The aim of the present study was to determine the mean size and age of migrating salmon, Salmo salar L., and sea trout, Salmo trutta L., smolts in three rivers of the Nemunas River drainage area from different climatic sub-regions in Lithuania. Research indicates there were significant differences of migrating fish age and size between stocked and natural smolt populations. In natural populations salmon and sea trout smolts aged 2+ dominated, but up to 82% of all migrating smolts of the stocked population in Siesartis River were aged 1+. The artificially-reared salmon and sea trout smolts in the Siesartis River were significantly larger than wild fish in the 1+ age group of migrants, and, accordingly, they reached smolt size and were ready to migrate earlier. The high density of salmonid parrs in the Siesartis River and intraspecific competition could have also prompted 1+ age group smolts to migrate. It is hypothesized that the phenomenon of early smolt migration in the Siesartis River could have been a local adaptation to the long migration route, or it could have resulted from density-dependent competition caused by imbalanced stocking programs.
