STIMULATION OF SPAWNING BEHAVIOR IN CASTRATED RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) BY 17-ALPHA,20-BETA-DIHYDROXY-4-PREGNEN-3-ONE, BUT NOT BY11-KETOANDROSTENEDIONE

The onset of spawning behavior in male salmonids can be closely correl ated to circulating levels of the progestin 17 alpha,20 beta-dihydroxy -4-pregnen-3-one (20-P). However, evidence that 20-P plays a causal ro le in the development and maintenance of spawning behavior in male sal monids is largely lacking. The purpose of this study was to investigat e the effect of 20-P implants on the spawning behavior in castrated ra inbow trout in the presence of a sexually active female. Mature rainbo w trout were placed in five experimental groups: sham-operated and cas trated implanted with either Silastic capsules containing 20-P, 11-ket oandrostenedione (11-KA), 11-KA + 20-P or empty capsules. Five to 6 we eks after implantation, all males were tested individually in a stream channel in the presence of a nesting ovulated female. Following behav ioral observations, blood samples were taken from all tested males. Im plants of 20-P and 11-KA resulted in increased plasma levels of 20-P a nd 11-ketotestosterone, respectively. Sham males exhibited the stronge st behavioral response, both in the frequency of quiver bouts and atte ntion toward the female. Castrated controls and 11-KA males showed no quiver response and reduced attention toward the female, while the beh avioral responses shown by the 20-P and 20-P/11-KA males were intermed iate. The results indicate that 20-P, but not ll-androgens, plays a si gnificant role in the stimulation of spawning behavior in male rainbow trout. (C) 1994 Academic Press, Inc.