Researchers at the University of Plymouth have developed a technique that will help fish-food producers find the right formula to produce healthy looking salmon and trout.

Paul Russell, senior scientific officer in the department of biological sciences at the University of Plymouth, said the fish-food industry needed an accurate means of identifying the right balance of natural pigments to be added during production.

Dr Russell said: "The aim is to ensure that the fish looks good. Pigmentation of salmon and trout also has many important biological functions such as protecting the eggs when laid in polluted water, and deposits in the skin help to protect the fish from sunburn."

With researcher Stephen Lagocki, Dr Russell has developed a way to measure the chromaticity of fish flesh using a Leica Quantimet 570 Image Analyser.

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The analyser measures the distribution of colour and the chromaticity of the fish flesh after they receive diets containing different pigments.

"The colour of the fish fillets and steaks can be changed by varying the amount of pigments fed to the fish, but it has always been a difficult thing to get exactly right," Dr Russell said.

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"Colours are compared with a specialist colour chart, but what normally happens is that there is no exact match. We have gathered enough information to advise producers of the correct dietary components needed to yield a fish of an exact colour," he added.