Management of Atlantic salmon in the Simojoki river, northern Gulf of Bothnia: effects of stocking and fishing regulation

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The endangered natural stock of Atlantic salmon ( Salmo salar L.) in the Simojoki river, northern Baltic Sea, has been managed primarily by fishing regulation, and by stocking with parr and smolts. The Finnish national regulation of salmon fishing in the Baltic Sea has changed considerably during recent decades: regulation was slight until 1996, strict in 1996–1997 and moderate from 1998 onwards. The wild/reared ratio was lower in smolts than in returning multi-sea-winter (MSW) spawners at the river mouth, indicating at least two times higher marine survival of wild smolts compared with reared ones. The catch-per-unit-effort (CPUE) of MSW spawning migrants in the trap net fishery of brood fish at the river mouth differed significantly between the slight, strict and moderate regulation periods, the CPUE being highest under strict regulation. The river catches of salmon were significantly lower during the period of slight regulation than later, while recoveries of tagged salmon in the Simojoki river were highest during the period of strict regulation. The results suggest that strict regulation of fishing is a very effective way of safeguarding the salmon stock, but even a short relief in the opening dates of the sea fishing season significantly reduced the spawning run into the river. The considerable proportion of reared fish among the ascending salmon indicates that stocking has increased the spawning run into the river. This has been important, especially during the years of low natural reproduction.