Growth, sexual maturation, cataracts, and blood biochemistry in diploid and triploid Atlantic salmon post-smolts reared at one of eight constant temperatures from 3.0 to 20.5 °C for 140 days

More Atlantic salmon ( Salmo salar ) post-smolt production is occurring at stable temperatures of 10–17 °C today, compared to the fluctuating ambient conditions used in the past. This increases growth, but also triggers unwanted sexual maturation. The latter can be avoided by using triploid females as they show no gonadal development during their life. However, triploids are believed to have a lower temperature optimum and suffer more from cataracts than diploids. Here, we assess the performance of mixed sex sibling circa. 260 g diploid and triploid post-smolts reared under one of eight temperatures from 3.0 to 20.5 °C (in increments of 2.5 °C) for 140 days in seawater and on continuous light. We assessed growth, sexual maturation, ocular cataracts, liver and heart size, and blood parameters associated with osmoregulation, anaerobic metabolism, and oxygen delivery. Cataract severity increased with temperature > 15.5 °C in diploids and > 5.5 °C in triploids, with the triploids at 20.5 °C being euthanised due to their severity. In male and female diploids, and male triploids, the prevalence of puberty increased with temperature. There were no pubertal diploids ≤5.5 °C, but all were pubertal ≥18 °C. In diploid females, the prevalence of puberty was 0 % ≤13 °C, but reached 76 % at 20.5 °C. Triploidy reduced the prevalence of male puberty to 75 % at 18 °C, whereas no triploid females entered puberty. In both ploidies, growth over the entire trial period was greatest at 10.5 °C, in which immature fish reached an average weight of circa. 1.2 kg. In general, triploids were significantly heavier than diploids ≥10.5 °C due to the higher levels of male maturation in diploids, but there was no ploidy effect on growth at any temperature in immature female fish. Triploids had larger livers (by 8 %) irrespective of temperature, whereas they tended to have smaller hearts ≤15 °C (1–7 %), but heavier hearts ≥18 °C (1–8 %), compared to diploids. Irrespective of temperature, plasma lactate was generally 17 % higher, but plasma chloride 1.5 % lower, in triploids. However, other ploidy effects, such as reduced haematocrit and haemoglobin (by up to circa. 30 %) in triploids, were time and temperature dependent. In conclusion, i) 10.5 °C was the optimal temperature for growth, ii) stable high temperatures induce puberty in post-smolts, and iii) triploids showed similar growth to diploids, lower levels of sexual maturation, but generally had a higher prevalence of cataracts. • In both ploidies, growth was optimal at 10.5 °C. • In diploids, increasing temperature led to more sexual maturation. • In triploids, no females matured, but male maturation increased with temperature. • Triploids had higher cataract scores than diploids with increasing temperatures. • Ploidy effects on physiology were generally temperature dependent.