Cartoon of a man fishing in a pond, and complaining that there aren’t any fish left to fish.
The pond is in the form of a map of the world
The pond in the cartoon is a metaphor for the oceans of the world.
The concept is that people have in the past (and even today) thought of the earth’s oceans as being almost endless and full of unlimited supplies of fish. While in fact the oceans are relatively small, especially when the rising human population puts so much pressure on the fish stocks