Quantifying the effect of predators on endangered species using a bioenergetics approach: Caspian terns and juvenile salmonids in the Columbia River estuary

Abstract

We estimated the consumption of juvenile salmonids (Oncorhynchus spp.) and other forage fishes by Caspian terns (Sterna caspia) nesting on Rice Island in the Columbia River estuary in 1997 and 1998 using a bioenergetics modeling approach. The study was prompted by concern that Caspian tern predation might be a substantial source of mortality to out-migrating juvenile salmonids from throughout the Columbia River basin, many populations of which are listed as threatened or endangered under the U.S. Endangered Species Act. The bioenergetics model used estimates of the energy requirements of the tern population and the proportion of tern energy requirements met by various prey types. The resulting estimate of the number of juvenile salmonids consumed by Rice Island Caspian terns was 8.1 million (5.9–10.4 million) in 1997 and 12.4 million (9.1–15.7 million) in 1998. Tern predation rates on juvenile salmonids were substantial, representing up to 15% of the juveniles to reach the estuary from some listed populations. Nevertheless, based on simple age-structured models of salmonid populations, it appears unlikely that management of Caspian tern predation alone would reverse salmonid declines. Management to reduce tern predation could, however, contribute to a comprehensive strategy to recover imperiled salmonid populations in the Columbia River basin.

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).…”

Seasonal Juvenile Salmonid Presence and Migratory Behavior in the Lower Columbia River

“…Avian predation studies have shown a greater susceptibility to terns by hatchery steelhead and yearling Chinook, indicating they have a greater tendency to reside near the surface of the water (Collis et al 2001;Roby et al 2003).…”

Foraging Patterns of Caspian Terns and Double-crested Cormorants in the Columbia River Estuary

“…Diet composition data were not available for all colonies in all years due to funding and logistical constraints. Years when diet composition was evaluated included: 2003and 2008-2009at Brooks Island, 2003at Knight Island, and 2003and 2008-2009 at Eden Landing E-10.…”

“…The accuracy of visual identifications was verified using voucher specimens and photographs. We assumed that prey items brought back to the colony by breeding adults were representative of the overall diet of Caspian Terns at that particular colony Roby et al 2003).…”

“…The diet composition of Caspian Terns at each colony over the entire breeding season was based on the average across all two-week periods. Further details on the methodology used in this study are presented in Collis et al (2002) and Roby et al (2003).…”