Impacts of Avian Predation on Juvenile Salmonids in Central California Watersheds

Abstract

Central California coho salmon (Oncorhynchus kisutch) are endangered and steelhead (O. mykiss) are threatened under the U.S. Endangered Species Act. As part of local monitoring efforts, Passive Integrated Transponders (PIT tags) are used in population biology and marine survival studies of these stocks. Since 2006, PIT tags have been recovered on Ano Nuevo Island (ANI), an important breeding site for several species of seabirds and marine mammals. The objective of this study was to assess magnitude and effects of avian predation on juvenile salmonids in central California. Objectives were accomplished by 1) identifying predators, timing of predation, and quantifying predation using PIT tag recoveries and Mark-Recapture modeling, and 2) using behavior of a predator (the Western Gull) to examine susceptibility of juvenile salmonids to predation. Between 2006 and 2010, 252 unique PIT tags were recovered on ANI in areas that indicate Western Gulls were the primary predator depositing tags on ANI. Predation by gulls occurred during downstream migration of juvenile salmonids or immediately following ocean entry. Mark-Recapture modeling indicated PIT tags from 1-4% of tagged salmonids were deposited on ANI from watersheds in central California. Minimal temporal overlap between gulls and salmonids occurred at area watersheds, and observation of predation was rare; however, predation was 100% when overlap occurred. Predation by Western Gulls was an appreciable source of mortality for central California salmonids and should be considered in future management plans for these species.