The purpose of this study was to investigate the mechanism(s) of the preference of sexually mature male goldfish for sexually mature female goldfish. First, the effect of reproductive condition on the sexual behaviour of male goldfish was examined to determine the mechanism(s) of the behavioural preference of mature males for mature females. Second, the effect of reproductive condition of the male on the response of the olfactory bulb electroencephalogram (EEG) to biological olfactory stimuli was examined for the electrophysiological correlates of the behavioural mechanism(s). The sexual behaviour of male goldfish is affected by reproductive condition. Regressed male goldfish do not demonstrate sexual behaviour. The sexual behaviour of mature male goldfish appears to be regulated by two mechanisms. A general tendency of the mature male to chase female goldfish seems to be regulated by an all-or-none mechanism which is activated when spermiation (release of sperm into the sperm duct) occurs in the male. A second mechanism appears to regulate the preference of mature male goldfish for sexually mature female goldfish. The preference for mature females is not directly dependent on spermiation since mature (spermiated) males with gonadosomatic indices (GSI) below 3.5 do not chase mature females preferentially. Mature males with a GSI above 3.5 chase mature females significantly more than regressed females, and a marked peak in the preference for mature females is observed for males with GSIs of 3.5-4.0. An earlier behavioural study suggested that the preference of mature males is a response to a pheromone from the mature females. Therefore, it might be expected that the neural activity of the olfactory system of mature male goldfish would respond differentially to an olfactory stimulus derived from mature female goldfish. The electrophysiological studies of this thesis indicate that a selective response to the stimulus derived from mature females is not evident in the olfactory bulb EEG of mature males. However, the amplitudes of the spontaneous EEG and the responses of the EEG to olfactory stimuli are significantly greater in the olfactory bulb of mature male goldfish than in that of regressed male goldfish. The EEG amplitude is correlated with the maturation of the testes, and gonadal hormones and/or other hormones may affect the EEG. The significance of the increase in EEG amplitude with reproductive condition is discussed in relation to the mechanisms of sexual behaviour. / Science, Faculty of / Zoology, Department of / Graduate

Goldfish (Carassius auratus) of 15-28g were maintained at 23 C and subsequently were intraperitoneally injected with saline (controls) and varying doses of Triiodothyronine (T3). Following the injection, the fish were placed in a thermal gradient and temperature selection and activity monitored for 30 min. There was no- significant difference between the responses of the experimental fish to the 2 different doses of T3. However, there was a significant difference in response between the experimental and control groups (P < 0.001). The decreased temperature selection occurred early in the test runs, while the increased activity began later. Larger goldfish (175-205g) received 0.5 μl intracerebral injections of 1.5-3.0 ng T3 in the anterior brainstem via an indwelling cannula injection system. These fish responded to the injections by selecting cooler water. The magnitude of the response was related to the distance of the injection site from the preoptic/anterior hypothalamic region. I conclude that increased plasma levels of T3 lead to the selection of cooler water. This effect appeared to be mediated via the anterior brainstem. Injected T3 also produced hyperactivity. This latter effect developed more slowly and appeared to be more diffusely mediated.