The term overfishing refers to the practice of catching so many fish that the ability of the fish to sustain itself is in jeopardy. This can result from catching too many parent fish which prevents ample breeding from taking place. It can also result from just catching too many of the species until they are completely gone.
Examples of overfishing include:
The cause of overfishing is largely due to the large number of commercial fishing fleets and the methods of fishing which are making it impossible for the ocean to sustain the existence of fish in these fishing areas. As a result of commercial overfishing, some species are extinct or approaching extinction.
The solution to overfishing is to stop poor fishing practices and to better monitor exploited populations of fish. Individuals can help by remaining informed on overfishing and supporting sustainable practices, namely by purchasing fish that have been sourced in a sustainable fashion.
Overfishing is a "Tragedy of the Commons" problem. It is better for each individual person to overfish to benefit himself but it would be better for all the fisherman collectively if everyone refrained from overfishing to sustain the population.