The recovery of Loch Ewe seabed

Through the operational life of fish farms, environmental effects are limited and managed by a robust regulatory framework underpinned by environmental standards that ensure the protection of the seabed and biodiversity. There is limited information available however on how the seabed responds following the cessation of fish farming operations. The aim of the study, which was conducted alongside delivery partners Benthic Solutions and Applied Genomics, and supported by Salmon Scotland, was to assess the environmental condition and trajectory of seabed recovery following the closure of the Loch Ewe fish farm.

“Isle of Ewe had been in operation since 2005. Prior to its closure in late 2020, the site had a lot of diverse features present within the area which warranted the designation of the Wester Ross Marine Protected Area, which goes to show that Mowi sites can coexist alongside protected areas quite successfully,” explains Connie Fairburn, Senior Environmental Analyst at Mowi Scotland.

“The aim of the three-year Isle of Ewe post closure monitoring project was to assess the trajectory of benthic recovery upon cessation of farming operations. It is believed to be the first of its kind in Scotland.”

The project included the monitoring of samples from the ‘allowable mixing zone’ around the fish farm which is equivalent to the area extending 100 metres from the pens in all directions. The extent of the mixing zone around farms is limited by the regulatory framework and, to illustrate, the mixing zone for Loch Ewe fish farm equated to 0.3% of the surface area of Loch Ewe.

At the boundary of the mixing zone, seabed environmental standards for ‘Good Ecological’ status have to be met. The same sample locations, extending in four directions from the location of the fish pens, were monitored between 2020 and 2023. In 2020, monitoring confirmed that the Isle of Ewe fish farm complied with all seabed environmental standards prior to its closure. This was the first monitoring survey conducted by Mowi under the new regulatory approach, incorporating a tighter environmental standard for the spatial extent of mixing zones, that was introduced by SEPA in 2019.

In 2020, 67% of sampling locations met the conditions for ‘Good Ecological’ status. In 2021, this percentage increased to 80%; in 2022, it increased to 93%; and in 2023, 100% of sampling locations met the conditions for ‘Good Ecological’ status.

Dr Iain Berrill from Salmon Scotland commented: “The project has been really important because it has shown us, and it has reiterated, that the seabed will recover. But it’s done more than that. It’s told us how quickly that it will recover and it’s shown us that it recovers very, very rapidly. And that reiterates our position on our understanding that our impacts are only temporary and they’re not long lasting.”

Reflecting on the results, Ian Wilson, Founder of Benthic Solutions, says: “We looked at how the seabed recovered in multiple different ways, not just seabed sampling with benthos and macro invertebrate analysis, but we also integrated an environmental DNA analysis which covers a much bigger profile of the biodiversity. We also did habitat mapping and photography.

“The results we received from the data were initially very, very positive. The first survey, although it was meant to be a year afterwards, was actually only six months after the site was decommissioned. We found that the distance to where the biodiversity shifted significantly halved within those six months.

“Even though the fish farm industry has been going for 50 years in Scotland, the data on recovery of sites post decommissioning is very limited and I think this is the first project of its type that shows the depth of information and shows how quickly that environment recovered.”

Connie Fairburn continues: “A healthy environment is imperative to ensure the optimal growing conditions for salmon.

“There are many misconceptions about the environmental impact of the aquaculture industry. However, this project provides solid evidence that seabeds can recover upon cessation of fish farming operations.”