How can we deal with silent but potentially deadly river invaders?

Our rivers and chalkstreams are under threat like never before and we are in grave danger of failing to react to the increasing damage caused by the number of non-native invasive species that are causing havoc on many of our already vulnerable waterways

Under the surface of our rivers lives an alien world, where new, foreign animals are rapidly establishing homes. Many of these species, including American Signal Crayfish, Himalayan Balsam and Demon shrimp look harmless and in some cases even attractive. However, these invaders change river processes and exploit food resources often to the detriment of our own, native wildlife. It is of high importance that we react quickly, before invasive species cause more havoc on our already vulnerable waterways. These critters could be coming to a river near you!

Paul Knight, CEO of fisheries charity Salmon & Trout Conservation UK (S&TC UK) said: “With increasingly stretched budgets, we are in danger of taking our eye off the ball. The only way to establish whether new or existing invasive species are starting to take hold in our rivers is for more intensive monitoring, but this is not happening at the level needed to detect problems.”

S&TC UK are currently the only organisation carrying out detailed scientific invertebrate monitoring of many rivers in England and Wales down to species level through the national Riverfly Census.

Paul Knight explains, “Through our Riverfly Census, we are working with Dr Nick Everall from Aquascience who has established that the Demon Shrimp is silently taking hold of the river Churnet in Staffordshire with disturbing ease and appears to be wreaking havoc on the indigenous invertebrate community.

“The results from three-minute kick-sweep samples taken at one Churnet site on five occasions from January 2014 to October 2016 are stark. The demon shrimp population has boomed, rising from zero in 2014 to 442 in October 2016.”

Paul continues, “Without early species-level monitoring like this we are blind. A lack of monitoring renders us clueless as to where invasive species like the Demon shrimp are and how rapidly they are spreading. This highlights how important high-resolution monitoring and good biosecurity are to stop the spread of invaders and safeguard our rivers.”

The American Signal Crayfish is another invader whose population is completely out of control. The six-inch-long killing machine has already annihilated the smaller native White Claw Crayfish from most of the waterways in the south of England. It’s a voracious predator that will eat almost anything it finds, including plants, invertebrates, snails, small fish and fish eggs. They also act as vectors of diseases, perhaps most notably crayfish plague, and this is lethal to the five indigenous European crayfish species.

But most worrying of all is the Gyrodactylus salaris (Gs) parasite that has not yet arrived in our river systems, but without vigilance it could devastate complete river catchments.

Paul Knight, who recently attended a NASCO meeting, explains how the impact of this species on Norwegian rivers has been a devastating blow.

He said, “Gyrodactylus salaris (Gs) is a tiny parasite, 0.5 mm long, which is lethal to Atlantic salmon. It has been responsible for the total loss of wild Atlantic salmon populations in some Norwegian rivers. The only way to treat the issue has been to kill everything in infected river catchments with Rotenone – a toxic poison. “

As yet, Gs has been kept out of the UK and Ireland, but if it was to arrive here, it would be a disaster. Unlike Norway, it is inconceivable that our rivers would be allowed to be poisoned, and so wild salmon stocks, already under threat in England and Wales, could become extinct in impacted rivers.

Paul Knight concludes, “The best way of making sure that the UK and Ireland remains free and to protect our rivers from yet another devastating invader is to stop Gs from getting here in the first place. Live fish movements are already banned from infected countries, but anglers, canoeists, and those managing our rivers can play their part by observing the basic biosecurity rules of – Check, Clean and, Dry all equipment which might transport the parasite. The best contingency plan is to ensure that we never have to use one!”

Salmon & Trout Conservation UK have been campaigning to improve the level of monitoring of our rivers across the country so that early warning signs can be detected and resolved. As well as its Riverfly Census, which monitors 22 English and Welsh rivers down to species level, S&TC also runs an advanced River Invertebrate Identification and Monitoring course, helping those who care about their river environments to become ‘Citizen Scientists’ who are properly trained to benchmark their own local rivers down to species level. This gives an accurate measurement of water quality and the effects of different forms of pollution on local waterlife.

Paul Knight, says, “With belts being tightened, we are not paying enough heed to our rivers. Without species level data on what was and what is now in our rivers, we do not know how wild fish, invertebrates and other important native aquatic species are faring. Crucially, we do not know how badly our rivers are being damaged by non-native species. Under the current system this could be a tragedy waiting to happen, which is why local anglers, catchment managers and conservationists are so vital in our battle to keep rivers clean and healthy.”

Salmon & Trout Conservation UK are keen to hear from those that have concerns about invasive non-native species on their rivers and should get in touch by visiting the S&TC UK website at to learn more about the Riverfly Census and the River Invertebrate Identification and Monitoring course. Alternatively, Follow Invasive Species Week on social media by keeping an eye on @CheckCleanDryGB and #InvasivesWeek.

Notes to Editors:

Salmon & Trout Conservation UK (S&TC UK) is a fisheries charity established in 1903 to address the damage done to our rivers by the Industrial Revolution. Since then, S&TC UK has worked to protect fish stocks and the wider aquatic environment for the public benefit. Our charitable objectives empower us to address all issues affecting fish and the aquatic environment, supported by evidence from our scientific network.

Gyrodactylus salaris (Gs) is a small parasite (0.5mm in length) infecting the skin, gills and fins of fish and is capable of rapid multiplication causing serious physical damage to Atlantic salmon parr. It has been responsible for significant mortalities (up to 98%) of wild Atlantic salmon populations in Norway, and as a result some salmon stocks have been lost completely. In Scotland, Gs is a listed disease that must be reported under Schedule 1 of the Aquatic Animal Health (Scotland) Regulations 2009. The U.K. is currently recognised as being free from Gs although evidence exists to suggest that our Atlantic salmon populations are highly susceptible to both infection and mortality.

Gs is restricted in its distribution to Europe. It occurs naturally in some Baltic areas of Finland, Sweden and Russia where some Atlantic salmon populations tolerate the parasite. Widespread distribution in Norway has resulted following the accidental introduction of the parasite in the 1970s. The parasite is also common within farmed rainbow trout populations, where it has a much lower impact and is capable of going undetected. Gs has been identified in Denmark, France, Germany, Portugal, Spain, Italy, Poland and Macedonia. The UK and Ireland are the only areas known to be free of the parasite. Across Europe many countries have an undetermined status and it is likely that the parasite is present in more countries than is presently known.

Invasive Species Week: From March 27 – April 2 organisations across Britain are coming together to support Invasive Species Week – and you can take part too. From disposing of garden waste responsibly, recording and reporting invasive non-native species, to volunteering with an invasive species local action group, there are plenty of ways of ensuring that you do your bit to help tackle this key wildlife issue. You can find more information about events being held this week, and ways in which individuals can take part, on the GB Non-native Species Secretariat website:, or by following Invasive Species Week on social media – just keep an eye on @CheckCleanDryGB and #InvasivesWeek.