Another Life: The plague threatening Ireland’s native crayfish

My trout-angling father-in-law, Seamus, pointed them out to me once from his boat in the shallows of a bay on Lough Mask: miniature mud-coloured lobsters scrambling away into darker shadows of their own. That was getting on for half a century ago. So I am relieved to see that, after several surveys by trapping, snorkelling and stone-turning scientists, someone spotted them in Mask again in 2006.

The day we're without our native freshwater crayfish, the white-clawed Austropotamobius pallipes, we'll have lost a species near extinction in most of Europe.

There are nearly 600 kinds of freshwater crayfish in the world, which might seem more than enough to be going on with. But about a third are now threatened by human development: dams in Mexico, urban sprawl in the United States, drought from man-made climate change in Australia.

The threat to Ireland's crayfish, long feared and now explosively present, is a fatal fungal disease brought in, via Britain or Europe, from America. Aphanomyces astaci is what killed more than 600 crayfish last month in a stretch of the River Bruskey near Ballinagh, in Co Cavan. A previous outbreak in the 1980s wiped out most stocks in the Boyne Valley, several midland lakes and some tributaries of the Shannon.

The crayfish plague is carried by North American and Australian crayfish species imported into the UK and Europe, either as meatier, more marketable farmed food or as more colourful pets for home aquariums. None had to be released into our rivers to bring the disease to Ireland: just its spores were enough, washed off from boats, waders, fishing baits or landing nets of visiting British or Continental anglers.

The chief villain is the American signal crayfish, Pacifastacus leniusculus, waving red patches on its claws. Along with carrying the plague, it has outcompeted and exterminated the white-clawed crayfish in a great many English and Welsh catchments. It was imported to the UK via Sweden in the 1970s for aquaculture, and as the dramatic impact of its escapes became clear, the first warnings about introductions were heard in Ireland.

These came notably from Julian Reynolds, a freshwater ecologist at Trinity College Dublin, who had been lobbied by farmers seeking to import signal crayfish into Co Louth. Authorities north and south have joined in resisting any introductions, but the aquarium species, such as the attractively marbled "marmorkrebs", another plague carrier, are available online.

More than 30 years of special study has made Dr Reynolds an international crayfish authority. Apart from Liechtenstein and Andorra, Ireland is the only European country where the white-clawed crayfish is not threatened by alien imports, and the "keystone" role of native populations has made their survival an international conservation issue. Reynolds joined some 40 scientists in a global picture of decline published earlier this year.

A dramatic example of invasive competition has been that of the red swamp crayfish, Procambarus clarkii, originally from Mexico, intensively farmed in the United States and now on every continent except Antarctica and Australia. Agile and adaptable, it was released into the marshland of southwest Spain and has since expanded through the whole Iberian Peninsula and across southern Europe.

Its reproduction is so prolific that the female tucks four times as many eggs under her tail as are produced by the average 12cm Irish crayfish. By 2006 it was listed among the top 10 of Europe’s most invasive species, with the highest number of different ecological impacts. It not only eats everything in sight – fish, snails, tadpoles, insects – but its burrowing habits cause banks to collapse. It also carries the crayfish plague.

Ireland’s crayfish does most of these things, too, but more selectively and in balance with its watery ecosystem. Its grazing of bigger water plants helps to bring more sunlight into the depths of lakes, and it uses shelter under cobbles and boulders as readily as burrowing into banks. All this has matured and evolved over the 800 years or so since, as is strongly conjectured, the white-clawed crayfish was brought to Ireland by 12th-century French monks with Friday’s dinner in mind.

What has suited it especially on this island is the limestone of so many lakes and lowland rivers, offering calcium for its exoskeleton.

Its general picture today is similar to that of England’s 50 years ago, but a general decline continues. Of 26 lakes in the Republic where crayfish had been recorded, 15 still have stocks today, with some recovery from the 1980s plague

So far unquantified is the toll on crayfish taken by Ireland’s invasive American mink, which, together with herons, otters and pike, must be reckoned as regular predators. Indeed, taken with crayfish plague, grey squirrels and sudden oak death, imports born in North America have not always been the best.