BIRD PREDATION ON YOUNG SALMON IN THE COLUMBIA RIVER ESTUARY
Daniel D. Roby, Associate Professor and Assistant Unit Leader,
Oregon Cooperative Fish and Wildlife Research Unit,
U.S. Geological Survey-Biological Resources Division, and Department of Fisheries and Wildlife,
Oregon State University, Corvallis, OR 97331-3803.

Virtually every evolutionarily significant unit (ESU) of anadromous salmonid (Oncorhynchus spp.) in the Columbia River Basin is currently or soon will be listed under the Endangered Species Act of 1973. Colonial waterbirds (i.e., terns, cormorants, and gulls) may be important predators on juvenile salmonids in the lower Columbia River. Consequently, the Oregon Cooperative Fish and Wildlife Research Unit at Oregon State University and the Columbia River Inter-Tribal Fish Commission initiated a study in 1997 to assess the impacts of fish-eating birds on the survival of juvenile salmonids in the Columbia Basin during out-migration. The objectives of this study were to (1) estimate the size of fish-eating waterbird colonies in the lower Columbia River and determine population trends, (2) estimate the number of juvenile salmonids consumed by these populations, (3) identify the factors that influence avian predation rates on smelts, and (4) recommend ways to reduce avian predation on smelts, if warranted by the study results.

There were nine major colonies of fish-eating birds that nested on islands in the lower Columbia River during the 1997 and 1998 breeding seasons. Most of these island colony sites are unnatural, created by either the dumping of dredge material or rising water levels associated with mainstem dam impoundments. Population censuses indicated that the number of fish-eating colonial waterbirds totaled about 170,000 individuals, a substantial increase over previous estimates (Table 1).

Rice Island, a dredge material disposal island in the Columbia River estuary (Figure 2), supported the largest known Caspian tern (Sterna caspia) colony in North America (over 16,000 birds in 1997), which had grown by over 600% since the colony originated in 1987. In 1998, the tern colony had increased by about 25% over 1997 estimates to cat 20,000 birds. Nesting success at the Rice Island Caspian tern colony was only cat 5% in 1997, due mostly to predation on eggs and chicks by glaucous-winged/western gulls (Laws glaucescens X L. occidentalis). In contrast, nesting success in 1998 was ca. 40%.

Two colonies of double-crested cormorants (Phalacrocorax auritus) at East Sand Island and Rice Island in the estuary (Figure 2) are the first and second largest on the entire Pacific coast of the U.S. and Canada and also appear to be growing. The nesting period for these colonies (mid-April to mid-July) generally coincides with the period of smolt out-migration.

Diet analysis indicated that juvenile salmonids were an important part of the diet of fish-eating colonial waterbirds in the Columbia River estuary (Table 6). Caspian terns appeared to be most dependent on salmonids (ca. 75% of the diet), followed by double-crested cormorants (ca. 24% of the diet) and glaucous-winged/western gull hybrids (ca. 11% of the diet). The large California and ring-billed gull (Laws californicus and L. delawarensis) colonies up-river relied less on juvenile salmonids as a food source compared to fish-eating waterbirds in the estuary (Table 6), perhaps due to measures implemented at Columbia River dams to reduce bird predation.

Juvenile salmonids were especially prevalent in the diets of fish-eating waterbirds in the Columbia River estuary during May (Figure 7). Steelhead smolts were most prevalent in Caspian tern diets during early May, followed by coho smolts in late May - early June, and then chinook smolts in late June - late July (Figure 8).

Over 2,000 salmonid smolt PIT tags were found on the Rice Island Caspian tern colony by visually searching, and we estimated that over 30,000 PIT tags have been deposited there over the last nine years. The recovered PIT tags indicate that steelhead smolts were consumed in greater proportion to availability than other salmonid species, and that juvenile salmonids of hatchery origin were consumed in greater proportion to availability than wild smolts (Figure 11).

We used a bioenergetics modeling approach to estimate the numbers of juvenile salmonids consumed by the Rice Island Caspian tern colony in 1997 (Figure 3). Model-based estimates were that 6 - 25 million juvenile salmonids were consumed by Caspian terns, or approximately 6 - 25 % of the estimated 100 million out~ migrating smolts that reached the estuary in 1997 (Tables 14 and 16). Preliminary analyses suggest that the number of juvenile salmonids consumed by Rice Island Caspian terns in 1998 was 8 - 30 million, an increase over 1997. In addition, preliminary estimates of the number of juvenile salmonids lost to cormorants and gulls in the estuary are in the millions.

The magnitude of Caspian tern predation on juvenile salmonids in the Columbia River estuary has been cause for considerable surprise and concern among fisheries and wildlife managers in the Pacific Northwest. How could losses of smolts to birds, especially to one species of fish-eating bird nesting in one colony in the Columbia River estuary, be so high? Is this level of avian predation the norm, or does it represent an aberrant situation reflecting a highly perturbed ecosystem? We think there are four observations that relate to the current situation. First, the Columbia River estuary has experienced declines of forage fish stocks that would, under other circumstances, provide alternative prey for fish-eating birds such as terns. Second, most of the salmonids consumed by Caspian terns at the Rice Island colony were raised in hatcheries, and the proportion of hatchery-raised smolts in the diet of terns exceeds what would be expected based on availability. Third, juvenile salmonids that survive the out-migration to the estuary most negotiate dams, slack water impoundments, and other obstacles in their efforts to reach the sea. The cumulative stress associated with this migration likely enhances their vulnerability to tern predation. Finally, the Caspian tern colony on Rice Island is now the only known colony of its kind along the coast of Oregon and Washington, and Rice Island represents one of the few, if not the only suitable nesting habitat for this species along the coast of the Pacific Northwest. This mega-colony has coalesced at Rice Island because there are few other options.

