SALINITY EFFECTS ON OXYGEN-CONSUMPTION, GILL NA-ATPASE AND ION REGULATION IN JUVENILE COHO SALMON(,K+)

The metabolic response of juvenile coho salmon Oncorhynchus kisitch to different salinities was examined, using whole-animal oxygen consumpt ion rates and gill Na+,K+-ATPase activities as indicators of osmoregul atory energetics. Coho salmon smelts were acclimated to fresh water (F W), isosmotic salinity (ISO, 10 parts per thousand) and sea water (SW, 28 parts per thousand) and were sampled for up to 6 weeks for plasma levels of cortisol, glucose and ions (Na+,K+,Cl-), gill Na+,K+-ATPase activity and oxygen consumption rates. Following an initial adjustment period, plasma constituents in SW fish returned to near-FW values, in dicating that the fish were acclimated to SW by day 21. Gill Na+,K+-AT Pase activities on days 21 and 42 were lowest in ISO, higher in FW and highest in SW. This result is consistent with the idea that less ener gy would be required to maintain ion balance in an isosmotic environme nt, where the ionic gradients between extracellular fluid and water wo uld be minimal. Oxygen consumption rates of swimming fish (1 body leng th s(-1)), however, did not differ significantly between the three tes t salinities after 6 weeks. The results of this study suggest that the metabolic response of juvenile salmonids to changes in salinity is de pendent on life-history stage (e.g. fry v. smelt), and that oxygen con sumption rates do not necessarily reflect osmoregulatory costs. (C) 19 98 The Fisheries Society of the British Isles.