Sometimes salmon is genetically modified, though it varies from company to company. Currently genetically modified foods are still quite controversial because the long-term effects of it on the human body has not been adequately studied. Salmon is typically genetically modified so that the fish grow in both the summer and the winter to market size. There are both benefits and potential drawbacks to genetically modifying things like crops, animals, and even fish.
While the effects of GM foods on the human body have not been studied enough for any solid conclusion, they can be very effective at helping crops to grow strong and making them resistant to certain bugs that can hinder the growing process. As far as salmon is concerned, genetically modifying them can help them to grow in different times throughout the year which was not a possibility years ago.