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Zebra Mussels and divers

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Zebra Mussels in Hand

Many divers and boat users will be aware of the arrival of Zebra Mussels into our freshwater rivers and lakes. These light and dark striped shell bivalves have gained a ferocious reputation for destroying host environments and affecting freshwater ecosystems. As divers, the transportation of Zebra Mussels is something we should be consciously aware of when transporting boats and equipment between freshwater systems.

Zebra Mussels are native to the Aral and Caspian Sea area of Eastern Europe and were first described by the Russian Zoologist Pallas in the 18th century. Since then this little thumbnail sized mussel has managed to spread to most freshwater waterways of Europe, reaching Britain in 1824 and the Great lakes of America in the later 1980’s. In this time they have wreaked havoc in the colonised waterways, blocking water abstraction pipes for both boats and treatment plants and descimating ecosystems.

In Ireland, Zebra Mussels did not seem to make an appearance until the 1990’s, when they were noted on the Lower Shannon close to Lough Derg. Whilst their exact method of introduction is unknown, it is generally believed that they were transported either in the bilge of second hand vessels brought into Ireland from Britain or Continental Europe

Since their arrival into the country they have caused extensive damage. They have led to the decline of the native mussel species as well as reduced spawning numbers of fish species. Zebra Mussels attach themselves to hard surfaces from where they filter phytoplankton. The surfaces they attach themselves to vary from rocks, to sand to timber to plastic to native mussels. When attached to native mussels they adhere to the outside of the shells, preventing them from opening and closing, thereby denying them the opportunity to feed or respire. In Ireland, Zebra Mussels have no known parasites, predators or disease and so they reproduce unaffected by outside threats. The colonisation of many areas by Zebra Mussels has led to the removal of the native mussel species in that area and their covering of large areas of riverbed, up to 700,000 in one square metre has affected the ability of salmonoid species to spawn.

A number of cases have also been recorded where zebra mussels have damaged boat engines, bocking water intake pipes, leading to engine overheating. In infected areas, it is generally accepted that zebra mussel infestation tends to affect stationary material more easily than moving machinery. Colonisation of boats tends to occur on stationary boats more easily than frequently operated vessels.

Zebra Mussels have several features which make them optimum invader exploiters. These include byssus threads which they use to attach to surfaces as well as high production levels, up to 40,000 larvae are produced by each female. The larvae are dispersed into the water column and can be transported via water currents and other sources such as boat hulls or bilge water. Zebra Mussels can survive for up to one month out of water and can survive in low saline waters. These adaptions make the zebra mussel threat and possibilities of their transportation by divers a real possibility.

To date, Zebra Mussels have colonised large areas of the Shannon-Erne waterway and a number of inland lakes but they have not been found in any of the Great Western Lakes (Corrib, Mask, Carra, Conn and Cullen) until very recently. Extensive education programmes have generally been credited with this, however the ever present threat of introduction should be brought to the attention of all aquatic users.

The versatility and adaptability of Zebra Mussels is something of which all CFT divers should be aware. This is especially relevant when boats and equipment are being used in freshwater environments such as rivers and lakes for both recreational diving and Search and Rescue operations. As previously stated Zebra Mussels can survive out of water for up to one month. Both the bivalve and its larvae can survive in BCD’s, regulators, attached to the hulls or even in the hulls of boats. The only effective method of removal of these bivalves is through thorough cleaning of all equipment used in infected waters. Immersion in salt water can also be an agent for removal but it should not be considered as a primary removal form as they can survive in low saline solutions which can be present in boat hulls. The thorough cleaning of all equipment immersed used in infected areas in waters in excess of 40°C., the flushing of engine coolant systems and the draining of the bilge all boats should be something that all clubs and users should consider as standard practice when undertaking activities in infected areas. Obviously this cleaning should be carried out away from the river/lake shore and from waterways leading to unaffected streams, rivers and lakes. In addition to this, any weeds should be removed as they may contain adult Zebra Mussels or their larvae.

In an effort to prevent the spread of Zebra Mussels into the Western Regional Lakes, a campaign called the Western Regional Zebra Mussel Control Initiative has been initiated. It aims to educate all aquatic users of the threat of the spread of these organisms and provide them with information on how to combat the threat. As part of this intiative, Dr. Aoife Thornton has been appointed as Zebra Mussel Education Officer. She is available to discuss divers questions relating to the preventative measures which can be employed to minimise the risk of further infections and can be contacted during office hours at 091-509063 or by email athornton@galwaycoco.ie

Zebra Mussels colonisation of boat ladder.

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