Scotland Kilchoan Native Oyster Restoration Project

The native oyster population has been in a significant decline due to overfishing, diseases, and changes to water quality by pollution and agricultural runoff. Native oysters play a crucial role in the inshore marine ecosystem. A single mature oyster can filter up to 240 litres of water a day, they provide a nursery for a substantial number of marine species and store carbon in their shell and in the seabed via their faeces. Due to the significant impact and ecosystem services they provide, native oysters are often referred to as ecosystem engineers.

A baseline survey, conducted in 2021, had shown a drastic decline in the native oyster population in Loch Melfort. Thirty sites were surveyed for native oysters. Ten out of the 30 sites had traces of native oysters such as shells and only five out of the 30 sites still had a remnant population of live native oysters.

In 2020 we applied for the licence to develop a native oyster restoration project in Loch Melfort. Since the license being issued in March of 2021, we have grown oysters in our nursery in the loch. Sourced at 10–12 mm in size, from different hatcheries across the UK the oysters are fragile and sensitive to harsh environmental conditions and predation. In the oyster nursery these juvenile individuals are protected from storm surges and predation while growing and getting acclimatised to the conditions in Loch Melfort. The nursery is in an area with plenty of water flow and nutrient availability for the oysters to feed on.

In 2021 our nursery had a capacity of 28,000 oysters. In 2023 a licence application for a second nursery site was submitted and since March 2024 the combined nurseries can hold 117,000 native oysters.

Once the oysters reach a size of 30 mm or 10–12 grams they are released on to the pre-surveyed sites along the shores of Loch Melfort. By the end of 2025 we have had a total of four release events resulting in 50,500 oysters released across four different release sites in Loch Melfort.

The Kilchoan Native Oyster Restoration project is a community based project…

For the implementation of this restoration project, we follow a community-based approach. By involving volunteers there is a shared sense of ownership with shared responsibilities. This engagement goes beyond being recipients of conservation efforts; community members actively participate and contribute to the restoration process. Volunteers are involved with the grading and cleaning of the nursery cages but also help with release and follow up monitoring events. Every four months during spring tides we visit our release sites and collect data regarding the growth and mortality rates. The monitoring is a critical aspect, providing valuable data and insights that contribute to a better understanding of project success and inform adaptive management strategies. In 2024, in collaboration with Seawilding, we redefined the objectives of our follow-up monitoring survey. This resulted in more targeted and relevant monitoring protocols, now focusing on the density of oyster beds rather than mortality rates. These updated protocols are being implemented across various restoration projects throughout Scotland.

Considering the sensitivity of a juvenile released oyster we estimated that 50% of our released oysters will survive and reach the targeted, sexually mature weight of 50 grams. Current data (October 2024) has shown that some of the released oysters have reached 110 grams, up from the 10 gram release weight in 2021. We are also seeing an average mortality rate of 33% across the four release sites which is considered as an overall success. One of the key measures of success for the project is the reproduction of our released oysters and the recruitment of spat, as well as their settlement in Loch Melfort. Spat refers to juvenile oysters in the pediveliger stage, during which they are still free-floating in the water column. In September 2024, a particle distribution model was developed to predict spat settlement patterns. This model will help us define where spat are likely to settle, with plans to ground-truth the predictions in the future. We have already observed an increase in wild juvenile oysters at the release sites. However, it is still too early to determine whether these oysters are offspring of the oysters released by KMT.