© Nofima
Rainbow trout share many characteristics with salmon and are often compared with this dominant farmed species in Norway. While farmed rainbow trout accounted for 6 percent of salmonid sales in 2024, salmon accounted for 94 percent, according to figures from the Norwegian Directorate of Fisheries.
“Much of the knowledge we have on salmonids comes from research on salmon. But the species are not the same, so it is important that we obtain specific knowledge about rainbow trout,” said Marta Bou, a scientist at Nofima, in a press release.
The omega-3 fatty acids EPA and DHA are important nutrients for salmonids. At the same time, marine fatty acids are a limited resource that must be used efficiently. Studies in salmon have shown that higher levels of EPA and DHA can improve fish health, robustness and quality. Nevertheless, little is known about how high levels of omega-3 affect rainbow trout in commercial sea-cage farming. For this reason, Nofima has participated in research on feed effects in Cargill’s research licence, which is operated by Hofseth Aqua. In this work, the scientists have examined in depth the mechanisms by which omega-3 fatty acids act in cells.
Feeding trials in sea-cages
In a full-scale sea-cage trial, the effects of a standard level and an elevated level of omega-3 were tested. The fish were stocked in cages at three different sites at the west coast of Norway, at different times of the year. The scientists assessed growth, welfare, survival and fillet quality. In parallel, they carried out an indoor comparative study of rainbow trout and Atlantic salmon at Nofima’s research station at Sunndalsøra, where they investigated whether there were differences between salmon and rainbow trout in uptake and metabolism of omega-3 fatty acids.
“We documented clear differences between salmon and rainbow trout. Rainbow trout store more omega-3 in the fillet than salmon given the same feed. At the same time, extra omega-3 did not result in better health, survival or other production benefits in rainbow trout under these conditions, as it does in salmon,” said Bou.
Enough omega-3 for consumers
The study showed that trout given standard feed had around 20 mg EPA+DHA per gram of fillet, while trout given the elevated level had around 26 mg per gram of fillet. Salmon, given the elevated level, ended up with just over 16 mg per gram of fillet. According to the European Food Safety Authority (EFSA), an average adult needs 250 mg EPA+DHA per day in their diet.
“If I eat 125 grams of rainbow trout fillet, I cover my entire daily requirement when the fish has been given standard feed. Even though I get more than my daily requirement with the high omega-3 feed, that does not necessarily mean that this level represents good overall resource use,” said Bou.
Cargill underlines the importance for rainbow trout research: “There are many similarities between salmon and trout, and over the years a great deal of knowledge has been transferred between the species. The results from this research project show how important it is to invest in research on rainbow trout,” commented Terje Utne, who is responsible for field trials at Cargill.
