New case study: integrating catchment and loch models to support sustainable freshwater aquaculture

Freshwater lochs are vital to Scotland’s salmonid farming sector, but they are also delicate ecosystems shaped by a range of natural and human influences. Many lochs are naturally low in nutrients, meaning even small changes in phosphorus levels - from rainfall, land use, managed water flows or aquaculture - can affect water quality. Understanding how much production a loch can support throughout the year is essential for both operators and regulators. 

Led by the University of Stirling, the other partners were Mowi Scotland, Dawnfresh Seafoods, SEPA and the Aquaculture Stewardship Council. The total project value was £563,283.

The project set out to improve how Scotland assesses nutrient loading in freshwater systems. Rather than relying on loch-only models or isolated monitoring data, the team developed an integrated approach that connects phosphorus coming from whole catchments with phosphorus generated by fish farming activity. 

Four Highland lochs were used to test the method. Each has long-term monitoring data and represents different hydrological conditions, including several with regulated water flows. Using nationally available land use and climate datasets, the project estimated how much phosphorus each catchment naturally delivers to its loch. These results were combined with detailed aquaculture production data and run through three established loch models, including one capable of capturing seasonal changes. 

The results showed that all four lochs remain within the low-nutrient (oligotrophic) range and that the integrated models produced credible predictions of both annual and seasonal phosphorus levels. Lochs with more heavily managed hydrology were harder to model, highlighting an important consideration for future regulatory assessments. 

The approach is already being used in licence applications and site planning, offering a clearer, evidence-based understanding of how natural and aquaculture-related nutrient inputs interact. By linking catchment and loch processes, the work supports environmentally responsible freshwater farming and provides a transferable tool for wider use in Scotland and beyond. 

The full title of this project is ‘Integrating catchment and loch models to assess phosphorus loading in freshwater salmonid aquaculture’.

Read the case study