I would doubt that there are no other fish for them to eat as nature maintains a balanceed percentage of predators to none predators.
I have a lake near me where people think there is no fish at all but it still contains pike, perch and brown trout.
Over the past few years I have seen carp, up to a surprising size and large shoals of bait fish on my fish finder while looking for features.
Despite this it is very rare to see a fish 'top' to give the game away that fish are present, perhaps someone smarter than me can tell me why this should be.