Evolving Conceptual Models for Understanding the Upper San Francisco Estuary

Abundance indices for four pelagic fishes (native delta smelt and longfin smelt and introduced threadfin shad and age-0 striped bass) in the upper San Francisco Estuary declined rapidly and unexpectedly to record low levels starting in 2002. In 2005, the Interagency Ecological Program (IEP) formed an interagency team to evaluate the potential causes of what has come to be known as the Pelagic Organism Decline (POD). Since then, the POD team has orchestrated a large, multi-disciplinary investigation. A series of three conceptual models has been central to developing the POD investigation and to synthesizing and communicating results. These complementary conceptual models help investigate and tell the “POD story” from different perspectives. A “basic POD conceptual model” was introduced in 2006 and groups the effects of potential drivers of the POD into four categories (previous abundance, habitat, top-down effects, and bottom-up effects). The emerging conclusion is that the POD was caused by multiple and often interacting drivers. Successful management solutions will likely also have to address multiple drivers. “Species-specific conceptual models” were introduced in 2008. They show how the major drivers differ for each of the four POD fish species, and how they differ in relative importance during different life history stages or seasons. The results can inform management actions for individual POD species.  The most recent conceptual model posits that the POD represents a rapid ecosystem “regime shift” that followed a longer-term erosion of ecological resilience. The POD regime shift story may inform management strategies aimed at shifting the ecosystem into a more desirable state and improving long-term ecosystem resilience. Together, the POD conceptual models offer insights into potential adaptive management strategies aimed at multiple drivers and their effects on specific species, communities, and the whole ecosystem. In addition to providing new conceptual models for understanding the ecology of the upper San Francisco Estuary, the POD investigation also serves as a model for successfully pairing sustained research, analysis, and synthesis efforts with long-term monitoring of environmental drivers and biological communities to inform policy and management solutions.