Monday, October 15, 2007
TEN days ago a public meeting was held in a hotel in Galway to discuss the threat posed to Lough Corrib by the invasion of an alien pond weed called Lagarosiphon major.
But the agenda was rather overshadowed by the surpriseannouncement that another alien invader, the zebra mussel, had just been found in the lake.
Zebra mussels are small, striped freshwater mussels native to rivers and lakes in Turkey and the Ukraine. For over a century they have been spreading around the world. Mostly they travel stuck onto the hulls of ships and boats. They are tough creatures that can tolerate journeys through salt water and even survive out of water for days or possibly weeks.
They were first found in Ireland in 1997 in Lough Derg on the lower Shannon. It’s believed that they came in on the hull of a motor cruiser imported from Britain. Since then they have spread over much of the country, despite intensive efforts to contain them.
Particular emphasis was placed on keeping them out of the great western lakes of Corrib, Mask and Carra because of their international importance as wild trout fisheries. Unfortunately this importance also results in a lot of angling boats arriving on trailers during the trout season. This is probably how they got into Corrib.
The infestation is confined to the north western part of the lake in the Oughterard area and the Fishery Board, aided by a study group in NUI Galway, will try and prevent them spreading. It’s unlikely they’ll succeed. Zebra mussels can live for over five years and in this time produce millions of offspring. Typically there is a massive population explosion when they first colonise a waterway and then the population reduces slightly.
When they first arrived in Ireland there was a lot of worry that they would clog up water pipes causing water-cooled power stations to close, posing difficulties for municipal water supplies and affecting motor boats. I have been able to find little evidence of this actually happening so far.
I do know a few boat owners who have been astonished by the number of mussels they found on the bottom of their boats when they had them lifted out of the water. There are a few on my own boat and the owner of a large steel boat based on the Shannon told me he found more than a million on his — though I don’t know who counted them. But this sort of infestation can slow a boat down and affect its fuel consumption.
What is a much bigger worry is the ecological impact of the invasion.
Nobody seems to be quite sure what will happen. Our native freshwater mussels are much larger than zebra mussels and some species are endangered. In Lough Ree I have found the shells of dead native mussels covered in live zebra mussels. I have no proof that the alien species killed the native one but it is possible. All mussels are filter feeders that live on algae. A native mussel that had to compete with a dozen zebra mussels stuck to its shell could have starved to death because all the algae had been filtered out of the water before it got to its own filters. And there could be many other ecological repercussions.
Zebra mussels do some good as well as harm, particularly in lakes, like Lough Derg, that have too much algae. They are also a food source for some species of fish, such as bream. Only time will tell whether the good will outweigh the bad.
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