By Irma Kallio-Nyberg, Matti Salminen, Irma Saloniemi, Laura Kannala-Fisk
Fisheries Research, Volume 96, Issues 2-3, March 2009, Pages 289-295, ISSN 0165-7836, DOI: 10.1016/j.fishres.2008.12.009.

We examined the factors underlying the deteriorating marine survival of a hatchery stock of Atlantic salmon, Salmo salar. The stock originated from the salmon population of the Neva River, which flows into the Gulf of Finland in the Baltic Sea. During 1981-2005, about 134,000 Carlin-tagged Neva salmon smolts were released in the Gulf of Finland and the Bothnian Sea. We investigated the effects of fin damage, smolt length, sexual maturity (precocious maturation), smolt year, the time (days) between tagging and release and the release date on survival. The survival of the released smolts clearly decreased towards present. In earlier years, larger smolts tended to survive better than smaller ones, but the advantage of a large smolt size decreased in the later years of the study. No significant effect of dorsal fin damage on survival was detected. On average, immature smolts survived better than precocious males, and the proportion of precocious males increased over time. However, the proportion of precocious males among the tagged fish did not explain the decreasing trend in marine survival. Release year was the most important predictor of the long-term decreasing trend in marine survival. The optimal tagging time varies: in some years smolts tagged in the autumn survived better, while in other years spring taggings were more advantageous. For most years, a late release (in May or June) was more favourable than an early one (in April). As the releases were mainly carried out quite early in the spring in the last 13 years of the study, an unfavourable timing of release may partly explain the lower marine survival recorded during this period.
Keywords: Atlantic salmon; Baltic Sea; Hatchery; Recapture; Precocity; Fin damage

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