Precocious Sexual Development in Hatchery-Reared and Laboratory-Maintained Male Steelhead Trout (Salmo gairdneri)

Abstract

The incidence of precocious steelhead trout (S. gairdneri) was determined in several Idaho [USA] hatchery steelhead populations. The range of incidence of precocious males was 0-64% of the sampled populations. This variation appeared to be related to age of the trout, water temperature and photoperiod. The incidence of precocious males greatly exceeded the incidence (0.1% of the population when encountered) of precocious females. Precocious male steelhead trout are apparently lost from the migrating smolt population and display mating behavior similar to sea-run adult male steelhead. Precocious males maintained in laboratory conditions remained sexually mature for at least 6 mo. of observation. This was reflected by mean gonadal weights and mean gonadosomatic indices from precocious male steelhead which differed significantly from nonprecocious juvenile male steelhead maintained under similar conditions. No consistent pattern of growth was observed among laboratory-maintained trout.