Another Life: All the oysters we could want – just the wrong kind, in the wrong place
The oysters of Ireland were once a native mollusc. Now Pacific oysters have invaded, with potential ecological pitfalls
Seabed: would too many oysters spoil the mix of life? Illustration: Michael Viney
Once upon a time, the oysters of Ireland were a native mollusc, thriving in the island’s more sheltered bays. At Clew Bay, in Co Mayo, in the late 1800s, some six million oysters a year were dredged for export to Britain. On the east coast, where the beds stretched for 50km, Arklow alone was selling nearly 3,000 tonnes a year.
Oysters need company for reproduction, especially Ostrea edulis, the European native with one flat shell, whose individuals change their sex from time to time. Clouds of male sperm need to find the gills of ripe females, and as overfishing thinned the chances of fertilisation, wild populations began to collapse. Techniques for management and cultivation took time to perfect, and even in the late 1900s only the oyster beds of Tralee Bay, Galway Bay and Clew Bay were producing a worthwhile return to local fishermen.
Then came the spread of parasites carrying disease. Most damaging was Bonamia ostreae, travelling to Europe from North America with stock of the new Pacific oyster Crassostrea gigas – fatter and juicier in its twin-cupped shells. Devastating the native oysters of Brittany and elsewhere in Europe, bonamiosis reached Cork and spread right up the west to Lough Swilly. By 2002, Ireland and the UK were the only countries outside of Spain and France to produce more than 200 tonnes of flat oysters a year.
Last month’s Clarinbridge native oyster festival could still celebrate the start of the season with a feast of Ostrea grown in Galway Bay, and Ireland is part of a European research programme to solve the Bonamia problem and restore populations of the native mollusc. Meanwhile, most of Europe’s oyster production remains with the Pacific C gigas, of which Ireland produces 7,000 tonnes a year, worth about €20 million.
This species, indeed, was the saviour of the European industry, especially in France, where it has gone on to spawn naturally in many warm bays. When the Pacific oyster was brought to Ireland, in 1972, the state’s shellfish laboratory at Carna, in Connemara, found it not to breed in Ireland’s cooler waters. Oyster farms sprang up round the coast using imported spat.
That, of course, was before the warming of the seas took hold. Even in the 1970s, Pacific oysters were forming stable and growing colonies away from the oyster farms of Brittany. Farther north, in the shallow Wadden Sea (at the southeast of the North Sea), their spread has concerned conservationists in Germany, the Netherlands and Denmark, for whom it is “one of the most spectacular biological invasions in this ecosystem”. Smothering the mussel beds on the huge tidal flats of the Wadden, the oysters threaten the food of great flocks of eider ducks and other migrant birds.