Recovery of Coded Wire Tags at a Caspian Tern Colony in San Francisco Bay: A Technique to Evaluate Impacts of Avian Predation on Juvenile Salmonids

Abstract

We recovered coded wire tags (CWTs) from a colony of Caspian terns Hydroprogne caspia on Brooks Island in San Francisco Bay, California, to evaluate predation on juvenile salmonids originating from the Sacramento and San Joaquin rivers. Subsamples of colony substrate representing 11.7% of the nesting habitat used by the terns yielded 2,079 salmonid CWTs from fish released and subsequently consumed by terns in 2008. The estimated number of CWTs deposited on the entire tern colony was 40,143 (ranging from 26,763 to 80,288), once adjustments were made to account for tag loss and the total amount of nesting habitat used by terns. Tags ingested by terns and then egested on the colony were undamaged, and the tags’ complete numeric codes were still identifiable. The CWTs found on the tern colony indicated that hatchery Chinook salmon Oncorhynchus tshawytscha trucked to and released in San Pablo Bay were significantly more likely to be consumed by Caspian terns than Chinook salmon that migrated in-river to the bay; 99.7% of all tags recovered were from bay-released Chinook salmon. Of the CWTs recovered on the tern colony, 98.0% were from fall-run Chinook salmon, indicating a higher susceptibility to tern predation than for the spring run type. None of the approximately 518,000 wild Chinook salmon that were coded-wire-tagged and released in the basin were recovered on the tern colony, suggesting that the impacts on wild, U.S. Endangered Species Act–listed Chinook salmon populations were minimal in 2008. Overall, we estimate that 0.3% of the approximately 12.3 million coded-wire-tagged Chinook salmon released in the basin in 2008 were subsequently consumed by Caspian terns from the Brooks Island colony. These results indicate that CWTs implanted in juvenile salmon can be recovered from a piscivorous waterbird colony and used to evaluate smolt losses for runs that are tagged.

Received January 25, 2010; accepted December 13, 2010

ACKNOWLEDGMENTS

This project was funded by the U.S. Army Corps of Engineers (USACE), Portland District. We especially want to thank Paul Schmidt and Geoff Dorsey (USACE) for their assistance and support during the study. We thank Kevin Neimela and Patricia Brandes from the U.S. Fish and Wildlife Service, Armando Quinones and Clint Garman from the California Department of Fish and Game, and Kari Burr from the Fisheries Foundation of California for providing support and information on CWT releases. We thank Gary Shugart of the University of Puget Sound for providing some initial guidance on ways to recover CWTs from tern nesting material. We are grateful to Steve Bobzien of East Bay Regional Parks District for granting access to Brooks Island and for his long-standing support of our research. A special thanks to Tim Marcella, Jarvi Kononen, and Angela Whitney for their assistance in recovering CWTs as part of this study.