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


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.

The unexpected reproduction of the Pacific oyster could make ecological problems for Ireland, too. This made it a ready candidate for Simbiosys, a major, wide-ranging investigation of “sectoral impacts on biodiversity and ecosystem services” for the Environment Protection Authority, led by Prof Jane Stout of Trinity College Dublin. Aquaculture was among them, along with roadside landscaping, energy crops and windfarms, and the final report is now downloadable from the EPA.

The aquaculture research team led by Tasman Crowe of University College Dublin found self-sustaining populations of “feral” Pacific oysters at 18 locations, most densely in Lough Swilly and Lough Foyle (up to nine per square metre), but also in the Shannon Estuary and Galway Bay. They have often formed up to 500m from oyster farms, almost always settling on mussel beds or other hard surfaces. Covering the tops of boulders, they greatly inhibit the growth of reefs built by the honeycomb worm, Sabellaria alveolata. These reefs serve a rich mix of other marine life and are protected around Ireland by the EU habitats directive.

Tests and experiments in Lough Foyle and Lough Swilly found the feral oysters self-sustaining and genetically independent from existing oyster farms. Allowed to spread, they can take up an estuary’s carrying capacity, changing its biodiversity, phytoplankton and food webs. Although they are harvested in some areas, they need urgent action to clear them, says the Simbiosys report, before dense reefs of uneconomic twisted shells are formed. Oyster farms, meanwhile, should be growing triploid oysters, which do not reproduce and also grow faster.

What was so wrong, I wonder, with our pristine bays and estuaries in the time before people began fishing oysters, and our native Ostrea edulis were building reefs all of their own, loaded with mussels, sponges and seaweeds and teeming with baby fish? Carpets of oysters could even have kept the bays free of today’s harmful algal blooms.

But climate change hasn’t finished with the molluscs. On the Pacific northwest coast of the US, the rising acidity of the ocean is blamed by scientists for decline in the growth and survival of baby Crassostrea gigas. And sustained Siberian winters in western Europe could kill them with cold. As in the life of the oyster, perhaps, it isn’t all open and shut.

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