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

Abstract

Seawater tolerance of Atlantic salmon ( Salmo salar) smolts acclimated at 8°C in fresh water was tested in seawater challenge tests at 2°C, 5°C, 11°C, 14°C and 17°C, and compared to a test at 8°C. Two tests were used, a 24-h, 35‰ and a 24-h, 40‰ salinity test. In 35‰ salt water, there were no differences in plasma Cl − concentrations in the range 5–14°C (mean values 140–144 mM). An increase in plasma Cl − level was found at 2°C and 17°C (mean values about 155 mM). The mean Cl − concentration at 8°C in 40‰ test was 10 mM higher than that at the same temperature in 35‰ salinity. An increase in plasma Cl − level was found when the temperature shifted from 8°C to either 5°C or 14°C (mean values 171 mM), and an even larger increase was recorded at 2°C (mean value 209 mM, 75% mortality) and 17°C (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. However, differences from the acclimation temperature larger than 4–6°C are not recommended, especially when the seawater temperature is lower than 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 ability of smolts, and may possibly be used to distinguish differences in hypoosmoregulatory capacity of smolts not readily obtained from tests in full-strength sea water.