N-3 long-chain polyunsaturated fatty acids in fish physiology: From aquaculture to economic, ecological and public health challenges.

N-3 long-chain polyunsaturated fatty acids (n-3 LC PUFA), particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), are essential molecules to fish physiology, influencing their development, metabolism, immunity, behavior, and reproduction. However, fish have a limited ability to synthesize these fatty acids endogenously, and must obtain them through diet. The dietary availability of these molecules is increasingly challenged by both ecological and aquaculture constraints linked to climate change and to sustainability of ressources (e.g. fisheries). Currently, global changes - including ocean warming - may reduce the availability of these fatty acids in marine food webs, raising concerns for fish population dynamics and aquaculture sustainability that still largely depends on forage fish. In this review, we first summarize the metabolic pathways and tissue distribution of n-3 LC PUFA in freshwater and marine fish, highlighting differences in bioconversion capacities. We then explore the physiological and behavioral consequences of varying dietary n-3 LC PUFA levels in aquaculture feeds and natural environments, including effects on growth, locomotion, cognition, metabolic performance, oxidative status, immune response, and reproductive investment. We also review current alternatives to fish meal and fish oil, such as plant, insect, microbial, and genetically modified sources, and discuss their potential to meet fish nutritional needs. Altogether, this synthesis underscores the current challenge of n-3 LC PUFA dietary shortage for fish health, aquaculture production and nutritional security of human population, and identifies knowledge gaps that must be addressed to ensure both ecological resilience and sustainable aquaculture development in a rapidly changing world.