The number of fish in the world is always changing due to reproduction, predation, human consumption and environmental factors, among other reasons. Thus, it cannot be known exactly how many fish there are in all.
What is known, though, is that there are 31,300 different species of fish, and scientists are still discovering previously unknown species all the time. For comparison, the total number of species of vertebrates, to which fish also belong, is 62,305. This means there are more kinds of fish than the number of species of all the other vertebrates combined. Starfish and jellyfish are not real fish and thus are not included in the figure for fish.