Male dominance, plasma hormone concentrations, and availability of milt in male rainbow trout (Oncorhynchus mykiss)

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Abstract
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 females in laboratory spawning channels. Gonadotropic hormone (GtH II), the gonadal steroids 11-ketotestosterone, testosterone, and 17α,20β-dihydroxy-4-pregnen-3-one, and milt volumes increased in dominant males in response to stimuli provided by a nesting female. Subordinate males in the same location did not show endocrine or milt responses. Following the removal of the dominant male of each pair, plasma hormone and milt levels increased in the previously subordinate males. This result indicates that endocrine and spermiation responses depend upon close-range sensory and behavioural interaction with the sexually active female. Treatment with testosterone did not cause a change in behaviour or social status of smaller males predicted to be subordinate.