The Origin and Distribution of the Freshwater Fishes of the British Isles

The origin and distribution of the freshwater fishes of the British Isles is discussed. With the exception of the secondary freshwater species which have gained entry to lakes and rivers by way of the sea and are correspondingly widely distributed, fresh- water fishes are indigenous only to the river catch- ments bordering the eastern English Channel and the southern North Sea. All the indigenous primary freshwater fishes are believed to have entered England during the existence of a land bridge connected to continental Europe which was severed probably at the end of the Boreal period, at about 7500 years B.P. Their wide distribution outside these areas is due directly or indirectly to anthropogenic factors. The only indigenous freshwater fishes in Ireland are those which have entered from the sea; all others have been introduced by man. Several relict populations of species, which entered lakes during the early post- glacial period and which were subsequently isolated by changes in land level relative to the sea, are also discussed.