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NIFES Uncovers Effects of Plant Diets for Salmon

NORWAY - For the first time, a study has examined how feeding Atlantic salmon high levels of both plant proteins and vegetable oils would affect the fish.

This experiment resulted in two kilos of salmon protein generated per kilo of fish meal protein in the feed. The study showed a slight reduction in salmon growth, even though all known nutrient requirements of the fish were covered by the feed.

In a study carried out by the National Institute of Nutrition and Seafood Research (NIFES), salmon were given a feed where a high proportion of fish meal and fish oil were replaced by plant proteins and vegetable oils. This is the first time that a feeding study of this kind has been carried out on Atlantic salmon.

"A fish diet comprising 70% plant oils and 80% vegetable proteins means that two kilos of salmon protein are produced for every kilo of fish meal protein in the feed. In other words, a good net production of high quality fish protein is obtained from plant proteins", says Dr. Bente Torstensen, senior scientist at NIFES.

Previous studies at NIFES have shown that salmon can be given a feed containing 100% vegetable oils without any reduction of fish growth when the source of protein in the feed is fish meal. NIFES has previously shown that 90% of the fish meal in the feed can be replaced by plant proteins without affecting the growth of the salmon as long as the source of fat in the fish diet is fish oil.

Reduced weight

The experiment used salmon in seawater weighing 0.35 kg and gave them feed containing either 100% fish meal and 100% fish oil (control group), 35% vegetable oils and 80% plant proteins, 70% vegetable oils and 40% plant proteins or 70% vegetable oils and 80% plant proteins. The fish were fed for 12 months until they were 3.9 kg. The salmon which had received feed containing the highest mixture of plant raw material - 70% vegetable oils and 80% plant proteins - experienced a 12% lower growth during the first three months compared to the control group. There was no corresponding weight reduction in the other groups during the first three months.

- Over the entire period of the study, the fish fed the highest proportion of plant raw material experienced a weight reduction of 17% compared to the control group. The weight reduction was largely due to a lower feed intake during the first three months of the study, says Torstensen.

Further investigations of protein and lipid interactions

The other dietary group which received a high plant protein level - 35% vegetable oils and 80% plant proteins - also experienced a reduced growth in the last study period, but not in the fist period of the study.

"This could indicate that interaction effects between plant proteins and vegetable oils in the feed may be the cause of the reduced feed intake in the group with the highest level of plant raw material. The mixture of raw materials may, for example, have made the feed less palatable" , says Torstensen, adding that a number of analyses are underway in order to investigate the reasons for the reduced growth.

"We are now studying whether the combination of vegetable oils and plant proteins may affect the salmon’s metabolism.

"The feeds used in the study contained a mixture of raw materials with the addition of amino acids to ensure that the salmon’s need for key fatty acids and essential amino acids was covered."

The study is part of the EU Aquamax project (Sustainable AquaFeeds to Maximise the Health Benefits of Farmed Fish for Consumers), which is coordinated by NIFES.