Relative Vulnerability to Avian Predation of Juvenile Salmonids Tagged with Passive Integrated Transponders in the Columbia River Estuary, 1998–2000

Abstract

Caspian terns Sterna caspia and double-crested cormorants Phalacrocorax auritus that colonize dredge-spoil islands in the Columbia River estuary prey upon millions of juvenile Pacific salmonids annually. We estimated the relative vulnerability of various salmonid stocks to these predators by using data from passive integrated transponder (PIT) tags detected on these colonies; 96,382 tags were detected from the 1998–2000 migration years. On tern colonies, detection rates were highest for tags from steelhead Oncorhynchus mykiss and lowest for tags from yearling chinook salmon O. tshawytscha. However, detection rates on cormorant colonies were similar for tags from steelhead and coho salmon O. kisutch but lower for tags from yearling chinook salmon. Analyses based on migration history showed tags of transported fish were frequently detected in lower proportions than those of their counterparts that migrated in-river, the pattern being most pronounced in steelhead. Analyses based on origin (hatchery versus wild) showed similar detection proportions for the tags of wild versus hatchery steelhead on both tern and cormorant colonies. In contrast, 3.1% of hatchery versus 1.1% of wild chinook salmon tags previously detected at Bonneville Dam were detected on a colony, the greater vulnerability of hatchery fish being more pronounced on tern colonies. These tags accounted for 11.5% of steelhead, 4.6% of coho salmon, and 2.6% of yearling chinook salmon detected at Bonneville Dam, the last downstream impoundment encountered by seaward migrants. These estimates of predation are minimal because detection efficiency was not 100% and tags from many salmonid prey were not deposited on a nesting colony.