Improved water clarity on Lough Derg, the largest of the Shannon lakes, and the fourth largest in Ireland, has certainly been dramatic during the last couple of years. This improvement has been attributed to the presence of Zebra mussels (Dreissene polymorpha) - the distinctively patterned, non-native, freshwater bivalve molluscs similar in form to marine mussels. They are also tiny, about the size of a 10p piece. Their chance discovery during the summer of 1997 in Lough Derg came about during studies of the lake's coarse fish population.
At that time, it was decided the mussels had been already present, undetected, for at least two to three years. With Lough Derg's waters appearing cleaner, some anglers decided the Zebra mussels, filter feeders with the ability to settle in mass concentrations, were welcome visitors. There was also the feeling that some well-meaning zoologist may have tipped a bucket load of them into the lake and the increased water quality was the result.
According to Dr Kieran McCarthy of the Zoology Department at the National University of Ireland, Galway, the exotic zebra mussel, with its range of striking shell patterns, is not quite the crusading conservationist we may have thought. It is, instead, "a notorious fouling organism with potential densities of hundreds of thousands per square metres capable of causing serious economic damage by blocking pipes", as well as damaging submerged structures, industrial equipment, and turbines. Even more problematic are the resulting serious changes in lake ecosystems, such as the removal of green algae. This holds profound consequences for wildlife, fish species and aquatic plant life.
Zebra mussels are highly fertile. A mature female may produce from 40,000 to more than 1 million eggs per year. Colonialising at speed, they are also extremely tenacious and invasive. Their discovery in North America in 1988 and apparent invasion of all five of the Great Lakes and their connecting waterways including the Erie Canal, the Hudson, the Mississippi and Ohio, was greeted as an expensive ecological and economic disaster in the making. These oval-shaped, angled animals attach themselves to a wide range of man-made structures, as well as other animals, such as the much-larger swan mussel (Anodonta cygnea) by secreting durable elastic strands, or byssal threads.
Native to the Black, Caspian and Aral Sea drainage basins of eastern Europe and western Asia, the zebra mussel has spread to much of central and northern Europe during the past 200 years through the development of canal networks. Another contributory factor was increased shipping due to the 19th century industrial revolution. Zebra mussels were first recorded in Britain in 1824 at London docks and are still absent from areas not serviced by canals.
Their arrival in Ireland is certainly linked to canals. Many were attached to the bottom of boats introduced into the system - such as British boats purchased by Irish buyers, or boats brought to Ireland for holiday cruising. According to an Irish Naturalists Journal report in 1997, boats inspected that year at marinas in Athlone, Coosan Bay in Lough Ree and Carrick-on-Shannon all revealed the mussels were being rapidly dispersed by holiday cruisers travelling within the Shannon's navigable waters and associated canals.
Improved water quality is an advantage, albeit superficial. Although bream and eel in Lough Derg are known to feed on zebra mussels, considering the mussel numbers, this is hardly a population control. Of great concern is the mussel colonisation of the spawning gravels used by the endangered pollan, a rare native Irish fish and glacial relict that may well finally become extinct. Last year I wrote about the endangered freshwater pearl mussel (Margaritifera margaritifera). It is a native Irish animal, possibly the most ancient and certainly the most long-lived. The zebra mussel is not native and its rampaging presence has already infiltrated the Shannon navigation system and the Erne waterway.
"Such bio-invasions are increasingly common," says McCarthy. "Once they arrive in a particular lake, such as Lough Derg, there is little we can do to control them. However, increased public awareness can limit their spread to other river systems. Anglers have helped prevent the spread of these mussels to the west of Ireland lakes."