Taurine is not an essential amino acid for grow-out Atlantic salmon

Norway: Fish meal used in fish feed contains high levels of taurine, which is not the case for plant proteins. What are the consequences when the levels of plant protein in the diet are high?

New research at NIFES (The National Institute of Nutrition and Seafood Research) shows that the farmed salmon`s requirement for taurine is met when the feed contains sufficient amount of the precursor amino acids.

Fish meal contains high levels of the sulphuric amino acid taurine. Today’s grow-out feed for Atlantic salmon contains up to 50 % plant protein, due to limited access of fish meal traditionally used in fish feed. Taurine is absent in plant proteins, which raises the question; is farmed Atlantic salmon supplied with sufficient amounts of taurine when salmon is fed on a diet with a high level of plant protein?

Research at NIFES shows that salmon synthesizes taurine from such diets when levels of the amino acids methionine and cystein are sufficient. Why taurine? Taurine play an important role in human vision and brain development as well as in absorption of lipids from the gastrointestinal tract. Taurine is synthesized from amino acids methionine and cysteine which involves an enzyme. Infants for instance have a very low level of this enzyme and are therefore naturally supplied with taurin through breast milk.

An amino acid is called “essential” when it must be provided through the food. It has been assumed that taurine also is an essential amino acid for salmon fry in fresh water and for grow-out salmon in seawater. Taurine is a beta- amino acid of which unlike the alpha amino acids is not incorporated into proteins. Sufficient amount of taurine In a feed trial, grow-out salmon weighing 0.5 kilograms were fed feed which contained a high level of plant protein at five inclusion levels of the amino acid methionine.

Besides different levels of methionine, nutrient composition was equal in all diets. Based on the metabolites from methionine degradation in the liver, the salmon`s requirement for this amino acid was determined. The liver is important for the proteinsyntheis and degradation. - When the requirement for methionine was met, methionine in excess was converted into taurine, says Marit Espe at the Aquaculture Nutrition Research Programme at NIFES.

- The higher the levels of methionine in the diet, the higher the levels of taurine was observed in the salmon`s liver. This suggests that salmon fed diets with low taurine as plant protein based diets has the ability to produce sufficient amounts of taurine assuming that it is supplied with enough methionine. If the feed also contains cysteine, the need for methionine is lower than previously thought, "she says. Recommended levels of methionine for Atlantic salmon range from 2.2 to 2.3 grams of methionine/100gram protein in the grow-out feed.