Effects of a temperature shift on seawater challenge test performance in Atlantic salmon (Salmo salar) smolt

Seawater tolerance of Atlantic salmon (Salmo salar) smolts acclimated at 8 degreesC in fresh water was tested in seawater challenge tests at 2 degrees C, 5 degreesC, 11 degreesC, 14 degreesC and 17 degreesC, and compared to a test at 8 degreesC. Two tests were used, a 24-h, 35 parts per thousand and a 24-h, 40 parts per thousand salinity test. In 35 parts per thousand salt water, there were no differences in plasma Cl- concentrations in the range 5-14 degreesC (mean values 140-144 mM). An increase in plasma Cl- level was found at 2 degreesC and 17 degreesC (mean values about 155 mM). The mean C l- concentration at 8 degreesC in 40 parts per thousand test was 10 mM high er than that at the same temperature in 35 parts per thousand salinity. An increase in plasma Cl- level was found when the temperature shifted from 8 degreesC to either 5 degreesC or 14 degreesC (mean values 171 mM), and an e ven larger increase was recorded at 2 degreesC (mean value 209 mM, 75% mort ality) and 17 degreesC (mean value 194 mM). The results indicated that smolt had the ability to hypoosmoregulate over a wide range of temperatures when transferred to full-strength sea water. Ho wever, differences from the acclimation temperature larger than 4-6 degrees C are not recommended, especially when the seawater temperature is lower th an in fresh water. The use of a higher salinity than that of full-strength sea water may give a more comprehensive picture of the hypoosmoregulatory a bility of smolts, and may possibly be used to distinguish differences in hy poosmoregulatory capacity of smolts not readily obtained from tests in full -strength sea water. (C) 2001 Elsevier Science B.V. All rights reserved.