DOWNSTREAM MIGRATION OF IMMATURE SALMON (SALMO-SALAR) SMOLTS BLOCKED BY IMPLANTATION OF THE ANDROGEN 11-KETOANDROSTENEDIONE

The effect of androgen treatment on individual downstream migratory activity of 2-year-old Baltic salmon (Salmo salar) smolts was studied in a circular stream channel during the natural smolt migration period. Fish were tagged with passive integrated transponder (PIT) tags and implanted with silastic capsules containing the androgen 11-ketoandrostenedione or empty capsules. 11-ketoandrostenedione gives increased blood plasma levels of 11-ketotestosterone through peripheral conversion. The migratory activity of individual fish was monitored automatically by means of a PIT-tag system. In androgen-treated fish, the number of downstream passages per day during peak migration was reduced to about 10% of that of control fish. It is concluded that androgens seem to prevent downstream migration, the development of hypo-osmoregulatory ability, silvering, and the decrease in condition factor associated with smolting. Rather, androgens seem to promote retention of freshwater adaptations and increased growth in weight during spring.