Early Life History of the Arctic Gadids Polar Cod (Boreogadus saida) and Ice Cod (Arctogadus glacialis)

Early life-history stages of polar cod (Boreogadus saida) and ice cod (Arctogadus glacialis) co-occur on Arctic shelves and are are nearly identical in morphology. The two species were sampled in southeastern Beaufort Sea from April to August of 2004 and 2008. A stratified subset of 2126 of the 10565 gadids collected was identified to species by genetics and/or the size of the nucleus of the lapillar otolith. Based on size in month of capture, 8-9% of the overall cods were assigned to Arctogadus. Identified Arctogadus were longer at hatch and larger than Boreogadus from April to June. Under the ice in spring, Arctogadus was associated with the ice-water interface while Boreogadus avoided the interface. The feeding incidence of larvae collected under the ice was lower in Boreogadus (14%) than in Arctogadus (88%). Both species shared the same protracted hatching season peaking during the maximum production of ice microalgae. At lengths <15 mm, Boreogadus preyed primarily on Pseudocalanus nauplii, and Arctogadus on cyclopoid nauplii. At lengths >15 mm, the two species shared a similar diet. A lower mortality rate was found in Arctogadus and may be partly explained by its larger size and its propensity to hunt at the ice-water interface.