There are at least 25,000 species of fish known to science and new species are being discovered all the time at the rate of several hundred a year.
They range in size from the whale shark that can reach 12 metres (40 feet) to the dwarf goby that only grows to 1 centimetre (0.4 inches).
Although freshwater lakes and rivers make up only 0.001% of the water on our planet, over 40% of all fish species are found in these habitats.
Most of the marine fish species (those that live in the sea) are clustered in the shallow areas of our oceans, not far from the land, where the food supply is at its most abundant.
Many species of fish are today threatened with extinction. Over fishing, pollution, habitat destruction and the introduction of alien competitor and predator species by man have already caused the extermination of numerous species across the globe.
At Chester Zoo many of the 100+ species we keep and breed are threatened with extinction in the wild. For some species, such as the cichlids from Lake Victoria and Lake Barombi Mbo in Africa and the live-bearing goodeids from Mexico, our breeding programmes may be their only chance of survival.