SEASONAL AND DIEL VARIATIONS OF VARIOUS BLOOD PARAMETERS IN ATLANTIC COD (GADUS-MORHUA) AND AMERICAN PLAICE (HIPPOGLOSSOIDES-PLATESSOIDES)

Seasonal and diel variations of different blood variables were studied in Atlantic cod (Gadus morhua) and American plaice (Hippoglossoides platessoides) subjected to natural fluctuations of salinity and temperature in the St. Lawrence estuary. Significant seasonal fluctuations were found for all the parameters measured (plasma osmolality, Na+, Cl-, K+, glucose, cortisol, and blood hemoglobin and hematocrit). In cod, the seasonal effect on blood hemoglobin was only detected through its interaction with time of day. In this species, all parameters also varied on an hourly basis, although these changes only represented a small proportion of the variance explained by the statistical model. In plaice, only plasma osmotic composition and blood hematocrit did not show diel variations, while the hour effect was difficult to characterize. In general, though our results indicate that both species acclimated to test conditions, American plaice seemed to cope less efficiently than Atlantic cod with winter conditions. In this respect, the Atlantic cod represents a better candidate for coastal aquaculture in this area.