Signal warning about the dangers of alien crayfish

As a boy, writes Damine Enright, I enjoyed catching crayfish in the River Suir near Thurles, Co Tipperary. River-prawns, I called them. I was in short pants at the time.

A young Hereford fattening on peppery watercress in the River Ver, Hertfordshire. Will pepper steaks result?

While my parents enjoyed the fairways of the local golf course, I waded the river and gathered for myself an education in the water birds, fish and frogs that teemed in the Suir and the wetlands along the banks.

Health and safety wasn’t an issue. My parents would drop me off at the gates to Lady’s Well, and collect me at nightfall. They didn’t seem to worry that I’d drown — at times, they might well have thought it would be a relief. Meanwhile, with rolled-up pants legs, thigh deep in the Suir, I witnessed gravel-beds blanketed with migrant black lampreys heading upriver, found stone loaches beneath the stones, eels galore, gudgeons and crayfish.

Since then, my only sighting of these freshwater crustaceans has been in rice-field ditches in Spain, and in UK rivers. When I lived in London, I often took my children on Sunday summer outings to the River Chess in Hertfordshire. There, splashing about in the shallows, we would sometimes capture the creatures with our bare hands. We released them but, in fact, they make a tasty meal. That was in the 1970s and 80s. Then, there was only one crayfish species in the Chess, the native White-clawed, Austropotamobius pallipes.

Last summer, visiting the same gravel-bottomed river, I sat on the banks and, this time, watched my grandchildren do the wading. Amongst the crays they caught (in relative abundance) that day, not a single one had the pale claws and sharp shoulder-spine that I remembered. The newcomers were bigger than those my sons and daughters, or I, remembered. The claws had bright red undersides and a distinctive white stripe between ‘finger’ and ‘thumb’.

I’d heard about an American species colonising British rivers, wiping out the natives with a fungus-like disease to which they are, themselves, immune, and by their larger size, and sheer aggression. I learned that, by the mid-1990s, in the case of the pretty Chess and many other chalk-streams, their arrival had spelt the extinction of entire white-clawed crayfish populations. American signal crayfish (Pacifastacus leniusculus) had taken over many rivers.

Changes can happen fast in the natural world. Signals, introduced to the UK in the 1970s for the restaurant trade, and briefly farmed in the 1980s, climb well and can walk long distances overland. Escapees from the farms quickly spread throughout England, Wales, and north to Scotland, annihilating their indigenous cousins, depleting the invertebrate river-life upon which fish feed, and damaging riverbanks.

They have since been proscribed; environmental agencies and angling associations advise that they be killed on sight. That evening, my family and I did the environment a favour. After leaving them in the freezer to numb up, we cooked and ate the catch. They were meaty and like prawns, but not salty.

Following the devastation, signal crayfish have wrought on water life across Europe, there is now serious concern that they will turn up in Irish rivers. Their most likely route is through Northern Ireland; Britain has not only the signal variety, but six other alien crays, and each carries a different lethal fungus. Northern Ireland environmental bodies, such as the Lough Neagh Wetlands Action Plan, monitor lakes and rivers. So far, the signal crayfish has not been found.

Ireland is now the only country in Europe that does not hold alien crayfish species. The fact that significant populations are found in isolated lime-rich lakes reduces the threat. Such populations are rarely found in the UK or Europe.

Ironically, our only outbreak of the killer disease occurred in relatively isolated Lough Lene, Co Westmeath, in 1987 but did not result from the arrival of American crays; rather, the fungus, related to potato blight, was probably brought in by overseas anglers carrying spore on wet fishing gear or using imported crayfish bait. Recurrence is suspected elsewhere, surveys having indicated diminishing numbers in several midland lakes. However, Ireland still holds some of the most important European stocks, and National Parks and Wildlife Service directives warn aquarists, pet shop owners and anglers against importing foreign species. To date, the warning has been heeded.

The family and I later walked beside a tributary of the River Ver, also in Hertfordshire, where the flow was choked with watercress. A young Hereford bullock was standing belly-deep in the water, contentedly eating all before it. Peppery-steaks, flavoured with signal crayfish perhaps?

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