Alien crayfish could wipe out native species
Signal crayfish carries 'plague' that has already destroyed stocks in Bruskey River
An invasive crayfish has been named the number one potential threat to Irish wildlife and would push native varieties to extinction if they accessed Cavan's waterways.
A 'Horizon Scanning' workshop attended by 23 international experts identified the top 40 invasive species most likely to arrive, establish and spread in Ireland in the next decade. The North American signal crayfish was considered most likely to invade and cause significant ecological impact.
Brian Nelson, an Invertebrate Ecologist with the National Park and Wildlife Service (NPWS), was among the experts to highlight this potential threat posed to the protected white-clawed crayfish in our waterways.
"They are more tolerant in living in different types of water," the Fermanagh native says of the signal crayfish, "so they become much more common and they have a very serious effect on biodiversity - they eat virtually everything in ponds and they burrow into banks, and cause huge problems in canals in Europe."
He adds: "There are places in Europe where they have invaded and the wildlife, fish and everything just disappears, and you just get a population of crayfish - they end up just eating each other. "
There's no way for non-native crayfish to naturally enter our waterways. They have to be brought here intentionally. A larger species than its European counterpart, the signal crayfish was purposely introduced onto the continent to be reared commercially as a food produce. One of the main threats of them arriving on these shores is through the pet trade. However, the signal crayfish is among five non-native crayfish which have been named in legislation making it illegal to bring them into the Republic.
"We definitely don't want them in because it would mean any possibility we have of recovering our native crayfish populations would be zero," explained Brian Nelson.
Total kill
While that prospect is alarming enough, the north American species poses a second threat.
"They carry what's known as 'the plague'. This is a fungal type organism which lives on the thick shell of he North American signal crayfish and they can survive infection," explains Brian. "Once it gets into European crayfish, it gets into the body of the animal and it kills the animal within about five or six days - and it's a total kill. They can't survive that infection."
Crucially, you do not get signal crayfish that are not infected by the plague.
While the signal crayfish remains a hypothetical risk for Ireland, sadly the plague is a very real problem already. It has been detected in ten waterways across Ireland, including the Bruskey River in Cavan. It was first detected here in autumn 2015, and it has devastated the native crayfish population.
The most northerly site of the river at which detections have been made is near Crossdoney. The river meanders south into Loch Gowna, which doesn't have any crayfish populations. Brian recalls visiting the Bruskey River with the local NPWS staff and observing the infected crayfish still alive, and a few days later was asked to do a piece with RTE Radio.
"I went back to that site and they had actually died in that time. The numbers of our own crayfish can be quite huge - you don't see them because they hide away and tend to come out in night time, but whenever you see them dead you realise how many crayfish there actually are - they can be extremely abundant animals."
Like the Bruskey, the Annalee and Cavan Rivers also run into River Erne but fortunately, according to Brian, "they seem to be okay, but we need to check again this year".
How the plague entered our waterways remains unclear.
"You normally only get the plague where you have the North American crayfish, but plague has arrived in Ireland and we don't have those species."
DNA testing of the Bruskey River strain of plague revealed an interesting fact: "We know the Bruskey strain is different from two or three other infections throughout Ireland - so whatever has brought this into Ireland has been different probably each time, and the suspicion is it's either boats, fish, or fisherman's gear - if you bring in gear that has water from an infected catchment - this could be in Britain or in the continent - that has the potential for bringing the plague into a river. We're certainly not pointing fingers at anybody, we're not blaming anybody, it's an unknown thing."
Guidelines are available at the Biodiversity Ireland website which outline the 'Check, Dry, Clean' protocol - the most effective way for waterway users to prevent their equipment from causing plague contamination.
Infected
Removing the plague from a river takes time due to its relentless nature.
"The organism transmits spores which get carried in the water and so long as they can meet another crayfish within 48 hours then the infection keeps going. The only way we can possibly get rid of that crayfish plague infection is if all our native crayfish die, we get no north American crayfish in that area, then hopefully that will mean that river is now clear of plague and you can put back your native crayfish."
Five years after the plague was detected in Bruskey River, it remains categorised as infected as the authorities have not yet established if the entire population of native crayfish has been infected and died.
The Marine Institute in Galway routinely test water samples for the plague organism. A more definitive test is to put a crayfish in a cage and place it in the river and see if they survive.
"If there's any infection in the water it will reach them," explains Brian.
The Bruskey will be tested again this year with the grim hope that the native crayfish have all died, thus generating a clear result. At that stage the work of replenishing the waterway with new batches of indigenous crayfish can commence.
Of the signal crayfish Brian concludes: "That is really why that species came out at number one: because of its potential to keep carrying the plague infection, you can never recover our native crayfish population."