The San Francisco Bay is the most important diving duck wintering area, harboring nearly 50% of several diving duck species annually counted along the Pacific Flyway. Greater and Lesser Scaup and Surf Scoters are in decline due to unknown causes, and food is hypothesized as a main limiting factor in wintering areas.  At the last SFBJV science subcommittee meeting, USGS researcher Dr. Susan De La Cruz gave a status report on an ongoing project in collaboration with Dr. Jim Lovvorn of Southern Illinois University and the non-profit organization, Oikonos, on carrying capacity modeling of benthivore diving ducks in San Pablo Bay.

Model projections will ultimately inform the nationwide process led by the North American Waterfowl Management Plan to fine-tune population objectives used for evaluation of management and conservation delivery efficacy. Carrying capacity refers to the maximum number of bird days that can be supported by the food supply in a site used by migratory birds. Starting out as modeling effort for the entire SF Bay, available data constrained it to San Pablo Bay shoals. Utilizing 10 years of studies in San Pablo Bay, monitoring marked diving ducks and assessing subtidal foraging on benthic invertebrates between October and January, the project utilizes an individual behavior based model to assess the number of scaup and scoter bird days this sub-bay can support. The model focuses on the highest densities of all three species, excludes intertidal areas used by shorebirds, and is based on adequate invertebrate surveys available during 1990, 1993, 1999-2001. It looked at habitat partitioning among species with the invasive clam Corbula as the dominant bivalve prey. Model outcomes will include: Part I. Estimated threshold prey densities, and Part II. Estimated duck-use days for San Pablo Bay. Future questions to address include: How will sea level rise and associated geomorphic changes influence habitats and Corbula prey densities? How will inclusion of data on other predators (i.e. fish data) affect current threshold projections?

For more information on this project please contact Dr. De La Cruz at  This e-mail address is being protected from spambots. You need JavaScript enabled to view it