There are several questions there.
With regard to the round goby, it is a complicated tale, let's just say. It's a fairly small fish, four to eight inches long, and it is food, obviously, for any of the piscivorous fish in the lakes that would encounter and feed on fish that small.
That would include northern snakehead, if we had them. They are known to be very predatory fish, so I expect that if we did have them, you would have several invasive species interacting at that level as well. I expect that northern snakehead would be food for other fish where they occur, especially when they're small, but they're a fair-sized freshwater fish. They can be up to 15 pounds, at least, so they may not have all that many predators, certainly when they're in their adult stage.
I think the point of all this is that we're bringing a species from another area and putting it into an ecosystem that has had hundreds of thousands of years of stability, where everything has come into a natural equilibrium. It's not easy to always predict exactly how a new species in that ecosystem will find its niche and the extent to which it would be predator and/or prey. I think that's one of the elements we try to focus on when we're doing a risk assessment, especially if we're trying to determine if it can successfully establish.
There is always the element of the unknown there, because you're introducing something that may not have a lot of natural predators, in some cases, which would contribute to their invasiveness or their ability to colonize a new ecosystem.