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Ecosystem Monitoring and Assessment Program 

Forage Fish Distribution and Diet in the Eastern Bering Sea

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Fall 2014
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refer to capton
Figure 1.  Map of eastern Bering Sea study area with individual Bering Sea Project (BSP) regions (numbered) consolidated into domains (shaded) for this study.  Dashed line represents delineation between northeastern Bering Sea (NEBS) and southeastern Bering Sea (SEBS).

Climate warming has impacted the southern extent of sea ice leading to many changes in ocean conditions and food webs in the eastern Bering Sea (EBS). We explore how these changes have affected two key forage fish species, capelin (Mallotus villosus) and Pacific herring (Clupea pallasii), examining the effects of climate change on this commercially important ecosystem in the EBS.

Catch per unit effort (CPUE) data from surface trawls, size, and diet of capelin and Pacific herring were collected during a series of warm and cold years during fisheries oceanographic surveys conducted in the EBS from mid-August to early October 2003 through 2011 (Fig. 1). Overall, catches for both species were higher in the northeastern Bering Sea (NEBS) relative to the southeastern Bering Sea (SEBS), irrespective of temperature conditions. Capelin catches were lower during warm years than during cold years; Pacific herring catches were less variable between warm and cold years (Fig. 2) . Capelin and herring lengths remained relatively constant between years. Capelin lengths were similar among oceanographic domains, while herring were larger in domains further offshore.

Diets for both species were significantly different between climate regimes (Fig. 3). Large crustacean prey comprised a higher proportion of the diets in most regions during cold years. Walleye pollock (Gadus chalcogrammus) contributed more than 60% to the diets of Pacific herring in the southern Middle Domain and more than 30% in the northern Middle domain during warm years.  A switch to less energetic prey for these forage fishes during warm years may have implications for fitness and future recruitment. The shifts in the distribution and lower biomass of capelin in the EBS could lead to disruptions in food webs and energy pathways in this complex marine ecosystem.

By Alex Andrews, Wes Strasburger, Ed Farley, and Jim Murphy



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