Acclimation of Atlantic salmon (Salmo salar L.) smolts to 'cold' sea waterfollowing 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 f resh 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 sw itched to ambient (approximately 7 degrees C) in all groups. The following parameters were monitored on days 8, 29, 50 and 72 after transfer: plasma o smolality and electrolyte concentrations, gill Na+-K+ ATPase activity, indi vidual feed intake and growth. By the end of the experiment cumulative mort ality was 18.1%, 12.5% and 5.0% in the groups of smelt transferred to sea w ater at 20, 40 and 6 degrees C, respectively. Plasma osmolality and the con centrations of chloride and sodium were inversely related to water temperat ure, 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 f or seawater-acclimated salmonids. Gill Na+-K+ ATPase activity increased in all groups of fish following transfer to sea water, the rate of increase be ing 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 s ea 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 a t 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 high est temperature. The results indicate that Atlantic salmon smelts are more tolerant of low seawater temperatures than earlier believed, and the negati ve effects of low temperature upon feeding and growth do not seem to be dir ectly related to an impaired ability of the fish to hypoosmoregulate. (C) 1 998 Elsevier Science B.V. All rights reserved.