Cyril J. Michel

Diel movements of out-migrating Chinook salmon (Oncorhynchus tshawytscha) and steelhead trout (Oncorhynchus mykiss) smolts in the Sacramento/San Joaquin watershed

We used ultrasonic telemetry to describe the movement patterns of late-fall run Chinook salmon (Oncorhynchus tshawytscha) and steelhead trout (O. mykiss) smolts during their entire emigration down California’s Sacramento River, through the San Francisco Bay Estuary and into the Pacific Ocean. Yearling hatchery smolts were tagged via intracoelomic surgical implantation with coded ultrasonic tags. They were then released at four upriver locations in the Sacramento River during the winters of 2007 through 2010. Late-fall run Chinook salmon smolts exhibited a nocturnal pattern of migration after release in the upper river. This is likely because individuals remain within a confined area during the day, while they become active at night and migrate downstream. The ratio between night and day detections of Chinook salmon smolts decreased with distance traveled downriver. There was a significant preference for nocturnal migration in every reach of the river except the Estuary. In contrast, steelhead smolts, which reside upriver longer following release, exhibited a less pronounced diel pattern during their entire migration. In the middle river, Delta, and Estuary, steelhead exhibited a significant preference for daytime travel. In the ocean Chinook salmon preferred to travel at night, yet steelhead were detected on the monitors equally during the night and day. These data show that closely related Oncorhynchus species, with the same ontogenetic pattern of out-migrating as yearlings, vary in migration tactic.

Sensitivity of survival to migration routes used by juvenile Chinook salmon to negotiate the Sacramento-San Joaquin River Delta

Populations of juvenile salmon emigrating from natal rivers to the ocean must often traverse different migratory pathways that may influence survival. In regulated rivers, migration routes may consist of a network of channels such as in the Sacramento-San Joaquin River Delta, or of different passage structures at hydroelectric dams (e.g., turbines or spillways). To increase overall survival, management actions in such systems often focus on altering the migration routing of fish to divert them away from low-survival routes and towards high-survival routes. Here, we use a 3-year data set of route-specific survival and movement of juvenile Chinook salmon in the Sacramento-San Joaquin Delta to quantify the sensitivity of survival to changes in migration routing at two major river junctions in the Sacramento River. Our analysis revealed that changes in overall survival in response to migration routing at one river junction depended not only differences in survival among alternative routes, but also on migration routing at the other river junction. Diverting fish away from a low-survival route at the downstream river junction increased population survival by less than expected, given the difference in survival among routes, because part of the population used an alternative migration route at the upstream river junction. We also show that management actions that influence only migration routing will likely increase survival by less than actions that alter both migration routing and route-specific survival. Our analysis provides an analytical framework to help fisheries managers quantify the suite of management actions likely to maximize increases in population level survival.