Effects of temperature regimes on proportion of early maturation and the activation of the BPG axis in male Atlantic salmon (salmo salar L.) postsmolts

Abstract

Early maturation of Atlantic salmon (salmo salar L.) male postsmolts is a growing concern for the Norwegian aquaculture industry due to an increasing occurrence in intensive rearing facilities, negatively impacting growth, welfare, and survival after transfer to sea. The neuroendocrine pathway to maturation is not well known in fish, and finding key hormones involved in the regulation could give insight to how, and when, temperature exerts influence on the decision to mature early. The experiment consisted of rearing four groups in duplicates under four different temperature profiles (15, 15-8L, 15-8E, and 8) from May 24th to November 2nd, 2022. Atlantic salmon in the four treatments (900 parr, initial weight ~50g) was reared in flow-through under continuous light (LD24:0, LL) with a 5-week winter signal (LD12:12) introduced on the 28th of July 2022 to promote developmental events. Two groups were kept at constant temperature: group 15 at 15°C and group 8 at 8°C. Two groups experienced a decrease in temperature from 15°C to 8°C simultaneously to change in photoperiod: group 15-8E at the start of WS and 15-8L at the end of WS. Body weight, condition factor (K), gonadosomatic index (GSI) and transcript levels from the diencephalon brain region of gonadotropin releasing hormones (gnrh2, gnrh3), deiodinase type 2 (dio2b), kisspeptin receptor (gpr54), and gonadotropin inhibitory hormone (gniha) were assessed. Results showed that rearing individuals at 15°C allowed for higher energy accumulation, leading to 100% of male maturity in mid-September after WS, while there was no maturation seen in the group reared at 8°C. There were little physiological differences between the two temperature regimes (15-8E, 15-8L). Results suggest that reduction in temperature decreases physiological development, leading to a lower percentage of maturing individuals than the constant 15°C treatment, however, the timing of reduction in temperature from 15°C to 8°C did not make a significant difference in biometry. Results from the gene transcripts revealed temperature to influence expression, showing a trend between genes in the lower temperature treatment (8°C) following increase in photoperiod after WS, while elevated temperature appeared to have a disruptive effect on the BPG axis causing early maturation. The results of this thesis further supports the use of less intensive water temperatures in industry production in order to reduce occurrences of early maturation. The clear effect of temperature on sexual maturation remains a mystery at the neuroendocrine level. Results suggest a disruption of the BPG pathway at elevated temperatures, but the picture of regulatory factors remains unclear.