Chub invader stakes its claim on our lakes

Another Life: One rainy day in 1889, a pike angler from Britain was fishing Cork's Blackwater River and using as live bait two small, bright silver fish he had brought carefully from England. The flood in the river caught the bait tins and swept them away (or that was his story), thus adding roach and dace to the freshwater species of Ireland.

Plus ça change. . . Earlier this year, two well-known English pike anglers on their way to fish in Ireland were intercepted by Customs at Holyhead, acting on a tip. Roach and carp intended for live bait were found in well-concealed tanks in their vehicle. In both England and Ireland, moving live freshwater fish about without a licence is illegal. Ireland bans possession and use of fish for live bait, and Scotland has just produced a draft bill to the same end.

Along with common worries about spreading fish pathogens and parasites, Ireland's special concern is with protecting prized salmon and trout from introduced coarse fish that eat them or their young, outbreed them and compete for their food. The salmonids (Arctic char is another) were the prime inheritors of our empty waterways when they swam in from the sea as the last ice age thawed. With human population, the rivers and lakes were progressively infiltrated by introductions of new food fish from Britain.

Giraldus Cambrensis, touring in the 12th century, noted Ireland's lack of pike, perch, roach, gudgeon, tench and minnow. All of these, along with carp, bream, dace and rudd, figure in the modern list of species. The arrival and multiplication of the 21st recruit - the chub, Leuciscus cephalus - was confirmed earlier this month from the River Inny, downstream from Ballymahon in Co Longford and part of the interconnecting Shannon system of rivers and lakes.

There had been reports of chub in the Inny over several years, but this fisheries board survey team netted more than 30, male and female. They were descendants of fish either brought in as live bait and discarded at the end of the day, or deliberately released to add to Inny's coarse angling attractions. The river already offers pike, bream, roach, tench and coarse-fish hybrids, along with trout.

Chub (otherwise the loggerhead) is one of the larger fish of the thick-lipped carp family - anything over 5lb is considered a worthy catch - and its gleaming, brassy scales look well in photographs. It is also quite inedible, being soft and full of bones and its culinary appeal not helped, perhaps, by Izaak Walton's recommendation of a large black slug as bait for this "fearfullest of fishes". It swims in small shoals that come as eagerly as park ducks to crusts scattered on the water, but vanish in a flash at the slightest hint of movement, or a suspicious silhouette on the bank. In winter it sinks deep beneath undercut banks and overhanging trees, but will still take a bait - behaviour that may have made it a candidate for extending the midlands angling season.

More than 50 years of controlling pike in the prime angling lakes has been Ireland's longest-running venture in conservation management. Intensive culling in Corrib, Mask and Carra is intended to restore the lakes to ecosystems dominated by the salmonids. But many rich alkaline lakes have been taken over by coarse fish and their hybrids.

The prolifically-breeding roach, in particular, have displaced trout stocks, sometimes almost to the point of extinction. After introduction to northern lakes in the later 1900s, huge populations built up in Lough Neagh, offering spectacular world records in coarse angling, then later crashed in a massive parasite infection. In what has been called "hyperpredation", the lake's roach provide winter feeding for cormorants, which in turn have been predators on young trout and salmon in surrounding rivers. In Lough Erne and elsewhere, roach have displaced rudd and interbred with bream.

Even the retiring tench, browsing on the bottom of lakes, can change a trout habitat for the worse. Churning up sediment, it clouds the water to the detriment of aquatic plants and thus dragonflies and snails, and its stirrings release phosphates that lead to eutrophication.

Such domino effects were described in the recent all-Ireland Quercus report on the impact of invasive species, which specifically listed chub as a fish that could cause problems if introduced from Britain. Its fears are now echoed by the Shannon Region Fisheries Board. For the trout of loughs Sheelin, Derravaragh, Ennell, Ree, Allen and Derg, it is one more new neighbour they would certainly have done better without.