Effect of water oxygen level on performance of diploid and triploid Atlantic salmon post-smolts reared at high temperature

Sterilization by triploid induction prevents interbreeding between escaped farmed salmon and wild stocks, but reduced performance of triploids at high seawater temperatures has been reported. As high temperature may be followed by limited oxygen (O2) supply in net cages, this study compared the effect of reducing O2 from 100% to 70% of air saturation (termed hypoxia) on parameters of production performance (feed intake, growth, feed conversion ratio, mortality), and physiological status (plasma K+, Cl−, Na+, osmolality, glucose, creatinine (Cr), bilirubin, triacylglycerol (TAG) and alkaline phosphatase (ALP) concentrations) in triploid versus diploid Atlantic salmon kept at high seawater temperature (19 °C). Two triplicate groups of diploid and two triplicate groups of triploid Atlantic salmon post-smolts were acclimated to 10 °C and 100% O2 before experiment start up. During the experiment, temperature was maintained at 10 °C for 10 days, increased to 19 °C over 9 days and kept stable at 19 °C until the experiment ended (day 51). From day 22 to 51, the O2 level was reduced from 100% O2 to 70% O2 in one diploid and one triploid group. The abbreviated group names are 2N100, 2N70, 3N100 and 3N70. Triploidy led to reductions of feed intake and growth, and this effect was amplified by reducing O2 from 100% to 70% O2. Analyses from blood samples drawn on day 51 show that plasma levels of Cl−, TAG, ALP and bilirubin were lowered in triploids in general, and that plasma Cr levels trebled and plasma K+ levels dropped in triploids subjected to 70% O2 for 29 days. Mortality was also significantly higher in the 3N70 group. According to these effects, the following order of production performance is suggested at high seawater temperature (best to worst): 2N100 > 2N70 ≥ 3N100 > 3N70. An interesting difference in the behavior between diploid and triploid fish was observed during the experiment: triploids generally moved against the tank water current, ram ventilating, as opposed to diploids, which displayed normal gill ventilation and were in part moving along with the current. The inability of triploid Atlantic salmon to withstand high temperature in combination with moderate hypoxia could set limitations to the geographical distribution of triploid salmon farming.