Pre-winter distribution and habitat characteristics of polar cod (Boreogadus saida) in southeastern Beaufort Sea

Abstract

Polar cod was shown to form dense under-ice winter aggregations at depth in the Amundsen Gulf (southeastern Beaufort Sea). In this paper, we verify the premises of the aggregation mechanism by determining the distribution and habitat characteristics of polar cod prior to the formation of winter aggregations. Multifrequency split-beam acoustic data collected in October–November 2003 revealed that polar cod split into two distinct layers. Age-0 polar cod formed an epipelagic layer between 0 and ~60 m depth without any clear large-scale biomass trend. In contrast, adult polar cod tended to distribute into an offshore mesopelagic layer between ~200 and 400 m that shoaled into a denser (1–37 g m−2) benthopelagic layer on sloping bottoms (between 150 and 600-m isobaths) along the Mackenzie shelf and into the Amundsen Gulf basin. Concentrations peaked in the Amundsen Gulf where estimated total biomass reached ~250 kt. Both age-0 and adult polar cod distributed in the warmer waters (>−1.4 °C). We hypothesise that polar cod concentration over slopes is governed by the combined actions of (1) local currents concentrating both depth-keeping zooplankton and polar cod at the shelf-break and basin slopes and (2) trophic association with these predictable topographically trapped aggregations of zooplankton prey. During freeze-up, these slope concentrations of polar cod are thought to constitute the main source of the observed dense under-ice winter aggregations. The hypothesis of active short-distance displacements combined with prevailing mean currents is retained as the likely aggregation mechanism.