DEVELOPMENT OF HYPOOSMOREGULATORY CAPACITY IN ARCTIC CHAR (SALVELINUS-ALPINUS) REARED UNDER EITHER CONTINUOUS LIGHT OR NATURAL PHOTOPERIOD

Two groups of Arctic char (Salvelinus alpinus) were reared in freshwater (5-6-degrees-C) under either continuous light (LDL) from first feeding (March) or LDL until July and then natural photoperiod (NDL, 70-degrees-N). Direct transfer to seawater (5.5-degrees-C, 35 ppt) in February resulted in both groups exhibiting increases in blood plasma osmolality, Na+, and Mg2+ concentrations and a significant decrease in muscle water content. When tested in May, an improvement in seawater tolerance was evident in both groups. In June, only the NDL fish showed further improvements in hypoosmoregulatory capacity, since they exhibited only minor fluctuations in plasma constituents and muscle water content following direct transfer to seawater. Increased body size could partially explain the improved seawater tolerance in the experimental groups. Acclimation to brackish water prior to transfer to 35 ppt seawater in June improved seawater tolerance only in fish reared under continuous light. The results indicate that the seasonal increase in photoperiod stimulates the development of hypoosmoregulatory capacity whilst the fish are still resident in freshwater.