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Geotextile mats are curbing threats of weed invader in Ireland

Case Studies | October 7, 2009 | By:

A non-native weed is choking off some of best fishing lakes in Ireland and disturbing water supplies in the west of the country.

Biodegradable geotextile mats are providing the solution to tackling one of the most serious problems affecting the Lough Corrib ecosystem in decades. The jute matting has been successful in trials conducted by the Central Fisheries Board (CFB) on the bed of Ireland’s second-largest lake.

And its purpose? Suffocation of the invasive and all-pervasive “curly-leaved African waterweed,” Lagarosiphon major, which has spread aggressively across bays in the lough since it was first detected on the 42,000-acre (17,000-hectare) freshwater habitat in 2005.

Lagarosiphon major is native to southern Africa and is sold as a pond plant in many local garden centers. The submerged plant forms a thick matted canopy that deprives water of oxygen and poses a significant threat to other freshwater species.

At Lough Corrib in far west-central Ireland, the future of an international salmonid angling location is at stake, according to the CFB. Left uncontrolled, the weed destroys habitats, contributes to flooding, and can wipe out angling and other lake-based tourism activities, Byrne said.

As one of the few large alkaline lakes remaining in western Europe, Lough Corrib can still support significant stocks of wild salmonid fish. Some 14 of its habitats and six species are listed on Annex I and Annex II of the EU Habitats Directive, making it a Special Area of Conservation (SAC). During the mayfly-fishing season, an average 16,000 fishing boats take to Lough Corrib’s waters.

CFB scientists say that they are particularly worried now, however, given the “environmental and economic havoc” that Lagarosiphon major caused during a 40-year span in New Zealand.

The weed can grow to 20ft (6m) in depth and propagates by fragmentation. It grows so densely that swans were nesting on it last year. Colonized areas are prime sites for perch and roach spawning. This puts further pressure on the brown trout and salmon fishery for which Corrib is famous worldwide.

Stem fragments of the plant are dispersed by wind, boat traffic, fishing equipment, and waterfowl, according to the CFB, which is providing scientific research support to the Western Regional Fisheries Board (WRFB).

Some 113 infested sites on the Corrib were identified last year—a 12-fold increase in four years. And some observers believe that figure may be an underestimate.

Curiously, the plant’s life cycle in Ireland is unusual, in that it grows rapidly in winter, rather than summer, unlike other areas in Europe. One CFB staffer commented: “The Irish version thinks it is still in New Zealand!”

This has influenced the cutting program currently applied by the WRFB and costing about $853,500 (€600,000) annually.

Cutting during the winter months is proving productive. Last winter, a reinforced workforce harvested the weed full-time, and took the equivalent of 2,640 truckloads out of some 10 infested sites. Deep cutting by trailing knives has focused on Rinerroon Bay, carpeted by the weed last year, which has been cleared of more than 3,306 tons (3,000 tonnes) of the weed.

The geotextile breakthrough has been carried out by the CFB using biodegradable textile matting to deprive the plant of light. Not only does the matting kill the weeds, it permits seeds and spores of native species to grow through and rehabilitate the habitat.

Divers working on the project initially tried plastic geotextiles, but the jute variety is easier to handle and to secure in position. And these actions are essentially “low impact,” as widespread spraying of herbicides across an entire SAC, which is also the main drinking water source for Galway city, would not be practical nor safe.

The scientists say they realize that they cannot “geomat” the entire lake, but intend to target key sections to prevent the weed from spreading to the southern part of the lake.

Ironically, an EU Life program allocation of $2 million (€1.4 million) to fund further research into the weed’s rapid propagation cannot be spent due to the current embargo on staff recruitment in state agencies.

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