I assume we are talking about sun-rendered....not pressed or dehydrated fish oil, yes?Huge difference!
You can rot down the entire fish or use the "rack", which is what you have left after you fillet the fish. There is fat that runs throughout the fish, including the fillets. Some fish ate more oily that others. Trout, salmon, carp...all are oily fish.
Bass and panfish? Not so much. They make a "juice" not a fish oil. Just like when people say "crawdad or crawfish oil"...Is there fat on a crawdad?
Honestly....the man wanted a simple answer. So there's my perspective....and that comes from someone who makes fish oil.