Unnatural visitors

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Every hour enough plastic to fill two bin bags is washed into our oceans from canals and rivers.

Killer shrimp, minks and Aesculapian snakes: Trust ecologist Dr Mark Robinson explains why some invasive species are causing havoc on our waterways.

KIller shrimp on white background Killer shrimp, copyright GBNNS

The country’s canals thrive with life, but not all of it is welcome. From mink and snakes to terrapins and the dreaded demon shrimp, these non-native species add a splash of exotica to the UK’s waters, but can also have a negative effect on the environment around them.

"Not all alien species are invasive species," explains Dr Mark Robinson, the Canal & River Trust national ecologist. "A classic example is the little owl. That was introduced in the 19th century and is very much loved. You could never remove them from the landscape. It depends on the impact they have whether they are classed as invasive. Often, because they are not part of the ecosystem, they derive an advantage and can exploit things in a way native species cannot."

A recent arrival is the colony of Aesculapian snakes discovered on the Regent’s Canal in London, possibly escapees from the nearby zoo. These highlight both the benefit and drawback of the canal. The snakes are likely to stay fairly localised because the only way they will get anywhere is along the canal. The waterways are great habitat corridors as they allow wildlife to travel into the heartland of our cities, but they can also allow invasive species to spread more widely.


How they got here: Escapees from mink farms as well as deliberate releases by animal rights activists. They might have been here since the 50s, but many people would be glad to see them gone.

Problems: Mink can swim and are ferocious hunters. They are blamed for the decline of the native water vole but also feed on crayfish, ducklings and fish.

Solutions: The increase in otters on waterways, caused partly by habitat improvement, is helping limit the mink’s territory. "Good otter numbers push out mink in certain areas, but we’re never going to get rid of mink in this country," says Robinson, who says the Trust do some mink control but very little as it barely makes a dent in the population. "They are here to stay."

American signal crayfish

How they got here: Imported to the UK for seafood in the 1970s, this voracious creature soon began to take over their new territory.

Problems: The American crayfish is large and vicious, threatening the native crayfish in direct competition as well as spreading crayfish plague. "If I’m being pessimistic, in 25 years we won’t have any white-clawed crayfish because the Americans will have taken over," says Robinson. "It’s a desperate situation."

Solutions: Attention is focused on creating protected locations for the native crayfish in reservoirs and higher ground. Crayfish can be eaten but trapping is not allowed to take place on canals. "They taste like scampi," says Robinson. "It’s not something I’d go back for."

Chinese mitten crab

How they got here: Medium-sized burrowing crabs with furry claws (resembling mittens), which stowed away on boats and have been in the UK for at least 70 years around the south and east coasts.

Problems: They can cause problems when they burrow into the banks and undermine them. However, the crabs’ breeding cycle requires them to return to estuarine conditions so they only travel a certain distance from the sea.

Solutions: One idea is to harvest the crabs and ship them back to China, where they are considered a delicacy but stocks are low thanks to the appearance of the larger Japanese crab.

Killer shrimp

How they got here: Originally from the Black Sea, they have spread to the UK since the 1990s probably via commercial shipping.

Problems: Fast breeders, hardy and – as the name suggests – stone-cold killers, these shrimp are having a damaging impact on their native cousins as well as insect life. The shrimp can even survive outside water for up to six days. "There’s also a species known as the demon shrimp," says Robinson. "They are both large, non-native, voracious shrimp that eat our small invertebrates."

Solutions: The focus is currently on identifying the scale of the population as well as implementing a clean-and-dry campaign, which means keeping gear clean to ensure that people don’t transfer the shrimp from one waterway to another.


How they got here: Legend has it that American terrapins arrived in the wake of the Teenage Mutant Ninja Turtles craze. When the critters outgrew their tanks – or their welcome – they were dumped in the nearest waterway.

Problems: They can scratch fish and eat invertebrates, but they are in quite low numbers and generally not an issue on the canals.

Solutions: Time. Terrapins cannot breed in the UK as the weather is not warm enough for the eggs to survive, so the population should die out naturally. "All the evidence is that they don’t breed here," says Robinson. "Some young ones have been seen in London, but even if they breed in the odd location it won’t be a problem."

Zebra mussel

How they got here: These molluscs, originally from Russia, are believed to have arrived on ship hulls and have been on the network since the 1820s.

Problems: They can attach themselves to canal gates and sluices, hindering operations.

Solutions: Removing them by hand takes time but is done as other methods are sought.


How they got here: Decorative plants introduced by Victorians that have rapidly spread in the easy conditions offered by canals. These include duckweed, water fern, floating pennywort and Himalayan balsam.

Problems: Some simply take over land around them, others damage the water itself. Duckweed and water fern starve the light and oxygen, so kill off other plant and are also a health and safety risk as they make the water look like something you can walk on. Floating pennywort is a major problem and costs a fortune to remove without making a real dent. Himalayan balsam is an even bigger problem than Japanese Knotweed because it spreads ferociously.

Solutions: Invasive plants are sprayed and removed but still remain one of the biggest problems facing our waterways.

Written by Peter Watts

Last date edited: 7 September 2016