Non-native game fish are frequently stocked to support fisheries. In most places, the stocking only continues if the non-native is already established, or won't be able to spread from the stocked water. Many managers have tried to reduce the practice, only to deal with complaints and such from angry anglers. In some locations, it works--non-native have been eliminated in some streams to allow native species of concern to stabilize populations. It's tricky.
Of course, a lot depends on who you talk to. There really are very documented cases where an invasive species totally eliminated a native--and the documented cases that exist tend to be on fragile landscapes with limited population diversity. It's hard to put a moral finger on the right side here--is it more important to have species diversity, or more important to have limited species that evolved 'locally'? It's well documented that species spread through a vatiety of vectors. Human ships and cargo, pets, and indeed human beings themselves are now just a new vector. While I don't support intentional introductions, I don't know how much effort we can put into removing an established species (think rainbow trout in the west, or those peacock bass in Florida). Most removal processes are pretty lethal to everything in the water: plants, inverts, fish.
