The invasion of British waterways

North american crayfish

Image source: Trevor Renals, GB Non-Native Species Secretariat
Our riverbanks are being invaded by non-native species

British waters are under attack! Non-native species of plants and animals are invading our waterways, destroying native river inhabitants and destabilising ecosystems.

Anglers are the first line of defence. According to the Angling Trust we already spend 100,000 hours every year combatting the invasive non-native species (INNS) that impact fishing. But there’s still more we can do to slow the onslaught and defend our waterways.

We take a look at five of the “Most Wanted” invasive species and give you some top tips on invasion prevention!

The Signal Crayfish

signal crayfish

Image source: Angling Trust
Beware the omnivorous signal crayfish

Introduced to the UK as a high-end seafood, the signal crayfish is the one enjoying the meal now. Treating British waterways as its own personal buffet, these large crayfish not only attack our native white-clawed crayfish, but will kill and eat small fish, fish fry, and fish eggs.

The signal crayfish is a disaster for indigenous wildlife. It transmits a “plague” which kills the native crayfish and displaces native fish, including Atlantic salmon parr and stone loach. Even worse, by burrowing into riverbanks, the signal cray erodes the aquatic habitats of other creatures and renders streams to shallow for fish to swim in.

The topmouth gudgeon

topmouth gudgeon

Image source: Creative Commons/ Wikipedia
Small but deadly, the topmouth gudgeon can be mistaken for a sardine

Don’t let its small size fool you. According to the Environment Agency (EA), the topmouth gudgeon is a “tiny yet destructive” fish, which carries a disease that can kill Britain’s native salmon and trout. This wily sardine-lookalike dominates the rivers and lakes it spreads to, even out-competing carp.

Accidentally introduced to the UK from Asia in the 1980s via the aquatic trade, the topmouth gudgeon quickly spread to 23 sites around the country.

But this is one of the success stories. The EA is battling back and the topmouth gudgeon is steadily losing ground. With a bit of luck it will soon go the way of the fathead minnow and the black bullhead catfish, both of which now no longer exist in the UK. Victory is possible!

The killer shrimp

killer shrimp

Image source: CanalRiverTrust
The killer shrimp is aptly named, often killing without eating their prey

Killer shrimp are deadly to freshwater invertebrates like native shrimp and also to small fish, often slaughtering without even eating their victims. They also kill any insect they encounter, including damselflies and water boatmen, reducing the amount of food available for fish.

The watery assassins come from the Black and Caspian seas but migrated here aboard commercial shipping. First spotted in the UK in 2010, the Killer Shrimp’s insatiable appetite and fast breeding cycle make them them one of most damaging invasive species not just in the UK but across Europe.

Floating pennywort

Floating pennywort

Image source:
Floating pennywort has almost completely covered this stretch of river

The pretty green leaves may look nice, but floating pennywort is an angler’s worst nightmare.

This invasive plant thickly carpets British waterways with its crinkled, kidney-shaped leaves, driving away fish and other river residents by blocking out the light and depriving the water of oxygen.

And, to make matters worse, the plant grows up to 20 cm a day and can spawn from even a tiny piece of root making it difficult and expensive to remove.

Originally imported to the UK in the 1980s to prettify ponds, floating pennywort is now considered so destructive that in 2014 the UK Government banned its sale here in the UK.

The sunbleak, or motherless minnow


Image source: Adrian Pinder/ Mike Ladle
The sunbleak moves in large shoals and can outcompete much larger fish

Small in stature the sunbleak may be, but you’ll be amazed the harm one small fish can cause. As Mike Ladle points out, it’s a fish that punches way above its weight:

“They… compete for food with other fish – particularly the young of rudd, roach or bream… [they] devour the fry, hatchlings or particularly the eggs of larger species.”

Sunbleak reproduce at lightening speed, rapidly colonising the rivers and moving in large shoals. Depressingly destructive, this fish is yet another example of a foreign invader originally imported to the UK to prettify fishponds.

What can you do?

Check, Clean, Dry

Image source: GB Non-Native Species Secretariat
Make “Check, Clean, Dry” your new mantra

To stop the spread of invasive species, remember three words: “Check, Clean, Dry”. Here’s what you should do each time you finish fishing for the day – or – move from one fishing spot to another:

CHECK: Check all your fishing gear for any living organisms.

CLEAN: Clean everything you took fishing, including your footwear, clothes and gear.

DRY: Make sure everything is completely dry before you use it again.Some invasive species can survive for days in damp conditions.

Help spread the word. The GB Non-Native Species Secretariat (NNSS) produces posters, signs, and other materials about the campaign that you can put up at your local angling club.

If you think you’ve got invaders in your local stretch of river, head over to the Angling Trust and NNSS websites to look at some of the mugshots on display. Can you identify a non-native? email your report to the NNSS or fill out the form.

Have you had any encounters with river invaders? Head over to our Facebook page and tell us your stories from the front lines!