Exploring Drivers of Pelagic Fish Population Decline in Bay-Delta

Science Center Objects

USGS scientists develop spatially explicit ecosystem model to explore changes pelagic fish species population and distribution in the Sacramento-San Joaquin Delta and Suisun Bay.

Longfin Smelt

Longfin smelt, male, 141 mm FL. Photographed on March 7, 2008 at the Tracy Fish Collection Facility, Tracy, CA. (Credit: Rene Reyes, U.S. Bureau of Reclamation. Photo: Rene Reyes, U.S. Bureau of Reclamation)

Prior to 2000, the four most abundant resident pelagic fishes in the study area included two native species, delta smelt (Hypomesus transpacificus) a federal listed endangered species, and longfin smelt (Spirinchus thaleicthys) a state listed endangered species, and two introduced species, threadfin shad (Dorosoma petenense) and age-0 striped bass (Morone saxatilis). Beginning in the early 2000's, abundance indices of these four pelagic fishes showed concurrent declines to record or near record lows (Sommer et al. 2007). The most alarming aspect of this decline was that it occurred during a series of relatively wet years when these fishes would be expected to experience good survival and increasing populations (Sommer et al. 2007). The multi-species decline, termed the Pelagic Organism Decline (POD), has been verified by statistical analyses (Thomson et al. 2010) and is under investigation by the Interagency Ecological Program (IEP) and others, including multiple state, federal, and university researchers. Because of the intense interest in the POD species, particularly delta smelt, little effort has been invested in trying to understand multi-species interactions in the study area, which may help elucidate causes for POD and lead to better understanding of ecosystem processes.

The basic objective of this proposal is to utilize a spatially explicit ecosystem model for the Sacramento-San Joaquin Delta and Suisun Bay to explore hypotheses regarding causes for the POD. Specifically, we use the spatially explicit form (Ecospace) of the Ecopath with Ecosim (EwE) modeling framework to model biomass and trophic pathways (food web) and how those pathways may have changed in response to changes in the physical-chemical environment and introductions of new species. The completed model will also provide a useful tool for simulating the outcomes of management actions being considered for the study area.

This project utilizes the EwE modeling framework developed by researchers at the University of British Columbia's Fishery Centre (Christensen and Pauly, 1992). The starting point is an EwE model for the study area by Marissa Wulff (formerly Bauer) (Bauer 2010). This proposal will involve:

  • Updating the existing model with new data sources
  • Developing the spatial framework in collaboration with NOAA collaborators
  • Incorporating Monte Carlo simulations in collaboration with NOAA collaborators so that sensitivity of the model to parameter uncertainties can be determined
  • Evaluating hypotheses regarding causes of POD and possible beneficial effects of proposed management actions