Regional and climate-driven factors affecting the migrations of sockeye salmon (Oncorhynchus nerka) smolts in Alaska

Abstract

The downstream migration of salmon smolts is triggered by a combination of responses to multiple environmental indicators, imposed on internal circannual rhythms, that result in variation between populations in timing. Using long-term data on daily smolt counts and associated environmental variables from multiple sources across Alaska, I tested the hypothesis that variation in migration timing between different sockeye salmon populations could be explained by differences in local environmental conditions. I first analyzed the peak, width, and interannual variation of the smolt migration period for eight populations from southwestern and southcentral Alaska. I then modeled the median emigration date and width of the emigration period as a function of stream temperature, air temperature, and precipitation to assess how each population responds to combinations of seasonal environmental cues that serve as indicators for the onset of downstream migration or conditions in the recipient environment (sea surface temperature). Lastly, I used the results of my analysis to identify spatial variation in the response of distinct salmon populations to environmental factors and identify trends in the migration timing of those populations. Through my analysis, I found that sea surface temperature and freshwater temperature are the primary environmental factors that control median smolt emigration date, while the width of the smolt emigration window is influenced primarily by freshwater temperatures and precipitation, with significant site-by-site variation.