We probably don't have to preach to you about the wisdom of throwing back fish, particularly the big, robust prize catches. If you throw them back, you give them the chance to live, mate and produce equally robust progeny. Be sure to learn the techniques espoused by catch-and-release experts, such as using a circle hook, which is less likely to catch a fish's gut and improves its chances of survival upon release.
But there are some instances when you can help preserve or improve the aquatic ecosystem by not throwing back certain fish. We're talking about invasive species, which have become a major problem in some waterways, gobbling up food and displacing native species. For example, if you're fishing on the Delaware River in New Jersey and you hook a nonnative flathead catfish, don't release it. Fish and game officials ask that you inform the state Bureau of Freshwater Fisheries, which is working with federal wildlife officals to track and prevent the invasive species' spread.