When it comes to identifying and dealing with aquatic invasive non-native species, can there be anyone better equipped than a bunch of motivated, knowledgeable anglers?
Well, sometimes, yes. But everyone has to start somewhere, and it never hurts to know more about the actual and potential threats to your favourite river, newly-restored or not. And when you finally decide to get professionally qualified to trap, spray and otherwise meet invasive aliens head on… that’s possible too.
If you’re reading this anywhere but the UK (and that’s quite likely in light of this site’s international readership), your key problem species may vary, but one principle will almost certainly remain the same: take a catchment-scale approach to detection, control and eradication if you can.
Successful non-native invasive species are highly mobile, adaptable and opportunistic – which means that if they’ve reached one part of your river’s catchment, they’re probably elsewhere too. So let’s start getting to know our main enemies…
Invasive aquatic invertebrates
American signal crayfish (Pacifastacus leniusculus)
Britain’s first American signal crayfish were recorded in 1975, several years after they’d been introduced to Europe to help boost native crayfish harvesting operations damaged by crayfish plague. Too late, signal crayfish were found to be carriers of the same fungal plague, which kills native British white-clawed crayfish within weeks of infection. But the new arrivals spread quickly via professional aquaculture and amateur bucket biology (despite the famous benefits of hindsight, it’s still almost impossible to believe that these introductions were officially encouraged by the Ministry for Agriculture, Fisheries and Food!)
By burrowing into soft banks, signal crayfish increase erosion and siltation of in-stream gravels. Convincing recent research by the Ribble Rivers Trust indicates that signal crayfish also threaten fish more directly – not just by eating their eggs and competing with them for food, but by driving them out of streambed nooks and crannies into areas where they’re more vulnerable to predation.
Adult crayfish prey on their younger relations as indiscriminately as upon caddis and other slow-moving aquatic invertebrates, so removing large crayfish may even be counterproductive. The latest thinking suggests the possibility of engineering signal crayfish out of certain reaches by replacing their favourite vertical soft earth banks with harder, sloping gravel edges, but there’s still no reliable means of control apart from stopping them reaching your river in the first place.
Killer shrimp (Dikerogammarus villosus)
Native to the areas surrounding Black and Caspian Seas, Dikerogammarus villosus has been marching steadily across western Europe since the Rhine-Main-Danube canal was opened in 1992. The species was first identified in the UK in September 2010 in Cardiff Bay, Grafham and Eglwys Nunnyd reservoirs, and had also been spotted in the Norfolk Broads by spring 2012.
Growing up to 30mm long, killer shrimp are much larger than the UK’s native gammarus species, and derive their popular nickname from their habit of indiscriminately killing and shredding prey without eating it. Nymphs, leeches, and fish eggs and fry of all species are all subject to attack from Dikerogammarus’ outsize mandibles, contributing to scientists’ fears that vulnerable species may be driven into local extinction.
Dikerogammarus can live in rivers, canals and lakes, tolerating low dissolved oxygen levels, a wide range of substrates and even 20% salinity. Environment Agency tests show that they can also “survive in the moist fold of a wader for up to 15 days”. Apart from stringent biosecurity measures, no effective means of control are currently known…
Zebra mussel (Dreissena polymorpha)
Evolutionarily linked to Dikerogammarus villosus in the Black Sea region, zebra mussels are thought to benefit killer shrimp by filtering nutrients out of the water column and depositing detritus in the substrate around their colonies – providing them with an extra food source and helping them outcompete other species. By filtering nutrients out of the water column, zebra mussels deprive juvenile fish of food (on the other hand, increased water clarity can also help underwater plants to photosynthesise effectively, providing habitat for a different aquatic community).
Dreissena polymorpha were first reported in the UK in 1824, spreading rapidly from Cambridgeshire to London, Salford and Edinburgh. Adult females are massively prolific, each producing up to 1 million eggs per year, and the species can colonise many substrates including water company pipeworks: in 1989 the town of Monroe, Michigan, lost its water supply for 3 days as a result of zebra mussels clogging the city’s water intake pipeline.
