Acclimation of Atlantic salmon (Salmo salar L.) smolts to 'cold' sea water following direct transfer from fresh water

Acclimation of Atlantic salmon (Salmo salar L.) smelts to sea water at low temperatures has been studied in groups of fish transferred directly from fresh water (6 degrees C) to sea water at 2 degrees, 4 degrees and 6 degrees C. Temperatures were maintained until day 54 when water temperature was switched to ambient (approximately 7 degrees C) in all groups. The following parameters were monitored on days 8, 29, 50 and 72 after transfer: plasma osmolality and electrolyte concentrations, gill Na+-K+ ATPase activity, individual feed intake and growth. By the end of the experiment cumulative mortality was 18.1%, 12.5% and 5.0% in the groups of smelt transferred to sea water at 20, 40 and 6 degrees C, respectively. Plasma osmolality and the concentrations of chloride and sodium were inversely related to water temperature, but values for all groups (osmolality: 324-344 mosM kg(-1); [Cl-]: 147-162 mM; [Na+]: 164-171 mM) fell within the range considered to he normal for seawater-acclimated salmonids. Gill Na+-K+ ATPase activity increased in all groups of fish following transfer to sea water, the rate of increase being correlated with water temperature. Feed intake and growth were very low for the first few weeks following the transfer of the fish from fresh to sea water, but increased thereafter. On day 50 the percentages of nonfeeding fish were 23%, 5% and 1% in the groups of smelt transferred to sea water at 2 degrees, 4 degrees and 6 degrees C, respectively. Accordingly, highest rates of feed intake and growth were recorded for the fish held at the highest temperature. The results indicate that Atlantic salmon smelts are more tolerant of low seawater temperatures than earlier believed, and the negative effects of low temperature upon feeding and growth do not seem to be directly related to an impaired ability of the fish to hypoosmoregulate. (C) 1998 Elsevier Science B.V. All rights reserved.