Modern Asian aquaculture relies on intensification of pond and cage farming to support increased production volumes and improve production efficiencies. However, increasing stocking densities has direct effects on animal stress levels, suppresses immune response, and has indirect effects on fish due to excess nutrient loading and reduced water quality conditions. Outdoor farming has to face changing environmental conditions such as fluctuating temperatures and salinity, with day-length and light intensity affecting natural productivity.
With disease-causing pathogens ubiquitous in the environment, animals already under stress exhibit weakened barrier defences and compromised immune system functionality and responsiveness, such that problems with disease and low survival rates are common and almost inevitable. All of these stress factors combined with exposure to pathogens often result in conditional changes of vitamin and amino acid requirements.
Water-soluble vitamins are co-factors for enzymes, are essential for amino acid metabolism, and are needed for almost all other metabolic processes. B-complex group vitamins are required for responsiveness to numerous stress factors, and are indispensible for cellular energy synthesis and storage. Vitamin C has great importance supporting cellular antioxidant defence against pro-oxidants and oxidative stress, and detoxification of xenobiotics and toxins in the liver. Fat-soluble vitamins have many inter- and intra-cellular functions such as metabolic regulation, antioxidant defence of membranes and similarly offer protection from pro-oxidants and oxidative stress.
Amino acids, being essential building blocks for synthesis of all proteinaceous tissues as well as metabolically active proteins, hormones, and antioxidant enzymes, are also indispensible for cell proliferation needed to support cellular immune response and wound repair. Free amino acids have a role in osmoregulation and cellular and tissue homeostasis. When normal energy supplies are depleted, a number of amino acids may be catabolized to provide a source of localized energy for immune system activity as a last resort.
In fish, a strong immune response usually involves strengthening of barrier defences (mucus, enzymes and antimicrobial substances) as well as the mobilization of phagocytes and macrophages that, using reactive oxygen species (ROS) both as a killing agent and a signaling molecule, increase internal oxidative stress requiring antioxidant defence.
Consideration of the nutrient composition of aquafeeds with the knowledge that requirements are dynamic rather than static in relation to stress, immune defences and physiological responses to environment will improve the capability of our farmed animals to adjust, adapt and survive all threats.