Western Irish Sea Mud Belt

What is a mud belt? 

Although not as charismatic as a kelp forest, coral reef, or a lush seagrass bed, marine mud belts may not be pretty but are a highly important ecosystem for marine conservation. 

Marine muds are geological features of marine deposits and can be found close to the shore and deep into the sea. They play a vital role in climate regulation in the form of carbon sequestration (the capture and storage of atmospheric carbon dioxide), elevate important fisheries productivity, and create important habitats for key marine species.

Where is the Western Irish Sea Mud Belt?

To the east of the Isle of Man, the Irish sea is generally very shallow (no deeper than 40m), even as shallow as 2m on the sandbanks between the Point of Ayre and Cumbria (even up to 10 nautical miles offshore). 

In contrast, off the west coast, the sea is up to four times deeper (reaching depths of up to 125m). Here, the seabed has formed into a deep bowl-like depression, that acts as a sink where fine mud derived from organic matter (mainly originating from phytoplankton) slowly drifts to the seafloor. This area of depression and mud accumulation is known as the Western Irish Sea Mud Belt (WISMB).

Role in Carbon Storage

Offshore mud habitats are critical for carbon accumulation and storage. The soft-sediment environments store significant amounts of organic carbon over long timescales.

DEFA’s recent Blue Carbon Report found that marine sediments, especially muds, are a critical sink for marine carbon, capable of storing it for thousands of years. The WISMB is the largest single carbon store found within Manx waters with the amount of carbon captured and stored annually in the mud equating to around 7% of Manx carbon emissions.

Processes such as tidal flow and water stratification (formation of distinct, stable layers in a water body) are crucial in the dynamics for the formation and carbon storage of mud habitats. Here, in the waters between the Isle of Man and Ireland, a gyre (large scale circular patterns of surface circulation) forms during the spring and early summer, trapping cooler bottom waters and influencing nutrient and organic matter movement. This gyre promotes nutrient exchange at depth, delivering essential organic carbon to the seabed. 

Ecology

Muddy habitats in Manx waters are highly biodiverse, with rare species such as the Slender Sea Pen (Virgularia mirabilis) found here. This fascinating species forms beautiful upright, leaf-like structures from colonial polyps (lots of individual animals) which can grow 60cm tall. The mud belt is also home to Langoustine (Nephrops norvegicus), also known as “scampi”, which are of high importance for commercial fisheries.

The high levels of primary production, due to the important accumulation of organic matter, provides food and nutrients to support a well-structured food chain within the marine ecosystem. In 2025, the WISMB was declared to be an “Important Shark and Ray Area” of global significance by the IUCN’s Shark Specialist Group because of its importance to breeding Spurdog (Squalus acanthias) and Tope (Galeorhinus galeus), species which are “Vulnerable” to global extinction and “Critically Endangered” respectively. With additional data from many years of shark tagging, we are now recording strong numbers of these special sharks within the WISMB.

Other species of shark recorded on footage so far include the Small-spotted Catshark (Scyliorhinus canicular) and the “Near Threatened” Nursehound (Scyliorhinus stellaris).

Importance to Fisheries

As mentioned previously, langoustine thrives in a well-conserved and managed mud habitat. Previously, Manx langoustine had been exclusively caught by vessels based in Northern Ireland, often using highly destructive fishing methods including the use of heavy sea-dragging gear. The use of such methods significantly reduces both biodiversity (through by-catch) and bio-abundance of nutrients, disturbing sediment and important carbon storage.

Sadly, Manx waters are one of the most physically disturbed in the entire North-East Atlantic. In recent years DEFA have therefore instigated temporary fishing closures in three large areas, together covering a whopping 211.5km² of seabed. This very welcome trial is to determine to what extent mobile fishing activity is negatively impacting organic carbon stocks and burial rates within the sediment. 

At the same time, DEFA have cleared the way for a far more sustainable pot-fishery for langoustine, which our Manx-based industry is now able to benefit from for the first time, delivering a multitude of benefits: climate, nature, economy and food security. To date, DEFA are reporting a rapid and significant increase in the size class of langoustines being landed, highlighting the near-immediate economic and ecological benefits of targeted fisheries management measures.

Going forward, MWT will be working closely with DEFA, the Department of Agriculture, Environment and Rural Affairs (DAERA) of Northern Ireland, Ireland-based eNGOs and the Manx Fish Producers’ Organisation to realise the permanent economic, climate and ecological potential of the WISMB. We look forward to working closely with the Manx Government to explore the best ways to ensure the area’s full ecological recovery whilst continuing to deliver ever-more sustainable and higher quality local food. 

WISMB Survey Method

As is clear from all the previous information provided, it is of increased importance to comprehensively survey the WISMB to inform policy to maximise its potential as a haven for the economy, climate, and ecology of the Isle of Man.

The WISMB surveys use a non-destructive method of Baited Remote Underwater Video (or is in simpler terms – BRUV). The BRUV devices include a camera attached upon a weighted structure which is baited to attract anything in the area (usually with some irrefusable mackerel). 

The devices are dropped to the seafloor at certain locations, both within and outside the mud belt and the temporary fishing closure areas, as to investigate the differences in these areas. From the time they find the bottom of the seafloor, the devices are left for at least one hour to allow any marine organisms to become habituated with the device and to ensure proper survey for any species that may be in vicinity. The devices are attached to buoys with fishing rope so they can be easily identified when returning to collect them. The footage is then stored on an SD card for later analysis by the Manx Wildlife Trust marine team, with all species and substrate composition recorded.

At the same time, a dedicated team on the survey vessels are on hand to survey for seabirds, pinnipeds (seals), cetaceans (whales & dolphins), and any other marine organisms which could range from giant Basking Sharks (Cetorhinus maximus) to jellyfish floating by. The importance of the mud belt is not only that of what we see on the seafloor but its vital role in creating a strong nutrient cycle which holds up a whole marine ecosystem. 

To date, surveys have been very successful, with strong numbers of Grey Seal (Halichoerus grypus) and Harbour Porpoise (Phocoena phocoena), along with sightings of Common Dolphin (Delphinus delphis), Risso Dolphin (Grampus griseus), and Basking Shark. Seabird records have also been boosted impressively with some exceptional transect counts of Manx Shearwater and European Storm Petrel.