Life-history patterns in a southern population of Atlantic salmon

Juvenile Atlantic salmon Salmo salar in the river Esva, Asturias, north-wes t Spain, developed a bimodal growth pattern during their first growing seas on. Segregation between the two modes was apparent by late autumn. All fish in the upper modal group (UMG) grew throughout the year and migrated downs tream in the following spring (by April) at the age of 14 months. Some lowe r modal group (LMG) fish (56% of the main stem, 31% and 50% of the two trib utaries) silvered like smelts and apparently migrated downstream the same s pring, although c. 1.5 months after UMG fish. Larger LMG fish appeared more likely than smaller ones to migrate. LMG fish did not grow in winter, but they grew fast between March and May. Maturing male parr were detected firs t at 5 months old in July, and they occurred initially among the faster gro wers, >6.8 cm long. In spite of this, length of maturing males did not chan ge from late summer and throughout the winter, whereas that of non-maturing UMG and LMG fishes continued to diverge from September onwards. These find ings suggest that favourable conditions for growth (very early start of a l ong growing season) at the southern limit of the species' range may influen ce the life-history pattern of this population. (C) 1999 The Fisheries Soci ety of the British Isles.