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From Jeremy Brown, Institute for Agriculture and Trade Policy

Your editorial of 17 January is on solid ground castigating the salmon farming industry, but like your letter-writer Rowan McCartney (31 January, p 38), you fail to differentiate your omega-3 polyunsaturated fatty acids adequately.

Wild salmon and wild fish in general remain the best bet, not only for lower levels of contaminants but also for nutritional benefits. While other omega-3s with fewer carbon molecules, principally alpha-linolenic acid (ALA), are abundant in linseed and flaxseed oils, as McCartney points out, only a marine-based diet will yield significant amounts of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA).

It is on DHA and EPA that research into health benefits has focused. ALA appears to function as a substitute in the absence of DHA and EPA, and may be converted into them at very low levels. The matter of synthetic isomer DHA is still speculative, and the same health benefits have not yet been demonstrated.

Wild fish and algae remain the best sources of DHA and EPA. Farmed fish can indeed be given feed derived from wild fish to duplicate this, but as the industry increases the amount of grain derivatives in feed in an effort to reduce costs, the levels of DHA and EPA decrease, with a corresponding rise in less beneficial omega-6 fatty acids.

Given the unique nutritional benefits to be derived from wild fish in our diets, there is all the more reason to manage our oceans sustainably, rather than converting them into industrial feedlots.

Bellingham, Washington, US