Nearly all species populations operate at the edge of starvation. More food? The local population increases to match the availability of nutrition in the environment.
When you're a fish and a big ol' hunk of food comes your way, too big or not, you're going to grab it and figure out later how to engulf it. That's just instinct. There are even fossils showing the deadly results of glluttony (fish skeleton inside a fish skeleton). It's just instinct from millions of years of survival.
Hmmm, time to have lunch!
v/r, N-A