Freezing resistance of caged Atlantic cod (Gadus morhua) during a Newfoundland winter

Abstract

Closure of the Atlantic cod ( Gadus morhua) fishery in Newfoundland has prompted considerable interest in cod aquaculture. However, the low temperatures and ice that characterize Newfoundland inshore waters during winter can be lethal to cod; thus, cage sites must be chosen with care. This study examines the overwintering conditions of caged cod located at two commercial aquaculture sites in southeast Newfoundland. Approximately 95% of the cod at these two sites overwintered successfully in landfast ice-covered cages, where water temperatures were below -1.2 8C for at least 1 mo, and fell to minimum temperatures of -1.58C at one site and -1.78C at the other. Mean blood plasma freezing points of cod sampled from these sites were -1.2 and -1.118C. Thus, cod can be overwintered in sea cages at temperatures below their freezing points (undercooled) — a situation that would normally be considered lethal, since contact with ice crystals would nucleate freezing in the undercooled cod. It is hypothesized that their survival was due to the thick covering of ice that prevented turbulence from driving surface ice crystals deep into the cage.