Relationship between gill Na+,K+-ATPase activity and downstream movement in domesticated and first-generation offspring of wild anadromous brown trout (Salmo trutta)

The relationship between smolt status and downstream movement following rel ease was investigated in two stocks of hatchery-reared anadromous brown tro ut (Salmo trutta). Yearlings from a domesticated stock (DS) and first-gener ation offspring (F1) of wild anadromous trout were held under identical con ditions from August 1997 until the following spring, where they developed s molt characteristics as judged from increasing gill Na+,K+-ATPase activity. Presmolts (low Na+,K+-ATPase activity), smolts (high Na+,K+-ATPase activit y), and desmolts (regressed Na+,K+-ATPase activity) were released on three occasions into the River Salten. Using both dye-marked and radiotagged fish , downstream movement was monitored by either trapping 3 km downstream (dye -marked fish) or radiotracking on a daily basis. The experiments showed a p ositive correlation between smolt status (gill Na+,K+-ATPase activity) and downstream movement. Gill Na+,K+-ATPase activity may therefore be used as a n indicator of migratory readiness in brown trout. F1 and DS trout had the highest migration frequency when released as presmolts and smolts, respecti vely. Despite smaller size, F1 trout had similar or better survival than DS trout after release. Our data suggest that initiation of downstream moveme nt is influenced by an interaction between the previous physiological devel opment of the fish and a discrete level of water discharge or water tempera ture.