Hopefully our Last Oyster Blog convinced you that we need native oysters back in the coastal water of the UK. Their benefits for local biodiversity and carbon sequestering capabilities make them an excellent candidate for marine restoration. However, reintroducing Native oysters is not as simple as chucking some over the side of a boat. There are two main challenges every native oyster project has to overcome. The first is a lack of broodstock left in the wild. The second is a lack of suitable habitat for oysters to settle.
The Broodstock Challenge
Firstly, what is brookstock? This is the population of breeding oysters that can replenish a wild population. Oysters are broadcast spawners. Males release sperm into the water column which is taken in by the females to fertilise eggs held inside their shell. These fertilised eggs develop into larvae which are then released into the water column. Due to overfishing and reduced water quality there are no longer enough adult oysters producing larvae to create a new oyster reef in the estuary.
To solve this issue we will be using oyster nurseries. These are cages hung in the water that contain adult oysters. Every year when spawning season comes around these nurseries will pump huge numbers of little oyster babies (larvae) into the tees estuary. Each female can produce up to 1 million larvae a year! These larvae will then float through the estuary looking for a nice place to settle and develop into adult oysters.
The Habitat Challenge
Even with millions of larvae in the estuary we still need to overcome the lack of suitable habitat before a reef will form. Native Oysters are gregarious, meaning that larvae prefer to settle where there are already oysters present. At the very least, larvae need a hard surface to attach to so they can grow. In the Tees estuary there is a limited supply of hard surface on the seafloor that oyster larvae can grow on.
To promote larvae from our nurseries settling on the seabed we will place large amounts of cultch into the estuary. This consists of a mixture of shells and gravel to provide a nice surface for larvae to grow. This process is known as seabed restoration.
With a combination of oyster nurseries and the deployment of cultch, we hope we will be successful in reintroducing native oysters into the Tees estuary. The nurseries will provide large amounts of oyster larvae and Cultch will provide a habitat they can call home.