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.

Developing a broader scientific foundation for river restoration: Columbia River food webs

“…In the Columbia, linked trophic and population models have been essential in understanding the scope of predation by northern pikeminnow (Ptychocheilus oregonensis) and nonnative predators in the mainstem river reservoirs (59)(60)(61)(62), impacts of predation by gulls (Larus spp.) and Caspian terns (Hydroprogne caspia) on migrating juvenile salmon (63)(64)(65)(66), impacts of nonnative mysids and lake trout on kokanee and native salmonids in lakes (28,67), complex species interactions (68), and stage-specific growth and survival of some juvenile salmon populations during freshwater and early marine rearing (69,70). General statistical and population models have been used to explore density dependence and carrying capacities in lakeand stream-rearing populations (71)(72)(73)(74).…”

Survival Estimates for the Passage of Spring-Migrating Juvenile Salmonids through Snake and Columbia River Dams and Reservoirs, 2008.

“…Predation is one factor that unquestionably directly affects survival of migrating smolts (Collis et al 2002). Avian piscivores are abundant along the Columbia River downstream from its confluence with the Snake River, and bird population sizes and consumption rates are well monitored.…”

Migratory Behavior and Survival of Juvenile Salmonids in the Lower Columbia River and Estuary in 2009

“…Two of these islands were particularly important to survival studies of fish migrating through the study area. Rice Island, a dredge spoils site at rkm 35 contained over 16,000 breeding pairs of terns, which were estimated to be dependent on salmonids for 74% of their diet (Collis et al 2002). Subsequent relocation efforts successfully moved a majority of these birds to East Sand Island, another dredge disposal site at rkm 10, where a colony of about 8,500 breeding pairs was established by 2002.…”

“…In addition to the terns, Ryan et al (2005) cited presence of a colony of about 8,000 breeding pairs of double-crested cormorants on a 15,000-m 2 area of rock jetty attached to East Sand Island. The colony of cormorants on Rice Island had decreased from 1,082 birds in 1998 (Collis et al 2002) to no nesting pairs by 2002 ).…”