Effects of premature seawater transfer and fasting on plasma growth hormone levels of yearling coho salmon ( Oncorhynchus kisutch) parr

Abstract

The influence of starvation and premature seawater transfer on plasma growth hormone (GH) concentration in yearling coho salmon parr, was studied. Juveniles were separated into four groups: (1) unfed fish in fresh water (FW), (2) fed fish in FW, (3) unfed fish in sea water (SW) (24%.), (4) fed fish in SW. Plasma sodium concentration, GH and cortisol levels were analyzed after 0, 1, 3, 4, 5 weeks of exposure. Hypoosmoregulatory ability seemed to be impaired after long-term exposure of the juveniles to SW: after 5 weeks, plasma sodium concentrations in SW groups were significantly higher than those in FW groups. The cortisol levels increased slightly in response to transfer from FW to SW as well as to starvation. Plasma GH levels were elevated in unfed FW fish but not in unfed SW fish after 3, 4, and 5 weeks of starvation. In fed SW groups, the increase of plasma GH levels was greater than that of the unfed FW groups. Analysis of variances of plasma GH levels showed the influence of salinity to be greater than that of fasting. It is suggested that the main reason for the high GH levels in ‘stunted’ salmon may arise from premature transfer to SW rather than from starvation.