THE ONTOGENIC DEVELOPMENT OF BODY SEGMENTS AND SEXUAL DIMORPHISM IN BROWN TROUT (SALMO-TRUTTA L)

Nonmigratory brown trout (Salmo trutta L.) from northern Spain showed no between-sex differences in total length or mass and only moderate, but significant, dimorphism in some morphometric traits. The main dimo rphic features were the sizes of the abdomen (larger in females) and h ead (larger in males, including the upper jaw), relative to each other and also to the post-anal length, the latter being considered a contr ol for body size. Least-squares linear regressions of head length on a bdomen length were homogenecus in both the slopes and the intercepts f or immature fish of both sexes, but mature males exhibited significant ly steeper slopes than mature females. Within each sex the slopes for immature and mature fish were homogeneous, and mature fish of both sex es had higher intercepts than immature ones. Therefore, immature indiv iduals were monomorphic and mature individuals of both sexes diverged from the juvenile pattern, each in a particular fashion: males acquire d a greater relative head length (particularly a longer jaw) and femal es a greater relative abdomen length. Sexual selection for larger head size relative to body size in males and fecundity selection for large r abdomen size relative to body size in females are hypothesized to be important in the evolution of sexual dimorphism in brown trout.