Adrean, L.J., D.D. Roby, D.E. Lyons, K. Collis, and A.F. Evans. 2012. Potential effects of management on Caspian tern (Hydroprogne caspia) predation on juvenile salmonids at a colony in San Francisco Bay, California. Transactions of the American Fisheries Society 141:1682-1696. (S)
Abstract San Francisco Bay is a proposed relocation site for some of the Caspian terns (Hydroprogne caspia) currently nesting at the world’s largest colony for the species in the Columbia River estuary and consuming salmonids listed under the U.S. Endangered Species Act (ESA). However, several runs of salmonids (Oncorhynchus spp.) listed under the ESA occur in San Francisco Bay and managers are concerned that increased Caspian tern predation may pose a threat to the recovery of these salmon. We used a bioenergetics modeling approach, employing estimates of tern energy requirements and proportions of energy supplied by various prey types, to estimate consumption of juvenile salmonids by Caspian terns nesting on Brooks Island in central San Francisco Bay during 2008 and 2009. Estimated salmonid consumption was ca. 205,000 smolts (95% CI: 175,000–245,000 smolts) in 2008 and ca. 167,000 smolts (95% CI: 144,000–191,000 smolts) in 2009. The inter-annual difference in smolt consumption was due to the smaller size of the tern colony and lower nesting success in 2009. Estimated predation rates on ESA-listed Central Valley spring-run Chinook (0.1%) were lower than those on unlisted fall-run Chinook (1.0%). Continuation of the current downward trend in the number of Caspian terns nesting on Brooks Island, and resulting reductions in salmonid predation, would not be sufficient to reverse salmonid declines in San Francisco Bay. The proposed enhancement of the Brooks Island Caspian tern colony to 3,000 individuals would at most cause declines in annual population growth rates (λ) of 0.28% for fall-run Chinook and 0.02% for threatened spring-run Chinook, assuming mortality from tern predation is 100% additive.