One of our research objectives for 1998 field season was to test the feasibility of potential methods to reduce predation on smelts by Caspian terns, including translocating the colony to a previous colony site on East Sand Island, close to the mouth of the Columbia River. Results from the 1998 field season suggest that moving the Caspian tern breeding colony from Rice Island to East Sand Island may be an effective method to mitigate losses of smalls to terns in the estuary.

East Sand Island is about 13 miles down-river from Rice Island and 6 miles up-river of the mouth of the Columbia River. A greater diversity of forage fishes are available to fish-eating birds in the vicinity of East Sand Island compared to Rice Island. In 1998, double-crested cormorants nesting on East Sand Island consumed a much smaller proportion of juvenile salmonids (ca. 10%) than cormorants nesting on Rice Island (ca. 55%). Caspian terns in the estuary foraged mostly within five miles of the breeding colony at Rice Island, and 90% foraged within 13 miles of the colony. Attempts in 1998 to attract Caspian terns to nest at a new site in the estuary (Miller Sands) using decoys and an audio playback system were successful. Finally, Caspian terns formerly nested on East Sand Island in the mid-1980s, and still frequently roost on the island.

These research results suggest that translocating the Caspian tern colony from Rice Island to East Sand Island is a feasible short-term management option for reducing tern predation on juvenile salmonids. Longer term management may include attracting portions of the current Rice Island Caspian tern population to nest outside the Columbia River estuary. Caspian tern colonies that formerly existed in Willapa Bay, Grays Harbor, and Puget Sound in the State of Washington are no longer extant, and there is evidence that these former colonies have coalesced to form the very large Rice Island colony. Re-establishing these colonies may provide considerable benefits for salmon restoration in the Columbia River Basin and reduce the vulnerability of the tern population to catastrophic events, like oil spills. Management action focusing on tern predation in the estuary may be an effective and efficient component of a comprehensive plan to restore salmon to the Columbia River Basin.

The Interagency Avian Predation Working Group, which includes representatives from the U.S. Army Corps of Engineers, National Marine Fisheries Service, U.S. Fish and Wildlife Service, Oregon Department of Fish and Wildlife, Washington Department of Fish and Wildlife, Bonneville Power Administration, Columbia River Inter-Tribal Fish Commission, Oregon State University, USGS-Biological Resources Division, and USDA-Wildlife Services, was formed in May 1998 to develop a plan for mitigating the impact of avian predation on juvenile salmonids in the lower Columbia River. At a recent meeting of the Working Group, it was decided to proceed with plans to attempt to relocate the Rice Island Caspian tern colony to a former tern colony site on East Sand Island. This would involve a combination of efforts to attract the terns to nest on East Sand Island and dissuade them from nesting on Rice Island. The former would consist of (1) habitat modification on a portion of East Sand Island to provide the bare sand nesting habitat preferred by terns, (2) placing several hundred Caspian tern decoys on the new colony site to attract terns to land, (3) setting up several audio playback systems on the new colony site to simulate the acoustic environment of a tern colony, and (4) assure that avian predators (gulls, crows) are prevented from disrupting early attempts by terns to breed at the new site. Efforts to dissuade terns from nesting at their current colony site on Rice Island will probably consist of vegetating the current site so that bare sand substrate is no longer available. If this approach fails, additional efforts to move the colony may consist of attracting natural predators (e.g., eagles, gulls, crows) or harassing the terns as they roost on Rice Island at night, prior to the initiation of egg-laying.

Responsibilities of the different agencies in implementing a plan to reduce avian predation in FY 99 have been discussed and agreed upon, both at the Executive-level and within the Working Group. The Oregon State University/Columbia River Inter-Tribal Fish Commission research team will (1) coordinate the efforts to attract the terns to East Sand Island, (2) conduct the monitoring and evaluation to determine the efficacy of management in reducing tern predation on smelts, (3) test the feasibility of other potential management actions to further reduce tern predation on juvenile salmonids (e.g., bird deterrent devices in foraging areas), and (4) continue to monitor other avian predator populations that may be targeted for management in the near future. To complete the M & E tasks assigned to the research team in FY 99, we will need an additional $204,000 beyond what has already been tentatively approved for funding by the Northwest Power Planning Council. The additional funding is needed to (1) conduct a radio telemetry study as part of the monitoring and evaluation of the management of the Rice Island tern colony, (2) conduct necessary repairs to project boats, and (3) cover the additional personnel costs needed to complete the work. In addition, $50,000 will be required to create Caspian tern nesting habitat on East Sand Island, $32,000 will be required to attract the tern colony to nest on this new habitat on East Sand Island, and at least $60,000 will be required to vegetate the current tern colony site on Rice Island.

The proposed management action of translocating the Rice Island Caspian tern colony to East Sand Island has the potential to save 3 - 12 million smelts that have reached the estuary and would otherwise have been consumed by terns. Monitoring and evaluation of that plan is critical for adaptive management of the problem. For example, if radio telemetry tells us that terns continue to forage at up-river locations, we can quickly identify foraging hot spots for avian predators that can be managed in season, if necessary.

It is unclear that any alternative sources of funding may be available in time to order needed equipment and supplies necessary for translocation of the Caspian tern colony to East Sand Island or for radio telemetry of terns nesting on East Sand Island, crucial to the monitoring and evaluation of the management action. We ask the Committee for assistance in identifying additional potential sources of support for effecting the translocation of the Caspian tern colony ($142,000) and for the monitoring and evaluation ($204,000) of management initiatives aimed at reducing losses of juvenile salmonids to birds in the Columbia River estuary.