Put forage fish on your plate as well as into salmon, say researchers

Consumers should eat less farmed salmon and more of the fish that are often used in salmon feed if they want to extract the most nutrients from their seafood, a new study has concluded.

Researchers analysed the flow of nutrients from the edible species of wild fish used as feed - such as mackerel, anchovies, and herring - to the farmed salmon they were fed to. They found a decrease in six out of nine nutrients in the salmon fillet – calcium, iodine, iron, omega-3, vitamin B12 and vitamin A, but increased levels of selenium and zinc.

They said most wild ‘feed’ fish met dietary nutrient recommendations at smaller portion sizes than farmed Atlantic salmon, including omega-3 fatty acids which are known to reduce the risk of cardiovascular disease and stroke.

“Whilst still enjoying eating salmon and supporting sustainable growth in the sector, people should consider eating a greater and wider variety of wild fish species like sardines, mackerel and anchovies, to get more essential nutrients straight to their plate,” said the study’s lead author, Dr David Willer, of the Zoology Department at the University of Cambridge.

Small changes

In the UK, 71% of adults have insufficient vitamin D in winter, and teenage girls and women often have deficiencies of iodine, selenium, and iron. Yet while, 24% of adults ate salmon weekly, only 5.4% ate mackerel, 1% anchovies and just 0.4% herring, the researchers reported.

“Making a few small changes to our diet around the type of fish that we eat can go a long way to changing some of these deficiencies and increasing the health of both our population and planet,” said Willer.

However, the experience of trade body Salmon Scotland is that such changes are unlikely to happen. Faced with a choice of fish that are all good for you, consumers will opt for what they like.

“Public health advice is that we should all be eating oily fish, like salmon, more often,” said Dr Iain Berrill, head of technical at trade body Salmon Scotland. “There is nothing stopping people from eating other oily wild-caught fish like mackerel, anchovies, or herring, but as this study makes clear most people don’t choose to do so. By comparison, farm-raised salmon is the UK’s most popular fish and demand at home and abroad continues to grow.”

The wild fish studied included Pacific and Peruvian anchoveta, and Atlantic herring, mackerel, sprat, and blue whiting – which are all marketed and consumed as seafood.

They found that these six feed species contained a greater, or similar, concentration of nutrients as the farmed salmon fillets. Quantities of calcium were over five times higher in wild feed fish fillets than salmon fillets, iodine was four times higher, and iron, omega-3, vitamin B12, and vitamin A were over 1.5 times higher.

Wild feed species and salmon had comparable quantities of vitamin D.

Zinc and selenium were found to be higher in salmon than the wild feed species. The researchers say these extra quantities are due to other salmon feed ingredients and are a real mark of progress in the salmon sector.

Retaining nutrients

“Farmed salmon is an excellent source of nutrition, and is one of the best converters of feed of any farmed animal, but for the industry to grow it [the salmon] needs to become better at retaining key nutrients that it is fed. This can be done through more strategic use of feed ingredients, including from fishery by-products and sustainably sourced, industrial-grade fish such as sand eels”, said Dr Richard Newton of Stirling University’s Institute of Aquaculture (IoA), whose team also included Professor Dave Little, Dr Wesley Malcorps and Björn Kok.

“It was interesting to see that we’re effectively wasting around 80% of the calcium and iodine from the feed fish – especially when we consider that women and teenage girls are often not getting enough of these nutrients”.

Nutrient metric

Willer said such numbers have been underacknowledged by the aquaculture industry’s standard model of quoting Fish In: Fish Out (FIFO) ratios rather than looking at nutrients.

The researchers would like to see a nutrient retention metric adopted by the fishing and aquaculture industries. They believe that if it was combined with the current FIFO ratio, the industry could become more efficient, and reduce what they say is a burden on fish stocks that also provide seafood.

Efficient converter

The allegation that the salmon industry is using too many food-forage fish is contested.

“Our sector uses only a tiny fraction of the fish meal and oil that’s produced, and we only use marine ingredients derived from wild fish that are responsibly sourced from managed fisheries,” said Berrill.

“Scottish salmon is an efficient converter of feed with a feed conversion rate of around 1:1.1, meaning it takes just over 1kg of feed to produce 1kg of food.

“Alternative marine ingredients, including trimmings from other species, reduce our use of fish meal and oil from wild sources to produce millions of healthy nutritional meals every year.”

Read the full study, Wild fish consumption can balance nutrient retention in farmed fish, in the journal Nature Food. 

The research was funded by the Scottish Government’s Rural and Environmental Science and Analytical Services Division (RESAS), a Royal Society University Research Fellowship, a Leverhulme Trust Early Career Fellowship, a Henslow Fellowship at Murray Edwards College, and the University of Cambridge.