Intraspecific variation in the seasonal reproductive timing of Pacific salmon (Oncorhynchus sp.) has importantimplications for the resilience of salmon and for organisms in freshwater and terrestrial communities that dependon salmon resources. Stream temperature has well known associations with salmon spawn timing buthow 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 across36 watersheds in the Wood River basin in southwest Alaska. We found that the environmental conditionsin the first two axes of a principal components analysis (PCA) explained 76% of the variation in summer temperatureamong streams and 45% of the variation in spawn timing of sockeye salmon. The average habitatcharacteristics of streams that characterized three spawn timing groups of sockeye salmon were significantlydistinct from one another. Sites supporting early spawning populations tend to have steeper and smallerwatersheds, while late spawning populations occur in streams draining large, lower gradient watershedswith lakes in the drainage network. Finally, we show that stream temperature and spawn timing amongstreams have little spatial correlation across the landscape, thereby producing a fine-scale mosaic of spawntiming across the river basin. These results demonstrate that geomorphology and hydrology interact toproduce a heterogeneous thermal template for natural selection to influence salmon spawn timing acrossriver basins.
