Aliens on the Exe – Pacific oysters

Pacific oysters, as their name suggests, are native to the Pacific Ocean, but they have been a familiar sight in the UK since the 60’s. They were first introduced in hope of finding an alternative species to supplement the shellfish industry following the decline of our native oyster. When they were first introduced, they were not considered capable of proliferation as water temperatures in excess of 20°C are necessary for reproduction. However, rising sea temperatures, warmer summers and milder winters, combined with their extremely fast growth and reproductive rates (up to 200 million eggs in a single spawning) has led to the establishment and spread of feral populations in the UK. Since then, the Tamer and Yealm estuaries have become heavily inundated, with many other Devon estuaries heading that way.

But what’s the problem? Well, Pacific oysters are considered an invasive non-native species. Their razor-sharp shells can ribbon-cut the unwary. Yet, it’s when they cement together to form ‘bio-reefs’ that they become an even more serious problem. These bio-reefs can change the ecosystem entirely by smothering intertidal gravel and mudflats, making it difficult for birds and young fish to forage on these rich and important feeding grounds. Not only this, they can cause a serious obstruction to the shore affecting recreational activities such as dog walking, kayaking, boating and much more.

Following reports of Pacific oysters being present on the Exe, our Acting Estuary Officer, Jay, along with volunteers, began to investigate their presence by conducting scoping visits, and undertaking the early stages of mapping and quantifying the oysters. Initial findings brought to light the substantial numbers of feral Pacific oysters that have worryingly become established across much of the Exe Estuary. In fact, the abundance survey, conducted near Cockwood Harbour, revealed that there are as many as 14 per 0.25m2. Perhaps more worryingly, is this is likely to increase over the coming years due to climate change and increased opportunity for larval settlement.

Fortunately, the Exe is not yet (at least from initial findings), at the ‘reef formation’ stage. However, there is a significant sense of urgency to try and mitigate against this growing population before it becomes both environmentally undesirable and economically unfeasible to do so. Although the solutions are not straight forward or clear, it is essential to continue surveying this invasive species, so the extent of their presence and impact on the estuary can be fully understood and hopefully minimised.

Read the full report here.

If you have any questions or are interested in Pacific oysters on the Exe, please get in touch with the Estuary Officer at