Responses of Pacific Salmon Populations to Climate Variation in Freshwater Ecosystems
Daniel E. Schindler and Lauren A. Rogers
Abstract.—The effects of changing climate on salmon populations depend on the species and life history of interest, local expressions of climate change, characteristics of habitat, and the adaptation of specific populations to geographic variation in habitat characteristics. Here we review some of the key changes in climatic conditions that have affected freshwater ecosystems used by Pacific salmon in the recent past and summarize how these will be further impacted by future changes in temperature and precipitation patterns. Recent climate change has affected some well-studied populations allowing for some generalization about climate impacts on specific populations. One recurrent response to warming in freshwaters is a positive growth response in juveniles and an acceleration of the freshwater component of life histories, although these responses are unlikely to occur at the southern range boundaries for salmon. In addition to substantial latitudinal variation in recent and expected climate impacts on salmon ecosystems, there is increasing evidence of regional variation among population responses to the same overriding regional changes in climate. Biocomplexity, defined as the variation in habitat characteristics and its associated suite of locally adapted populations, provides a portfolio effect to salmon stocks. A portfolio effect results from weakly correlated dynamics in the component populations of a salmon stock. A stock characterized by a high diversity of populations and their associated dynamics, is less sensitive to the variation in an individual population, compared to a stock with low diversity. This portfolio effect provides resilience to salmon fisheries in the face of ongoing climate change because fisheries integrate across the component diversity within stocks. While many of the future characteristics of freshwater habitats remain highly uncertain in the face of ongoing climate change, protection of diverse networks of viable habitat and the stock diversity that maps onto this landscape diversity, is one obvious strategy to ameliorate the effects of future climate change on salmon stocks.