We compared smolt physiology of Atlantic salmon stocks, that migrate from freshwater stream environments to a freshwater lake (Lake Saimaa, Finland), from the Neva River (Russia) to the brackish water of the Baltic Sea, and from the Teno River (Finland/ Norway) to the full-strength seawater of the Arctic Ocean. The three fish stocks were reared under common garden conditions, and ion homeostasis, seawater tolerance, activities of gill and kidney Na,K-ATPase and [ , and Mg 2+ ion concentrations of blood plasma decreased during smoltification. α1a and α1b isoforms of gill Na,KATPase decreased and increased, respectively, during smoltification. Although small differences appeared at some points in time, the seasonal pattern and magnitude of changes in smolt characteristics were generally similar in the Saimaa salmon and in the anadromous stocks of the Neva and Teno salmon. It is suggested that the strong smoltification process of the landlocked Saimaa salmon is somehow necessary for migration of the young salmon from river to lake and therefore has been retained in evolution.

We examined whether gene expression in the young salmon (Salmo salar) gill differs in relation to the salinity of their migration habitat by comparing three salmon stocks: (1) fish that migrate from a river system to Lake Saimaa, (2) fish that migrate to the brackish waters of the Baltic Sea, and (3) fish that migrate to the full-strength salinity of the Arctic Ocean. Transcripts of the gill tissue were measured at three successive developmental stages (parr, smolt and postsmolt) using the cDNA microarray in fish reared under common conditions. The changes in gene expression were qualitatively and quantitatively similar in the three stocks irrespective of the salinity of the natural growing habitat. This suggests that the parr-smolt transformation in the gill tissue of the landlocked fresh-water salmon stock is similar to the seawater migrating salmon. The transformation of the gill to a hypoosmotic organ in the freshwater salmon has been retained in evolution, possibly due to its adaptive role as a signal for migration from a relatively poor-growth environment of the river to a more productive lake habitat.

Through the recent increases in abundance, pikeperch (Sander lucioperca) has not only become a valuable target for recreational and commercial fishing, but also the most important top predator in many lakes. Generally oligotrophic Lake Höytiäinen (283 km2) sustains one of the most economically valuable pikeperch stocks in Finland, and its annual fisheries yield (up to 5.5 kg/ha) is exceptionally high among Finnish lakes. Here, we followed seasonal movements of fish using conventional tagging, and studied short‐term and long‐term diet of pikeperch by stomach content and stable isotope analyses to examine if the movements were motivated by prey availability. A clear seasonal migration pattern was observed: in summer, 80% of the tag recoveries were obtained from the shallow and mesotrophic northern and western areas, whereas in autumn and winter, the deep and oligotrophic southern area dominated in the recaptures. The diet of pikeperch consisted of the most abundant fish species in the lake, perch (Perca fluviatilis) and roach (Rutilus rutilus), as well as insects. Stable isotope analysis indicated a shift from a dominance of pelagic food sources to littoral food sources during the growing season of pikeperch. Our results suggest that the pikeperch migrations facilitate efficient material transfer from the mesotrophic part while the deeper and oligotrophic part offers a favourable overwintering environment.

Silvering of the skin, reduced condition factor, elevated gill Na + , K + -ATPase activity and well-developed capacity to regulate the osmotic and ionic balance in sea water were observed in 1 and 2 year old hatchery-reared Saimaa landlocked salmon Salmo salar m. sebago during April-June. Loss of hypoosmoregulatory ability and gill Na + ,K + -ATPase activity was observed earlier in 2 year than 1 year old fish. Coincident with changes associated with smolting both age groups showed diminished osmoregulatory capacity in fresh water. Slow growth during May-June may also be attributed to osmoregulatory difficulties in fresh water. These results support the suggestion that the developmental changes at smolting are seasonal and unrelated to any salinity changes and the idea of smolting as evidence of maladaptation of the fish to fresh water.

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