Ecosystem engineers — sea anemones, barnacles, and more — all find places to make a home in the bustling high-rise living reef.
Oyster shells are often seen washed up on beaches: rough on the outside, smooth and shimmering on the inside.
Oysters are familiar, even though they are a mollusc that lives in shallow coastal habitats. Many of us know oysters from restaurant menus — but these are mostly farmed pacific oysters, a species native to China, Japan and Korea, introduced here for cultivation in the 1970s.
In the Red Sea, archaeologists have discovered evidence of oysters being harvested in great quantities from the wild some 125,000 years ago, which is about 100,000 years before humans even started any kind of farming. In North-West Europe, native oysters have been thriving in shallow waters all around the coast since long before the hunter-gatherers who first settled here arrived on these shores.
Mesolithic hunter-gathers who lived in Ireland before the first farmers were also keen on oysters, as evidenced by the great number of oyster shells found in ‘kitchen middens’ — the waste heaps uncovered in archaeological excavations.
Their popularity throughout the ages is hardly surprising. Oysters are a high protein food and have always been an abundant and relatively accessible delicacy.
Oysters are also an important species in marine ecosystems. Each little oyster grows slowly, filtering feeding nutrients out of the seawater. It takes years for an oyster to build up volume, accumulating layers on its shell-like tree rings.
Together, communities of tens of thousands of native oysters make up enormous offshore oyster beds, also called oyster reefs, where old dead oyster shells accumulate and new generations of living oysters keep extending the reef. Over time, these reefs become enormous solid structures in the shallow waters off the coast, capable of spanning several hectares.
Benefits of oyster reefs
Much in the same way that tropical coral reefs are home to an enormous variety of life, native oyster reefs here provide habitat for a huge variety of other marine creatures. They are the scaffolding for seaweeds to anchor, the shelter for crabs to live in, and the substrate for living marine sponges. Sea anemones, barnacles, and tube worms all find places to make a home in the bustling high-rise living reef. Predators such as starfish, winkles and whelks graze on and around the reefs.
Such bustling communities in turn provide a habitat for fish to spawn safely. Juvenile fish tend to stay close to living oyster reefs for protection from tides, storms and predators. One study carried out in Strangford Lough found that oysters increase the biodiversity of marine habitats quite impressively, supporting as many as 78 other species.
And because oysters are filter feeders, they help improve water quality by cycling nutrients and removing suspended sediment. This is why our native reef-forming oysters are known as ‘ecosystem engineers’ and are considered a keystone species in the marine environment.
The ‘biogenic’ reefs (structures made by living organisms) are the basis for entire ecosystems. Without the reefs, an array of lifeforms simply cannot survive the harsh conditions of open water.
But in the 19th century, the fondness for oysters in growing cities, both here and in England, meant that the demand for oysters grew intensely. New harvesting techniques meant oysters could be dredged from reefs in vast quantities. 3,000 tonnes were taken through Arklow alone each year for export to Britain.
London fish markets sold hundreds of millions of wild-harvested oysters through the 1860s, 70s and 80s, many of them harvested from wild oyster reefs in the Irish Sea. As a consequence, natural oyster beds, which for thousands of years had been self-sustaining, were decimated, and oyster populations collapsed by the end of the 19th century. Not only the oyster reefs were gone, but communities of marine life supported by the oyster reefs simply collapsed.
Oyster harvesting now
Skipping forward a few thousand years and oysters remain a popular food. Except now there is no longer an oyster fishery on the Wexford and Wicklow sandbanks and native oysters are rarely harvested. Instead, we farm Pacific Oysters on trestles in oyster farms in shallow bays and inlets all around the coastline.
Oyster farming, when carried out within the carrying capacity of the local environment, can be a low impact form of aquaculture. It depends on clean estuarine waters as oysters are filter feeders.
However, when a significant proportion of a sensitive intertidal habitat is covered with oyster trestles, shore birds who are dependent on those same areas can suffer displacement. When bird species, already to the pin of their collar trying to survive, lose out on feeding areas, the consequences can be significant.
The cumulative impacts of many shellfish farms over large parts of protected sites is a challenging issue that, so far, Ireland hasn’t managed to address.
Another concern among conservation scientists is the naturalisation of introduced Pacific oysters in Irish ecosystems. As the climate warms and sea temperatures are on the rise, Pacific oysters from oyster farms are settling into estuarine habitats here, becoming naturalised. in inlets such as Lough Swilly, Lough Foyle and Strangford Lough, all legally protected conservation sites.
Some consider the Pacific oyster an invasive alien species. Meanwhile, remaining native wild oyster beds are in urgent need of protection. Now, recognition is growing that oyster reefs do so much more for marine ecosystems than we ever imagined: cleaning water, providing fish nurseries, even locking away carbon from the atmosphere.
Following the success of oyster reef restoration projects across the world, a group of pioneering individuals are pursuing the restoration of native oyster reefs in Ireland. A project called Native Oyster Reef Restoration Ireland (NORRI) is developing collaborative approaches to restore native oyster reefs and other associated degraded marine habitats. If the project is successful, there is no reason why these thriving offshore ecosystems can’t be restored in Irish waters.
Oysters punch way above their weight in providing the conditions for a bountiful sea, so restoring native oyster reefs would have significant positive effects on inshore ecosystems. Beginning in the waters off Arklow, where historically there was a thriving oyster fishery, this project offers hope for whole webs of marine life and, in turn, for coastal communities.