Application of a Particle Tracking Model for San Francisco Estuary Striped Bass Eggs and Larvae

The San Francisco Estuary (SFE), inclusive of the Sacramento/San Joaquin Delta (Delta), is a highly urbanized estuary, where recent (ca., 2000) declines in several pelagic fishes have led to resource issues of national significance. Numerous factors have been implicated in the decline of SFE striped bass, including habitat changes, water diversions, invasive species, and toxic effects. To help address some of these issues, we developed a particle-tracking model to track the fate and transport of striped bass eggs and larvae in the Delta. The model was built upon an existing hydrodynamic model, the Delta Simulation Model 2 (DSM2). In this research we investigated the relative effects of natural mortality (i.e., the combined mortality from water temperature and settling) and water diversion related mortality on striped bass eggs and larvae.

To examine egg and larval mortality, the temporal, spatial and magnitude of spawning events was inferred from detailed striped bass egg and larval survey data collected in 1990. First, we assumed the temporal distribution of eggs followed a normal distribution over the April-July period. Next, we interpolated spatially to obtain the number of eggs spawned in each channel at each hour. Finally, along with DSM2 hydrodynamic data from 1995, 2001, and 2003, we ran the particle-tracking model to represent the fate and transport of striped bass eggs and larvae during selected water years. Natural mortality of eggs and larvae was represented in the model by a temperature-dependant mortality function and eggs also experienced mortality as a function of settling (determined though model calibration to empirical survey data).

Overall egg through larval mortality (eggs surviving through 20-day post-hatch larvae) was 98.6%, 98.1% and 98.9% in a wet, dry and average water year. Natural mortality, due to the combination of temperature-dependant and settling mortality observed during the wet, dry and average water years, were responsible for 91.4 %, 89.6% and 89.5%, respectively, of the overall mortality. Agricultural water diversions accounted for 0.2 %, 0.5% and 0.3%, in each respective water year, of the overall mortality. Large scale water exports by the Central Valley Project and State Water Project accounted for 0.5 %, 0.6% and 1.2%, in each respective water year, of the overall egg through larval mortality. Lastly, reducing water exports during the wet year did not significantly impact the overall mortality. Reducing water export operations during the dry and average years reduced the overall mortality up to 0.2%.