SURVIVAL OF NORTHERN ATLANTIC COD (GADUS-MORHUA) EGGS AND LARVAE WHEN EXPOSED TO ICE AND LOW-TEMPERATURE

Freeze resistance of eggs and larvae of Atlantic cod (Gadus morhua) from the northern cod stock was investigated to determine whether ice contact could affect survival during the spring spawning season off Newfoundland. Egg and larval homogenates did not appear to contain antifreeze proteins (mean freezing points -0.78 and -0.88-degrees-C, respectively). However, cod eggs did not freeze at -1.8-degrees-C in icy aerated seawater, could be undercooled to -4.0-degrees-C in ice, and froze at temperatures between -4.1 and -17-degrees-C; freeze resistance depended on the integrity of the chorion. Larvae withstood undercooling to -1.8-degrees-C, provided they were not brought into direct contact with ice crystals. If directly touched with ice, larvae froze at -1.36-degrees-C (feeding stage) or -1.34-degrees-C (yolk-sac), approximately 0.5-degrees-C lower than would be expected from the freezing temperatures of their body fluids. The nature of their external epithelium and delayed development of sensitive gill structures below 0-degrees-C may contribute to larval freeze resistance. Cod eggs and larvae are found in spring off Newfoundland and Labrador, when sea temperatures can be as low as -1.8-degrees-C and ice cover extensive. While cod eggs are remarkably freeze resistant, such environmental conditions may cause freezing mortalities in larval cod.