Salmon or trout from the wild have pink or red flesh because of their diet,
which includes crustaceans or insects. However, fish raised in ponds or sea
pens often have pale pink flesh due to the feeds that they take in. Thus,
fish farmers add dyes derived from petroleum to the fish feed to improve the
fish flesh coloration. Scientists from Royal Holloway University of London
and partners developed an environment-friendly alternative to achieve the
pinkish in fish flesh. Instead of making dyes from petrochemicals, they
genetically engineered tomatoes to produce high levels of ketocarotenoids or
high value pigments used in food and feed.

The researchers used Moneymaker tomato variety that had been genetically
edited to add bacterial DNA associated with producing ketocarotenoids. Then
they crossbred the modified tomatoes with another variety that expresses
elevated levels of beta-carotene. This led to a new variety of tomato with
high levels of ketocarotenoids. The fruits were freeze-dried and powdered
then mixed into fish feed. The researchers found that the fish absorbed
roughly twice as many ketocarotenoids from tomato as they did from the same
amount of petrochemical-derived dye.

[www.pnas.org]