Thanks Onion.
Bones are just calcium, the easiest way to get it. Since they are made to eat fish (and this pretty much goes for all predatory fish) their stomach acid is strong enough to break the bones down and extract the calcium. The marrow is also VERY good for them, since it helps maintain their own bone marrow, and that's where some of the components for the blood comes from (If I remember correctly)
As a matter of fact, that's why I give my dogs natural bones from pork or beef (range and organically fed) I actually cut the bones out of my dinner and they get them. It's a heck of a lot better for them than anything I can get out of their bag of food.
Before anyone flames me for that, what do you think dogs/wolves ate before they were domesticated?
ANYWAYS,
Think of the bones, scales, and other yummy tidbits of a fish as sugar, fish fats and oils as kool-aid, and red/white meat fats as oil.
As for the stomach acid, it's now water.
The fish products (listed above) are broken down by the stomach acid, like kool-aid and sugar dissolve when mixed with water, even the chunks of sugar, or those powdery balls of kool-aid mix, right? Because they were made to dissolve in water. Well, your stomach acid was made to dissolve the things you eat.
if you mix water and oil, what happens? It seperates, no matter how you stir it. That's animal fat in your fish's stomach, and since it doesn't digest there, it goes on to your fish's body, and has nowhere to go to be used up.
I'm sure these metaphors and examples of this is that are annoying by now, But I want to make sure the point is easily understood by all who are reading this post. I know you got it the first time, but some people may not, so I'm using a different tactic to fully explain the difference.
That way, if they get it, nobody gets annoyed with this question being repeated over and over, and it saves us all a little bit of time and effort. ^^