Migration routes and behaviour of Atlantic salmon and sea trout around operational wind turbine arrays in the Moray Firth, Scotland

Summary

Offshore Wind Developments (OWDs) are set to expand substantially around Scotland over the next decades to meet ambitious carbon reduction targets and meet Net Zero by 2045. A robust evidence base is needed to inform the spatial planning of OWDs to ensure minimal negative impacts on marine biota, including anadromous salmonids for which there are substantial knowledge gaps (Diadromous Fish ScotMER Receptor Group, 2024).

Possible impacts of OWDs on salmon and sea trout migrating through the marine environment may be acute, derived from construction and decommissioning activities (e.g. underwater noise and vibration, sediment disturbance and habitat changes), or chronic, i.e. persisting while the wind farm is operational. The latter may result from exposure to electromagnetic fields (EMFs), flicker and shadow effects from rotating turbine blades, along with on-going habitat changes and elevated predation risk around pile foundations and scour protection structures.

A fish tracking study was conducted in 2023 (Y1) and 2024 (Y2) which aimed to: 1) Determine the overlap between emigrating salmon post-smolt migration routes and OWDs in the North-West Moray Firth, 2) Quantify the duration and characteristics of post-smolt swim paths within two operational OWDs, and 3) Investigate patterns of residency and behaviour of adult sea trout within an operational OWD.

Salmon smolts (n = 208) captured in the River Wick were fitted with acoustic transmitters and tracked through an array of receivers within the lower river, harbour, marine bay and two adjoining OWD sites: Beatrice and Moray East. Additional receiver lines projecting from the north and south edges of the Beatrice OWD intercepted migration pathways.

Adult sea trout (n = 69) captured from tidal pools and the coast (near River Wester outlet) were fitted with acoustic transmitters, translocated and released either into the Beatrice OWD or a nearby ‘control area’ outside of the development. Over the following months, sea trout movements and behaviour were tracked throughout the Beatrice and Moray East OWDs and compared with the control area.

On average, 44% of salmon post-smolts did not head directly north after leaving Wick Bay to take the most likely shortest straight-line route to their northerly feeding grounds, but instead swam east, south-east, and to a lesser extent, directly south.

A total of 33 salmon post-smolts (19% of those that entered the Moray Firth) entered the wind turbine zone. Known residency was typically of short duration, with post-smolts spending a median of 36 minutes (maximum 5 h) among the turbines and most swim paths indicated direct transit, generally in an easterly to south-easterly direction.

A small number of salmon post-smolts were in the vicinity of, and potentially in, the turbine zones for extended periods: up to 22 h in Y1 and 51 h in Y2. Three individuals exhibited convoluted swim paths with multi-direction movements.

Translocated sea trout predominantly dispersed outside of the receiver array within a few days, although a substantial proportion of individuals in both Y1 (35%) and Y2 (42%) persisted in the area for over a week, moving extensively with swim tracks of up to 385 km, and re-entered the detection zones multiple times and up to 4 months after first detection.

Sea trout exhibited frequent diving behaviour and extensive use of deep waters (40–76 m). No differences were detected in sea trout behaviour and residency while in Beatrice OWD compared to the control area, suggesting that fish were unlikely to have adopted this deep diving behaviour in response to the turbine structures.

While acknowledging inherent limitations of the detection array and tracking technology, there was no evidence to indicate that either salmon post-smolts or adult sea trout were subject to high levels of marine predation within the OWDs or the wider study area.

The spatial and temporal patterns of salmon and sea trout movement observed in this study aid the prediction of encounter rates and understanding of the potential effects of wind turbines on salmonids as they undertake their juvenile and adult migrations. The findings should be used to inform Environmental Impact Assessments and contribute to the conservation of these imperilled species.

Recommended Citation

Piper, A.T. (2025). Migration routes and behaviour of Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) around operational wind turbine arrays in the Moray Firth, Scotland. ZSL Report.