Mortality risks in Baltic salmon during early migration was estimated through a sequential release experiment. Effects of time of release and size of fish on survival rate were studied.
A protected transfer to the sea and an acclimatization prior to release increased the recapture rates by 1.6 to 2.0 times compared to fish released in the river. Furthermore, fish with a delayed release had a 2.8 to 5.0 times higher recapture rate than smolts released in the river.
I found a strong positive correlation between the size of the fish and recapture rates during all three experimental years. Mortality rates peaked during the downstream migration and entry in to the sea. The weekly risk of mortality during the two first weeks was estimated to be 27.8%. Thereafter the mortality risk declined rapidly to 6.1% per week during the following 8-9 weeks. From mid September until the end of November the estimated mortality rate was only 3.5% per week.
Baltic salmon appears to migrate at a sub-optimal size with respect to survival during migration. A gain in survival by a larger size during migration,could be obtained by a prolonged freshwater residency. However this is considered to be outweighted by the option of an accelerated growth rate in the sea.