At a recent meeting of Sustainable Water Network (SWAN) David Lee of Cork Environmental Forum was asked to write a series on Invasive Species in Ireland.
This is his first article – on Zebra Mussels.
An “Invasive Species” is defined as a species that is
(1) non-native to the eco system under consideration and
(2) whose introduction causes or is likely to cause economic or environmental harm or harm to human health.
Invasive species can be plants, animals and other organisms e.g. microbes. Human actions are the primary means of species introductions.
The Zebra mussel is a small, non-native mussel originally found in Russia. In 1988 this animal was transported to North America in the ballast water of a transatlantic freighter and colonized parts of Lake St Clair.
In less than 10 years, zebra mussels spread to all five Great Lakes and into the Mississippi, Tennessee, Hudson and Ohio River Basins. Many inland waters in Michigan are now infested with zebra mussels. Only one lake was infested in 1992: today there are over 100. Zebra mussels are very successful invaders because they live and feed in many different aquatic habitats, breed prolifically each female produces 1 million eggs per year, and have both a planktonic larval stage and an attached adult stage.
Young zebra mussels are planktonic. They are the diameter of a human hair and invisible to the naked eye. Because of their size, they are spread easily by water currents and can drift for miles before settling. Adult zebra mussels are 2 inches in length and attach to hard objects and remain stationary. They often attach to objects involved in human activities, such as boats and boat trailers, and are inadvertently moved from one water body to another by people.
Zebra Mussels in Ireland
It is now generally accepted that zebra mussels were first introduced to Ireland as adults attached to the hulls of boats in 1988 and transported by sea ferry from Britain. Alternative hypotheses include the suggestions that they may have been transported as larvae in ballast water to Limerick docks; they may have been accidentally imported with aquarium fish, aquatic plants or in commercial fish transporting trucks etc.; or they may have been deliberately introduced be somebody who considered that the effects on eutrophicated Irish lakes would be beneficial.
Dr Dan Minchin claims the most likely means for their introduction was from used craft coming from Britain or the Netherlands. These were imported for private use on the Shannon Navigation and almost certainly were established on the lower part of Lough Derg following the immersion of a zebra mussel fouled hull. Larvae from here could be carried 20km downstream to become entrained within the Limerick Docks.
Under the Community Customs Code (EEC1992) used craft could be imported into Ireland without value added tax on entry from January 1993 onwards. In England it became necessary to have a certificate of competence for second-hand boats. This resulted in increased sales of second-hand boats from England to Ireland. These changes created an invasion window and an increase in the quantity of dispersal vectors. It is likely that zebra mussels were able to invade Irish waters due to this combination of events, which had not occurred in the previous 170 years.
Over the period 1997-2001, managers of companies who transport and import boats (transportation of boats using wide load trailers is a specialised service) were interviewed. In addition, locations in Britain, the Netherlands and Germany, where boats had been lifted from the water into trailers and directly imported and re-immersed in Ireland were recorded. Contacts were then made with boat marinas in Ireland likely to receive used imported boats and were selected and examined for the presence of zebra mussels; three boats were examined upon arrival in Ireland and one boat was examined 3 months after re-immersion in Irish waters. Date, origin and destination of importation were noted. Zebra mussels were sampled by scraping a 2x2m area of the hull surface with a 15cm wide steel blade. Shell lengths of living mussels obtained from the first three boats were measured and mussel densities were estimated according to a logarithmic scale. The shell lengths of dead and living zebra mussels attached to the hull of the fourth boat, examined after 3 months, were measured to estimate mortality related to overland transportation and post-invasion growth of surviving mussels.
The potential of zebra mussels to cause economically significant damage through their effects as fouling organisms cannot be understated. Organisms are capable of causing serious economic damage by obstructing pipes, and by damaging submerged structures, industrial equipment, turbines etc. Numerous examples of pier structures, navigational buoys, anchor chains, boat hulls etc., have been observed with zebra mussels which will significantly add to maintenance costs.
