Many fish farmers in temperate climates raise Atlantic salmon (Salmo salar). Salmon are carnivores, fed pellets made from (among other things) wild fish. Wild fish are getting scarcer so a team of scientists, led by Erling-Olaf Koppang at the Norwegian Veterinary Institute, is feeding vegetable oils to farmed salmon. A few days ago they published this report of what they’ve found so far.
Why should we care? It’s become standard health advice that we humans should eat plenty of oily fish such as salmon. We’re advised to do this because these fish contain ‘long-chain omega-3 fatty acids’. Awkward name, eh? But salmon tastes good and it does us good.
The trouble is, salmon are carnivores. People catch small wild marine fish containing those health-giving fatty acids. The small fish are then processed into fishmeal and fish oil to make pellets for fish farming (aquaculture). So as we eat more and more farmed fish, we’re exploiting more and more wild fish. This isn’t sustainable.
As small wild fish become overfished, some people are developing the genetically modified (GM, genetically engineered) oilseed crop which I nicknamed Omega Camelina. In that post I linked to a film of Johnathan Napier, who leads the Omega Camelina team, explaining the new GM crop. Prof Napier says that without GM, ‘plant oils can’t substitute for fish oils.’ They don’t have long-chain omega-3 fatty acids. That pretty much destroys the claims about eating plant foods to get those essential nutrients.
But why not feed vegetable oils to farmed salmon, replacing some of the fishmeal and fish oil? Prof Koppang tried feeding oils from olive (Olea europaea), rapeseed (oilseed rape, canola, Brassica napa) and soybean (soya, Glycine max). The salmon grew but their intestines became abnormal. The folds that grow on the inside of fish intestines weren’t so deep in the (partly) veg-fed salmon as they were in the fish-fed salmon.
Folded intestines let salmon absorb nutrients from their feed. This particular study doesn’t tell us whether the (partly) veg-fed salmon grew so fast as other salmon. It doesn’t tell us whether their meat would be equally nutritious to human consumers. If I see answers to those questions, I’ll show them to you.
[Edit] Looking again at the comments on my Omega Camelina post, I see my fellow blogger Kay Hortographical explaining which kinds of vertebrate can convert the short-chain fatty acids found in plants, to the long-chain fatty acids we humans gain by eating salmon. Marine fish can’t do it, if I understand correctly.
But is the Atlantic salmon a marine fish? It spends most of its lifecyle in the sea, but swims up rivers to spawn. So I don’t know whether it can convert short-chain fatty acids into long-chain ones. If it can’t, I predict that when it’s fed vegetable oil its meat won’t be very rich in long-chain fatty acids. If I’m right, the (partly) veg-fed salmon might be cheaper to raise than fish-fed salmon, and more sustainable, but less nutritious for the consumer.