Development of farmed fish: a nutritionally necessary alternative to meat.

The projected stagnation in the catch from global fisheries and the continuing expansion of aquaculture is considered against the background that fishmeal and fish oil are major feed stocks for farmed salmon and trout, and also for marine fish. The dietary requirement of these farmed fish for high-quality protein, rich in essential amino acids, can be met by sources other than fishmeal. However, the highly-polyunsaturated fatty acids eicosapentaenoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3) present in high concentrations in fish oil are essential dietary constituents for marine fish and highly-desirable dietary constituents for salmonids. Currently, there is no feasible alternative source to fish oil for these nutrients in fish feeds. Vegetable oils rich in linoleic acid (18:2n-6) can partially substitute for 20:5n-3 and 22:6n-3 in salmonid and marine-fish feeds. However, this is nutritionally undesirable for human nutrition because the health-promoting effects of fish-derived 20:5n-3 and 22:6n-3 reflect a very high intake of 18:2n-6 relative to linolenic acid (18:3n-3) in Western diets. If partial replacement of fish oils in fish feeds with vegetable oils becomes necessary in future, it is argued that 18:3n-3-rich oils, such as linseed oil, are the oils of choice because they are much more acceptable from a human nutritional perspective, especially given the innate ability of freshwater fish, including salmonids, to convert dietary 18:3n-3 to 20:5n-3 and 22:6n-3. In the meantime, a more judicious use of increasingly-expensive fish oil in aquaculture is recommended. High priorities in the future development of aquaculture are considered to be genetic improvement of farmed fish stocks with enhanced abilities to convert C18 to C20 and C22 n-3 polyunsaturated fatty acids, enhanced development of primary production of 20:5n-3 and 22:6n-3 by single-cell marine organisms, and continuing development of new species.