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.

Previous studies of the spatial pattern of stream invertebrate drift have focused on spatial variation at microhabitat scales or landscape scales, or temporal variation over diel or seasonal scales. None have examined consistency in spatial variation over longer time scales (>1 year). This study examined invertebrate drift density and composition at fixed locations (terminal ends of 10 riffles) each month at day and night along a 1 km reach of a 2nd order stream over a period of nearly 2 years. Consistent differences in the density of macroinvertebrate drift between riffles over 2 years were observed. The only habitat characteristic observed to be related to invertebrate drift density was the length and size of riffles above sampling sites, with larger and longer riffles producing the highest drift densities. Consistent differences in the supply of drifting macroinvertebrates along a stream reach may have implications for the supply of colonists to substrate patches and the profitability of feeding positions for drift-feeding fish and other predators.

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We examined diel variation in prey selection by giant kokopu, Galaxias argenteus, in relation to diel variation in the abundance of drifting invertebrates in a second order stream in Otago, New Zealand in summer. Clear differences in both the quantity and composition of prey consumed was evident. Greater numbers of aquatic insects were consumed at night relative to the day, whereas more terrestrial insects were consumed by day relative to the night. Giant kokopu were selective in the prey consumed. Trichoptera dominated giant kokopu diet at night even though drift samples collected at night were dominated by Deleatidium mayflies. Results suggest that giant kokopu feed actively during night and day in summer, and that prey selection is influenced by interactions between factors including prey availability, prey characteristics and light.