Hammy Hamster, Roderick Rat and some of the other rodent regulars from the BBC television series Tales of the Riverbank first tasted freedom in 1973. After filming the final episode of the popular children’s programme, some of the crew released a number of animals into the Isle of Wight countryside.
Sadly ‘Ham’ and ‘Rod’ were never heard of again, but Monty (or whatever his name was) the Mongolian gerbil and his clan found the place to their liking—a fair second best to their native Gobi desert. They established a breeding colony of around 100 ger-bils which lived for about three years.
Such animal liberations are more common than you might think. The wildlife of the British Isles – generally speaking rather sparse by comparison with that of mainland Europe – is regularly topped up with outside introductions. Some species, such as zander and wels catfish, were deliberately introduced to Britain and have established colonies – and they are still spreading or being spread by anglers.
In addition to these well known introductions from mainland Europe, a number of exotic fish and other animals have been released, some to breed and establish temporary populations, others to die out quickly.
Many are simply fish released by aquar-ists who have presumably tired of their pets. Some are quite amazing, such as a colourful South American cichlid of the genus Cichlasoma, netted in an Epping Forest pond in 1990, or the South American catfish found dying in a Thames tributary in 1989 – which briefly caused excitement because local fishery staff thought it was a young sturgeon. It is unlikely that either of these would have survived their first British winter, though.
Some tropical species, however, can survive if they are lucky enough to be released into a suitable habitat.
A population of Poecilia reticulata gup-pies managed to survive for several years in the 1960s in the River Lea at Hackney, East London. There were two vital factors in their favour. Firstly, they were living in the heated effluent produced by Hackney’s electricity generating station. Secondly the main river was so polluted that no native fish could survive — the guppies were living in a predator-free, competitor-free world.
The luxury East End life-style of the yuppy guppies came to an end when the River Lea was cleaned up during the rebuilding of the local sewage treatment works; sticklebacks moved into the Hackney area and ate the guppy fry, while more or less simultaneously the power station was shut down and the warm water supply ceased.
A better known example of a British guppy population concerned the Church Street stretch of the Canal at St. Helen’s, Merseyside, in which the stock of a local aquarium shop was dumped in the early 1960s.
The St. Helen’s canal received the heated effluent from Pilkington’s glassworks and many of the fish survived and some bred. Guppies were common for a number of years and platies and mollies (other popular tropical fish) were also present. However, the most exciting fish for local anglers was the cichlid Tilapia zilii, a native of western Africa, which grew up to 15cm (6in) long in the canal.
A slightly more alarming capture was an albino specimen of the walking catfish Clarias batrachus, caught there by an angler. Once landed on the bank, it proceeded to shuffle along the towpath towards its captor!
All these tropical fish can only survive while a source of hot water continues. Cold water exotics do not need such luxury. The goldfish Carassius auratus is another pet fish which has become widespread. It is caught or seen from time to time in ponds and rivers, particularly in southern parts of the British Isles. Some goldfish survive for years and establish breeding populations. However, the dice are loaded against goldfish – their bright gold colour and bulk is as good as wearing a label saying ‘eat me’ to pike and herons.