From polar night to midnight sun: photoperiod, seal predation, and the diel vertical migrations of polar cod (Boreogadus saida) under landfast ice in the Arctic Ocean

The winter/spring vertical distributions of polar cod, copepods, and ringed seal were monitored at a 230-m station in ice-covered Franklin Bay. In daytime, polar cod of all sizes (7–95 g) formed a dense aggregation in the deep inverse thermocline (160–230 m, −1.0 to 0°C). From December (polar night) to April (18-h daylight), small polar cod <25 g migrated into the isothermal cold intermediate layer (90–150 m, −1.4°C) at night to avoid visual predation by shallow-diving immature seals. By contrast, large polar cod (25–95 g), with large livers, remained below 180 m at all times, presumably to minimize predation by deep-diving mature seals. The diel vertical migration (DVM) of small polar cod was precisely synchronized with the light/dark cycle and its duration tracked the seasonal lengthening of the photoperiod. The DVM stopped in May coincident with the midnight sun and increased schooling and feeding. We propose that foraging interference and a limited prey supply in the deep aggregation drove the upward re-distribution of small polar cod at night. The bioluminescent copepod Metridia longa could have provided the light needed by polar cod to feed on copepods in the deep aphotic layers.