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Return migration of Atlantic salmon, Salmo salar L., was studied in the Gulf of Bothnia, northern Baltic Sea, by a mark-recapture experiment and catch records from commercial trap-nets. Coastal salmon fishing is regulated by delayed opening of the fishery in consecutive regions based on the assumption that the wild fish migrate before reared ones and the migration is unidirectional and continuous from south to north. Neural network modelling suggested that the migration does not progress linearly from one regulation region to another, but shows variation between origin and sea age among and within regions. Further evidence of the non-linear migration included a noticeable part of salmon on their way to two major estuaries first visiting the northern-most Bothnian Bay before turning back south. Salmon returning to the different homing sites in the north showed no differences in run timing in the southern Gulf whereas the same individual fish showed differences in catch accumulation further north. Run timing estimates indicated only a slight tendency towards earlier migration for wild salmon compared with reared fish.

J. 2006. Size of wild and hatchery-reared Atlantic salmon populations in the northern Baltic Sea estimated by a stratified mark-recapture method. e ICES Journal of Marine Science, 63: 1477e1487.We estimated the total size of the Atlantic salmon (Salmo salar) population complex (wild and cultured) in the Gulf of Bothnia, northern Baltic Sea, using a stratified mark-recapture method. In 2001In , 1970 salmon were captured by the commercial trapnet fishery and tagged with external arrow tags. A total of 349 tagged fish was later recaptured among 65 180 salmon screened for tags. Recoveries were gathered by the commercial trapnets and by screening fish entering counting facilities in rivers and broodstock fisheries at the river mouths. In addition, tags were recovered from recreational river catches. Our estimates suggest that the total size of the migrating population in both years was about 230 000 fish. Proportions of wild and cultured salmon differed between the two years. In 2001, the proportion of wild salmon was 37%; the corresponding figure for 2002 was 62%. Based on estimates of wild salmon smolt production and the number of released smolts, the estimated proportion of cultured smolts that survived the feeding migration and returned to the Gulf of Bothnia (2e4%) was approximately 2.5e4.5 times lower than that of wild smolts (9e10%).