Splendour of our sea is under threat

The strangest place to be, on our wild stretch of shore, is at a jagged cape of rock where the ocean turns into Clew Bay. Here, at Emlagh Point, a weave of Silurian strata is worn into a maze of dark ravines, some fantastically crowned with turrets and spires.

Wandering there at low tide the other afternoon, a dying sun flashing like tinsel on the wet, black rock, I came upon a huge steel ball, red with rust and wedged into a crevice. A hole through the ball was for threading it onto the footrope (actually a strong steel cable) of a seabed trawl, helping to weigh it down and bounce the net clear of reefs and boulders.

Hundreds of these balls have reached our western coast, lost from trawler gear at distant reaches of the continental shelf and ultimately tumbled ashore in storms. Mayo fishermen call them "mountain rollers" and attribute them to Russian trawlers.

But more and more of all bottom trawls are fitted with heavy steel rollers and bobbins - "rockhoppers", as they are more usually called. Many also have "ticklers" - heavy lengths of cross-connected chain that scour the seabed, flushing shrimp or flatfish into the path of the net. Its cod-end, filling with fish along with rocks and mud, is towed for kilometres across the bottom.

The mouth of a rock-hopping trawl is held open by a pair of massive otter-boards, spread by the force of water as the trawl is towed. A beam trawl, weighing 13 tonnes, is held open by a long steel beam that scrapes along the seabed; it, too, is fringed with chains. Other gear, such as dredges for scallops, oysters and crabs, rake the seabed sediments like a ploughman's harrow.

This battering, bulldozing and scarifying of the seabed used to be confined to relatively shallow waters of the continental shelf. But as stocks of traditional food-fish decline, bottom trawling is moving out across the continental slope, to depths of 1,400 metres or more. Rockhopper trawls, global positioning systems and fish finders are bringing big trawlers to the most remote seamounts, never fished before.

The amount of damage to the seabed depends, of course, on the weight of the bottom gear - increasing all the time as trawlers grow in size - and the intensity of trawling. An area equal to the world's entire total of continental shelves is trawled every two years - a huge disturbance to the ocean biosphere. Within that global figure, some densely fished areas are trawled over and over: in the southern North Sea, for example, some spots are trawled 400 times a year.

Closer to home, the richest breeding grounds of Nephrops, the "Dublin Bay prawn", in the northern Irish Sea are swept up to five times a year by the Irish fleet alone. Beam trawls leave deep scars in the mud, flattening the seabed and filling in the little lobster's burrows. Some once-common species in the intensely fished western Irish Sea have virtually disappeared. This area may never recover, but remain an artificial, man-made community limited to species that can tolerate severe disturbance.

There are no impact studies on what trawlers are doing to the benthic life of our Atlantic continental shelf, but an EU-funded project, in conjunction with the fishing industry, is seeking alternatives to smashing up the seabed. Trials with otter trawls off Ireland's south-west coast are focused on changes meant to keep the nets off the bottom and lessen the drag of debris.

The impact of trawling is reviewed in a study by two American marine scientists, Les Watling and Elliott A. Norse, just published on the Internet by the US Marine Conservation Biology Institute (www.mcbi.org/btrawl/wnpaper.html).

It compares the effects of trawling to those of clear-felling virgin forests, wrecking the seabed's vital habitats and reducing biodiversity. "The sea's equivalent of ancient forests are becoming cattle pastures by default," say the authors.

Some of the devastation is obvious: crushing and burying marine organisms and exposing them to predators. But the consequences are much more complex and far-reaching. The physical structures of the seabed are like sheltering cities as much as forests: the more intricate they are, the more they teem with life and food. Natural rocky structures, even to the smallest pebbles, are elaborated by the animals themselves, in towers, tubes, turrets and reefs built of shelly calcium.

In deep sea, free from disturbance, these structures grow slowly, created by animals that may live for centuries. The branching "trees" of large sea fans growing on the fishing banks off Nova Scotia could be 500 years old. Among the creatures they hide from predators are young cod - a species in crisis on both sides of the Atlantic.

Television films about deep-sea exploration make fascinating viewing, yet, half a century after Cousteau, we still know very little about the highly structured seabed ecosystems which nurture Europe's fish stocks. In 1998, a team of Irish petroleum geologists reported an amazing discovery in the Porcupine Basin, west of Co Kerry - coral reefs in deep, cold water. Yet this is where we go on dragging bigger and bigger trawls, like ploughing the earth blindfold, from an aeroplane.

It has been a battle to regulate fishing by quotas, even when everyone knows the need. But quotas are no longer enough. We need no-trawling zones to allow seabed communities to recover. Conservation of key terrestrial habitats, such as forests and wetlands, is already a European goal. Even for economic notions of "sustainability", we must extend it to oceans.