Life-history transitions among Atlantic salmon (Salmo salar) morphotypes

Abstract

Morphotype distribution in Atlantic salmon populations is a critical determinant in the migration dynamics and rearing success of this species. The achievement of smolting status depends on the individual opportunities to reach the upper mode of the bimodal size distribution in the first winter. Growth of the morphotypes between 10 and 22 mo post-hatch was investigated. For this, an individual tagging system was applied on two replicates of parr from a Sella River sample (northern Spain). The smolt-like parr of the first winter (37%) became smolts in the first spring. Meanwhile, dwarf parr (25%) and precocious males (6%) did not smolt and became precocious males in the second winter. Medium-sized parr of the first winter (32%) became smolt-like parr (19%) or precocious males (13%) in the second winter. The average smolt-like sizes in the first and the second winters were 11.39 cm and 14.97 cm, respectively. These results indicate that for the Sella River sample (i) only upper modal fish smolt in the first spring, (ii) 1 + smolts are mainly females (77%), and (iii) smolting does not depend upon a population threshold size.