Mink make their mark

A few miles up the coast, where Clew Bay retreats to the shelter of its islands, some friends have been improving a big, leafy garden on an inlet, at a wheelbarrow's length from the shore.

A hollow at one end of the land, a soggy spot at spring tide, seemed a good place to make an artificial lake. It was scooped out with a digger a couple of years ago: the hole was half the size of a football pitch, with a couple of hillocks left as islands topped with some of the peaty spoil. The lake was planted with bulrushes, and then, when the water cleared, stocked with rainbow trout.

Last year, wild mallard were queueing up to nest. Two separate females used the bigger island and each brought a brood of nine ducklings down the same grass slide and led them away to the sea. The trout, meanwhile, were regularly fed: 15 of them, fattening up to 3 lbs-something each.

Their success could be judged this winter as the fish began to turn up dead on the islands, their bellies ripped out, the bodies disappearing by the next day. Within a month, all 15 were gone.

About half-way through the carnage, holes began appearing high up in the soft, grassy humps of the islands. I was shown them on a frosty morning, when the lake's flotilla of decoy ducks, pinioned by ice, seemed more than usually inanimate.

This is a classic shore for otters, near the mouth of the Owenwee salmon river. It was something of a miracle that my friend's trout had gone undetected for so long. An initial, partial meal, then a return in the night to polish it off, is the trademark, indeed, of the otter. But the holes in the islands, so newly excavated, were far too small for an otter, and much too large for a rat.

Mink, then, it had to be. It is a few years since the first west-coast sightings, outriders of the inevitable. Another decade and they will be everywhere. Mink only rarely take fish this big, and trout are usually too fast for them. But these particular fish had nowhere to go.

One detail of note is that only in Ireland has the feral American mink, Mustela vison, been shown to dig its own dens. In North America, it almost invariably uses burrows made by muskrats. In Britain, it prefers rabbit burrows, or uses holes under waterside trees or in stone walls and screes, or in reed beds.

It uses this sort of squat in Ireland, too - often a number of dens within its hunting range. But in the 1980s, investigating mink in the Midlands for the Wildlife Service, Dr Chris Smal found four dens clearly dug by the animals themselves in soft, peaty soils along the banks of the Glore and Inny. Three of the dens had two entrances, one above the other to provide for winter floods, and the burrow width was 811 cm.

My Clew Bay friend donned waders and crossed to his little islands. He found them criss-crossed with pathways between cosy curling-up-places in the long grass. There were no fewer than 15 burrow openings, each about 8 cm across, but all had been abandoned at a hand's depth in the soil. Perhaps it felt too wet. Perhaps it dawned on the mink (just one, they are solitary in winter) that the rainbow trout bonanza was fading and there was no point putting down roots in the middle of a lake populated only by wooden ducks. At any rate, he seems to have left, remembered only by a few rain-washed scats packed with tiny bits of fish-bone.

Now my friend's concern is for the mallard, when they return, and I can offer no great comfort. Mink are opportunists, like most predators, and the eggs and young of birds nesting on islands are especially at risk.

My own particular worry, now the mink have reached us, is for a colony of sandwich terns that pack wing-to-wing in summer on a little raft of an crannog in a lake behind the shore just north of me. The terns at Our Lady's Island Lake in Co Wexford have survived the mink and other predators only by the most determined electric fencing and wardening. Neither will be there to reassure the sandwich terns of Cross Lough, Killadoon, already a shrill and jumpy bunch of birds.

It is the sudden flurries of autumn and winter killing, often by juvenile mink pushing out into virgin territory, that revive the creature's notoriety from year to year. Ireland's zoologists may have been a little too ready with the long ecological view, discounting the shock of the initial impact of mink on unsuspecting duck, coots and moorhens, especially on small rivers.

There will, true enough, be "equilibrium" in time, as the mink space themselves out to match their food supply. Already in the Midlands, where crayfish, coarse fish, frogs, rats and eels all bulk out the mink's varied menu, waterbirds have learned to build their nests in safer places. In Ireland's mild climate, mallard can afford to extend their breeding season, laying another dozen eggs to make up for those the mink has stolen. Teal, on the other hand, already thinly scattered as breeding birds, may vanish altogether from the margins of many moorland pools.

When populations have settled down, the density of mink in Ireland is likely to be much less than it is in Britain. There, otter numbers had crashed because of pesticide pollution, and multiplying mink found the rivers wide open to them. Today, there are 100,000 mink in Britain but only 7,500 otters, most of them in Scotland. Now that the otter is increasing again, it is said to be ousting the mink from some of its old rivers in England, and even killing it in fights.

However, on the west coast of Ireland, probably the otter's most secure stronghold in Europe, the picture is more likely to be one of co-existence. Where the two species compete on the west coast and Hebridean islands of Scotland, the otter gets its pick of the fish, while the mink is forced to find more of its food on land. It's one reason why the mallard, on her mossy throne in the lake below the reek, will need to make yet another nine copies of herself.