The reproductive behaviour of male and female Salmo salar was studied in a specially constructed arena.
The Results showed that body size, cardiac-somatic index and relative kype size were all correlated with male dominance rank. Dominant males approached ripe females more frequently and ultimately mated more frequently than subordinates. Males repelled by females from the spawning beds had smaller adipose fins. No correlation was found between female aggression and male dominance, body size or cardiac-somatic index. The results suggest that male salmon establish a dominance hierarchy during the spawning period and that intra-sexual selection has favoured the evolution of a status signalling system, whereby males signal their dominance rank by their relative kype size. Dominant males having conspicuous secondary sexual characteristics are able to mate more frequently because they can occupy ripe females. As females seem to select their mates on grounds of the relative size of the adipose fin, inter-sexual selection is also presumed to have affected the evolution of male secondary sexual characteristics.