Sustainable Exploitation Rates of Walleye: The Role of Climate and Density-Dependent Life History Traits

We present a model that describes how plasticity in life history traits dictates the ability of a population to sustain the additional mortality imposed by exploitation.  We apply this model to estimate sustainable exploitation rates for walleye populations of North America, spanning a climatic range of 1000 to 4000 ^oC growing degree days (GDD).  Climate is responsible for much of the among-population variation in walleye life history traits (e.g., growth, natural mortality, maturation, and reproductive investment).  Application of a thermal age concept (i.e., age x GDD) permits us to factor out the climate-driven differences among populations.  We then use both among-population and within-population analyses to show that the remaining variation can be used to bound the density-dependent response ranges of life history parameters that support sustainable exploitation.  Our results suggest density-dependent variation in walleye growth rate can be as large as 2-fold.  Assuming that early survival increases with growth and the age of entry to the fishery equals age of maturity, this level of growth compensation will support fishing mortality that is approximately equal to the natural mortality rate.  Because natural mortality increases with GDD, the model predicts sustainable rates of exploitation are higher in warmer climates.