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Comparison of coastal and river releases of Atlantic salmon smolts in the river Kokemaenjoki, Baltic Sea

Comparison of coastal and river releases of Atlantic salmon smolts in the river Kokemaenjoki, Baltic Sea

ICES Journal of Marine Science 55(6): 1071-1081

ISSN/ISBN: 1054-3139

Paired releases of Carlin tagged fish were carried out in the river Kokemaenjoki, Bothnian Sea, in 1991-1993, to compare the survival and migration of Atlantic salmon (Salmo salar L.) smolts released at two different sites, off the estuary and at Nakkila, 55 km upstream. The eight different smolt groups, divided between the two sites, ranged in mean weight from 27 g (1 yr) to 260 g (2 yr). The size of the smolt groups were planned according to previous experience of the expected rate of recapture to allow the detection of a deviation of 20-25% from the initial 50:50 (coast:river) stocking ratio at a 5% significance level and 90% power, which requires 279 and 180 recaptures for each group, respectively. A 25% deviation corresponds to 40% mortality/55 km of river migration, which is somewhat less than reported in the literature. In all, 20 000 smolts were released. Tag recoveries were collected until the end of 1996 (total number 2544). For all six groups of 2 yr smolts (120-260 g), the numbers recaptured were as high or higher than expected, exceeding the limit of 279 recaptures. For the two groups of 1 yr smolts (27 and 34 g), the rates of recapture were lower than expected (only 20 and 14 recaptures, respectively), and therefore the planned statistical analysis was not possible for these groups. For all paired releases of 2 yr fish, the results were similar irrespective of the release site. First, the distribution of recoveries over the years was homogenous for coastal and river releases at the 5% significance level (G-test for independence), suggesting that it is appropriate to add up all recoveries over time. Secondly, the total amounts of recoveries did not differ significantly from the original stocking ratios (coast:river) at the 5% significance level (G-test for goodness of fit), indicating that the release site did not affect the survival of 2 yr smolts. Due to extremely intensive sea fishery, 99.5% of adult recoveries with a known location (total 1695) were reported from the sea area. Only eight fish (0.5%) were captured in the Kokemaenjoki, and none in other rivers. It was thus not possible to test properly for differences in homing and straying between the two release sites. The migration pattern in the sea area was not affected by the release site. To improve the survival rates and the cost effectiveness of sea ranching, salmon smolts are released directly into the sea in the vicinity of several Baltic rivers. The present study challenges the economic justification of such releases in the River Kokemaenjoki. For large 2 yr smolts, comparable survival rates in this river system may be attained in upstream releases as well.

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Accession: 010357481

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