LIFE-HISTORY TRANSITIONS AMONG ATLANTIC SALMON (SALMO-SALAR) MORPHOTYPES

Morphotype distribution in Atlantic salmon populations is a critical d eterminant in the migration dynamics and rearing success of this speci es. The achievement of smelting status depends on the individual oppor tunities to reach the upper mode of the bimodal size distribution in t he first winter. Growth of the morphotypes between 10 and 22 mo post-h atch was investigated. For this, an individual tagging system was appl ied on two replicates of parr from a Sella River sample (northern Spai n). The smelt-like parr of the first winter (37%) became smelts in the first spring. Meanwhile, dwarf parr (25%) and precocious males (6%) d id not smelt and became precocious males in the second winter. Medium- sized pan of the first winter (32%) became smelt-like parr (19%) or pr ecocious males (13%) in the second winter. The average smelt-like size s in the first and the second winters were 11.39 cm and 14.97 cm, resp ectively. These results indicate that for the Sella River sample (i) o nly upper modal fish smelt in the first spring, (ii) 1 + smelts are ma inly females (77%), and (iii) smelting does not depend upon a populati on threshold size.