Circulating growth hormone levels in Atlantic salmon smolts following seawater transfer: effects of photoperiod regime, salinity, duration of exposure and season

Atlantic salmon juveniles in fresh water, both on simulated natural photoperiod (LDN) and continuous light (LL) regime, were transferred to fresh water (controls) and different salinities (28, 35 and 40 parts per thousand) at five different dates over the period of parr-smelt transformation (February to June). Survival was noted and plasma samples obtained after 12, 24, 48, 96 and 192 h of salinity exposure. Survival and plasma sodium levels were used to assess hypoosmoregulatory ability, and plasma growth hormone (GH) levels measured to elucidate the possible role of this hormone in the salinity tolerance of smelting salmon. The fish on LDN photoperiod had markedly elevated plasma GH levels as well as improved hypoosmoregulatory ability in June, whereas these changes were inhibited by the LL regime. Salinity exposure caused a transient increase in circulating GH levels. The response was complex, both in regard to the date of exposure, the photoperiod regime, and the level of salinity. The data indicate that elevated plasma GH levels in smelting Atlantic salmon in freshwater are of importance for the subsequent hypoosmoregulatory ability in seawater. However, it is questioned whether the transient increase in GH following salinity exposure represents a specific short-term hypoosmoregulatory role. (C) 1998 Elsevier Science B.V. All rights reserved.