Effect of elevated temperature on estrogenic induction of vitellogenesis and zonagenesis in juvenile Atlantic salmon (Salmo salar)

Fertility and embryo survival rates are often low in eggs from thermally challenged Tasmanian Atlantic salmon, partly due to a reduction in plasma 17β-estradiol (E₂) levels. We used juvenile Atlantic salmon to assess whether hepatic tissue remains responsive to stimulation by E₂ at the higher temperatures sometimes encountered by Tasmanian salmon during summer. E₂ administration stimulated vitellogenin (Vtg) and estrogen receptor alpha transcription at 14°C and 22°C, although induction of Vtg occurred more rapidly at 22°C. Consequently, plasma Vtg levels increased and reached a plateau more quickly at 22°C. Zona pellucida (Zp) B and C transcription was significantly lower in E₂-treated fish at 22°C relative to 14°C. This shows that the Vtg gene is E₂-responsive at high temperature unlike Zp B and C genes that displayed traits of thermal inhibition. Therefore, estrogen replacement therapy in adult salmon may offset some, but not all thermal inhibition of reproductive function.