Density, aggregation, and body size of northern pikeminnow preying on juvenile salmonids in a large river

Predation by northern pikeminnow Ptychocheilus oregonensis on juvenile salm onids Oncorhynchus spp. occurred probably during brief feeding bouts since diets were either dominated by salmonids (>80% by weight), or contained oth er prey types and few salmonids (<5%). In samples where salmonids had been consumed, large rather than small predators were more likely to have captur ed salmonids, Transects with higher catch-per-unit of effort of predators a lso had higher incidences of salmonids in predator guts. Predators in two o l three reservoir areas were distributed more contagiously if they had prey ed recently on salmonids, Spatial and temporal patchiness of salmonid prey may be generating differences in local density, aggregation, and body size of their predators in this large river.