Alien invasion threatens our waterways

CONCERNS voiced by anglers and conservationists about pondweed — both the Canadian and African varieties — in some of our most famous lakes highlight the impact of many invasive, alien species in Ireland.

The fast-growing pondweed has become a nuisance in Lough Lein in Killarney and Lough Corrib in the west, among others, and is just one of hundreds of unwelcome flora or fauna species that have taken root here.

Mink and zebra mussels are other unwelcome invaders which can have devastating consequences for other wildlife.

The campaign to stop the spread of the zebra mussel to inland waterways is being stepped up.

Giant hogweed, found on the banks of rivers and streams, is another example of the estimated 800 invaders which have come here in the past 200 years. About one-third of these are now part of the established Irish flora and relatively few pose a threat to native vegetation. However, some, including the rhododendron, can have a very damaging impact on native plants such as oak and yew trees. Bodies such as the Central Fisheries Board are becoming more worried about the effects of pondweed, which more commonly occurs in lakes, ponds, canals and slow-flowing water on the other side of the Atlantic.

Pondweed was introduced to Ireland, in 1836, as a fragment on alog imported from Canada. It spread rapidly to Europe and is a pest in many parts of the world. Some people use it to decorate garden ponds.

Anglers can no longer cast in parts of Killarney’s lakes because their lines and hooks get caught in the green weed, which continues to spread from the Ross Castle shoreline.

Harvesters have been used to drag the weed out of Lough Corrib but one option under consideration in Killarney is the use of pesticides, though this would have to be handled with extreme care, given the sensitivity of the environment in question.

Harry Clifton, who knows the lake intimately, believes the weed was at its worst in the winter just passed. He fears it will choke the whole lake if it continues to expand at the present rate. According to those who should know, continued cutting will weaken the thick, green pondweed and may lead to its disappearance. There are also several methods of mechanical control — removal by hand, raking or dredging.

Even more serious problems are caused by the zebra mussel, which not only destroys freshwater ecology but also blocks water abstraction pipes and damages boat engines.

The thumbnail-sized mussels, with easily identifiable stripes, are native to the Aral and Caspian seas, but can be found in many freshwater, inland waterways in Europe. They reached Britain in 1824 but did not get to our shores until the mid-1990s. It is thought they entered the Shannon on boats brought in from Britain. Many lakes here are now at risk of invasion by this pest, which causes major changes to the food chain in waters.

Dr Aoife Thornton, a zebra mussel education officer in the western region, says ecological changes are already obvious in the Shannon/Erne waterway where native mussels can’t feed because zebra mussels attach to the outside of native mussel shells. “The native mussels are prevented from opening and closing their shells, so they starve to death,” she says.

One of Dr Thornton’s tasks is to make boat owners, anglers and wildfowlers aware of the threat posed by the zebra mussel. At first, the aim was to protect all vulnerable western lakes but the campaign is now gaining momentum nationally. Zebra mussel warning signs have been erected around all the great western lakes asking boat owners to ensure no mussels are attached to the hull of their craft.

“Once zebra mussels fall off a boat hull, for example, a new site of infestation may result,” Dr Thornton has written. “They can survive for up to one month out of water and are readily transported across land when attached to boat hulls and equipment, or when contained in bilge water that is allowed to drain out into an uninfested waterway.”