Colony Size and Diet Composition of Piscivorous Waterbirds on the Lower Columbia River: Implications for Losses of Juvenile Salmonids to Avian Predation

Abstract

We investigated colony size and diet composition of piscivorous waterbirds (gulls, terns, and cormorants) nesting on the lower Columbia River from the mouth (river km 0) to the head of McNary Pool (river km 553) in 1997 and 1998. The study was prompted by concern that avian predation might constitute a significant source of mortality to juvenile salmonids Oncorhynchus spp. during out-migration. The diet of California gulls Larus californicus and ring-billed gulls L. delawarensis nesting in colonies above The Dalles Dam (river km 308) included few fish and very few juvenile salmonids. The sole exception was a small colony of California gulls in which salmonids accounted for 15% (by mass) of the diet. Juvenile salmonids were, however, an important component of the diet of colonial waterbirds nesting in the Columbia River estuary. On Rice Island (river km 34), salmonids accounted for 74% (by mass) of the diet of Caspian terns Sterna caspia, 46% for double-crested cormorants Phalacrocorax auritus, and 11% for glaucous-winged−western gulls L. glaucescens × L. occidentalis. Juvenile salmonids were especially prevalent in the diets of colonial waterbirds on Rice Island during April and May. By comparison, juvenile salmonids were significantly less prevalent in the diet of cormorants and gulls nesting lower in the estuary on East Sand Island (river km 8), presumably due to the greater availability of marine forage fishes. Our results indicate that avian predation on juvenile salmonids in the lower Columbia River is more prevalent in the estuary than near the large upriver gull colonies. Furthermore, the high incidence of salmonids in the diets of Caspian terns, cormorants, and gulls nesting on Rice Island suggests that the impact of avian predation on survival of smolts may be reduced by discouraging piscivorous birds from nesting there, while encouraging nesting on East Sand Island and other sites nearer to marine foraging areas.