Oh we've done a great job at killing off fish...
We introduced the lionfish as an invasive species, which decimated the population of other fish beneficial to the reefs off the Florida coast. We've also done a great job at overfishing grouper on the East Coast...which is probably the one natural predator out there that could keep the lionfish numbers in check if we allowed their population to rise. If we put an end to commercial overfishing, the problem would at least begin to correct itself over time. I have a buddy in FL that goes spear-fishing for lionfish "for fun"...although plenty people do it, it's definitely not enough to keep things in check. A while back I read a report that said in order to just maintain the current population, over 25% of adult lionfish would have to be killed off on a monthly basis. Not an easy task with a lack of natural predators.