ANTIFREEZE PRODUCTION, FREEZE RESISTANCE, AND OVERWINTERING OF JUVENILE NORTHERN ATLANTIC COD (GADUS-MORHUA)

Four size groups of northern Atlantic cod (Gadus morhua) (juveniles < 15, 15-25, and 26-40 cm and adults > 50 cm) were investigated over a winter cycle for their ability to increase freeze resistance by producing plasma antifreeze glycoproteins. All juveniles had significantly higher plasma antifreeze levels than adults during the winter (January-March) under similar conditions of cycling temperature and ambient photoperiod. Highest winter plasma antifreeze levels and lowest plasma freezing temperatures were consistently found in the 15-25 cm group. Fish length and plasma antifreeze levels were inversely related from January to April, with the exception of the 0 + cod. All juvenile groups developed considerable freeze protection early in the winter cycle before the onset of freezing temperatures. However, the adults were not significantly protected until the end of January, when ambient temperatures had fallen below 0-degrees-C. The 15-25 cm cod were held in three cycling temperature regimes. Induction temperatures for antifreeze production in this size group appeared to be between 2 and 3-degrees-C, as compared with between 1 and 0-degrees-C for the adults. We suggest that these patterns of antifreeze production may have direct bearing on habitat selection and overwintering behaviour of the northern Atlantic cod at different stages of development.