Ocean-type juvenile Chinook salmon (Oncorhynchus tshawytscha) can be present in the nearshore areas of Lake Washington and Lake Sammamish, WA for 4–5 months (January–June) and may encounter nonnatal tributaries. The use of these tributaries is not well known. We determined the presence of juvenile Chinook salmon in 12 tributaries through nighttime snorkel surveys from March to June. At one heavily-used tributary, we determined their temporal and spatial distribution by conducting surveys every two to three weeks from February to June. Additionally, we determined whether delta areas of tributaries are used by juvenile Chinook salmon by comparing their density and diet to other lakeshore sites. Of 12 streams surveyed, juvenile Chinook salmon were observed in eight. The abundance of Chinook salmon appeared to be related to a variety of factors including proximity to the natal stream, stream gradient, and stream size. In an intensively-monitored stream, juvenile Chinook salmon were found primarily in shallow areas in February and March and then shifted to deeper pools as juveniles increased in size. Within the lake nearshore area, juvenile Chinook salmon commonly used delta areas of nonnatal tributaries and their abundance was frequently greater than other nearby shoreline sites. Diet analysis indicated nonnatal streams are also a source of prey for lake-dwelling juvenile Chinook salmon, especially during rain events. Nonnatal tributaries in lake systems appeared to be valuable habitat features for juvenile Chinook salmon and can function in a variety of ways, including providing both stream and delta habitat.
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