Environmental impacts on the gonadotropic system in female Atlantic salmon (Salmo salar) during vitellogenesis: Photothermal effects on pituitary gonadotropins, ovarian gonadotropin receptor expression, plasma sex steroids and oocyte growth

• Photoperiod accelerated the activation of the BPG axis during vitellogenesis.
• Females in cooled water had equally rapid gonadal development as in warmed water.
• Females in cooled water compensated with stronger up-regulation of fshb.
• Plasma levels of E2 were higher in cooled water to maintain rapid gonadal growth.
• Ovulation was compromised in the photothermally treated groups.

The gonadotropic system and ovarian growth and development were studied during vitellogenesis in female Atlantic salmon subjected to either simulated natural photoperiod and ambient water temperature (NL-amb), or an accelerating photoperiod (short day of LD8:16 from May 10) combined with either warmed (ca 2 °C above ambient; 8L-warm) or cooled water (ca 2 °C below ambient; 8L-cold) from May to September.Monthly samples were collected from 10 females/group for determination of transcript levels of pituitary gonadotropin subunits (fshb and lhb) and ovarian gonadotropin receptors (fshr and lhr), plasma sex steroids (testosterone: T and estradiol-17β: E2), gonadosomatic index (GSI) and oocyte size.Short day in combination with either warmed or cooled water induced an earlier increase in pituitary fshb and lhb levels compared with NL-amb controls, and advanced ovarian growth and the seasonal profiles of T, E2. By contrast only minor effects were seen of the photothermal treatments on ovarian fshr and lhr.The 8L-cold had earlier increase in fshb, lhb and E2, but similar oocyte and gonadal growth as 8L-warm, suggesting that the 8L-cold group tried to compensate for the lower water temperature during the period of rapid gonadal growth by increasing fshb and E2 production.Both the 8L-warm and 8L-cold groups showed incomplete ovulation in a proportion of the females, possibly due to the photoperiod advancement resulting in earlier readiness of spawning occurring at a higher ambient temperature, or due to some reproductive dysfunction caused by photothermal interference with normal neuroendocrine regulation of oocyte development and maturation.