COS 82-7
Influence of inflows into and exports from the Sacramento-San Joaquin Delta on survival and migration routes of Chinook salmon smolts
Migration from freshwater rearing habitats to the ocean is a critical process in the life cycle of Chinook salmon (Oncorhynchus tshawytscha). Survival during this journey is heavily dependent upon migration pathways and travel times, which are likely to be strongly influenced by hydrodynamics. The influence of flow characteristics on the fate of migrating smolts is of particular concern in the Sacramento-San Joaquin Delta in California, a heavily engineered estuarine system that supports two Endangered Species Act-listed O. tshawytscha runs. The hydrodynamics of this system are heavily influenced by water management projects, primarily the Central Valley Project (CVP) and the State Water Project (SWP), which deliver a large fraction of Delta water to municipal and agricultural users outside of the Delta. We employed an individual-based particle tracking model to explore the influence of CVP exports, SWP exports, and river inflows on the survival probabilities and pathways of Chinook smolts migrating through the Delta. Smolt fate was simulated for a range of scenarios drawn from historical flow data and spanning a range of export:inflow ratios; for a variety of release locations; and for a variety of hypothesized fish behaviors.
Results/Conclusions
We found that under a null model of passively drifting fish, fish fate was strongly linked to inflow rates. At low inflows, transport of smolts through the Delta was slow and their total distance traveled was great due to frequent reversals of direction caused by tidal fluxes. Mortality was primarily caused by predation, which was elevated due to the extended residence times. For fish released in the southern part of the Delta, near the export facilities, entrainment in the CVP and SWP pumping plants was substantial for all export rates, though predation-induced mortality was still dominant. Entrainment and predation were substantially mitigated by increasing inflow, which increased the smolts’ transport rates, thereby minimizing exposure to predators. Incorporating more realistic smolt behaviors, such as more complex route decision rules and active swimming in response to flow rates, salinity gradients, and time of day, substantially altered the qualitative outcomes. The influence of water exports on fish fate ranged from negligible to substantial depending on the assumed behavior. Our results reaffirm the influence of flow rates on smolt survival found in previous studies while underscoring the importance of an understanding of fish behavior for predicting the outcome of water management actions.