Exotic zebra mussels clogging up waterways

The dreaded zebra mussel (Dreissena Polymorpha), which was introduced into Ireland only a few years ago, has spread the whole length of the Shannon waterway.

The small shellfish, shaped like marine mussels, attach themselves to hard surfaces and because they congregate in such large numbers - as many as 100,000 per square metre - they can sink boats or buoys.

But the main problem created by the creatures, which grow to between 15 and 25 mm, is that they clog intake pipes, drains, screens, waterworks, power plants, hatcheries and other facilities.

In the Great Lakes system in North America they have forced the closure of power plants and caused great damage to lock gates. They also clog up the intake valves of boat engines which can cause overheating, damage to equipment and increased maintenance costs. However, the creatures do have a positive side, and for the first time in years they have prevented the growth of green algae on Lough Ree.

According to Shannon Fisheries Development inspector, Mr Matt Nolan, each mussel filters a litre of water a day and their presence has greatly improved the quality of the water on the Shannon lakes.

"But we are very concerned that they will change the whole ecology and aquatic system of the river and will most certainly do damage to other forms of life in the system.

"Unfortunately, they do not take out the phosphates from the water but they do filter it and that is why we have had no algae on the lake this year," he said.

"It is estimated that there are at least one trillion zebra mussels in Lough Ree at the present time, so we are having to keep a very sharp lookout for them.

"We have a feeling that they may be colonising the base of the lake and we do not know the long-term impact this will have on the system," he said.

He added that billions of mussels had been located in Lough Key near Boyle and it seemed inevitable they would spread to every system in the country.

Earlier this week he took The Irish Times on a boat trip just south of Athlone where he showed evidence of the spread of the mussels.

A steel sheet which had fallen into the water near the Hodson Bay Hotel shoreline, was coated with the mussels, which reproduce within a year.

The mussel releases thousands of microscopic young which swim weakly in the water and are moved downstream by currents. They settle after about three weeks and grow into the shape of a miniature adult.

He said the mussels live from three to five years and rapidly increase their numbers, so it is important not to assist their spread in any way.

"They arrived from the Black Sea and Caspian Sea attached to boats and ships following the building of canals and had established themselves in Britain by 1830.

"In 1986 they were introduced into North America in ships' ballast and they are causing very serious problems there now for the authorities," Mr Nolan said.

"Originally, it was thought they came here in ballast water from ships coming to Foynes but now we are almost certain that they were introduced here on the hull of a private boat brought in from England and put on the canal in Lowtown, Co Kildare."

Recently, he said, Foras na Mara, the Marine Institute, had issued guidelines to anglers and boat owners on how to prevent the spread of the mussels and the damage they can cause to boats.

"For instance, if a boat is left permanently in the water, these mussels can actually sink her by sheer weight of numbers. It has happened elsewhere," he said.

"We are recommending to anglers and boat owners to beach their boats at least three times a year and remove the mussels by using a paint scraper or pot scourer. Ideally, the boat should be washed down with hot water but cold water is better than nothing and the same should be done with boat trailers. Hull waxes and silicone anti-foulants are effective against them."