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

Four groups of juvenile Atlantic cod (Gadus morhua) were collected in summe r (1992) from four coastal sites of northeast Newfoundland. They were held together at seasonally ambient temperature and photoperiod and monitored ov er one winter for variation in a biochemical phenotype, antifreeze producti on capacity. Our results show that population differences in antifreeze pro duction capacity reflect the severity of overwintering conditions faced. Co d from three bays along the northeast coast (49 degrees 30'-47 degrees 30' N) exhibited very similar patterns of antifreeze production, suggesting tha t these environments demand from their cod inhabitants approximately the sa me high level of freeze resistance. However, cod from the most exposed nort herly site at the tip of the Great Northern Peninsula (51 degrees 30' N) de veloped antifreeze levels about 50% higher than those of the other three gr oups, confirming that the more extreme high-latitude environment demands ev en greater freeze protection for survival. Since 34% of Great Northern Peni nsula cod produced antifreeze levels outside the range of all other juvenil es, we suggest that the physiological distinctness of this population has e volved by genetic amplification of antifreeze production capacity rather th an by selective mortality. The existence of physiological population struct ure should be factored into strategies for sustainable fishing of this spec ies in northern waters.