Marine heat waves are becoming increasingly common as an outcome of climate change . Increased seawater temperatures beyond 18°C directly affects marine and aquaculture fish with detrimental effects on metabolism, physiology and homeostasis. In southern Australia, salmon farming operations observe a reduction in feed intake, along with compromised osmoregulation, flesh decolouration, condition factor and hepatosomatic index. Dietary macronutrient composition impacts energy and oxygen requirements during digestion, leading to the hypothesis that a highly digestible diet can be beneficial during periods of reduced feed intake at high temperature.
This presentation summarizes recent efforts in Australia to understand the biological impacts , develop high-throughput methods to measure thermal stress and details several dietary manipulation experiments that seek to address the impacts of thermal stress in salmon . A combined discovery and targeted proteomics and metabolomics approach was used to generate a panel of biological thermal stress indicators. These were applied to investigate the impacts of dietary carotenoid source or macronutrient (protein or lipid) energy source replicated thermal stress challenges.
The effects of diets during chronic thermal stress on fish performance (survival, specific feed rate (SFR), weight gain (WG), specific growth rate (SGR), feed conversion ratio (FCR)), flesh pigmentation, whole fish nutrient retention (protein, lipid and energy) and energetics (metabolic rate). The impact of diet at the tank level was assessed on post-digestion oxygen consumption rate (routine metabolic rate ; RMR) and acute critical thermal tolerance (CTmax). Performance of individual f ish was tracked using intraperitoneal tags and is highly recommended to overcome and understand high levels of individual variation.
Diets containing carotenoids from natural sources (bacteria and algae) significantly increased feed intake during peak thermal stress and significantly reduced liver heat shock and antioxidant biomarkers, but with no subsequent benefit on flesh pigmentation. Fish fed a higher proportion of energy in the form of protein resulted in significantly reduced performance (WG, SGR, FCR ) with associated reduction in visceral fat . Fish fed high protein diets at high temperature had significantly reduced protein and energy retention, coupled with significantly reduced RMR but without impact on Ctmax. Lastly, diet significantly reduced liver proteomi cs bio markers linked with the heat shock response and antioxidant pathways.
Along with tools to quantify stress levels in Atlantic salmon and measure the potential beneficial effects of diets deployed during peak thermal stress periods, this study supports several nutritional intervention strategies (fortification, support and recovery) that can be effective in mitigating the impacts of thermal stress and supporting physiological homeostasis and fish welfare.