November 27, 2013
Researchers from the SWFSC Fisheries Ecology Division’s Salmon Ocean Ecology Team undertook an experimental approach to quantify predation rates by Western Gulls on threatened juvenile steelhead populations in six central California coast watersheds.
Researchers used the recapture of unique tags, called passive integrated transponder (PIT) tags, on a gull breeding colony to identify the proportion of tagged steelhead that were consumed by Western Gulls and subsequently defecated or regurgitated on the colony. However, because many eaten tags may be deposited off-colony, researchers also conducted an experiment where they fed a known number of PIT tags to gulls to determine what proportion of eaten tags were subsequently detected on the colony.
Together, the results of these two datasets suggest gull predation may be as high as 30% in some watersheds during some years. This study reveals that avian predation can be a significant source of mortality for juvenile salmonids. Furthermore, these results suggest even a generalist and opportunistic predator such as the Western Gull can exert high predation pressure on juvenile salmonid populations, and highlight the need to consider sources of predation when managing for the recovery of imperiled stocks.
Osterback, Ann-Marie K., Danielle M. Frechette, Andrew O. Shelton, Sean A. Hayes, Morgan H. Bond, Scott A. Shaffer, and Jonathan W. Moore. 2013. High predation on small populations: avian predation on imperiled salmonids. Ecosphere 4(9): article 116 (21 p.).
Contact: SWFSC Fisheries Ecology Division, Salmon Ocean Ecology Team
Hundreds of Western Gulls congregate at shallow creek mouths to bathe, drink, and opportunistically feed on items that are transported downstream by stream flow, including outmigrating juvenile salmonids. (Photo taken at the mouth of Scott Creek, Santa Cruz County, California.)