Zebra mussels also threaten other species of mussel by latching onto them, as the most stable objects in silty substrates, before smothering or otherwise killing them. This behaviour has reputedly eliminated 2 species of native mussels from lakes in Ireland’s Shannon system, as well as many species from lakes in North America, while fish populations have been affected at Salford Quays. Intriguingly, crayfish, roach and water birds predate heavily on juvenile mussels, but the only reliable method of controlling adults is physical removal: Anglian Water apparently spends £500,000 on clearing zebra mussels from its treatment plants every year.
Other invasive invertebrates to watch out for: Chinese mitten crab (Eriocheir sinensis), other non-native crayfish
Himalayan balsam (Impatiens glandulifera)
Like so many other invasive plant species, Himalayan balsam was brought back to Britain as a Victorian plantsman’s trophy… and soon hopped over the garden wall into the wild. In 1855 it had already been recorded in our own countryside, and quickly got its first nickname of Mersey weed.
With up to 800 spring-loaded, floating seeds produced by each 3 metre high annual plant, together with the ability to shade out all competition before dying back in the winter to leave riverbanks bare and vulnerable to erosion, you’d find it difficult to imagine a more effective invader than Himalayan balsam. Worse still, it delivers a double whammy to native plant species by seducing bees and other pollinators away from their flowers with its own sweet scent.
Still, those shallow roots make it easy to pull up, it composts or dessicates easily if piled away from the river bank, and past seasons’ seeds persist in the soil for no more than 3 years. As projects on the Rivers Monnow and Kent are well on the way to proving, Himalayan balsam can be cleared on a catchment scale by starting at the top and working downstream before seeds set around July or August – pulling by hand, spraying extensive stands with glyphosate, or even strimming at a low level (with the cutter blades set below the plants’ first node to prevent regrowth from this point).
Using these techniques, it’s possible to clear large infestations very rapidly – especially if you take a leaf out of the Monnow Rivers Association’s book and encourage Passport Scheme anglers to pull 50 rogue plants each as part of their day out on the water! Future biological controls may also include a rust which attacks Himalayan balsam at both seedling and mature life stages.
Japanese knotweed (Fallopia japonica)
Having evolved on the slopes of Mount Fuji to break through hardened lava, Japanese knotweed is capable of growing through tarmac, concrete and even joints in drains at a rate of a metre per month, tolerating acid soil, heavy metal contamination and air pollution. Dense thickets up to 3 metres in height crowd out other plants before dying back below ground in the autumn: on riverbanks the fleshy but brittle stems can easily be broken off in spates, creating flood risk by lodging in culverts and other pinch points.
With its fearsome ability to propagate from the smallest fragment of root, stem or leaf, it’s fortunate that all Japanese knotweed plants in the UK are clones of a single female plant, introduced to the UK between 1825 and 1840, but Fallopia japonica also spreads via underground rhizomes at a rate of up to 7 metres per year. As a result, under the UK’s Environmental Protection Act (1990), it’s classified as controlled waste, and even earth containing fragments of roots and stems must be disposed of at a licensed landfill site.
Thanks to powerful economic imperatives, means of controlling Japanese knotweed are well understood. Research by the Wye & Usk Foundation suggests that the best time to spray with glyphosate is during late summer or autumn when the plants are starting to draw nutrients (and thus a dose of Roundup!) back down into their rhizomes in preparation for winter. Even better results are obtained by injecting individual stems with high concentrations of glyphosate, using specially-designed injection kits, and since 2011 DEFRA has approved the controlled country-wide release of species-specific sap-sucking psyllids (Alphalara itadori) by CABI as a biological control.
Giant hogweed (Heracleum mantegazzianum)
At the time of its introduction to Britain from the Caucasus mountains of Russia in 1893, giant hogweed was welcomed by gardeners for its “Herculean proportions” and “splendid invasiveness” (USPs which enthusiastic Victorian marketers also applied to Japanese knotweed and Himalayan balsam).
