Potential Effects of Management on Caspian Tern Hydroprogne caspia Predation on Juvenile Salmonids at a Colony in San Francisco Bay, California

Abstract

AbstractSan 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 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 fish. We used a bioenergetics modeling approach, employing estimates of tern energy requirements and proportions of energy supplied by various prey types, to estimate the consumption of juvenile salmonids by Caspian terns nesting on Brooks Island in central San Francisco Bay during 2008 and 2009. Estimated salmonid consumption was ∼205,000 smolts (95% confidence interval, 175,000–245,000 smolts) in 2008 and ∼167,000 smolts (144,000–191,000 smolts) in 2009. The interannual 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 salmon Oncorhynchus tshawytscha (0.1%) were lower than those on unlisted fall‐run Chinook salmon (1.0%). Continuation of the current downward trend in the number of Caspian terns nesting on Brooks Island and the 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 of 0.28% for fall‐run Chinook salmon and 0.02% for threatened spring‐run Chinook salmon, assuming that the mortality from tern predation is 100% additive.