Overfishing is catching too many fish at once, so the breeding population becomes too depleted to recover. Overfishing often goes hand in hand with wasteful types of commercial fishing that haul in massive amounts of unwanted fish or other animals, which are then discarded. As a result of prolonged and widespread overfishing, nearly a third of the world's assessed fisheries are now in deep trouble — and that's likely an underestimate, since many fisheries remain unstudied.