Variation In The Biogeography Of Gulf Of Alaska Micionekton As A Driving Mechanism For Observed Growth Trends In Pacific Salmon

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The adult body lengths and weights of many Pacific salmon stocks (Oncorhynchus spp.) are negatively correlated with sea surface temperature in the northeastern Pacific Ocean. The mechanisms behind the correlation may be due to (1) a physiological response of salmon to warming surface waters; or (2) shifts in the abundance or composition of prey species. Determining the relative importance of each mechanism is critical to predicting the effect of climate change on salmon populations. In this study, I summarize empirical studies of oceanographic conditions, salmon food habits, and salmon growth in the Gulf of Alaska during the 1950s, 1980s and 1990s. I use bioenergetics models to determine the variation in the factors determining salmon growth. In particular, I show a relationship between oceanographic boundaries, the southern limit of salmon distribution, and the northern limit of a micronektonic squid species (Berryteuthis anonychus), a dominant salmon prey item’ in the region. I use the area of species overlap to define a zone of 'highest salmon growth,' the size of which varies on interannual time scales. I calculate the area of squid/salmon overlap from oceanographic data for the years 1950-98, and show that variation in this overlap explains much of the negative correlation between sea surface temperature and adult salmon body weight. This suggests that biogeographic variation in food web structure, with micronektonic squid as a keystone species, is a mechanism that contributes to the interannual variation in salmon growth in the northeastern Pacific.