Why is fish oil only harvested from the liver?

The whole fish contains fatty acids including the skin and when fish oils are produced they will often use the whole fish in the process. The exception is cod liver oil which will provide a good source of vitamin A and vitamin D (even then they will often use the whole fish). The process of making most fish oils available on the market will use the whole fish and not just a small part of it. Through a process of steam heating, centrifuge and separation, the oil is refined out leaving a mass of protein and dry material that is used as fish meal for animal feed.