Constant High Temperature Promotes Early Changes in Testis Development Associated with Sexual Maturation in Male Atlantic Salmon (<i>Salmo salar</i> L.) Post-Smolts

Sexual maturation of post-smolts is a concern for Atlantic salmon producers, and its occurrence is dependent upon factors such as water temperature and energy availability, among others. The present study was designed to investigate the effect of different temperatures and feeding regimes on testis development and local regulation of spermatogenesis in salmon post-smolts. A 3 × 2 factorial design was used, with three temperatures (8 °C, 12.5 °C, and 18 °C) and two feed regimes (100% and 67% ration). Salmon (1800 parr, initial mean weight 23.1 ± 7.2 g) were reared from 28 October 2018 to 30 May 2019 in a freshwater flow-through system under continuous light (LD24:0), except for a 5-week winter signal (LD12:12) introduced on 4 February 2019. Testis histology, transcription of follicle-stimulating hormone receptor (<i>fshr</i>) and luteinizing hormone receptor (<i>lhr</i>), and transcription of testis proteins involved in spermatogenesis regulation such as gonadal-soma-derived factors 1 (<i>gsdf1</i>) and 2 (<i>gsdf2</i>), anti-Müllerian hormone (<i>amh</i>), and insulin-like growth factor-3 (<i>igf3</i>), were analyzed. Results showed that high temperature alone (18 °C), irrespective of the feed regime, promoted early presence of type B spermatogonia and reduced transcript levels of the proliferation-inhibitory factor <i>amh</i> in males still considered immature, of groups 18–100% and 18–67% that later matured in high proportion (~100%). This effect was also present to some degree in the group 12.5–100% (40% maturation), and absent in 12.5–67%, 8–100%, and 8–67% (groups with little or no maturation). Later, at onset of rapid testis growth, high temperature was linked to a pronounced downregulation of <i>amh</i> and of the self-renewal factors <i>gsdf1</i> and <i>gsdf2</i>, as well as to a pronounced upregulation of the proliferation-stimulating factor <i>igf3</i>. Overall, the present findings demonstrate that rearing salmon at high temperatures can stimulate an early activation of the brain–pituitary–gonad axis before actual onset of rapid testis growth, enhancing and accelerating the mechanisms that control initiation and progression of maturation, while the feed regime has a minor impact. This poses a challenge for current salmon aquaculture practices that use sustained high temperature to maximize growth, since these practices increase the risk of early post-smolt maturation.