High protein diets improve the growth of juvenile barramundi (Lates calcarifer) at elevated temperatures

Elevated temperatures and low aquatic oxygen levels are increasingly accompanying climate warming and can adversely affect the production and welfare of farmed fish. Both environmental factors and diet affect fish energetics. Still, little is known about the dietary effects on fish energetics at elevated temperatures and low aquatic oxygen levels and how that influences growth. Thus, a feeding trial was conducted to investigate the effect of two isoenergetic diets, high protein (HP, 57% crude protein and 9% total lipid) and high lipid (HL, 40% crude protein and 19% total lipid), on juvenile barramundi (Lates calcarifer, initial body mass: ∼ 3 g). Fish were acclimated for at least four weeks on the diets (three weeks at 35 °C) before measurements began (i.e.: measurements were taken between weeks four to six of the trial). Additionally, feed intake and maximum metabolic rate were tested under three levels of aquatic oxygen levels (normoxia >80, 60% and 30% air saturation). We found no significant dietary effect on feed intake, routine, maximum and postprandial peak metabolic rate. Gill ventilation rate was also unaffected by diet treatment. However, fish fed the HP diet had increased haematocrit and haemoglobin levels, improved growth rates, feed conversion ratio, and protein deposition relative to those fed the HL diet. Thus, this study demonstrated that a high protein diet enhanced key physiological traits such as oxygen-carrying capacity, potentially supporting improved juvenile barramundi growth at elevated temperatures.