Drinking in Atlantic salmon presmolts and smolts in response to growth hormone and salinity

Drinking rate in freshwater Atlantic salmon presmolts (about 0.1 ml/kg/h) was unaffected by daily injections of ovine GH (50 micrograms/fish) for a week but upon transfer to sea water an immediate and full drinking response was developed compared to saline treated fish (3.34 +/- 0.16 vs. 2.23 +/- 0.27 ml/kg/h). Smolting did not affect drinking rates in freshwater but after 7 days in sea water, salmon smolts imbibed 3.88 +/- 0.25 ml/ kg/h, significantly higher than the rate for saline injected presmolts (p < 0.05, one-way ANOVA), but not significantly different from oGH treated presmolts. Smolting and oGH treatment were without effect on plasma Na+ levels in freshwater fish and 7 days after transfer to sea water both groups showed a better regulation of plasma Na+ levels compared to saline treated presmolts. Atlantic salmon smolts showed higher levels of plasma Cl- than presmolts in freshwater, and after 7 days in sea water, both oGH presmolts and smolts showed significantly lower levels of plasma Cl- than saline injected presmolts. GH treatment in freshwater presmolts improved hypoosmoregulatory capacity following transfer to seawater and these results are discussed in relation to the physiology of smolting, and control of drinking.