Pesticides and the Decline of Pelagic Fishes in Western North America's Largest Estuarine Ecosystem

Assessing the cumulative effects of pesticides on aquatic species or ecological functions has been difficult for historical, legal, conceptual, and practical reasons.  This presentation will examine the connections and gaps between conservation science and toxicology in the context of current use (modern) pesticides and the decline of pelagic fishes in the San Francisco Estuary (California, USA).  The ecological status of this highly managed estuary, formed by the confluence of the Sacramento and San Joaquin rivers in California’s Central Valley, is declining in response to interacting anthropogenic factors including point and non-point sources of pollution, water withdrawals for agricultural and domestic use, land-cover change, altered flow regimes, and colonization by non-native species.  Abundances of smelt, striped bass, shad, salmon, steelhead, and sturgeon have diminished in recent decades – some stocks sharply since the early 2000s.  The Central Valley is one of the most productive agricultural regions in the United States, and this productivity depends in part on the use of more than 800 different pesticides, with hundreds of thousands of kilograms of chemicals (active ingredients) applied each year (2005-2008 data from California Department of Pesticide Regulation; http://www.cdpr.ca.gov/docs/pur/purmain.htm).  Complex patterns of pesticide use at the ecosystem scale, together with scientific uncertainty related to potential unintended impacts on fishes and their habitats throughout the San Francisco Estuary, illustrate the many challenges that non-point source pollution can pose for the management of large aquatic systems.