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Asian clams threaten Lough Derg fish stock

DENSE infestations of up to 10,000 individuals of a notorious aquatic invasive mollusc could seriously threaten large amounts of fish in Lough Derg, an Inland Fisheries Ireland (IFI) research officer has claimed.
Dr Michael Millane, research officer for the EU Life+ CAISIE Project admits the presence of the Asian clam population in Lough Derg is worrying for a number of reasons.
“Experience from other invaded waterways suggests that in Lough Derg, this highly invasive species could form dense infestations of up to 10,000 individuals or more per square metre, which may completely cover large areas of the lake bottom in a thick layer of clams up to 15cm deep. 
“The clams can filter large amounts of lake water and plankton daily, which can disrupt the natural food-web and this, through changes in food availability, ultimately may impact upon the composition of the resident fish population. 
“The presence of large amounts of clams can also block water abstraction pipes and shallow out rivers. In addition, as the Asian clam is content in both lake and river systems, it may cause similar impacts in the inflowing and out-flowing streams and rivers of Lough Derg. In these systems, dense Asian clam infestations have the potential to clog up any Atlantic salmon and brown trout spawning grounds,” he explains.
Mr Millane says, in essence, the clam acts as an “ecosystem engineer” as it can substantial and irreversibly alter the ecological character and habitat of the lake or river that it invades.
As Lough Derg is the most westerly known occurrence of the Asian clam in Ireland to date, he warns it may facilitate the further spread of this highly invasive species into other uncolonised waterways unless strict biosecurity and disinfection procedures are implemented by water users. 
Invasive species biosecurity guidelines have been produced by the EU Life+ Funded CAISIE project.
The Asian clam (corbicula fluminea) is a most unwelcome recent addition to the fauna of Irish rivers and lakes. Considering each clam can produce up to 70,000 juveniles each year, the potential for the enormous expansion of this population is apparent. In late 2010 and in 2011, populations of the Asian clam were recorded in the River Shannon at Banagher, Carrick-on-Shannon and Lough Derg.
The inadvertent introduction and transfer of the invasive Asian clam to uninfested waterways represents a major threat to Ireland’s habitats, native species and internationally renowned fisheries. The ecology of invaded watercourses can be dramatically altered and may become unsuitable for water-based amenity and recreational pursuits. At present, as water temperatures are increasing in our rivers and lakes, Asian clam populations are reproducing and releasing vast quantities of planktonic juveniles into the water.  The microscopic juvenile clams subsequently settle out of the water column, attaching to underwater surfaces using sticky threads. In order to limit the further spread of this highly invasive species, IFI is urging all water users, particularly anglers and boaters, to implement strict biosecurity measures including disinfecting all equipment that has been exposed to or used in waterways when moving from one area to another.                   
Dan Minchin of the Lough Derg Science Group explains the clam filters and breathes at the same time and occupies the soft sediments. It behaves very similar to a cockle and needs a lot of energy to grow and as a result uses a lot of oxygen.
During a hot period, when the colder water separates from the warmer surface waters, Mr Minchin points out the oxygen gets used up and this can result in the lower colder water becoming de-oxygenation, which can lead to “large-scale mortalities”.
The clam has been found in depths of 24m in Lough Derg, occurring within the colder water zone.
“Normally, Lough Derg is well mixed with wind turning the water over but in hot, still weather is when such a mortality event can happen.
“The clam is concentrated in the main water flow down the lake from the Portumna Bridge down to Goats Road and a further population occurs in the Dromineer region.
“I am studying populations this year with some help from Waterways Ireland. In 2011, densities of 3,000 per square metre were found below Portumna Bridge and about 300 per square metre in one region in Dromineer Bay.
“The population is expected to increase and expand to other regions within the lake,” he warns.
Mr Minchin explains the clam has a great reproductive and dispersal ability and there is no practical management method to deal with them once they have formed large populations.
However, he notes people can help in preventing its spread to other lakes by being careful about moving fishing and angling equipment in particular.
“The young stage can stick to equipment with a fine mucus, which can attach to nets including angling keep nets.
“It is suspected that the bags often referred to as ‘stick’ bags by anglers used to hold the keep nets might be responsible for its movements in some regions in Northern Europe and perhaps this may even have happened in Ireland.
“Cleaning and drying out fishing gear is the correct thing to do,” he adds.

 

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