Seasonal changes in the hypo-osmoregulatory ability of brook charr: the role of environmental factors

In winter, brook charr Salvelinus fontinalis demonstrated poor osmoregulato ry ability. Salt-water (SW) transfer resulted in sharp increases in plasma osmolality and chloride levels, a decrease in white muscle water content an d high mortality. A clear improvement of the hypo-osmoregulatory mechanism efficiency was observed in spring; by May or June, fish achieved full accli mation within 1 week of SW transfer. In early fall, the time needed to rest ore osmotic and ionic balance increased once again. A high level of stress was detected only in winter and fall, as indicated by plasma cortisol and g lucose concentrations. Great improvement in osmotic and ionic regulation wa s observed when brook charr were transferred to 8 degrees C SW in February and April while impaired SW adaptability was recorded in individuals transf erred to 2.5 degrees C SW in June and August. Temperature itself was more i mportant than a thermal gradient between fresh and salt water. Stress indic ators substantiate this temperature-dependent improvement of osmotic and io nic regulation in brook charr. The results support the hypothesis of a mino r contribution of cortisol to hypo-osmotic regulation in brook charr, as th e cortisol response always seemed to be associated with stress in the exper iments. (C) 2000 The Fisheries Society of the British Isles.