They come from beneath the water

The arrival of invasive exotic species of wildlife in Irish rivers, canals and lakes is provoking concern among experts because of the threat they pose to native species and habitats

THE PHRASE"invasive species" conjures up images of slimy creatures from science fiction B-movies terrorising a startled population. Think of those 1950s classics: Invasion of the Body Snatchers, It Came from Beneath the Seaor Day of the Triffids.

While the reality may be less sensational, a range of alien species is posing significant threats to Irish wildlife with, as yet unknown, long-term consequences. The latest example is the Asian clam – a native of countries including China, Korea, Japan and the Philippines – which was discovered in Irish inland waters for the first time in April and which has “terrified” the State’s fisheries officials.

The chance find was made by Pascal Sweeney, an environmental consultant, while working on the River Barrow at the village of St Mullins in south Co Carlow. He alerted Central Fisheries Board officials who dispatched a team of divers to the river. What they found has alarmed wildlife experts.

The board’s Dr Joe Caffrey, a leading expert on “invasive species”, said the exploratory dives revealed “a well-established colony” and recovered “hundreds of samples”. He fears that the clam has now gained a foothold in the Barrow and may already have spread to the other major rivers in the southeast, the Nore and the Suir, and even further afield.

Why does this matter? Dr Caffrey says the Asian clam poses a threat to salmon, trout and other native species, such as the freshwater pearl mussel, by upsetting the food chain in Irish rivers and fatally soiling the gravel beds used by fish as spawning grounds.

Controlling the spread of the “invader” is very difficult. The innocuous-looking mollusc has a yellow-green shell, grows to a length of about 5cm and breeds prolifically – a self-fertilising adult can produce up to 100,000 offspring in just a few years.

Ireland isn’t its only target. The first reported sighting of the Asian clam in Europe was on the River Rhine in the 1980s and it is now widespread throughout the Continent. In the US, where the clam was first spotted in the 1930s, it has been blamed for clogging up pipes in nuclear power plants resulting in multi-million dollar repair bills. And the little creature’s potential to cause havoc in cities by “re-awakening” inside the walls of concrete buildings could provide the script for a Hollywood horror film. The US Geological Survey has reported that where river beds are dredged for sand and gravel for use in making concrete, the Asian clams “have incorporated themselves in the cement” and then burrowed to the surface as the cement starts to set – thereby weakening the structure of buildings.

UNLIKE SOME TYPESof shellfish, the Asian clam lacks the ability to cling to the hulls of boats, so mystery surrounds its arrival in Ireland. Colette O'Flynn, an ecologist with the National Biodiversity Data Centre, which monitors all wildlife species, initially believed that it might have been deposited in the Barrow by someone "dumping" the contents of a fish tank. Imported shellfish and exotic plants are used to decorate the interiors of tropical fish tanks and are sometimes disposed of carelessly by owners. She appeals to members of the public with garden ponds and aquariums not to dump unwanted material in any stream, river, canal or lake but to instead "compost the material at home or else send it to a local landfill site to be dealt with "in an environmentally friendly way".

But Dr Caffrey believes the Asian clam, which is edible, may have been introduced to the river “deliberately” as a food source. He cites another example of “invasive species” being deliberately introduced to Irish waters, saying that earlier this decade, visiting English anglers intentionally introduced the chub (a non-native, freshwater fish) to Irish rivers. The uninvited guest quickly became a predator of native fish and forced fisheries’ officials to undertake a major eradication programme.

According to the Central Fisheries Board, “invasive species” are defined as plants and animals that have been introduced from outside their natural habitats and are non-native to an area. They can “take over the habitat of native Irish species very quickly, resulting in the decline or elimination of these native species, cause economic and environmental harm and may even adversely affect human health”.

The problem is global and its causes diverse – species can now easily “jump” continents due to increased travel and trade links, the growth of international shipping and sometimes sheer greed or carelessness. According to a recent UN report, there are now almost 11,000 varieties of “invasive species” in Europe.

Other species threatening Irish inland waterways include the Chinese mitten crab, which first arrived in Ireland at Waterford Estuary in 2005, feeds on native fish and can cause the subsidence of river banks. Another is a tiny prawn, the bloody red shrimp, normally found in the freshwater and brackish areas surrounding the Black and Caspian seas, which was discovered in Lough Derg in the summer of 2008.

SCIENTISTS AREalso concerned about alien plants – which can cause native species to be "ousted" and "dramatically" change the ecology of waterways, riverbanks and lake shores. Imports such as the water primrose may look good in an ornamental pond but can have "severe negative impacts for local biodiversity" if released into the wild.

And human health can also be directly threatened. The giant hogweed, brought back to Ireland from the Caucasus and replanted in the gardens of the landed gentry during the Victorian era, later “escaped” and is now growing wild along many river banks. Its sap can cause severe burns and permanent skin damage.

Even a single “invasive species” can lead to waterways becoming completely choked and impede navigation, angling and other water recreations.

One of the biggest headaches facing the authorities is to tackle the infestation of Lough Corrib by curly-leaved waterweed, which has caused considerable damage and changes to the ecosystems of the lake.

What can be done? Clean-up operations – such as those on the Corrib – are complex, time-consuming and expensive. The best cure is prevention. The authorities have appealed to river- and lake-users to immediately report sightings of unusual plants, fish and other unfamiliar creatures to fishery officials. They have also reiterated their plea to the owners of garden ponds and aquariums “never to dispose of the contents in the wild”.

A project co-funded by the EU and the Department of the Environment, Heritage and Local Government, called “Control of Aquatic Invasive Species in Ireland” (CAISIE, Life+) is being run by the Central Fisheries Board. The aim is to try to halt the arrival of new “invasive species” and limit the damage being caused by those already here. The programme includes developing and implementing new control and eradication methods; heightening public awareness through public meetings; and overhauling legislation.

However, the news isn’t all gloomy. One invasive species, the “Fairy fern” was introduced to Ireland as an aquatic plant for ponds and aquariums but escaped into the wild and spread rapidly. The sheer density of its growth impeded the passage of boats and threatened the viability of the Barrow as a navigable waterway. But the Central Fisheries Board has reported that: “thankfully this particular species died off spontaneously, perhaps due to grazing by native aquatic beetles”.

See caesie.ie