What is killing the serpulid reefs of Loch Creran?

The largest example of serpulid reefs in the world is in Scotland. But the reefs are dying, and the causes are unclear

Beneath the waters of a loch on the west coast of Scotland lies a globally significant ecosystem. Hundreds of marine worms gather together on the loch floor, their feeding tubes point upwards to feed off plankton, are crowned by feathery tentacles in bright red, pink and orange.

The worm species serpula vermicularis is found all over the world, but in most locations they live a solitary life. However, they can clump together to form reefs up to 1m across. Though this is rare globally, Scotland had two examples. In one, in Loch Sween, the reefs died in the 1990s for reasons unknown.

The other is Loch Creran, about six miles north of Oban in Argyll, which has the best developed and largest area of serpulid reefs in the world, growing at depths of six to ten metres, up to 75cm high and 1m across.

Squat lobster by serpulid tubeworm reef - photo credit John Aitchison

Research has shown that they can be home to over 70 different species. Orange sponges, colonial and solitary sea squirts, hydroids and seaweeds cling to the reefs, the only solid attachments in the muddy seabeds. Animals such as small spider crabs, squat lobsters, hermit crabs and starfish hunt between the tubes, while brittlestars shelter deep within them.

The presence of the reefs has led to the loch being designated a Special Area of Conservation (SAC), and a Marine Protected Area. The reefs are a priority marine feature, making them officially a conservation priority for regulator NatureScot.

Serpulid tubeworm reef - photo credit John Aitchison

In reality, they are in big trouble. Research in 2020 by NatureScot estimated that they had declined by 20% between 2005 and 2014-19. The reefs’ condition in 2001 was officially categorised as in “favourable” condition, but in 2020 this was amended to “unfavourable – declining” - the bottom of the scale used to describe the status of protected areas.

“They were vibrant gardens of living things, covered in other life. But now they're just dead tubes on the ground, all broken up – a striking contrast to how they were before,” says John Aitchison, a campaigner with the Coastal Communities Network (CCN).

Serpulid reef - photo credit John Aitchison

NatureScot blames the reef’s condition on a natural cycle of decline and recovery. But while reefs have declined, there has been no evidence of recovery, Aitchison points out. At a meeting with CCN in the spring, NatureScot confirmed that the reefs are declining all over the loch, rather than in a patchy way, Aitchison says.

Scottish nature conservation bodies promised to investigate potential causes of the reef’s decline in Loch Creran. NatureScot, the Scottish Environmental Protection Agency (SEPA), Marine Scotland and Crown Estate Scotland are now examining changes to the environment around the loch, and human activities taking place in the loch and surrounding area, which include a fish farm, processing facility and hatchery, and caravan park.

If this study gives cause for concern, they may then decide to sample the seabed, for example looking for residues from pesticides used by fish farms to kill sea lice, which cause physical damage and stress to the fish, and can lead to their death. SEPA would not commit to doing so at the meeting, Aitchison says.

Serpulid reef - photo credit John Aitchison

New risks

But while the investigation is ongoing – results of initial desk-top studies are expected by mid-2023 –planning applications to expand fish farming and related infrastructure are underway.

Gael Force Fusion (GFF), a manufacturer of marine equipment including plastic cages for fish farms, is planning to build a factory and launch slipway in a bay where serpulid reefs are found. Meanwhile, Scottish Sea Farms (SSF) is applying for permission to expand its hatchery with a 9,500m2 unit on the southern shore of the loch.

Documents associated with the planning applications express concerns that both developments could harm the reefs. A letter sent in February from NatureScot to Argyll and Bute Council, which will decide whether the developments can go ahead, stated that the proposal was likely to have a “significant effect” on the reefs if pollution entered the loch during construction.

“Reefs are present along most of the coastline above Barcaldine including in the vicinity of the proposal where they were previously absent for many years due to pollution of the seabed. It is therefore vital that water quality is protected in order to maintain the extent and distribution of the interest within the site,” the letter states.

However, if SSF provided a “high quality and project specific” surface water management plan with measures to prevent this, the regulator would not object to the project, it stated. SSF has said it will put a surface water management plan in place, but this has not yet been submitted to the council.

GGF’s proposal meanwhile has been flagged as potentially problematic by Graeme Smith, an independent marine biologist and expert on the loch commissioned by GGF to assess potential impacts of the development on the loch’s ecology.

He highlights potential damage to the reefs from various sources, including boat moorings, cage movements, spillage of pollutants during manufacturing or from fuel and oil from vehicles and non-biodegradable plastics being blown into the loch by the prevailing winds. These could be prevented by various actions such as carrying out manufacturing as far as possible from the loch, and installing fencing to catch any plastic waste blown by the wind, he states.

“There are factors during development of the site and subsequent manufacturing which could affect this species... A management-led approach to creating an in-house culture towards all manufacturing operations within a highly sensitive marine environment would be beneficial in reducing the ongoing impact on the environment,” it states.

GGF did not respond to a request for comment.

Serpulid reef - photo credit John Aitchison

Precautionary approach

The regulators’ investigation so far is at an early stage. A statement from NatureScot said that it was too soon to reach conclusions about impacts from any particular activity in relation to the reefs’ condition. However, so far there was no evidence to suggest that fish farming is harming the reefs, it stated.

“The declines observed in the reefs appear to be widespread, occurring throughout the loch rather than associated with the locations of fish farms,” it added.

In a statement, Scottish Sea Farms said that the loch was “vibrant”, and that reefs were not present in the same area to its farm. In addition, water samples taken in 2021-2022 found dissolved nitrogen levels “well within” the current best in class water quality standard under the Water Framework Directive, and that modelling carried out for its proposed hatchery extension indicated that nutrients would continue to be the case.

But modelling has not been carried out to test whether chemicals used by the fish farm including emamecetin benzoate had dispersed further away from the location of the fish farms, Aitchison says.

Campaigners are urging use of the ‘precautionary principle’ in relation to fish farming on the loch. This means that, where an activity or development poses a known risk, but evidence is lacking on its severity, the activity or development should be postponed until better information is available.

Applying this to Loch Creran would mean not allowing expansions to fish farming, and stopping use of chemicals at the existing farms, Aitchison says. But regulators, including SEPA and NatureScot are also obliged by law to promote economic growth, which makes them hesitant to use the precautionary principle, Aitchison believes.

In addition, the loch’s designation as an SAC means that developments need to prove “beyond reasonable scientific doubt” that they are not harming its features, he points out.

“This is at the heart of our complaint. It is not sufficient for NatureScot and SEPA to assume that the collapse of Loch Creran’s serpulid reefs are part of a natural cycle without evidence beyond reasonable scientific doubt that this is so,” he says.

Aitchison acknowledges that there could be multiple causes for the loss of the serpulids, including increased pressure from predators, increased pressure from squirts and algae, and changes in the plankton they eat.

However, such is the speed of the decline and the importance of the reefs internationally that the regulators should be acting with urgency, not dismiss it as being down to a natural cycle, which is “the easy way out of having to do anything”, he says.