Invasive animals and plants which have been released or seed themselves into the wild are causing havoc on the UK's rivers and canals, according to British Waterways.
Foreign pets such as terrapins, or garden plants such as giant hogweed, are damaging wildlife, and hampering access to banks and towpaths and maintenance of locks, bridges and channels, the agency said. Dealing with problem species costs British Waterways about £1m a year.
The organisation, which manages a 2,200-mile network of canals and navigable rivers, has drawn up a list of the worst-offending non-native species. It includes the Australian swamp stone-crop, the water fern, the zebra mussel, the Chinese mitten crab, the giant hogweed, the zander – a species of fish – and the red-eared terrapin.
British Waterways said gardeners should dispose of unwanted non-native plants in a responsible way.
Chris John, British Waterways' national ecologist, said: "Whilst not all non-native species are harmful, many pose real problems to our native wildlife, to boaters and to our historic channels, locks and bridges. This is very costly and diverts resources".