Winning a goldfish at a fairground, that childhood rite of passage, is threatening the survival of a native breed of carp, the Environment Agency says.

So many goldfish have been discarded into the wild that 18in-long crucian carp are being crossbred out of existence in Britain’s ponds, lakes and rivers.

Goldfish, which have a greater sexual appetite than their wild cousin, breed with carp so successfully that only hybrid fry are produced. Once goldfish are introduced to a pond, lake or a slow-moving river it is only a matter of time before the native crucian carp, Carassius carassius are replaced by crossbreeds. The Environment Agency is so alarmed by the invasion that it has urged owners to kill unwanted goldfish rather than release them into the wild.

Far from being the “humble” fish of popular perception, the agency says, the goldfish is a dangerous alien that could be more damaging than the grey squirrel and the American signal crayfish that have devastated native red squirrel and crayfish numbers. Goldfish that have been won as fairground prizes are often barely an inch long, and being kept in a bowl or small tank stunts their growth.

However, far from being too small for the carp to bother mating with, they grow rapidly once they are released into the wild to match and even outperform the native species.

They can reach almost 10lb — three times the size of a crucian carp — making it easier to compete for habitat and food.

Goldfish were introduced to Britain in the 17th century.

Some escape directly from garden ponds into streams but most, including both brown and gold varieties, are deliberately released by people who are no longer able or willing to care for them.

Paul Lidget, a fisheries adviser for the Environment Agency, said: “We are beginning to realise the extent of the problem with the brown and gold and the hybrid varieties. They are more commonplace than was previously thought and it’s largely down to households getting rid of unwanted pets.”

He added that releasing ornamental fish into the environment was an offence, but that many were unaware of the law.

Crucian carp

Explore UK News

• Crime News

• Education News

• Health News

• Scotland News

Times Recommends

• The bright side of this winter rain
• Story behind the picture: Snow Leopard, 2008
• Times Charity Appeal: The Place2Be