But while Heracleum mantegazzianum can reach 5 metres in height and colonise miles of riverbank thanks to the 50,000 seeds produced by each adult plant, these invasive tendencies aren’t the only problem. Giant hogweed sap contains a toxin called furocoumarin which causes phytophotodermatitis – sensitivity to ultraviolet light. Simply brushing against the plant’s hairy leaves and stems can be enough to transfer beads of sap to the skin, resulting in blistering and third degree burns which can require hospital treatment, and recur for years with every exposure to the sun.
According to the Environment Agency, the best way to control this dangerous umbellifer is by cutting the root with a spade 15cm below ground level whilst wearing full protective clothing. Chemical treatment by spraying or injecting glyphosate is also possible.
Floating pennywort (Hydrocotyle ranunculoides)
Floating pennywort arrived in the UK from North America in the 1980s as a plant for aquaria and garden ponds, and was first noticed in the wild in Essex in 1991.
Mainly colonising areas of slow-moving water, the long fleshy stems of Hydrocotyle ranunculoides grow up to 20cm a day, forming densely interwoven mats of floating vegetation which can double their size and weight in as little as 3 days. These die back to the margins in winter, but during the late summer growing season the whole water surface can be covered, cutting off light for other aquatic plants and leading to oxygen depletion for fish.
Like Japanese knotweed, floating pennywort can regenerate from tiny nodes of vegetation, so total eradication is difficult. Manual cutting and removal is temporarily effective (with a seine net stretched across the river downstream to catch floating fragments) but licensed spraying with an approved herbicide is probably the only permanent answer. Mixing glyphosate with adhesive oil helps to stop it running off the leaves and improves uptake into the plant.
Other invasive plants to watch out for: Australian swamp stonecrop (Crassula helmsii), Parrot’s feather (Myriophyllum aquaticum), Water fern (Azolla filiculoides), Water hyacinth (Eichhornia crassipes)
American mink (Neovison vison)
Since coypu were eradicated from East Anglia in 1989, American mink are currently the only fur-farm escapees living wild in the UK, but their predatory nature has ensured that they’re now Public Enemy Number One for conservationists nationwide.
According to the Game and Wildlife Conservation Trust, wild breeding populations of mink were established by 1956, and had spread across more than half of Britain by 1957. Neovison vison can have a major impact on biodiversity, killing a wide variety of prey including fish and birds, and contributing to a 90% decline of water voles in many areas.
Otters are thought to attack mink which invade their territories, but the most effective form of control is trapping: tracking the presence of mink via the use of clay pads in tunnels on specially designed mink rafts, then replacing the pad with a cage trap which must be visited on a daily basis. Successful trapping projects have been led by the University of Aberdeen in Scotland, and the Monnow Rivers Trust on the Dore and other Monnow tributaries, and water voles are bouncing back where their main predator has been removed.
- American Signal Crayfish (Pacifastacus leniusculus) by David Perez (Own work) [CC-BY-3.0], via Wikimedia Commons
- Killer Shrimp (Dikerogammarus villosus) by Silvia Waajen via www.onderwaterwereld.org
- Zebra Muscle (Dreissena polymorpha) by GerardM [GFDL or CC-BY-SA-3.0], via Wikimedia Commons
- Himalayan Balsam (Impatiens glandulifera) by H. Zell (Own work) [GFDL or CC-BY-SA-3.0], via Wikimedia Commons
- Japanese Knotweed (Fallopia japonica) by H. Zell (Own work) [GFDL or CC-BY-SA-3.0], via Wikimedia Commons
- Giant Hogweed (Heracleum mantegazzianum) by Huhu Uet (Own work) [GFDL or CC-BY-3.0], via Wikimedia Commons
- Floating Pennywort (Hydrocotyle ranunculoides) by Marcia Stefani (Uploaded by berichard) [CC-BY-2.0], via Wikimedia Commons
- American Mink (Neovison vison) by Wojciech Uszak (Own work) [GFDL or CC-BY-SA-3.0-2.5-2.0-1.0], via Wikimedia Commons
For further detailed photos and the latest information about all these non-native invasive species (and many more) please visit DEFRA’s GB non-native species secretariat’s identification sheets page.
Next time: Can you ever finish restoring a river?