The effect of salinity on desmoltification in Atlantic salmon

In order to determine which salinity is needed to prevent desmoltification in Atlantic salmon, Salmo salar, changes in the hypoosmoregulatory capacity of postsmolts reared at different salinities were monitored. Smelting occurred under natural li;:ht and temperature conditions, and was completed by mid-June. Thereafter the smelts were divided into six groups reared at different salinities (0, 10, 15, 21, 26 and 31 parts per thousand). The hypoosmoregulatory capacity was measured by 24 h seawater challenge tests. A decrease in hypoosmoregulatory capacity was evident in the groups kept at 0 and 10 parts per thousand, and 3 weeks after smelting there was a significant difference between the 0 parts per thousand group and the groups kept on 15 parts per thousand or higher. Two weeks later both the 0 parts per thousand and the 10 parts per thousand groups had regained a high hypoosmoregulatory capacity, indicating that desmoltification did not result in a lasting state of low hypoosmoregulatory capacity, but rather appeared as a transient process lasting no more than 5-6 weeks. (C) 1998 Elsevier Science B.V. All rights reserved.