Association between geomorphic attributes of watersheds, water temperature, and salmon spawn timing in Alaskan streams

Intraspecific variation in the seasonal reproductive timing of Pacific salmon (Oncorhynchus sp.) has important implications for the resilience of salmon and for organisms in freshwater and terrestrial communities that depend on salmon resources. Stream temperature has well known associations with salmon spawn timing but how stream and watershed geomorphology relates to the variation in salmon spawn timing is less understood. We used multivariate statistics applied to five environmental variables to compare conditions across 36 watersheds in the Wood River basin in southwest Alaska. We found that the environmental conditions in the first two axes of a principal components analysis (PCA) explained 76% of the variation in summer temperature among streams and 45% of the variation in spawn timing of sockeye salmon. The average habitat characteristics of streams that characterized three spawn timing groups of sockeye salmon were significantly distinct from one another. Sites supporting early spawning populations tend to have steeper and smaller watersheds, while late spawning populations occur in streams draining large, lower gradient watersheds with lakes in the drainage network. Finally, we show that stream temperature and spawn timing among streams have little spatial correlation across the landscape, thereby producing a fine-scale mosaic of spawn timing across the river basin. These results demonstrate that geomorphology and hydrology interact to produce a heterogeneous thermal template for natural selection to influence salmon spawn timing across river basins.