Spawning behaviour and sex hormone levels in adult and precocious brown trout (Salmo trutta L.) males and the effect of anosmia

Abstract

In the present experiment the behaviour and endocrine status of males of the brown trout, Salmo trutta L., (Salmoniformes: Salmonidae) were studied when males were kept in a stream tank with a nest digging female. Groups of mature adult males and precocious intact or anosmic male parr were placed with the nesting female so that the group resembled a natural spawning situation with big anadromous fish acting as dominant males and precocious parr acting as ''sneakers''. A control experiment was also run with only males without a female present. In intact parr there were significant positive correlations between the per cent of the total observation time spent with a female, milt volume, and plasma concentration of 17h,20i-dihydroxy-4-pregnen-3-one. Anosmic parr had significantly lower volumes of strippable milt and gonadal steroid hormone levels compared with intact parr. However, no differences were found in the control experiment. Significantly fewer anosmic parr attended and courted the nesting female and those anosmic fish that attended the female had significantly lower plasma levels of gonadal hormones. Intact parr also displayed a greater number of agonistic acts against other parr without any difference in fighting ability. No differences in aggression occured in the control experiment. In adult males together with a female, post-experimental gonadal steroid hormone levels were higher than pre-experimental levels. Positive correlations between aggression and androgen hormone levels were observed in adult males. No differences in plasma hormone levels were observed between adult males and intact precocious males. The results show that olfactory occlusion results in low steroid hormone levels and milt volumes in precocious males placed in a spawning situation. The courting behaviour was also affected by anosmia. Odours from the nesting female may have caused the enhanced plasma hormone levels and stimulated the males to attend the female.

Effects of copper on olfactory‐mediated endocrine responses and reproductive behaviour in mature male brown trout Salmo trutta parr to conspecific females

“…Among the other behavioural variables studied, the time of upstream swimming was reduced in exposed parr. There were some similarities with the results in the current study and those in Olsén et al (1998) with anosmic and intact male S. trutta parr. Those similarities are that intact male parr spent more time in close proximity to the female, completed more courting events and had higher levels of expressible milt than anosmic parr when placed with a female.…”

“…It is interesting that exposed male S. trutta parr in the stream tank had higher blood plasma levels of 11‐KT than the controls, but no differences were observed in frequency of aggressive acts. Previous studies have shown positive correlation between aggression and blood plasma androgen levels (Olsén et al , 1998). Sex hormone levels in plasma of the parr in the stream tank were much lower than in the priming experiment both in the exposed and the controls, c. 10 times lower of 11KT and half the concentration of 17,20β‐P, a difference that probably was caused by a difference in social environment.…”

“…The female behaviour (frequency of probing, digging and spawning) and the behaviour of the different males, both small and adult, close to her were recorded. The behaviours recorded are summarised in Table IV, and for further information see Olsén et al (1998).…”

“…The ability of males to detect both pheromones by way of olfaction is probably vital to reproduction. Olsén et al (1998) examined effects of anosmia (loss of the sense of smell) on male S. trutta parr's behaviour and sex hormone blood plasma levels when placed together with ovulated nest digging females and adult anadromous males. They reported that parr with blocked nasal passages had significantly lower hormone levels, less volumes of milt, were less aggressive and courted females less than control males.…”

Environmental influences on life history strategies in partially anadromous brown trout (Salmo trutta, Salmonidae)

“…But to reproduce, trout must return to freshwater and compete for access to suitable spawning sites and mates. Interference competition generally favours large individuals (Olsén et al, 1998); however, in small shallow tributaries, small resident trout may have higher fitness because these habitats are less accessible for large individuals. Interference competition generally favours large individuals (Olsén et al, 1998); however, in small shallow tributaries, small resident trout may have higher fitness because these habitats are less accessible for large individuals.…”