Scientists working for the ESB and University College Galway have found zebras on the lower reaches of the River Shannon - and they're none too happy about it. The zebras in question are not striped mammals but zebra mussels (Dreissena polymorpha) which have turned up in fairly large numbers in Lough Derg where the fish stocks were being investigated. Zebra mussels have been responsible for causing major problems at lock-gates, piers and buoys, and serious difficulties for hydro-electric schemes because they occur in extremely high density when they anchor themselves with threads to solid objects.
According to the ESB's Fisheries Conservation Report, the zebra mussels - which were restricted to the Black, Caspian and Azov Seas, until the 19 century - appear to have been introduced into the Shannon system three years ago.
In 1986, the mussels were introduced accidentally to the North American Great Lakes via ships' ballast water and since then they have spread dramatically throughout the system.
This spread is due to their high reproductive rate of around 30,000 eggs per female and the fact that these can be easily spread at the planktonic larval stage.
The ESB report said that studies of the zebra mussels have shown they can dramatically alter lake ecosystems and cause changes in algal communities, zoo-plankton and bottom dwelling organisms associated with water clarity.
"Some of the ecological changes are similar to those associated with reversal of eutrophication. However, other changes in the lake ecosystems have consequences for fish, water fowl and aquatic life in general," said the report.
"Zebra mussels act as fouling organisms on a variety of manmade structures such as lock-gates, piers, buoys and boats. Water intake structures for municipal or industry supply networks and hydroelectric plants in both Europe and North America have been seriously affected by these bivalves," it said.
"Hence the urgent need for information on the species and for development of control measures appropriate to local circumstances.
"Generating station managers in the Shannon basis will be advised of the possible consequences for cooling water systems, etc. An initial report on the species will be produced during autumn 1997," the report concluded.