Fish feed reformulation over the past 10 years has been motivated by the limited supply and high price of fish oil, the traditional source of health promoting long-chain omega-3 fatty acids, eicosapentaenoic (EPA) and docosahexaenoic (DHA), in farmed salmon.
Tocher et al found that although production of the global fed aquaculture industry increased from 15 to 35 million tons over the period 2000–2012, the level of fish oil used within the same period remained static at around 800,000 metric tons per year. "Aquaculture currently uses around 75% of the global fish oil supply with 21% going towards direct human consumption. The increasing demand from aquaculture and the nutraceutical and pharmaceutical industries, coupled with natural climatic events such as El Niño affecting both fish oil supply and prices, resulted in the salmonid industry, as the biggest user of fish oil (~60%), necessarily reducing the overall levels of marine ingredients in feeds," they noted.
Last month a BBC News article generated somewhat of a stir in the UK after it highlighted the declining levels of those omega-3 oils in farmed salmon, referencing research led by Professor Douglas Tocher at the University of Stirling.
The studycompared the fatty acid composition of over 3,000 Scottish Atlantic salmon farmed between 2006 and 2015. Tocher told BBC News: "About five years ago, a portion of Atlantic salmon of 130g was able to deliver three-and-a-half grams of beneficial omega-3. This is actually our weekly recommended intake. Now, the level of omega-3 has halved.
“Therefore, instead of eating one portion, we would need to eat two portions of farmed salmon per week.”
Alex Obach, managing director of Skretting Aquaculture Research Centre (ARC), Nutreco’s centralised aquaculture R&D department, told us reaction to the study should be measured:
“We should look at the key figures, not the overall trend. We welcome the findings of the University of Stirling study, particularly its emphasis on expressing omega-3 fats in farmed salmon in terms of quantity of grams of EPA and DHA per 100g of fish rather than the percentage of the fat.
“Yes, there has been a clear reduction in EPA and DHA levels in farmed salmon since 2006, in some cases one third or a half less, but EPA/DHA levels in farmed salmon are still higher than any other dietary source of omega-3, as per the levels mandated under EU regulation."
That EU directive states a food can claim to be a source of omega-3 fatty acids where the product contains at least 0,3 g alpha-linolenic acid (ALA) per 100g and per 100 kcal, or at least 40mg of the sum of EPA and DHA acid per 100g and per 100 kcal.
A claim that a food is high in omega-3 fatty acids may only be made where the product contains at least 0,6 g ALA per 100g and per 100 kcal, or at least 80mg of the sum of EPA and DHA per 100 g and per 100 kcal.
Better than wild?
“We would never [countenance] producing a [salmon] diet that is deficient in EPA and DHA.
“And the University of Stirling research also dismisses the idea that a lot of consumers have that wild salmon is healthier than its farmed counterpart,” continued Obach.
Though the Tocher el al study indicated the increasing use of economically beneficial fish oil substitutes such as rapeseed oil in aqua feeds has resulted in increased levels of ‘terrestrial’ fatty acids like oleic, linoleic and, to a lesser extent, ALA, the authors stressed DHA and EPA levels are still higher in farmed salmon when comparisons are drawn with its wild equivalent.
“When expressed in absolute terms, i.e. grams of EPA and DHA per 100g wet weight (ww) serving, which takes into account the lipid content of the flesh we find that, despite the reduction in EPA and DHA levels due to the increased use of plant ingredients in salmon feeds, farmed Atlantic salmon still provide a significantly higher amount of EPA and DHA than wild salmon, yielding 2.75, 2.21 and 1.36g per 100g ww flesh for salmon farmed in 2006, 2010 and 2015 respectively, compared to 0.76g per 100g ww flesh for wild salmon.
“Furthermore, despite the decrease in the levels, farmed Scottish salmon still delivers more EPA and DHA than most other fish species as well as all farmed terrestrial livestock.”
In terms of the average EPA and DHA in Norwegian farmed salmon filets, data from the Norwegian National Institute of Nutrition and Seafood Research (NIFES) shows:
2014: 1275 mg/100g
2015: 1145 mg/100g
On average, then, salmon filets in that market currently contain 1100 to 1300mg of those essential fatty acids per 100 grams.
But Tocher et al found, when looking at the International Society for the Study of Fatty Acids and Lipids (ISSFAL) recommendations, a minimum daily intake of 500mg to reduce the risk of cardiovascular disease, a single 130g portion of Scottish salmon farmed in 2006 would have been adequate to meet the 3.5g EPA/DHA weekly intake level set by ISSFAL, whereas, in 2015, they said this would have required two portions.
New EPA and DHA sources
Obach said, five years ago, he would have been alarmed at declining levels of DHA and EPA in farmed salmon because, at that stage, there were no alternative sources of fish oil derived DHA and EPA available to feed formulators.
“But today, we know how to ferment microalgae strains with high levels of DHA and EPA, and genetically engineered plant oils are also a promising omega-3 fatty acid source,” said the Skretting ARC MD.
So viable replacements to fish oil are within reach, and should be at commercial scale in three to four years, said Obach. “EPA and DHA will not be limiting nutrients in the future [regardless of the projected growth in aquaculture up to 2050],” he added.
Fatty acid model
Skretting has been refining what it refers to as a fatty acid model, which is all about determining at what stage of the salmon life cycle is it best to add in DHA and EPA, in terms of nutritional and performance value.
“We want to confirm whether is it better to add them when fish are growing fast and at what stage is retention of fat higher in the salmon. We are also exploring strategies to maximise the fish’s ability to elongate and desaturate plant-derived ALA to EPA and DHA and to see when adding much higher levels is better for DHA and EPA accumulation in the flesh,” explained Obach.
Rune Waagbø, director of research at NIFES, who has also been looking at the impact of declining fish oil levels in fish feed, told us: "NIFES has continuous research on this topic, both from a seafood composition point of view, but also related to fish welfare. Historical data show a decline in fillet EPA and DHA, but, at a certain feed level, other mechanisms influence the filet composition, [such as] own [endogenous] synthesis of EPA and DHA from the plant omega-3 fatty acid - linolenic acid 18:3 n-3 - in the salmon.”
Title: Impact of sustainable feeds on omega-3 long-chain fatty acid levels in farmed Atlantic salmon, 2006–2015
Authors: M. Sprague, J.R. Dick & D.R. Tocher