Omega-3 long-chain polyunsaturated fatty acids and aquaculture in perspective

In the 40 years since the essentiality of polyunsaturated fatty acids (PUFA) in fish was first established by determining quantitative requirements for 18:3n − 3 and 18:2n − 6 in rainbow trout, essential fatty acid (EFA) research has gone through distinct phases. For 20 years the focus was primarily on determining qualitative and quantitative EFA requirements of fish species. Nutritional and biochemical studies showed major differences between fish species based on whether C18 PUFA or long-chain (LC)-PUFA were required to satisfy requirements. In contrast, in the last 20 years, research emphasis shifted to determining “optimal” levels of EFA to support growth of fish fed diets with increased lipid content and where growth expectations were much higher. This required greater knowledge of the roles and functions of EFA in metabolism and physiology, and how these impacted on fish health and disease. Requirement studies were more focused on early life stages, in particular larval marine fish, defining not only levels, but also balances between different EFAs. Finally, a major driver in the last 10-15 years has been the unavoidable replacement of fish oil and fishmeal in feeds and the impacts that this can have on n − 3 LC-PUFA contents of diets and farmed fish, and the human consumer. Thus, dietary n − 3 in fish feeds can be defined by three levels. Firstly, the minimum level required to satisfy EFA requirements and thus prevent nutritional pathologies. This level is relatively small and easy to supply even with today's current high demand for fish oil. The second level is that required to sustain maximum growth and optimum health in fish being fed modern high-energy diets. The balance between different PUFA and LC-PUFA is important and defining them is more challenging, and so ideal levels and balances are still not well understood, particularly in relation to fish health. The third level is currently driving much research; how can we supply sufficient n − 3 LC-PUFA to maintain these nutrients in farmed fish at similar or higher levels than in wild fish? This level far exceeds the biological requirements of the fish itself and to satisfy it we require entirely new sources of n − 3 LC-PUFA. We cannot rely on the finite and limited marine resources that we can sustainably harvest or efficiently recycle. We need to produce n − 3 LC-PUFA de novo and all possible options should be considered.