The researchers studied the differences in fat content and quality between light and dark muscle in farmed rainbow trout. Dark muscle had more fat, mainly from triacylglycerols, while light muscle had more phospholipids with higher polyunsaturated fatty acids. Frozen storage led to increased free fatty acids and decreased phosphatidylcholine content, causing lipid deterioration. Fish from colder water had more polyunsaturated fatty acids but lower lipid stability during storage. Carotenoid supplementation did not improve lipid stability.