The extraordinarily densities up to more than 10,000m on some structures give an indication of the rapidity with which a serious fouling problem can arise in respect of Dreissena. Lock gates and associated water ducts will undoubtedly suffer from this problem, within the Shannon navigation system and associated canal network. The dock gates in Limerick have already become heavily infested, the hydroelectricity generating station at Ardnacrusha was among the first sites at which Dreissena was noted in the Shannon. Also within the Shannon system, other peat burning stations rely on the use of river water for cooling purposes and these in time will experience zebra mussel fouling problems. As the species spreads to other Irish catchments a range of other hydroelectricity, peat coal and oil burning power stations are likely to be affected to varying degrees. Water abstraction by industries, county councils and other local authorities, as well as by small local group schemes or individuals are likely to experience serious problems as a result of zebra mussel infestation.
Zebra mussels affect natural ecosystems both directly and indirectly. The greatest direct impact relates to the mussels feeding behaviour. They are filter feeders and process up to 1 gallon of water per mussel per day. During this process every particle in the water column is removed and either eaten by the mussels or wrapped in mucus and spat out. This feeding ability, in combination with high population densities, rapidly clears the water of even the largest lakes.
Unfortunately, the material removed from the water consists of other live animals and algae that supply food for larval fish and other invertebrates. In response to this changing food supply, populations of some animals have begun to decline. As the lakes clear, the brighter light levels cause aquatic plants to increase in number and size. This increased plant growth can be beneficial to some fish such as northern pike and to yellow perch. However the plants do cause problems for recreational boaters and swimming beaches, can increase taste and odour problems in drinking water supplies, and can block water intake pipes.
Within Europe some notable brown trout (Salmo trutta) angling lakes are found in Ireland. The main fishing period occurs when the burrowing mayfly hatch, other mayfly nymphs have different requirements. Their abundance causes trout to develop a feeding fixation and so are more easily captured. Impacts of zebra mussels on this recreational fishery are unknown. Young trout develop in small streams and rivers and so are unlikely to be affected until they enter
lakes where their behaviour could be modified. Zebra mussel druses and ‘carperts’ provide interstitial refuges and opportunities for detritivores such as amphipods that may become an important food source. Whether the ephemeropteran biomass is likely to change is not presently known.
Increases in water transparency, following zebra mussel filtration, leads to an alteration of the abundance and diversity of invertebrates. However, a greater deposition of enriched wastes is likely to lead to complex interactive processes that could modify certain fish populations. Visual predators such as the pike may have advantages in targeting prey, yet increases in water clarity will promote macrophyte cover in shallow water areas to provide refuges for small fishes.
Pike usually assemble in quiet water to spawn, where eggs stick to aquatic plants. The presence of zebra mussels on vegetation, where eggs are normally laid, may effect successful egg attachment and development. Opportunities may arise for the roach, an avid benthophagic predator with pharyngeal teeth that are well suited to feed on zebra mussels.
Bathing frequency increases in summer when water temperatures may exceed 20degrees this often takes place close to berthing areas where there may be raw sewage discharges. Zebra mussels in paddling and wading depths result in cuts and lacerations. During July 1999 and 2000 approximately half of the members of each bathing group surveyed had cuts. These wounds may be prone to a wide range of infections that could include Wiel’s disease, caused by the bacterium Leptospira interogans.
Possible future scenarios
It is inevitable the zebra mussel will spread to other water bodies in Ireland. To date its expansion has been controlled following information campaigns promoting a shared responsibility with known user groups. The difficulty is to maintain this interest over longer periods of time without having to rely on its discovery in a new lake or river and thereby generate a further public awareness. In Ireland, the species has potential for further spread because of extensive midland limestone deposits, providing suitable water quality. Future spread in Ireland will almost certainly result from leisure related activities.
The presence of uncolonised lakes near to infested areas carries a high risk and angling associations are well informed about this. In Lough Melvin there are three genetic stocks of brown trout: the ferox (Salmo trutta ferox), the gillaroo (Salmo trutta stomachius) and the sonaghan (Salmo trutta nigripannis). These spawn in different parts of the lake, not in rivers (Cabot 1999), with possible reproductive impacts due to zebra mussel colonisation. The whitefish Coregonus autumanalis pollan, in Loughs Derg and Erne, may also have reduced reproductive success as a result of fouling of substrata where eggs are laid.
