarchies under normal andreduced food supply. Under normal food supply conditions dominantfish were predominantly nocturnal and maintained large homeranges at night. Conversely, subdominant fish were diurnal andoccupied large home ranges by day but were generally not observedat night. When the food supply was limited, the utilizationof pools was determined by social rank. Dominant fish from eachpool increased diurnal activity, home range size, aggressiveness,and the capture of food items offered by day while simultaneouslyreducing the spatial and temporal activity and habitat use ofsubdominant fish. These results indicate that behavioral changesin large dominant fish influence and reduce the amount of resourcesavailable to subdominant fish.