One of the intersting aspects of at least some predators is that they not only need to eat but they need the proper anount of calories. I have raised fryer chickens that essentially starved to death with full crops. Food quality wasn't good enough for their metabolism.

Some orca population is Alaska crashed because they started eating sea otters. No fat, not enough calories. In Puget Sound, the Chinook have more fat (calories) than pink or chum. They can only eat so many pounds per day, and if the calories aren't there they starve.