Just to chime in on spotted (Kentucky) bass. They were "illegally" introduced to a mid sized lake near me. My family has fished this lake since it was built and the bass fishing was always mediocre. My father, who was an avid bass fisherman for many years, told me the biggest bass he ever caught out of there was a 3.75 pound largemouth many years ago. However, in the past year I've caught a 5.5 pound largemouth and a 6 pound smallmouth out of the same lake, as well as many 3-4 pound spots. I asked around (even though I can't find it officially) and the lake record bass is a 13 pound largemouth and a 9 pound smallmouth. Not to mention this lake also has an EXPLODING flathead and crappie fishery.
So, as was stated, mother nature will adapt and in most cases, thrive. However, when it comes to fish that mass reproduce with no natural predators and people don't want to eat, that's where your problem is (ie. Asian carp). Which, the way snakeheads protect their young along with their adaptability, could also make this a problem.










