The effect of low temperature on cod, Gadus morhua

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Abstract
The laboratory studies described in this paper involved over 200 cod. Fish held at controlled temperatures below 2°C for about 40 d in normal sea water (32–34°‰S) showed an increased δF (Ramsay-Brown method) and chloride content (coulometric titration) of their blood plasma. Muscle water content decreased suggesting that at least part of the blood plasma changes were due to dehydration. Cod kept in low salinity water (7–8°‰S) at low temperature showed a decreased δF and chloride content of their blood plasma. Muscle water content increased and the fish gained weight although they were not fed, suggesting that at least part of the blood plasma changes were due to hydration. There was no evidence to show that these changes were due to osmoregulatory failure. Cod survived for at least 80 d when held at 0°C and did not die when subjected to cold shock from 4°C to 0°C. The results suggest that the distribution of cod in arctic waters and the capture of dead cod in the North Sea during severe winters is unlikely to be related to a lethal effect of low temperature. Other experiments indicated that warm (10%C) low salinity (7–8°‰S) sea water may be lethal for cod, which suggested that their distribution in the Baltic Sea could be restricted during the summer. The effect of low temperature on cod is discussed in relation to condition factor, parasitism by Lernaeocera, photoperiod, freezing resistance, and the aggregation of cod at fronts between warm and cold water masses in the Svalbard area.

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