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Ecosystem Monitoring & Assessment Program

Spatial Distribution of Juvenile Chinook Salmon in the Northern Bering Sea During Warm and Cold Year Environments

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Summer 2015
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Recent sharp declines in Chinook salmon returns to western Alaska rivers have impacted local communities through curtailment of commercial and subsistence harvest. The first summer at sea is a critical period for Chinook salmon where survival is dependent on rapid growth in nearshore habitats, and attaining sufficient size and energy reserves prior to winter is essential. Therefore, a better understanding of the effect of climate change and variability on Chinook salmon early marine ecology is needed.

We examined physical and biological characteristics of the northeastern Bering Sea marine environment, the region where juvenile Yukon River Chinook salmon spend their first summer at sea, to improve our understanding of these characteristics on juvenile Chinook salmon spatial distribution. The oceanography and shelf dynamics between the north and south Bering Sea are markedly different above and below approximately lat. 60ºN. Sea ice extent and duration, and freshwater inputs from the Yukon River are substantially higher in the north compared to the south, resulting in large variations in oceanography between the northern and southern eastern Bering Sea (EBS), and between localized areas within the northern EBS.

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Figure 1. Due to variability between regions in the north Bering Sea for temperature anomalies, we use temperature anomalies in the southern Bering Sea as a proxy for northern Bering Sea warm and cold year categories. Years were categorized as warm or cold based on sea surface temperatures (top 10m averaged) from CTD profiles during late summer in the southern EBS (<60 °N).   

Generalized additive models (GAMs) were fit with environmental and biological covariates (temperature, chlorophyll a, salinity, silicate concentration, Pacific herring, and capelin) to investigate juvenile Chinook distribution changes between warm and cold years in the northern EBS. Two categories – ‘Surface’ (top 10m) and ‘Bottom’ (bottom 10 m) were examined for both warm and cold years (Fig. 1). Data were collected by the Center's Ecosystem Monitoring and Assessment  program on Bering, Arctic, sub-Arctic, integrated surveys (BASIS) during late summer/early fall in the eastern Bering Sea.

GAMs from both warm year categories showed juvenile Chinook farther north and offshore with broader overall distributions, while both cold year models revealed juvenile Chinook farther south and closer to shore, with more concentrated distributions (Fig. 2). Juvenile Chinook distributions varied between warm and cold years, both in terms of biomass and in terms of their model fits. In cold years, models of surface and bottom waters both explained more than 50% of the model deviance, while in warm years models scarcely explained 20% of the deviance. During warm years, poorer model fits and fewer significant environmental covariates suggest that such years may be characterized by more complex dynamics. Temperature and salinity were significant for both cold year categories, but not during warm year categories. Water column stability during summer, in addition to cascading effects on juvenile Chinook prey populations may both play important roles that warrant further investigation.

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Figure 2. Contours each show model-estimated relative abundance for juvenile Chinook distributions with lighter colors showing higher abundances, and darker colors showing lower abundances. Each category (warm surface, warm bottom, cold surface, cold bottom) revealed different combinations of significant environmental predictors (listed with each map).


By Jeanette Gann, Jordan Watson, Jim Murphy, and Edward Farley

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