MALE-DOMINANCE, PLASMA-HORMONE CONCENTRATIONS, AND AVAILABILITY OF MILT IN MALE RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

The relationships among social status, endocrine profile, and amounts of milt available by stripping were examined in pairs of male rainbow trout (Oncorhynchus mykiss) competing for access to sexually active fe males in laboratory spawning channels. Gonadotropic hormone (GtH II), the gonadal steroids 11-ketotestosterone, testosterone, and 17 alpha,2 0 beta-dihydroxy-4-pregnen-3-one, and milt volumes increased in domina nt males in response to stimuli provided by a nesting female. Subordin ate males in the same location did not show endocrine or milt response s. Following the removal of the dominant male of each pair, plasma hor mone and milt levels increased in the previously subordinate males. Th is result indicates that endocrine and spermiation responses depend up on close-range sensory and behavioural interaction with the sexually a ctive female. Treatment with testosterone did not cause a change in be haviour or social status of smaller males predicted to be subordinate.