Changes in the vertical distribution of age-0 walleye pollock (Gadus chalcogrammus) during warm and cold years in the southeastern Bering Sea

In the last 20 years, the southeastern Bering Sea has shifted its thermal variability to longer-term (4–6 years) ocean–ecosystem temperature stanzas. Age-0 walleye pollock (Gadus chalcogrammus) populations respond to thermal changes with horizontal (east–west) shifts in spatial distribution over the continental shelf, though there are limited data on whether thermally mediated vertical shifts in distribution also occur. Vertical shifts may impact predator–prey overlap between age-0 pollock and their lipid-rich prey, calanoid copepods and euphausiids, resulting in different feeding conditions that ultimately affect fish body condition prior to winter onset. For this study, we analyzed acoustic backscatter measured during acoustic trawl surveys over the southeastern Bering Sea shelf in cold years (2011, 2012) and warm years (2014 and 2016) to determine the vertical distribution of age-0 pollock. This study presents evidence that age-0 pollock changes in vertical distribution were related to changes in ocean annual temperature. Age-0 pollock went from occurring deeper in the water column during cold periods to being surface-oriented during warm periods, potentially exacerbating spatial mismatches between pollock and prey. We relate patterns in a vertical position to physical water column properties, feeding, and bioenergetic condition of collected pollock and discuss implications for recruitment success during different thermal oceanographic stanzas.