Mastercaster,

Perhaps one of the smart biologists will provide a more detailed reply, but meanwhile . . .

I think seals and sea lions eat the bellies of fish because that's were the organ meat is located. Fish flesh is great for protein and some fat, but the heart, liver, and pancreas are excellent sources of vitamins and minerals that are critical dietary components. Another, less biological reason is that they might be like anglers. When they catch a fish, they can't wait to catch another. Maybe they find it good sport, and it helps maintain their skill level.

Sincerely,

Salmo g.