Survival of Juvenile Chinook Salmon in the Yolo Bypass and the lower Sacramento River, California

Abstract

While our knowledge of the range of survival that outmigrating juvenile Chinook Salmon experience in different routes of the Sacramento-San Joaquin Delta has increased in recent years, few studies have focused on their survival during outmigration in the Yolo Bypass, the Delta's primary floodplain. The Yolo Bypass floodplain provides valuable rearing habitat and growth benefits to juvenile fish in flood years, and efforts are underway to improve access to the Yolo Bypass and the Toe Drain, its perennial navigation channel, under a broader range of flows and river stages than is currently possible. We compared variation in transit time, and estimated survival between different release groups of fish outmigrating through the Yolo Bypass or through migratory routes in the lower Sacramento River. Tagged late-fall-run juvenile Chinook Salmon were released in both systems in 2012 and 2013. There was no significant difference between the estimated cumulative probability of survival in the Yolo Bypass system and combined routes of the lower Sacramento River in either year (0.312-0.629 vs. 0.342-0.599, 95% credible interval in 2012; and 0.111-0.408 vs. 0.240-0.407, 95% credible interval in 2013, respectively). The Yolo Bypass had a higher coefficient of variation (CV) in travel time relative to the lower Sacramento River routes in both years (0.34 vs. 0.29 in 2012, and 0.44 vs. 0.34 in 2013). This work suggests that in relatively low water years, the estimated survival of outmigrating juvenile Chinook Salmon in the Toe Drain is directly comparable to routes in the lower Sacramento River, and that metrics of behavioral diversity can and should be incorporated into future telemetry studies.