Impacts of pesticides on prey availability and implications for growth of endangered salmon populations

Pesticides currently used in agricultural and urban areas throughout the Pacific Northwest are detected routinely in aquatic habitats that support ESA-listed salmonid populations. The presence of these pesticides raises concern that salmonid populations may be affected directly via physiological stress on individuals as well as indirectly via alterations to the food web. While direct effects of pesticide exposure have received some attention, there has been less focus on salmonid food webs and namely salmonid prey. Many pesticides are designed to kill invertebrates, and consequently exposure to pesticides can result in dramatic changes in aquatic invertebrate communities and in particular, the drift behavior of many taxa. Because juvenile salmonids are primarily drift-feeding, we are especially interested in how these changes affect prey availability and subsequent salmonid growth and survival. For instance, a brief exposure to a pesticide at a concentration sublethal to fish can reduce invertebrate densities for months and for significant distances downstream. To address indirect effects and evaluate their importance relative to direct effects, we have incorporated dynamics in prey availability into an existing individual-base model that is linked to a population growth model. While we will briefly present the full model, we will focus on indirect effects. Using published data that track invertebrate community responses and recoveries following pesticide exposure, we present a range of scenarios that illustrate when and why indirect effects may be as important as direct effects in limiting salmonid survival. How these effects may be measured in the field will also be discussed.