Spawning-Age Differences and their Temporal Trends in Wild and Sea-Ranched Atlantic Salmon Stocks, from Stock Mixture Data

Origin and age was determined for individual fish caught in offshore catches of Atlantic salmon stocks (Salmo salar L.) in the Baltic Sea over the years 2000–2009. DNA microsatellite loci and smolt age were used to probabilistically assign returning spawners to their stock of origin. Data for this study were based on approximately 2600 catch samples of the five most common wild and four sea-ranched, hatchery-reared stocks. Spawning age, and sex ratio differed both within and between these wild, and sea-ranched groups. The females were mainly (78.7%) two sea-winters old and the males usually (68.7%) only one sea-winter old. In both sexes, the mean age at maturity was lower in the hatchery-reared, sea-ranched stocks than in naturally reproducing stocks. In the 2000s, there was a weak decreasing trend in the male spawning age, but not in that of females. The sex-ratio of the spawners was female dominant in the naturally reproducing stocks, but male dominant in hatchery-reared stocks. Published historical data from two of the same rivers suggest that the majority of males were multi-sea-winter spawners in the 1930s, and variation in the age distribution of the spawners has become narrower and skewed towards a younger age in the present data (2000–2009) compared to the earlier situation.