Importance of migrating salmon smolt in Ring-Billed ( Larus delawarensis ) and California Gull ( L. californicus ) diets near Priest Rapids Dam, Washington

Ring-billed ( Larus delawarensis ) and California Gulls ( L. californicus ) have been implicated in depredations on migrating salmon smolt in the Columbia River. As part of a gull management program conducted in 1995 and 1996, we collected L. delawarensis (n = 120) and L. californicus (n = 45) near Priest Rapids Dam, Washington, and analyzed stomach contents to determine food habits and thus the importance of fish in gull diets. Percent volume measurements and index of relative importance rankings suggested a greater reliance on fish by L. californicus than by L. delawarensis . Peak percent consumption of fish by both species occurred in May, coinciding with peak salmon outmigration through Priest Rapids Dam; and for both species number of fish consumed by gulls was higher below Priest Rapids Dam. Gulls collected prior to, and after, peak smolt migration indicated low importance rankings for fish in both L. delawarensis and L. californicus diets. However, the importance ranking of fish in gull diets changed over time and was higher for both species as the smolt migration peaked in May.