The Columbia River Basin in North America, about the size of France, formerly produced billions of anadromous salmonids (salmon and steelhead) annually, all of which reach the Pacific Ocean via the Columbia River estuary. Currently, 13 of 20 Evolutionarily Significant Units (ESUs) of salmonids from the Basin are threatened or endangered under the U.S. Endangered Species Act, due primarily to over-harvest, degraded habitat, hydropower dams, and hatcheries. The largest breeding colony of Caspian terns in the world and the largest breeding colony of double-crested cormorants in western North America recently formed on an island in the Columbia River estuary. These colonies of native, protected waterbirds are products of (1) creation of artificial nesting islands, (2) reliable food supply produced by salmon hatcheries, and (3) loss of secure nesting sites and food resources elsewhere. Using bioenergetics models and recovery of smolt PIT tags on the bird colonies, we estimate losses of salmonid smolts to terns and cormorants are more than 10% of those that survive to the estuary. Fisheries managers view avian predation as one of several significant impediments to restoring threatened Columbia Basin salmonids. This knotty problem, which has pitted salmon conservationists against bird conservationists, cannot be resolved without active habitat management for both birds and fish.
Our research to measure the magnitude of avian predation on juvenile salmonids in the lower Columbia River and estuary began during the late 1990s. The goal of this work was to assess the impact of avian predation on recovery of ESA-listed salmonids. Our research indicated that annual smolt losses to avian predators were in the range of 15 – 20 million, a significant proportion of total out-migrating juvenile salmonids. Although most concern by fisheries managers over avian predation was focused on gulls foraging near dams, the vast majority of smolt losses to avian predators were caused by Caspian terns and double-crested cormorants nesting at large colonies in the estuary. These estimates of total numbers of smolts consumed by fish-eating birds were based on bioenergetics models, but recoveries of smolt PIT tags on waterbird colonies confirmed that smolt predation rates were highest for bird colonies in the estuary. Potential exceptions to this general rule are the relatively small colonies of Caspian terns and double-crested cormorants near the confluence of the Snake and Columbia rivers and Caspian terns nesting off the Columbia River at Potholes Reservoir; in some years predation rates on in-river migrants of Snake and Columbia River salmonid stocks have approached those of the large colonies in the estuary.
Fishery managers are embarking on an expensive and elaborate plan to create new colony sites for Caspian terns outside the Columbia Basin, with the intention of redistributing most of the terns that currently nest in the Columbia River region and reducing smolt losses to tern predation. This management plan is based on the hypothesis that (1) suitable colony sites are limiting and (2) food supply is not the primary factor dictating where terns nest. Our research results have provided support for both of these hypotheses; the availability of suitable nesting habitat for Caspian terns and other colonial waterbirds is the primary factor dictating where population-source colonies occur, but artificially enhanced food supplies (e.g., salmon hatcheries) can attract birds to nest in marginal habitat where colonies are population-sinks and fisheries conflicts arise. Habitat management to create or enhance colony sites where breeders are productive without relying on artificially enhanced prey can help strike a balance between the restoration of threatened fish stocks and the conservation of piscivorous colonial waterbirds. In addition to managing Caspian terns, a management plan to reduce the smolt impacts of double-crested cormorants that nest in the Columbia River estuary have been developed and are currently underway.