Testing hypotheses to explain a surprisingly large year-class of walleye pollock in an ice-free subarctic southeastern Bering Sea; a review of available data

Abstract

Several hypotheses link year-class-strength of eastern Bering Sea walleye pollock (Gadus chalcogramus) to climate variability. These focus on positive relationships with sea ice as a substrate for ice algae, as a water column stabilizer, and as a determinant of a cold pool. In 2018, sea ice was absent over the outer and middle Bering Sea shelf south of 60°N and therefore the 2018 year-class of pollock was expected to be small. However, the 2018 year-class of pollock is estimated to be the most abundant on record. We evaluated three, non-mutually exclusive hypotheses to account for the unexpected strength of the 2018 pollock year-class, 1) reduced predation on age-0 pollock, 2) reduced competition for zooplankton prey, and 3) cross-shelf transport of zooplankton in spring. We found that the northward movement of age-1 and older pollock out of the southeastern Bering Sea reduced the potential for predation on age-0 pollock over the southern shelf, as well as the potential for competition for zooplankton prey there. In contrast, the cross-shelf advection of zooplankton in spring did not appear to have influenced prey availability for age-0 pollock in late summer and fall. Thus, the northward movement of a large fraction of both age-1 as well as older pollock, not seen in the past, was at least a major contributor to the unique occurrence of an exceptionally strong year class in a warm year, 2018. As the Bering Sea warms, and if age-1+ pollock migrate northward after spawning in the southeastern Bering Sea as in 2018, it is possible that eastern Bering Sea pollock will maintain moderate to strong year-classes despite the loss of sea ice.