I've been to some smaller rivers where the chub have overtaken a whole stretch of river, these chub grow large and act as the main predator it seems. Some I've caught in the 1-2lbs!!! range. I figure this has happened to a shrinking trout population (angling, water temps), therefore opening the door to these ravenous thugs. Once this happens I figure it is nearly impossible for the little trout to compete and the chub choke out the trout. I feel too bad just killing a fish due to my desire to intervene and therefore I don't, but I'd like to think that if large chub populations were controlled, it could open up the rivers to the trout again.
It's definitely proof that trout populations are sensitive and are high in the river food chain.