Quantifying the effect of Caspian tern predation on threatened and endangered Pacific salmon in the Columbia River estuary

Abstract

Caspian terns Sterna caspia breeding in the Columbia River estuary exploit Pacific salmon Oncorhynchus spp. as prey, consuming millions of outmigrating juvenile salmonids annually. We analyzed recoveries of salmonid passive integrated transponder (PIT) tags from the East Sand Island tern colony to calculate predation rates (% of available fish taken) on 4 Columbia and Snake River steelhead O. mykiss Evolutionarily Significant Units (ESUs). A life cycle modeling approach was used to estimate potential increases in ESU population growth rate (λ) given potential reductions in Caspian tern numbers on East Sand Island. Reducing tern predation on steelhead ESUs by 50 - 100% increased λ from 0.8 to 2.5%, depending on the ESU and the reproductive contribution of hatchery fish, and assuming no compensatory mortality. This is comparable to survival improvements modeled for hydropower improvements in the basin but less than those modeled for harvest reduc- tions. Reducing avian predation as part of an effort to reduce all sources of mortality may assist in ESU recovery. A thorough understanding of such predator-prey relationships is needed to manage conflicts between predators and their threatened and endangered Pacific salmonid prey.