A non-native weed species is choking off some of our best fishing lakes – and disturbing water supplies in the west, writes LORNA SIGGINS
A SIMPLE BIODEGRADABLE geotextile mat may provide the solution to tackling one of the most serious problems affecting the Lough Corrib ecosystem in decades. The jute matting has been highly successful in trials conducted by the Central Fisheries Board (CFB) on the bed of the State's second-largest lake. And its purpose? Suffocation of the invasive and all-pervasive "curly-leaved" waterweed, Lagarosiphon major, which has spread aggressively across bays since it was first detected on the 17,000- hectare freshwater habitat in 2005.
Lagarosiphon major(African curly-leaved waterweed) is native to southern Africa, and is sold as a pond plant in garden centres across the State. The submerged plant forms a thick matted canopy which deprives water of oxygen and poses a significant threat to other freshwater species.
In Corrib’s case, the future of an international salmonid angling location is at stake, according to Dr Ciaran Byrne, newly appointed chief executive of the CFB. Left uncontrolled, the weed will destroy habitats, contribute to flooding and will wipe out angling and other lake-based tourism activities, Byrne says.
As one of the few large alkaline lakes remaining in western Europe, Corrib can still support significant stocks of wild salmonid fish. Some 14 of its habitats and six species are listed on Annex 1 and Annex II of the EU Habitats directive, making it a special area of conservation.
During the mayfly-fishing season, an average 16,000 angling boats take to Lough Corrib’s waters.
Dr Joe Caffrey, CFB chief scientist, is particularly worried now, however, given the "environmental and economic havoc" that Lagarosiphon majorcaused over a 40-year period in New Zealand.
“If there isn’t action, very quickly we will be saying Corrib RIP,” he says. “And it will have an enormous impact on watercourses throughout the State.”
The weed can grow to six metres in depth and propagates by fragmentation. Such is its density that swans were sighted nesting on it last year. Colonised 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, by boat movements, angling equipment and possibly wildfowl, according to the CFB, which is providing scientific research support to the WRFB.
Some 113 infested sites on the Corrib were identified last year – a 12-fold increase in four years. Caffrey believes that even this 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. “The Irish version thinks it is still in New Zealand!” Caffrey says.
This has influenced the cutting programme currently applied by the WRFB and costing about €600,000 annually.
Cutting during the winter months is proving to be far more productive. WRFB chief executive Dr Greg Forde deployed up to 15 per cent of staff last winter to full-time harvesting and took the equivalent of 2,640 truck loads 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 3,000 tonnes of it.
THE BREAKTHROUGH HAS been the research carried out by Caffrey and his team on use of biodegradable geotextile matting to deprive the plant of light. Not only does the matting kill the offending weed, but it permits seeds and spores of native species to grow through and rehabilitate the habitat.
Divers working with Caffrey initially tried plastic geotextile, but the jute variety is easier to handle and to secure in position.
“The beauty of this is that it has a low impact – we can’t go spraying herbicides across an SAC which is also the main drinking water source for Galway city,” he says.
“The matting has literally smothered the lagarosiphon,” Caffrey says. “Most interestingly it has also been colonised by some of the smaller, native plant species which returned once the alien weed was out of the way – growing through the fine pores of the matting.”
The scientists realise that that they cannot “mat” the entire lake bed area, but intend to target key sections to prevent the weed spreading to the southern end of the waterway.
Ironically, an EU Life programme allocation of €1.4 million to fund further research here into the pondweed’s rapid propagation cannot be spent due to the current embargo on staff recruitment in State agencies.