Variation in abundance and physiological status of juvenile chum salmon (Oncorhynchus keta) in relation to marine factors in Southeast Alaska

Abstract

Little is known about the mechanisms influencing the critical early life stages of juvenile chum salmon (Oncorhynchus keta) from coastal to offshore marine waters. There is mounting evidence to suggest that fluctuations in early marine conditions affect juvenile salmon physiological status and year class strength. We investigated relationships of a suite of marine factors at local, regional, and basin scales to the physiological status and abundance of juvenile chum salmon in northern Southeast Alaska (SEAK) from 1997-2013. Correlation analyses were used to identify potential mechanisms influencing year class strength. Marine factors at the local scale were correlated to the observed physiological status of juvenile chum salmon: average June/July wind speed was negatively correlated to weight-at-length residuals, sea surface temperatures in July were positively correlated with length, and the June mixed-layer depth was positively correlated to the energy density of juvenile chum salmon in July. Marine factors at the regional scale influenced juvenile chum salmon abundance: freshwater discharge was positively correlated whereas upwelling was negatively correlated with abundance, linking high abundance to characteristics of strong Aleutian Low (AL) climatic conditions. Comparisons of juvenile chum salmon physiological status were also made between: 1) SEAK habitats (Icy Strait and the Eastern Gulf of Alaska, EGOA), 2) stock groups (hatchery and wild), and 3) years 2010 and 2011 possible mechanisms influencing productivity of chum salmon. Between habitats, length of juvenile chum salmon did not differ. However, both weight-at-length residuals and energy density values were significantly higher in the EGOA, irrespective of year, indicating juvenile salmon allocate energy to somatic growth in Icy Strait, while the EGOA may serve as a habitat for juvenile chum salmon to store energy as lipids. Between chum salmon stocks, wild stocks were shorter and had higher weight-at-length residuals than hatchery stocks. Between years, the 2010 ocean year was associated with a strong AL that coincided with higher physiological status of juvenile chum salmon and relatively higher returning adult commercial harvests and ocean survival of hatchery fish compared to the 2011 ocean year. Our results suggest differences in juvenile chum salmon physiological status in 2010 and 2011 coincided with positive and negative anomalies of the coupled ocean-atmosphere system, which were linked to previous winter environmental conditions, and have the potential to be used as a predictive salmon management tool to forecast year class strength in SEAK.