Future fish feed: increasing immunity on salmon

Scientists at Nofima have developed a salmon feed that strengthens the immune defence among salmon so the survival rate after a natural outbreak of the IPN virus increases dramatically.

Professor Kjell-Arne Rorvik at Nofima Marine (formerly Akvaforsk) believes this is an example of the fish feed of the future.

"The best way of significantly improving fish feed involves finding and utilising components that collaborate with the fish's biology so that the energy in the feed is better utilised."

Provides more energy

When salmon weigh around 100 grams and are ready to be transferred to sea cages, they are referred to as smolt. It is clear that the biology is chanllenged when the salmon smolt are transferred from fresh water to sea water.

This major readjustment means the smolt have a poor appetite and the intake of energy-rich feed is often insufficient to maintain a good immune defence. The smolt enters a period of several weeks' duration with reduced weight and increased susceptibility to sickness.

The infectious pancreatic necrosis (IPN) virus is one of the aquaculture industry's major health challenges. The disease particularly affects smolt in the first weeks after transfer to salt water.

One method to enable smolt to have sufficient energy to fight off the disease is to add bioactive fatty acids to the feed. Tetradecylthioacetic acid (TTA) increases the salmon's ability to oxidize fat to energy.

Tested on humans then salmon

TTA has been tested on humans and is known as a remedy that reduces cholesterol, improves immune defence and increases the capacity for fat oxidation.

Scientists Kjell-Arne Rorvik and Magny Thomassen conducted TTA trials on smolt. The smolt were divided into three groups with each group receiving a different feed, one of which contained small amounts of TTA.

While in the sea cages, the salmon were exposed to a natural outbreak of IPN. As a result of the outbreak, the scientists recorded clear differences between the groups of fish that had consumed different feeds (see graph).

The results showed that the fish that consumed the feed containing TTA had a mortality rate of 2.3 % compared to 7.8 % for the fish that consumed normal feed.

"To a fish farmer, a reduction like this equates to good fish welfare," says Rorvik. "It will also be a major advancement if the industry can implement this simple step to reduce the mortality rate in the early sea phase."
This is a user-controlled innovation project part funded by the Research Council of Norway in collaboration with BioMar and Thia Medica. Before TTA may be utilised commercially, it must gain EU approval as an additive.

By using small amounts of TTA in the trial feed, the mortality rate after an outbreak of the IPN virus was reduced to one-third.

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Future fish feed: increasing immunity on salmon