Population differences in antifreeze production cycles of juvenile Atlantic cod (Gadus morhua) reflect adaptations to overwintering environment

Abstract

Four groups of juvenile Atlantic cod (Gadus morhua) were collected in summer (1992) from four coastal sites of northeast Newfoundland. They were held together at seasonally ambient temperature and photoperiod and monitored over one winter for variation in a biochemical phenotype, antifreeze production capacity. Our results show that population differences in antifreeze production capacity reflect the severity of overwintering conditions faced. Cod from three bays along the northeast coast (49.5°-47.5° N) exhibited very similar patterns of antifreeze production, suggesting that these environments demand from their cod inhabitants approximately the same high level of freeze resistance. However, cod from the most exposed northerly site at the tip of the Great Northern Peninsula (51.5° N) developed antifreeze levels about 50% higher than those of the other three groups, confirming that the more extreme high-latitude environment demands even greater freeze protection for survival. Since 34% of Great Northern Peninsula cod produced antifreeze levels outside the range of all other juveniles, we suggest that the physiological distinctness of this population has evolved by genetic amplification of antifreeze production capacity rather than by selective mortality. The existence of physiological population structure should be factored into strategies for sustainable fishing of this species in northern waters.