Recovery of Coded Wire Tags on a Caspian Tern Colony in San Francisco Bay
We recovered coded wire tags (CWTs) from a Caspian tern (Hydroprogne caspia) colony on Brooks Island in San Francisco Bay, California to evaluate predation on juvenile salmonids (Oncorhynchus spp.) originating from the Sacramento and San Joaquin rivers. Subsamples of colony substrate representing 11.7% of the nesting habitat used by 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 are 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, with the tags' complete numeric code still identifiable. The CWTs found on the tern colony indicated that hatchery Chinook salmon (O. tshawytscha) trucked to and released in San Pablo Bay, located roughly 25 km from Brooks Island, were 313 times more likely to be consumed by Brooks Island Caspian terns than Chinook salmon that migrated in-river to the Bay. Of the CWT recovered on the tern colony, 98% were fall-run Chinook salmon, indicating a high susceptibility to tern predation relative to spring, winter, and late-fall run-types. 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 impacts to wild, ESA-listed Chinook salmon populations were minimal in 2008. Overall, we estimate that ca. 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 and the tags deposited on the Brooks Island colony. Results indicate that CWTs implanted in juvenile salmon can be recovered from a piscivorous waterbird colony and used to evaluate smolt losses for those runs that are tagged.
- A.F. Evans, D.D. Roby, K. Collis, B.M. Cramer, J.A. Sheggeby, L.J. Adrean, D. S. Battaglia, and D. E. Lyons
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