There is a very interesting question. Why fish and not the best quality Irish Angus beef contains high levels of unsaturated fats? These fatty acids that are so good for our health.
In fact, this is very simple. It comes from the difference between the environments in which fish and beef or pigs leave. Oceanic waters are colder than pasture or barns. Furthermore, fishes are cold blooded creatures and mammals are hot blooded ones. Throw a thick beefsteak in the sea and it will congeal, its cells are designed to operate at the animal's usual body temperature around 40 degrees Celsius.
The cells membrane, energy reserves of a fish and everything they eat must remain fluid and workable at all time even if the environment surrounding them approach freezing temperature.
Their fatty acids are therefore very long and irregular and don't solidify into orderly crystals until the temperature gets very low.