Exploring the Impact of Avian Predators on Central California Salmonids

The working hypothesis of this project is that predatory birds, principally Western gulls, take a significant toll on federally listed coho and steelhead in Scott and Waddell Creeks. To test this, scientists captured, tagged, and radio-tracked 65 Western gulls, three common mergansers, a snowy egret and a kingfisher. Stable isotope signatures from blood and feather samples from these birds and 100 others (captured but not radio tagged) are being used to evaluate their diets. More than 6,000 salmonids have been tagged with PIT tags in the two creeks. In Scott Creek, down-stream movements and mortality rates are being monitored by PIT tag antennae arrays for hatchery and wild fish. On five trips to Año Nuevo Island, more than 200 egested PIT tags were recovered from seabird breeding areas. The tags are providing insights into the time of year salmon are most vulnerable to predation, as well as the age classes targeted by birds. Other seabird colonies are being scanned for egested tags, including the Caspian tern colony on Brooks Island in San Francisco Bay. The results, to date, reveal that gulls, but also other birds, are indeed impacting salmon. The researchers will soon begin testing the degree to which bird-exclusion devices can protect the fishes.