The waterways are under threat from a host of alien invaders
From galloping weeds to cannibal crayfish, beware of the natural predators
Purple-pink is not the usual colour associated with a Policeman’s Helmet. At the height of summer the dome-shaped flowers of Britain’s tallest growing annual plant, Himalayan Balsam, can dominate the canal bank.
A sweet scent wafts from the nectar in a short spur on the lower sepal attracting bees that are temporarily trapped in the flower, ensuring efficient pollination, before backing out. With just their back end visible, the charming local name of ‘bee-bum’ has arisen.
The succulent stems and sharply-toothed green leaves are characteristic of the genus Impatiens, named for the fast method of seed dispersal.
Explosive seed capsules hurl seeds more than ten metres and the plant has become very common. Welcomed in some waste areas, it is considered a pest where its robust growth suppresses native flora. This Asian beauty is an alien invader, first introduced to botanical gardens in c.1839 from India, it escaped to form waterway colonies by 1855.
Other invasive non-native wildlife are also causing alarm along our canals. Zebra mussels spread to Western Europe from Russia in the early 19th century as the canal systems were built, hitching a lift on the hulls of ships.
They are smaller than marine mussels, with a distinctive D shape, and ornamented with blue or brown alternating with yellow-white wavy bands.
Native freshwater mussels live partly buried in the sediment at the bottom of lakes and rivers, but Zebra mussels are able to attach themselves to hard surfaces by adhesive filaments (the byssus), the only freshwater mussels able to form colonies in this way.
They clog up waterworks and systems across the UK, impeding the working of lock gates and sluices.
Zebra mussels produce millions of microscopic larvae when water temperatures are above 12C (roughly May to September). The larvae drift in currents until settling on hard surfaces such as masonry and wooden posts where they develop into adults that live two to three years.
They are eaten by fish, crayfish and waterfowl, but in insufficient numbers to stop them from smothering native species and altering ecosystems by removing phytoplankton and essential nutrients.
Floating Pennywort has similar ecological consequences. Introduced to Britain in the 1980s by the aquatic nursery trade, its sale became illegal in 2014.
Dense floating mats of lush foliage are swamping waterways in the south-east of England and Norfolk and spreading into the Midlands, crowding out native plants and starving fish of oxygen.
The shiny, kidney-shaped leaves have crinkled edges and fleshy stalks and are interwoven by fine roots. The tiny white flowers are rare but, if present, can be seen between July and August. The combination of rapid growth rate – up to 20cm a day in late summer – and the ability of the plant to spread by fragmentation – a single piece can float away and regenerate into an entire plant – make control of Floating Pennywort especially difficult.
The restaurant trade introduced American Signal Crayfish in the 1970s as an alternative to lobsters. On wet nights the amphibious crayfish clambered out of farm ponds and walked overland to populate our waterways where they have now spread out of control, eradicating the smaller native White-Clawed Crayfish by taking over their territory, eating them, and spreading a deadly fungal disease.
You can see these unwelcome invaders along our canals where they burrow into the bank using their tail as a scoop. The damage, exacerbated by boat wash, can cause the banks to collapse. Active during the day, they are reddish-brown with large robust claws that are bright red on the undersides and have a white ‘signal’ patch near to the hinge.
They eat practically anything, being both scavengers and predators. They can grab plants, invertebrates, snails, and small fish with their claws and tear them apart.
Clipping off weed near the bed of the waterway is especially devastating to the environment.
The natural balance is increasingly fragile.