Relationships between smolt size, postsmolt growth and sea age at maturity in Atlantic salmon ranched in the Baltic Sea

Tagging data were used to examine the relationships between smolt size, post-smolt growth and sea age at first maturity for the short-migrating Neva strain of Atlantic salmon ( Salmo salar L.) ranched in the Bothnian Sea and the Gulf of Finland. The results provided evidence that post-smolt growth was influenced by both relative and absolute smolt size. For both sea-areas, 2-year smolts of small relative size within a release group grew more rapidly in the sea than did smolts of higher relative, but equivalent absolute size. The negative influence of increasing relative smolt size on marine growth was, however, outweighed by the stronger positive influence of increasing absolute smolt size. A 160-mm increase in smolt size (140–300 mm) resulted in an overall growth advantage of about 1 year. In the Bothnian Sea, the predicted mean length after 1 year in the sea was 288 ± 25 mm for 140-mm smolts and 560 ± 16 mm for 300-mm smolts. Under the more favourable conditions of the Gulf of Finland, the respective mean lengths were 369 ± 15 mm and 613 ± 12 mm. The sea age at first maturity was inversely related to both freshwater and marine growth rates. For both sea areas, large smolts yielded proportionately more grilse than did small ones. Smolt years with good post-smolt growth rates yielded more grilse than did years with poor growth rates. The overall level of grilsing was higher in the Gulf of Finland than in the Bothnian Sea. These results suggest that the relationships between smolt size, post-smolt growth and age at first maturity in the sea are influenced by the environmental conditions of the respective sea area. A framework explaining the links between smolt size, marine growth, survival and sea age at maturity in Neva salmon is presented for the Gulf of Finland and the Bothnian Sea.