Arrival Patterns and Movements of Adult Sockeye Salmon in Lake Washington: Implications for Management of an Urban Fishery

Abstract

The Lake Washington watershed in Washington State has several discreet breeding populations of sockeye salmon Oncorhynchus nerka that return over the course of several months and remain in the lake for up to 6 months before spawning. Depending on the arrival timing and distribution of sockeye salmon in the lake, in-lake fisheries might disproportionately exploit one population or another and might select individuals from within populations having traits that covary with arrival timing (e.g., age, size, and sex). We applied disk tags to adult sockeye salmon as they entered the lake in 2003 and 2004 and tracked individual fish with ultrasonic transmitters. Recovery rates of disk-tagged fish declined over the course of both seasons and were lower in 2004 than in 2003, suggesting that prespawning mortality was occurring because of high water temperatures. Arrival timing did not vary consistently between populations and was at most weakly correlated with life history traits (older fish tended to arrive earlier). Ultrasonic transmitters revealed that individual sockeye salmon were distributed throughout the lake in the early summer but gradually congregated near the mouth of their natal stream in the fall. Fisheries are therefore likely to exploit the populations in proportion to their overall abundance without selecting strongly for spawning date or life history traits, tending to minimize evolutionary effects within populations. However, the expansion of a hatchery on the river with the largest population has been proposed; if fisheries increase to take advantage of the enhanced population, they may have a disproportionate effect on the smaller wild populations in other tributaries.