37-year study shows Loch Arkaig salmon farm did not contribute to nutrient pollution
Water quality monitoring at a freshwater loch has shown that the level of nutrients was not affected by salmon farming over four decades.
The independent study, commissioned by salmon farming giant Mowi, followed water quality – specifically, the level of phosphorus and chlorophyll present – in Loch Arkaig, in the Highlands, where Mowi has been operating a salmon farm.
The study, which began in 1987, was run continuously for 37 years. Earlier this year Mowi announced that it would no longer be farming salmon at Loch Arkaig and now the company has compiled a report based on the water quality monitoring in the loch, to better understand the impact of operation of the fish farm.
Excess nutrients in freshwater environments – a phenomenon known as eutrophication – is a serious problem. Excess nutrients, primarily nitrogen and phosphorus, can come from various sources, including agricultural runoff, sewage discharge, and industrial effluents. The increased nutrient levels lead to a proliferation of algae and other aquatic plants, which can severely disrupt the aquatic ecosystem.
Stephen MacIntyre, Head of Environment at Mowi Scotland, said: “With the recent closure of our Loch Arkaig fish farm, it is only right to look back on our operations extending to almost 40 years, with a particular focus on the nutrient status of the water body, to better understand any potential impact from salmon farming.
“The data set which extends for 37 years shows that the geometric mean for phosphorus levels in Loch Arkaig has remained stable and within its trophic status. Whilst phosphorus level is the principal nutrient controlling the nutrient status of the loch, its values vary throughout the year due to land management in the catchment and rainfall levels. It is therefore important to invest in monthly sampling and establish a long running data set to establish a geometric mean.
“Loch Arkaig has been officially classified by SEPA as a water body of High Ecological status. This means that the water at Loch Arkaig is unaffected or virtually unaffected by human activity.”
Over the last 37 years, water samples have been taken regularly from Loch Arkaig by a team from the University of Stirling. These water samples were then analysed in the lab. The aim of the monitoring is to assess the nutrient status of the water body to ensure there is no change in ecological status with the key parameters analysed including total phosphorus (TP) and chlorophyll-a (Chl-a).
This continuous water quality monitoring programme is not unique to Loch Arkaig. This research is also being undertaken at Mowi’s freshwater farms at Loch Ness, Loch Lochy, Loch Garry, Loch Shiel and Loch Awe. The data is similarly used to monitor and track the nutrient status of these water bodies.
Stephen MacIntyre said: “Mowi’s continuous water quality monitoring programme is another example of Mowi leading the Blue Revolution. We are committed to farming salmon sustainably and, in addition to complying with all regulatory standards and conservation thresholds, we often go beyond our regulatory requirements by carrying out additional environmental studies to examine and understand our interactions with the water environments and habitats with which we coexist. Ultimately, we rely on a healthy water environment to rear heathy salmon.”
The scope of the water quality monitoring studies has been enhanced with additional monitoring requirements introduced to support Mowi’s farming accreditations under the Aquaculture Stewardship Council, which recently changed its requirements on water quality as part of the new Farm Standard due to launch this year. The long-term environmental data sets being generated from Mowi’s freshwater lochs will provide unique scientific value, the company said.
These findings on freshwater quality follow an earlier study by Mowi published in 2024, based on a three-year post-closure environmental study conducted around Mowi’s former Isle of Ewe fish farm on the north west coast of Scotland.
This study indicated that the seabed at Loch Ewe had made a good recovery following the cessation of fish farming operations at the site.
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