Top 8 invasive species to look out for when you paddle

This American native is almost twice as large as our common frog and is one of the most harmful invasive amphibian species. Not only does it feed day and night on a wide range of prey, but also carries a disease that has led to worldwide amphibian decline and several global extinctions. It occupies any type of still or slowly moving water, especially where aquatic and bank vegetation are abundant. Calm water that will not dry out and deep pools in rivers and streams are preferred.

This lobster-like freshwater species was introduced from America in 1975 to be farmed for food, but quickly escaped and spread rapidly through Great Britain. Growing up to 15cm long, these beasts are aggressive and breed faster than the native species. Since its arrival it has driven the white-clawed crayfish towards extinction through a crayfish plague fatal only to our native crayfish. It also burrows into riverbanks leading to erosion and increasing flood risk.

This crab takes its name from its large, white-tipped claws, which are covered in soft fur and resemble mittens. It was probably introduced as a food source and released, or arrived as a stowaway. Populations have now established in the River Thames, Medway and Ouse and have been recorded at several sites throughout England and Wales, including the Dee. The mitten crab creates burrows in muddy river banks which increases erosion and silt loading in the rivers and can lead to bank collapse. 

These stripy stowaways landed in Britain’s waterways on the hulls of ships from Eastern Europe and decided to stay. Growing up to 5cm long, these molluscs rapidly form large colonies that attach to almost any submerged hard surface, impeding the smooth running of canal gates and sluices among other things. Zebra mussels have a detrimental effect on the native swan mussel population and on fish populations when they take over spawning grounds.

Killer shrimp are considered to be one of the most damaging invasive species in Europe. Currently found in Cardiff Bay and Eglwys Nunydd reservoir (West Glamorgan) with potential to spread rapidly if not contained. They prey on a range of native animals, fish eggs and even young fish, but often kill prey and leave it uneaten. This alters the ecology of habitats they invade and could cause extinctions. Both shrimps and their eggs can survive for up to 15 days in damp conditions - which is why it's so important to Check Clean Dry.

This highly invasive freshwater weed from South America was originally introduced to Europe for its attractive yellow flowers. It can rapidly block waterways and take over ponds and lakes, forcing out native wildlife and causing flooding. Water primrose is currently only established in a small number of sites in Wales and England. Natural Resources Wales and The Environment Agency are working hard to eradicate existing populations.

Another invasive plant from North America, the American skunk cabbage was introduced in 1947 by ornamental plant collectors who admired its striking flowers. Once established, the plant can spread quickly. It grows up to 1.5m tall and prefers wet ground. Commonly found in wet woodlands, here it forms dense stands, crowding out native plant species and making its presence known by emitting a strong skunk-like odour.

This is probably the best known and most notorious invasive species and was introduced from North America. Floating pennywort can grow up to 20cm in a day - and up up to 15 metres in a single season - quickly covering whole water bodies, and harming aquatic wildlife by blocking out light and reducing the oxygen available in the water. The dense mats choke waterways and drainage systems, impeding water flow and increasing flood risk, while also making it hard for any craft to navigate the waterway.