All available evidence suggests that zebra mussels became established in Ireland following importation of boats with attached living zebra mussels. Subsequent inoculations within the navigation systems indicated that the likely means of dispersal to new water bodies in Ireland were spawning of attached adults on boat hulls and hull scrapings returned to the water. Larval transport by small boats was not considered to be a likely means of spread because volumes of water transported in small boats are small. Macrophytes were found near boat slipways, and these could constitute a source for their spread.
Recreational boats are an important dispersal vector for zebra mussels in Ireland and there is potential for rapid international spread. It is inevitable that the majority of the connected and navigable waterways in Ireland will become and their range will almost certainly expand will any new canal developments or canal restorations wherever the water quality is suitable for their development and reproduction. The reopening of disused canal link has been proposed between the current Erne System and Lough Neagh.
This would almost certainly result in the spread of zebra mussels to Lough Neagh, unless a population becomes established before the canal development is completed due to overland inoculations from boat hulls.
Several vessels with mussels on their hulls have been found in areas where zebra mussels were not known to be established. The main cause for concern is the overland transport of boats to lakes at more elevated levels within infested catchments and to other river systems. Those vessels posing the highest risk are large boats that are heavily fouled, but such movements are relatively infrequent compared with angling boats moved in relation to fly hatches. Recently, information leaflets in English, French and German describing how risks of infesting other waterbodies can be reduced have been distributed to boating and angling centres with the aim of increasing co-operation in managing zebra mussel spread.
Numbers of zebra mussels on boat hulls were probably sufficient to form new populations following spawning in areas where zebra mussels did not previously exist. This may have happened in 1998 in Acres Lake and possibly in Lough Key in 1997 and Lough Ree and Lough Erne in 1996. Vessels most likely to acquire large numbers of zebra mussels were those that remained idle for long periods of time. This would appear to be the case for barges, canal boats, private cruisers and lake-boats. Although zebra mussels were present on boat hulls on the Grand Canal, populations do not yet appear to have become established there.
In order to minimise the spread of zebra mussels to other waterbodies, all boat owners should scrape boat hulls to remove any zebra mussels and hose down the boat with hot water, if available. All bilge water should be drained and all weed removed from the boat and trailer. Outboard motors should be cleaned and boat hulls allowed to dry out if possible. Keepnets should be dried out or washed in hot water before use in other waterbody. Scrapings and water from hosing down etc., should be disposed of elsewhere and not returned to the waterbody where the cleaning is happening.
There seems to be a good level of public awareness about zebra mussels in Ireland, and this may have helped prevent the spread of zebra mussels to other lakes. Voluntary action has been taken by community organisations: in Lough Melvin vessels not normally resident in the lake were not allowed to be launched without a receipt showing the boat had been steam-cleaned or passed through a car wash. However zebra mussels may spread to other waterbodies as a result of illegal eel fishing activities. Nets are normally recovered at night and may contain sticks or weed with zebra mussels attached to them, which may not be noticed. These nets are moved between water bodies and may act as a dispersal vector for zebra mussels.
Invasive Species and European Directives & Conventions
Invasive non-native (alien) species are included as part of the assessment of pressures and impacts that will determine ecological status for the Water Framework Directive (WDF). It is widely recognised that invasive non-native species have the potential to compromise the achievement of good ecological status (GES, as per WFD) for waterbodies, and the conservation objectives to maintain or restore, at favourable conservation status for natural habitats and species as per the Habitats Directive. The obligation on the Irish Government to protect Natura 2000 sites and features, under the Habitats Directive, offers a further important driver in the fight against invasive non-native species.
As a signatory to the Convention on Biological Diversity (CBD) the Irish Government is required to as far as is possible and appropriate, prevent the introduction of, control or eradicate those alien species which threaten eco
systems, habitats or species. Signatories must report on what has been done to implement the Convention and how effective this is in meeting the objectives of the Convention.
Hoping this will be of interest to you and if anybody has anymore information I would be glad to add it to my document.
I wish to thank Dr Dan Minchin, for his help in putting this article together and The Shannon Regional Fishery Board for use of their website.