Marta Bou Mira, a scientist at aquaculture research institute Nofima and the Norwegian University of Life Sciences (Norges miljø- og biovitenskapelige universitet, NMBU), studied what minimum levels of marine omega-3 fatty acids are required in the diet to ensure that farmed salmon are healthy and growing well, under different environmental challenges and through different stages of life. The fatty acids she studied specifically are EPA and DHA, and fish oil is the main source of these fatty acids.
A persistent shortage of fish oil in the market means it is not desirable to use more marine omega-3 fatty acids than necessary in aquaculture feed.
In the study salmon were fed 14 experimental diets containing five levels of EPA and DHA ranging from 0 to 2 per cent, from start feeding until a harvest of four kilograms. Bou Miras’ work shows that it is economically profitable to include up to one per cent of marine omega-3 fatty acids in the diet, as this gives the highest self-production of marine omega-3 in salmon. The research team has previously shown that salmon have an ability to convert some plant omega-3 to marine omega-3 as compensation when the level in the feed is low.
The experiments showed that the level of EPA and DHA previously deemed adequate in feed is too low for salmon to maintain good health in the demanding environment provided by sea cages.
The lowest levels of omega-3 fatty acids in the feed led to structural changes in the bowel and spine and mortality for lice treatment at high seawater temperatures. The content of marine omega-3 in commercial feed is, however, well above the levels which gave negative effects in experiments.
The huge increase in salmon farming overt the last decade has led to increased demand for raw materials. Since the availability of fish meal and fish oil is limited, around 70 per cent of salmon feed is now made up of plant proteins and plant. This has led to a reduction in the level of healthy omega-3 fatty acids EPA and DHA in salmon tissue and organs. The proportion of plant ingredients in fish feed is expected to increase further in the future, which requires new knowledge about the salmon's need for the essential omega-3 fatty acids.
Marta Bou Mira holds a Masters degree in fish nutrition from the University of Barcelona and started her doctorate at NMBU and Nofima in 2013. She will continue her studies of salmon fat metabolism at Nofima in a post doctorate position funded by the Norwegian Research Council.