Modeling Invertebrate Community Resistance and Resilience to Assess Indirect Impacts of Pesticides on Endangered Pacific Salmon

Pesticides currently used in agricultural and urban areas are detected routinely in aquatic habitats that support populations of threatened and endangered Pacific salmonids. This raises concern that salmon may be affected directly via physiological stress as well as indirectly via effects on their invertebrate prey. Empirical studies have provided some insights to the mechanism of action of many of these compounds to the salmon populations of concern. While these direct effects of pesticide exposure have received some attention, there has been less focus on ecological interactions involving 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.  To address how pesticides may affect salmon indirectly, we have incorporated prey dynamics into an existing individual-base model that is linked to a salmonid population growth model. We simulate pesticide exposures and track the relative importance of 1) a potential prey spike due to catastrophic drift, 2) prey community resistance and 3) prey community resilience. Using published data that track invertebrate community responses and recoveries following pesticide exposure, we present a range of scenarios that illustrate when and why these ecologically relevant indirect effects may be as important as direct physiological effects in limiting salmonid survival.  For example, when aquatic communities are exposed briefly to certain pesticides at concentrations that are sublethal for fish but highly toxic to their prey (80% or more of the invertebrate community is killed) and prey recovery rates are slow (1% per day), prey availability is reduced for enough time to limit salmonid growth and survival, and significantly lower salmon population growth rates. Alternative scenarios will further illustrate how invertebrate community dynamics affect salmon population growth rates.