Survival of reared and wild Atlantic salmon smolts: size matters more in bad years

We compared the marine survival of Carlin-tagged wild and hatchery-reared Atlantic salmon smolts of the Simojoki river, northern Baltic Sea. All the reared and released smolts were the offspring of native spawners returning to the river. Reared smolts were adipose-fin-clipped and released from the hatchery several weeks before tagging. The wild and reared smolts were simultaneously caught and tagged at a smolt trap located at the Simojoki river mouth. The study was conducted in two years, 1991 and 1993, when post-smolt survival in the Baltic Sea was different. Tags were returned by fishermen and return rates were used to estimate the survival of the smolt groups. We applied generalized linear models with survival as response variable and the year, origin, and smolt size as explanatory variables. On average, wild smolts had a 4.5 times higher survival rate than reared fish of the same smolt size. The difference in observed tag recovery rates as such was only about twofold or less, as the larger size of the reared smolts compared with the wild ones compensated for their lower survival rate. The better survival of wild than reared smolts was more pronounced in the low-survival year (1993 smolt year class) than in the high-survival year (1991 smolt year class). (C) 2004 Published by Elsevier Ltd on behalf of International Council for the Exploration of the Sea.