The polar cod, Boreogadus saida, plays an important part in the food web of the Arctic seas. Young fish are preyed
on by birds at the sea surface in late summer and then by adult cod as they migrate to deeper, overwintering
grounds. Since the chance of survival is higher for larger individuals, selection pressure pushes hatching to occur as
early as possible, in late winter or early spring, to allow individuals to reach a larger size before migration begins.
The study hypothesises that early hatching occurs in under-ice river plumes, where the input of freshwater keeps
temperatures relatively high in winter (just below 0C) compared to -1.8C under the ice. These warmer
temperatures accelerate embryonic development and allow successful feeding when the larvae first hatch out.
To test this thermal refuge hypothesis, the researchers studied six oceanographic regions of the Arctic Ocean
ranging from inland and coastal seas, which are heavily influenced by rivers, to recurrent polynyas, which are areas
of open water surrounded by sea ice and with little freshwater input. Results showed that the hatching period
became shorter in duration and shifted from winter to summer with decreasing freshwater input. For example,
hatching started as early as December and January in the Laptev Sea, Hudson Bay and Beaufort Sea, where
freshwater input is high, and was delayed until spring (April or May) in Baffin Bay and the Northeast Water, where
freshwater input is minimal.
The scientists estimated individual fish lengths at the end of summer in the different regions sampled between 2003
and 2006 by measuring the thickness of annual growth layers in the inner ear just like tree rings. Lengths ranged
from less than 10mm to 50mm, corresponding to larvae hatched in July and those hatched in December and
January, respectively. Importantly, 97 per cent of the polar cod that reached a length greater than 35mm were from the
regions with high freshwater input.
The results from this study suggest that the projected effects of climate change in the Arctic Ocean, including earlier
ice break-up, more frequent winter polynas and freshwater-induced temperature changes caused by increased river
discharge, could enhance the growth and survival of polar cod by promoting conditions that favour winter hatching.
Further research is now needed to confirm the thermal refuge hypothesis by directly observing whether winter
hatching occurs within plumes of rivers under sea ice.
Climate Warming May Enhance Polar Cod Survival
Researchers have demonstrated that hatching in polar cod can begin as early as January in Arctic seas that receive large freshwater input, compared to April-July in seas with little freshwater input. Since early hatching leads to higher survival rates, the effects of climate change may enhance polar cods survival by favouring conditions that allow winter hatching to occur.
by Lucy Towers
