Historic map reveals how mussel farm is bringing shellfish reefs back to the Devon seabed

The UK’s first large scale offshore mussel farm is allowing shellfish reefs to return to parts of the seabed off England’s south coast for the first time in up to 150 years, a new study has revealed.

Researchers have spent the past decade examining the environmental impact of the farm, which was first established in Lyme Bay off the coast of South Devon in 2013.

In that time, the seabed beneath the farm’s ropes has begun to undergo a transformation as mussels from the lines are deposited on the seabed below.

This has seen it change from muddy sediment with limited biodiversity into reefs, which research has shown have the potential to benefit a number of commercial fish and crustacean species and the ecosystem more generally.

However, a historic map – recently uncovered by a Devon-based artist – has revealed this might in fact be a form of restoration rather than creating habitats never seen in the area before.

Dating from 1871, the French map describes a large area of the seabed – stretching from Torquay in the west and beyond Lyme Regis to the east – as being home to “rich shell beds”.

Scientists believe these are likely to have been oyster or mussel reefs that have long since been destroyed through the use of bottom towed fishing gear during the 20^th century.

With such practices banned following the designation of the Lyme Bay Marine Protected Area (MPA) – and the mussel farm’s three sites located above the old shell beds – it is allowing the region to become home to shellfish reefs once more.

The map was part of the Marine Biological Association's collection but now donated to artist Hannah Wisdom. She shared it with scientists at the University of Plymouth, who have been working with fishing communities along the Lyme Bay coastline to monitor the impacts of the MPA since 2008, and the mussel farm since 2013.

Dr Emma Sheehan, Associate Professor in Marine Conservation and senior author on the study, said: “When we first started to monitor the mussel farm in 2013, we hoped to find evidence of its potential to restore the health of the seabed. That is something we have shown consistently, with new species of marine flora and fauna regularly being seen within its ropes and on the seabed. However, to find out that such habitats were found in the region 150 years ago is really exciting. It shows that the farm is not only having a positive impact on the health of the ocean now, but also returning the seabed to some semblance of its former state.”

In the study, published in Science of the Total Environment, researchers also highlight how the shellfish reefs that have been recreated over the past decade are being used by a number of commercially important species.

Using a combination of acoustic tags and underwater monitoring equipment, scientists found that individual European lobsters (Homarus gammarus) remained beneath the farm for up to 283 days using both the farm anchors – and areas of seabed dominated by fallen mussels – for feeding and refuge.

By contrast, brown crabs (Cancer pagurus) showed little willingness to stay within the farm’s infrastructure and all tagged crabs had left the area of the farm under surveillance within nine days.

This, scientists say, could be evidence that the mussel farm is providing refuge and similar feeding opportunities for lobster and crab as their typical habitat on rocky reefs.

John Holmyard, Managing Director at Offshore Shellfish Ltd which manages the Lyme Bay mussel farm, said: “These findings show that an area of flat, muddy and degraded seabed can be restored into a productive, biodiverse habitat. Through the location of a carefully designed offshore mussel farm, we are creating an effect comparable to that seen from the statutory protection of a rocky reef area. It is enabling lobsters and crabs to feed and grow within the protection of the farmed area before they move out and provide further benefits to surrounding fisheries.”