Growth hormone response to seawater challenge in Atlantic salmon, Salmo salar, during parr-smolt transformation

Abstract

The relation between plasma growth hormone (GH) levels and hypo-osmoregulatory ability during parr-smolt transformation was studied in 2-year-old immature parr and previously mature male parr of two different Baltic stocks of Atlantic salmon ( Salmo salar). At several times between early May and the end of June fish were transferred to seawater (25‰ salinity) and sampled after 24 h. Among freshwater controls plasma GH levels increased only slightly in one stock during the smoltification period, whereas GH levels were significantly elevated in mid-June in the other stock. Seawater adaptability reached an optimum by the end of May to early June in one stock and 2 weeks later in the other one. Plasma GH levels in fish exposed to seawater were similar to those of freshwater fish until mid-May, but increased to significantly higher levels at the time of peak seawater adaptability. Changes in plasma sodium levels during seawater exposure were negatively correlated to plasma GH levels. At the time of peak seawater adaptability two groups were detectable among previously mature males: one group had plasma sodium levels comparable to those of immature fish, while the other had significantly higher sodium levels. In one stock plasma GH showed a similar bimodal distribution, where the group with high GH levels corresponded to low plasma sodium levels and the group with low GH levels had significantly higher sodium levels. The results indicate that some Baltic stocks of Atlantic salmon develop an ability to respond to seawater exposure with a rapid increase in GH levels during the parr-smolt transformation. It is suggested that increasing environmental salinity triggers the final development of hypo-osmoregulatory ability.