Testing times for the bottom-feeders

Another Life: A grey, high-pressure day, the air charged with moisture on the edge of drizzle, the sea held down by a thousand-and-something hectopascals. Spring tide emphatically out and pausing, barely breathing, away below what's left of the wreck these days: half a jawbone with a few black stumps.

Exposed, an enormous parade-ground of rippled sand, miraculously swept of stormy kelp (into cracks and corners and under the rug at the machair's fraying edges). Flocks of little waders - dunlin, sanderling - seem at a loss in so much barren space, fanning out to search a square metre each and then the next. Only when the dog puts them up do they remember their togetherness, wheeling around to alight (nothing so crude as land) at a new stretch of tawny distance. The dog makes great curlicues around my own, splay-footed, herringbone of a track.

At the end of the strand, where the dunes run into rock, the birds change. Offshore, a squadron of great northern divers keep to their usual winter station, a fathom or two from the nearest sand eel - something, anyway, that gets swallowed on the way up. Americans call them loons. To know why, you'd have to hear them on Arctic breeding lakes, howling at appalling jokes.

Closer to, on reefs awash with tumbling dabberlocks, a picket of oystercatchers confer, then silently disperse. I never see oystercatchers catching anything, still less hammering away as they're supposed to with those blunt orange picks of bills: perhaps they're just going to, or just have.

What brings them to this corner is the mussels: millions of tiny mussels cladding the exposed outcrops of rock in black chain mail. The great fall of the slack spring tide lets me walk around the ocean's side for once, where the sea anemones shrink into themselves and wait, and bright orange patches on the overhangs turn out to be crusting sponges, yielding to a prod. Higher up, in among the tough roots of wracks, I recognise the red leaves of dulse and stand there, chewing its salty cud, to count the mussels.

Not really - not for a moment. But mussels have this knack of mesmerising multiplicity, of clamorous countlessness in a manner unmatched by say, barnacles at a similar density. Because you have seen mussels at full scale, so to speak, piled at the edge of your plate, you feel bound to recognise the singleness of mussels, even when no bigger than your little fingernail and unlikely to grow much more. That is how, I suppose, they insist on their immensity of number when jammed together in a solid sheet several metres in extent.

In a cleft near the rock was a washed-up length of shredded rope, stuck at intervals with plastic battens, like the rungs of half a ladder. It reminded me that, round the corner in Killary Harbour, this is the month for harvesting cultivated mussels, heaving up the ropes that hang below lines of blue barrels and tugging off the clusters of plump Mytilus edulis.

Such abundance of recruitment makes the Killary mussel farmers, and their counterparts in Bantry Bay, enviably self-sufficient in natural resources. But in bays from Lough Foyle to Wexford Harbour, Clew Bay to Castlemaine in Kerry, the commercial bottom culture of mussels is now a big and growing industry that has to look outside its own waters for seed. In his recent book on Ireland's shellfish history, Alive Alive-O (Tir Eolas, 15.99) UCG's Professor Noel Wilkins gave some truly astounding figures for the tonnes of seed mussels dredged from natural beds in the Irish Sea to establish and sustain sheltered bottom production around the coast.

"One estimate suggests," he writes cautiously, "that upwards of 1,000 tonnes are taken each night during the summer season from the Irish Sea and a total allowable catch of 98,000 tonnes a year is now in force.

Alarm bells are already starting to sound. These Irish Sea banks are the ones that held "inexhaustible" stocks of oysters in the 19th century. We may find, when we have fished them out, that the mussel stocks are similarly 'inexhaustible'. . ." Mussels have plenty of natural enemies, chief among them the starfish, which tugs the mollusc's shell open just a crack and sends in its stomach to digest it. One gathering of starfish off the west coast of England, 1.5 kilometres long, was judged to have cleared about 4,000 tonnes of young mussels in four months. Crabs and dog whelks claim their share, too.

In the time when the shore above our strand was thick with cabins and potato ridges, "seed" mussels were sheared off the rocks with spades, for fertiliser. If ever men in longboats come to raid them now, I shall suspect that, somewhere, there's yet another crisis of sustainability.