This is a very interesting question. The livers of certain other types of fish yield oil which is even richer in fat-soluble vitamins than cod liver oil. For example, halibut liver oil is a richer source of vitamin A and vitamin D compared to cod liver oil. Although these other types of fish livers oils are richer in fat-soluble vitamin content compared to cod liver oil, they do not have the documented healing capabilities as the fish liver oil derived from the Gadus Morhua species of cod. This suggests the possibility that there is something more than just these fat-soluble vitamins which contributes to the health benefits of Gadus Morhua liver oil.