Cold, plague and extermination: life in our watery wild

ANOTHER LIFE:THE YELLOW-BELLIED SLIDER, Trachemys scripta scripta, is a brightly patterned turtle a little bigger than one's hand whose natural home is in the slow-moving rivers and marshes of Florida. A fisherman's catch of one in the Shannon's Lough Derg this spring supported gossip that such turtles, their welcome outworn as pets, are not infrequently being tipped into Ireland's rivers and canals.

“Wifey can’t stand” is one terse reason for abandonment embedded in the fascinating chat of the online social network Boards.ie. Emigration, moving house, marriage and arrival of human babies are other reasons for offers of cut-price yellow-bellied sliders. Better that, of course, than launching them into the Irish watery wild.

The turtles set adrift will not survive our current winters, a fact for which Ireland’s freshwater biologists are grateful. Between alien zebra mussels, Asian clams, Chinese mitten crabs and half a dozen rampant ex-aquarium waterweeds, our native freshwater life has more than enough to struggle with.

Another taste in aquarium species, however, could threaten Ireland's only type of freshwater crayfish, Austropotamobius pallipes. This is already in sharp decline in most of western Europe, ousted by competition or infection with disease by imported alien crayfish species, so that only Liechtenstein and Andorra, like Ireland, have the white-clawed crayfish (its common name) in any natural numbers. Most of the 10 aliens, from North America and Australia, were introduced by shellfish farmers seeking meatier species with immunity to the plague. Nearly all have now established themselves in the wild in Europe, reducing the native species or wiping them out.

Britain is especially overrun by the signal crayfish, Pacifastacus leniusculus(with red instead of white flashes on its claws). This North American species appears to be almost as nimble on land as in water. It carries the plague fungus and is progressively exterminating the white-clawed crayfish from most English and Welsh catchments; they have quite vanished, for example, from my home county of Sussex. Irish freshwater ecologists fear that, despite defensive efforts, the signal crayfish will enter Northern Ireland through illegal trade and spread through the rest of the island. The risks, including crayfish baits brought in by visiting anglers, are spelled out in a graphic leaflet from the National Parks Wildlife Service (download at npws.ie/publications/archive/ Crayfish_leaflet.pdf). But even more colourful or exotic crayfish are being offered online by the aquarium trade. An outstanding threat among the little "lobsters" now offered online by aquarium suppliers is the attractive marbled crayfish, or "marmorkrebs", an aquarium-bred strain of an American crayfish species, Procambarus fallax forma virginalis. The last word conveys its particular menace. Marmorkrebs is parthenogenetic – its eggs develop without male intervention – and continuously produces young. How many crayfish does a home aquarium need before offloading into the nearest stream or pond? Like virtually all American crayfish, P fallaxcan carry and transmit plague, and it is already naturalised in several places in Europe.

The white-clawed crayfish was probably brought to Ireland from northwestern France to augment the food of early monasteries. The biggest invertebrate in our lakes and rivers, growing slowly to about 12cm, it is now seen as a keystone species, its largely nocturnal appetite regulating the luxuriance of the bigger water plants, such as stoneworts, and populations of aquatic insects, tadpoles and small crustacea. It is eaten in turn, in large numbers, by otters, mink and herons.

Its need for calcium to build its shell through the many moults of a lifetime makes the limestone midlands the crayfish’s chief territory, but limestone bedrock takes it also into the western lakes and even tributaries of the Moy. Here, and in the catchments of the Corrib and Boyne, its protection in special areas of conservation is endangered by the statutory drainage-maintenance operations of the Office of Public Works.

The wide range of shelter used by the crayfish – as young ones, in shady shallows and among plant roots beneath the banks, and as adults, in hideaways among the cobbles of deeper water – creates problems for the OPW and its mechanical bucket-swingers. A fisheries-board report sympathised with its “real conservation dilemma” in trying to avoid severe damage to crayfish populations.

The OPW already has a 10-point environmental guidance scheme in which its machine drivers and crews are trained, designed to spare rivers, their banks and wildlife the destructive reshapings of the past. The fisheries-board report makes many practical suggestions, such as a preliminary banging of the water with the crane bucket to scare the crayfish out of reach, bigger holes in the buckets and longer pauses to let them pour out again, and hunts by the crew among the excavated spoil to find crayfish and put them back.

Eye on nature

I saw a nest of hundreds of caterpillars in a whitethorn bush. What are they?

Brian Nolan, Barna, Co Galway

From your photograph, they are caterpillars of the eggar moth; they spin a web to hide in by day and come out at night to eat the foliage.

I saw a bird like a peregrine that seemed to be hunting dragonflies at Lough Graney, in Co Clare. Do peregrines take insects?

Lily de Sylva, Cloughjordan, Co Tipperary

A peregrine falcon was more likely to be hunting nestlings in birds’ nests at that spot.

I saw a red kite gliding near Howth Summit, presumably the Wicklow bird, and also one near Malin, on the Inishowen Peninsula, probably from Co Down.

Frank Smyth, Sutton, Dublin

Recently I saw two large birds corresponding to the description and images of the red kite. I didn’t know they existed in Ireland. Gillies Macbain, Cranagh Castle, Co Tipperary

Red kite were introduced to Wicklow in 2007 and Down in 2008.

After thundery rain, hundreds of jellyfish were congregated at the outflow of the river into Kinvara Harbour. A few hours later they were all dead.

Pat McMahon, Kinvara, Co Galway

Jellyfish die naturally at this time of year.

Michael Viney welcomes observations at Thallabawn, Carrowniskey PO, Westport, Co Mayo, or viney@anu.ie. Please include a postal address