Sustainable fisheries do not reduce the overall number of fish, because the number of fish that are caught and killed does not ever exceed the birth of new fish.
At one point we thought that we could remove as many fish as we wanted from the oceans without any consequence. During this period, we drastically overfished some of our oceans and seas and reduced some populations to critically low numbers.
Some scientists think that as much as 85% of the world's fish populations have been overfished. Common examples are cod in the North Sea and sole in the Irish Sea and English Channel. Overfishing can cause a critical point in populations that means certain species cannot ever recover and will become extinct.
To address overfishing many countries are adopting a more sustainable strategy for fishing and have introduced fishing quotas which limit the amount of fish that can be caught and killed from specific species. The size of the gaps in fishing nets has also been increased to ensure that juvenile fish can reach reproductive maturity and have offspring before being caught.
These practices have meant that many fish populations are now recovering.