Gulls Eat Fish: Assessing Predation Risk of Juvenile Salmonids Using Radio-Telemetry

Recoveries of PIT tags from Año Nuevo Island (ANI; San Mateo County, California) indicate that predation by Western Gulls  may represent a significant source of mortality for juvenile coho salmon and steelhead originating in watersheds in central California. Whereas central California coho and steelhead populations have decreased in recent years, and are listed by the U.S. Endangered Species Act, the breeding population of gulls on ANI is increasing, likely because of access to anthropogenic food sources. To assess susceptibility of juvenile salmonids to predation by gulls breeding on ANI, we examined movements and foraging habits of Western Gulls at two Santa Cruz county watersheds (Scott and Waddell Creeks). During 2009, 40 Western Gulls were captured and fitted with VHF radio-transmitters. Tagged gulls were tracked along 55 km of coastline to examine movements of adult gulls in relation to stage of the breeding cycle, availability of juvenile salmonids as a source of prey, and alternative foraging sites. Seventy-one percent of radio-tagged Western Gulls that used ANI were detected at a landfill 25 km south of ANI indicating that anthropogenic food subsidies likely explain recent increases in Westerns Gull breeding on ANI.  In addition, most detections of radio-tagged gulls occurred within 25 km of ANI during stages of the breeding cycle when birds were most tied to nesting sites, which coincided with migration of juvenile salmonids from fresh to salt water. These results correspond with recoveries of PIT tags on ANI, which indicated that relative deposition rates of PIT tags from watersheds where PIT tags were deployed decreased with increasing distance from ANI. Data loggers were installed at two creek mouths (Scott and Waddell) to continuously record presence of tagged gulls and visual observations were conducted to document predation attempts by gulls. Western Gulls were present during daylight hours (90% of detections), whereas juvenile salmonids were present at night (90% of detections).  Observation of predation of juvenile salmonids by Western Gulls was rare; 20 predation events were observed during 198 hours of observations.  Although predation appeared to be limited by the low degree of temporal overlap between gulls and salmonids at Scott and Waddell Creeks, predation was 100% when overlap occurred.  Increases in the ANI breeding population combined with concurrent decreases in salmonid populations has resulted in levels of predation that may be unsustainable when combined with other pressures faced by these imperiled salmonid populations.