Circannual cycles of blood plasma freezing point and Na+ and Cl− concentrations in Newfoundland winter flounder (Pseudopleuronectes americanus): correlation with water temperature and photoperiod

Abstract

A 4-year field program was conducted on the winter flounder to correlate changes in plasma Na+, Cl−, and freezing-point depression with sea-floor water and sediment temperatures, day length, salinity, depth of capture, and observations on burrowing in sediments.Plasma Na+, Cl−, and freezing-point depression showed annual cycles with maxima in winter (January–April) (temperature −1.1 to −1.4 °C) and minima in summer (July–October) (10–14 °C). The change in freezing-point depression from summer to winter was about 0.65 °C, 20% of this was attributable to Na+ and Cl− and the remaining 80% to the presence of an 'antifreeze.' The data suggest that plasma 'antifreeze' appeared in November (4–6 °C) and disappeared during May (−1.0 to 3 °C). During the winter the flounder were found only in the deeper areas of the sampling site and were usually buried up to 12–15 cm in the sediments which were warmer (0.1 to 0.4 °C) than the seawater.The plasma Na+, Cl−, and freezing-point depression of winter flounder held in the laboratory for 7 days were always significantly lower than the field-sampled fish. The differences between these two groups was greatest during the summer, suggesting that the effects of 'stress' during capture differ seasonaly.