GILL NA-K

ABSTRACT. Gill Na+-K+ ATPase activity and body silvering were studied in spring in one- and two year old sea trout (Salmo trutta m.trutta L.), under hatchery conditions. Two groups were distingu-ished in each age-class: smolts and non-smolts. Smolts of both age classes showed higher ATPase ac-tivity, lower condition coefficient, and more distinct body silvering. One year old smolts were also longer, and their bodies contained more fat compared to non-smolts. In two years old smolts lower fat level was observed. Discriminant functions were proposed, taking into consideration ATPase ac-tivity and body silvering, to distinguish smolts from non-smolts. These functions may be used as a smoltification index