Nature's water filters 

The native or flat oyster (Ostrea edulis) is a sessile (fixed in one place), bivalve (2-part shell) mollusc. It is a filter feeder meaning it uses its valves to pump water across hair-like gill structures to filter out microscopic algae and small organic particles from the surrounding water. A single oyster can filter up to 200 litres of seawater (approximately a bathtub) per day, which can significantly improve water quality and clarity. By removing particles from the water column, the oyster can also increase light penetration to the sediment and promote the recovery of seagrasses, another threatened and valuable coastal habitat.  

Oyster reefs: supporting sustainable fisheries 

Left undisturbed, oysters will form reefs, these complex structures are formed when large numbers of living oysters and dead shells form an extensive biogenic habitat on the seafloor. These unique three-dimensional habitats created by native oysters support higher biodiversity and biomass of species than the surrounding sediment/seabed. Oyster reefs can also increase fish production by providing a protective nursery ground for juveniles, that acts as a refuge from predation and provides a source of food by increasing the abundance of prey. Protected restoration areas can provide spill-over of larvae that may seed and support sustainable fisheries. 

Oyster reefs as carbon sinks 

The drawdown of sediments together with the stabilising effect of the reef can also result in reefs acting as carbon sinks although the ecosystem benefit is complicated. However, existing oyster reefs can protect considerable stores of carbon that should be protected to avoid further releases of carbon into the atmosphere. Carbon sequestered and stored in the marine environment is called blue carbon. 

Native oyster decline in Northern Ireland

Extensive native oyster beds were abundant in European seas, and humans have been harvesting them since the Stone Age. However, it is estimated that oyster populations have declined by 95% since the 19th century, and now native oyster reefs are now one of the most threatened habitats in Europe. The key driver behind this huge decline is historic overfishing, however, habitat loss, disease, pollution and invasive non-native species are all contributors. Native oysters are also characterised by a slow growth rate and sporadic recruitment, which further limits their success. In Northern Ireland, extensive oyster beds are known to have existed in Carlingford Lough, Lough Foyle and Belfast Lough for several hundred years, but most stocks crashed during the late nineteenth or early twentieth century. With this decline, we have also lost the benefits they provide. Now only a small population exists in Strangford Lough, and recent evidence has found a small number of oysters have returned to Belfast Lough. 

Bringing back native oysters to Belfast Lough 

To support the natural recovery of the native oyster in Belfast Lough, Ulster Wildlife is installing 24 oyster nurseries in Bangor Marina, which will be operational by Spring 2022. 

An oyster nursery is a micro-habitat housing 27 mature oysters that will reproduce and release the next generation of oyster larvae to settle out on the seabed of Belfast Lough. An individual oyster can release up to 1 million larvae per year! The cages are hung in the water underneath the pontoons which provides the oysters with protection from predation. Similar projects have been established in other parts of the UK, but this is the first project of its kind in Northern Ireland, and it is funded by the DAERA Challenge Fund.  

With the challenges of the climate crisis, we need to protect and restore ecological systems that provide nature-based solutions and resilience to these challenges. Northern Ireland and the rest of the UK must significantly increase the ambition, scale and number of marine restoration projects. As well as establishing the first native oyster nursery in Northern Ireland, Ulster Wildlife has partnered with the National Oceanography Centre and the University of Hull to identify areas suitable for blue carbon habitat restoration using predictive habitat modelling methodologies. This will pave the way for Northern Ireland to actively create and restore seagrass, kelp, saltmarsh, and oyster beds, thereby increasing biodiversity, the potential for carbon sequestration and a host of other ecosystem services that support mankind.