Passive integrated transponder (PIT) tags implanted in Columbia River basin juvenile salmonids Oncorhyncus spp. were recovered from breeding colonies of Caspian terns Sterna caspia and double‐crested cormorants Phalacrocorax auritus on Rice Island, a dredge spoil island in the Columbia River estuary. Tags were recovered to assess the relative vulnerability of different salmonid species, stocks, and rearing types to avian predators. We detected 50,221 PIT tags at the two bird colonies, mostly from juvenile chinook salmon O. tshawytscha and steelhead O. mykiss raised in hatcheries; 72% of the total tags were from the tern colony and 28% from the cormorant colony. Tagged steelhead smolts were more vulnerable to predation by both bird species than were yearling chinook salmon. More than 15% of PIT tags from steelhead smolts that were available in the estuary in 1998 were detected at the bird colonies compared with 2% of PIT tags from yearling chinook salmon. The greater vulnerability of steelhead may reflect size‐dependent selection by avian predators. Salmonids listed under the Endangered Species Act and unlisted salmonids were equally vulnerable to predation by both terns and cormorants. Hatchery‐raised yearling chinook salmon were more vulnerable than their wild counterparts to predation by terns, a surface‐feeding species; however, hatchery‐raised and wild yearling chinook salmon were equally vulnerable to predation by cormorants, a diving species. These results suggest that hatchery‐raised yearling chinook salmon, and hatchery‐raised steelhead in some years, are more vulnerable to tern predation than wild fish because they have a greater tendency to reside near the water surface where terns forage.

No 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.