Osmoregulation, feed intake, growth and growth hormone levels in 0+ Atlantic salmon (Salmo salar L.) transferred to seawater at different stages of smolt development

Summary

We studied the physiology and endocrinology of seawater acclimation in two size-groups (body weight [b.wt.] at transfer 38 and 53 g, respectively) of 0+ Atlantic salmon (Salmo salar L.) smolts transferred to seawater (33‰) in autumn at three different times following the completion of the photoperiod-induced smoltification process. Smoltification was monitored by use of 24-h seawater challenge tests, and measurements of gill Na+, K+-ATPase activity and condition factor in freshwater acclimated fish. Parr–smolt transformation in large fish was slightly delayed compared to small fish. In late October 1999, when both groups displayed full smolt characteristics, each size group was transferred to four tanks (trial I) supplied with either running fresh water (FW) or running seawater (SW), both at 8.8 °C. In all groups of fish, individual feed intake, body weight, gill Na+, K+-ATPase activity, and plasma levels of chloride and growth hormone (GH) were monitored on days 3 and 30 following transfer. The same protocol was repeated with starting dates December 3 (trial II) and January 11, 2000 (trial III). At the start of trial III, both size-groups displayed reduced hypoosmoregulatory ability (seawater challenge tests) and decreasing gill Na+, K+-ATPase activity. Mortality was low (≤4.4%) in all groups during the trials. For the small SW-smolts, the highest rates of feed intake and specific growth (SGR) were recorded in trial I (SGR: 1.2% b.wt. day−1), whereas feed intake and growth of the larger SW-smolts were highest during trials II and III (SGR: 0.9% b.wt. day−1). Plasma chloride levels in small SW-fish on day 3 increased slightly with date of transfer (from 143 mM in trial I to 154 mM in trial III), but were within the normal range by day 30 across all size-groups and trials (140–144 mM). Following seawater transfer, plasma GH levels increased transiently, whereas gill Na+, K+-ATPase activity increased permanently in all groups. The different patterns of growth displayed by the two size-groups of fish following seawater transfer suggest that the optimal time for seawater transfer may depend on the body size of the fish. Fish transferred past their peak in smolt status showed high hypoosmoregulatory capacity and retained, at least in part, their ability to increase plasma GH levels and gill Na+, K+-ATPase activity following exposure to seawater. Growth rates in seawater may indicate that in commercial farming of underyearling Atlantic salmon, seawater transfer can be delayed for a considerable period of time after smolting, provided that seawater temperature is favourable.