Sometimes all the hungry fishes have no where else to look for food...
I'm even surprised how they got in there in the first place. There is an aqueduct near me. While it has been there for ages, the only big fishes are the ones that got through on the other side, the delta water side as a fry and added years, grew up big. With no known predator, even fishermen because they all would prefer to hit the big main water to fish than try in a dinky looking ducts, these fishes would just grow and grow. Now to add to the equation, every year, I would catches about a dozen small shaker stripers and I would introduce it to the ducts. I would catch them on my lure and toss it on the other side. Now whether they live or die, that I do not know. I don't even fish for them to know either. I used to catches like 50 or so slabs of crappies a day there but after I introduced them predatory fishes like the stripers and a few large mouth bass, different strain, I then would catches fewer and fewer crappies. Big fish eats little fish thus why.
My fishes that I caught were healthy so that means they are feeding well off them shads. They hangs out at the pumps and eats whatever critters that flows from the other side. Whatever not eaten will be saved for another day.
Fishing 101, look for running currents.