Gill Na+,K+-ATPase activity of hatchery chum salmon fry during freshwater rearing and acclimation to brackish water and its relationship to marine survival

The period of transition from freshwater to seawater has been shown to last 6-12 h for chum salmon Oncorhynchus keta fry in the brackish water of an estuary. In hatcheries, chum salmon fry are often reared in freshwater and released at larger sizes than wild out-migrants. From 1982 to 1984, we attempted to mimic the transition period in an estuary by adding seawater to freshwater-supplied raceways for 3, 12, or 24 h before release. We compared the gill Na+,K+-adenosine triphosphatase (Na+,K+-ATPase) activity in these groups and the freshwater control and tagged fish with coded wire tags to measure marine survival rates. Gill Na+,K+-ATPase activity increased significantly during the first 3 weeks of rearing in freshwater. Overall, enzyme activity increased from 9.8 +/- 1.7 (mean +/- SE) mumol inorganic phosphate (Pi) . mg protein(-1) . h(-1) initially to a peak level of 31.9 +/- 2.8 mumol Pi . mg protein(-1) . h(-1) in 1982 and from 10.4 +/- 1.4 mumol Pi mg . protein(-1) . h(-1) to 43.2 +/- 4.4 mumol Pi mg . protein(-1) . h(-1) in 1984. In 1984 the peak was followed by a decline, whereas in 1982 the peak was attained after a small decline the previous week. The enzyme activity of fish acclimated in brackish water did not differ significantly among groups or from that of the control group in either year. Marine survivals were not related to either enzyme activity or the length of acclimation in brackish water and tended to be higher in the controls. These findings suggest that chum salmon fry undergo a process similar to the parr-smolt transformation found in older and larger congeneric species and that acclimation to brackish water in the hatchery for 3-24 h before release does not improve survival relative to rearing in freshwater.