Bioenergetic Responses by Pacific Salmon to Climate and Ecosystem V ariation

Salmon growth can respond to changes in temperature, food availability, food quality, and activity. Cli- matic variability can affect one or more of these factors, because different climate regimes are associated with different temporal-spatial patterns of temperature, salinity, and other oceanographic features that can alter ocean distribution patterns of salmon and cause shifts in assemblages of other organisms. Consequently, climate vari- ability can simultaneously change the availability or productivity of exploitable prey, and the intensity of competi- tion or predation experienced by salmon at various stages of ocean life. Variability across multiple factors can potentially confound the understanding and prediction of salmon growth or survival. Bioenergetics models can account for changing thermal and food conditions explicitly, and are valuable analytical tools for isolating and evaluating the relative contribution of different factors (e.g., temperature, feeding rate, food availability, food qual- ity) to the consumption and growth of salmon during different life stages. Model simulations, coupled with data on growth trajectories, diet composition, and thermal experience, provide estimates of: 1) consumption rates on each prey (measures of both the importance of various prey to the energy budget of salmon, and the predation impact of salmon on prey species); 2) feeding rate as a proportion of the theoretical maximum consumption rate, a measure of relative food availability; and 3) growth efficiency, a measure of how much food was required to achieve the observed growth rate. We applied bioenergetics models to juvenile pink salmon in the Gulf of Alaska during years of low (2001) versus high (2002) ocean survival to examine feeding and growth performance between years while explicitly accounting for significant variability in stage-specific distribution, diet, growth, and consumption. From these simulations, we determined that higher feeding rates on pteropods, primarily during July-August 2002, ex- plained the higher growth rates and larger body mass of juveniles that were associated with higher stage-specific marine survival for juveniles in 2002. Current bioenergetics models for salmonids provide valuable diagnostic and analytical tools. However, as modeling applications become more predictive and demanding, modifications and improvements will be required to address important topics like behavior, variable activity costs, seasonal and ontogenetic energy allocation, and foraging models.