Arctic cod (Boreogadus saida) hatching in the Hudson Bay system: Testing of the freshwater winter refuge hypothesis

Buoyant Arctic cod (Boreogadus saida) eggs are found at the surface or at the ice-water interface in winter. While winter temperatures in saline waters fall below 0 degrees C, the temperature in areas affected by under-ice river plumes is slightly higher. Under-ice river plumes may therefore provide thermal refuges favoring the survival of the vulnerable early life stages of Arctic cod. Thermal refuges would allow early hatchers to survive, benefit from a long growing period, and add to the number of individuals recruiting to the adult population: These expectations define the freshwater winter refuge hypothesis. More than 42 rivers drain into Hudson Bay making it particularly well suited to test this hypothesis. Whereas the bulk of Arctic cod observed in Hudson Bay hatch between mid-April and June, some larvae hatch as early as January. We used two independent but complementary methods to test the hypothesis: (1) Lagrangian model simulations that traced back the planktonic trajectories of the sampled larvae and (2) measurements of the concentration of strontium-88 in the otolith cores. Throughout the Hudson Bay system, Lagrangian simulations revealed that early hatchers were more likely to hatch in lower surface salinities and that larvae reaching larger prewinter lengths were likely to have hatched near or within estuaries. Analysis of otolith microchemistry showed that larvae with low strontium-88 concentration in the otolith core, indicating a low salinity hatch location, had hatched earlier and thus had a longer growth period before freeze-up. These results show the potential for Arctic cod persistence in the Arctic where freshwater input is projected to increase and the ice regime is predicted to become more seasonal, provided that the surface temperatures remain below embryonic and larval lethal limits.