Nested scales of spatial heterogeneity in juvenile walleye pollock Theragra chalcogramma in the southeastern Bering Sea

Abstract

We sought to characterize the distribution of juvenile walleye pollock Theragra chalcogramma in an area of intense predatorprey interactions and to describe habitat features that lead to their observed distributions. The distribution of juvenile walleye pollock around the Pribilof Islands in the southeastern Bering Sea in 2008 and 2009 was patchy, at spatial scales rang- ing from a few meters to several 10s of kilometers. These patches, and the spaces between them, were hierarchically nested with small, dense patches clustered together into larger aggregations which were further aggregated over the region sampled. Vertical physical habitat structure affected the vertical distribution of juvenile pollock similarly in both years. However, despite largely similar physical characteristics in both years, there were significant differences between years in the horizontal distribution, patch structure, and abundance of juvenile pollock. In neither year did biological or physical environment characteristics explain the broad-scale variability in the horizontal distribution of juvenile pollock, but at small horizontal scales a behavioral compo- nent was evident, with fish changing their group coherence as conditions changed despite consis- tent group sizes. It is clear that if we are to understand the processes that drive the distribution of juvenile pollock, we must consider multiple scales of heterogeneity. Only then will we begin to understand the role of pollock in the ecosystem and be able to predict the consequences of large sources of variability such as climate change, a critical question in the rapidly changing physical and biological environment of the Bering Sea.