OPTIMAL TIME AND SIZE FOR SMOLT MIGRATION IN WILD SEA-TROUT (SALMO-TRUTTA)

A tagging programme, in which wild sea trout (Salmo trutta) were marked at the parr stage with internal tags in winter and recaptured during smolt migration in a trap in the following spring, was conducted for three seasons in a small stream in southwestern Sweden. Individuals that were initially smaller migrated later, grew more in length, and were smaller at migration than those initially bigger. Body length at migration decreased with migration time during the season for tagged as well as nontagged trout. A model for optimal time of and size at migration is proposed, based on the assumption that the expression (body length at the end of the season) x (survival over the season) is maximized and that migratory and postmigratory survival is positively size dependent. The qualitative predictions on the relationship between initial size and time of and size at migration are supported by the tagging data. Pre-and postmigratory growth rates are predicted to have opposite effects on migration time, so the migration timing is expected to depend strongly on the degree of dependence between these.