This is an assumption so if I'm wrong then please correct me.
I own a fish tank. There is a touchy balance between space availiable and the ammount of fish. In a fish tank, one fish can throw the balance, whether the fish prey upon another or not. Basically, the end result is some fish die. In a fish tank, it would not be of starvation, but because of stress from overcrowding, and fast buildup of toxic chemicals like ammonia. In a lake, it would take many more fish to do this, but in a lake it is worse because unlike a fish tank, there is a limited ammount of food. Fish are adaptable. I would like to compare this to what is happening in Elk Lake. Fishermen have reported catching carp on large rapalas trolled through the water. They were targeting trout, obviously. So the carp there appear to be adapting to the lack of their natural food, and learning to take small baitfish. Carp have different variants reated to diet. Carp feeding on plant matter develop thin lips, and carp feeding on foods like crayfish, (goodbye bass fishery) and other small animals develop thick lips. And we do know that invasive species can wipe out native species.
So if all this is true:
1. Each body of water can hold only a set number of fish.
2. Fish can adapt to eat different foods, taking away from other species' food sources.
3. Damn we don't want carp spreading!