Circum-arctic comparison of the hatching season of polar cod Boreogadus saida: A test of the freshwater winter refuge hypothesis

The hypothesis that salt-related differences in winter sea surface temperature dictate regional differences in the hatching season of polar cod Boreogadus saida is tested by contrasting hatch-date frequency distributions among six oceanographic regions of the Arctic Ocean characterized by different freshwater input. Consistent with the hypothesis, hatching started as early as January and extended to July in seas receiving large river discharge (Laptev/East Siberian Seas, Hudson Bay, and Beaufort Sea). By contrast, hatching was restricted to April–July in regions with little freshwater input (Canadian Archipelago, North Baffin Bay, and Northeast Water). Length (weight) in late-summer (14 August) varied from <10 mm (<0.01 g) in July hatchers to 50 mm (0.91 g) in January hatchers. An earlier ice break-up, more frequent winter polynyas, a warmer surface layer, and increased river discharge linked to climate warming could enhance the survival of juvenile 0+ polar cod by enabling a larger fraction of the annual cohort to hatch earlier and reach a larger size before the fall migration to the deep overwintering grounds. A further test of the hypothesis would require the verification that the early winter hatching of polar cod actually occurs in the thermal refuge provided by under-ice river plumes.