Fishing
In the past, fishing has been so good for getting money that people began overfishing (fishing too much) - a serious problem that does lots of damage (bad). Overfishing does not always mean extinction, but simply that a fish type has been harvested so that there cannot be as many of that kind of fish as before. As more boats are sent out to catch the fish, many population levels of a type of fish can drop. Then, there are not enough of that kind of fish left to have new kinds of fish.