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

1. Introduction

Increasing global energy demands coupled with recognition of the urgent need to reduce greenhouse gas emissions to counter anthropogenically driven climate change have led to widespread commitments to produce more “clean energy”. Marine renewable energy sources are an important component in this transition, with offshore wind energy production leading the rapid growth in this industry; global installed capacity reached approximately 83 GW in 2025 and is forecasted to grow to 285 GW by 2030 (GWEC, 2025).

The UK aims to achieve 50 GW of wind generation capacity by 2030 to help meet the Government’s emission reduction targets and provide greater energy security (International Energy Agency, 2023). Scotland, with its exposed coastline leading to plentiful and consistent wind generation potential, will play a fundamental role in meeting these goals, with the number of offshore wind developments (OWDs) set to expand significantly around Scotland over the next decade (Scottish Offshore Wind Energy Council, 2024). The Scottish Government aims to achieve net zero by 2045 and to generate 50% of Scotland's overall energy consumption from renewable sources by 2030 (Scottish Government, 2023).

There exists substantial knowledge about the impacts of long-established onshore wind energy developments on terrestrial organisms, with a particular focus on birds and bats (Arnett & May, 2016; Katzner et al., 2025; Laranjeiro et al., 2018). In comparison, research into the possible impacts of offshore wind developments on marine biota and particularly diadromous fish is in its infancy (Honkanen et al., 2024; Svendsen et al., 2022). While ecological studies of offshore oil and gas installations provide useful indication of the habitat changes and likely responses of biota to the construction of marine engineered infrastructure (Iorio-Merlo et al., 2023), the potential impacts of OWDs are wide-ranging and may include physical, acoustic, and electromagnetic effects across a range of taxa including marine mammals, birds, and fish (Gill et al., 2012; Harding et al., 2016; Malcolm et al., 2015).

Fish are key taxa of concern in the context of OWDs. Within this group, diadromous fishes are associated with the greatest knowledge gaps because of their relative scarcity and transient movements in marine environments (Bicknell, Gierhart, Newton, et al., 2025; Diadromous Fish ScotMER Receptor Group, 2024; Honkanen et al., 2024). The life history strategy of anadromous salmonids such as Atlantic salmon (Salmo salar) (salmon hereafter) and the migratory form of brown trout (Salmo trutta) (sea trout hereafter) which necessitates spawning in freshwater and migration to marine environments to optimise growth, renders them vulnerable to encountering OWD sites, potentially with multiple exposures (i.e. during both in and out-migrations).

The possible impacts of OWDs on salmon and sea trout fall into two categories: acute and chronic. Acute impacts derive from short-term construction and decommissioning activities and include heightened underwater noise, particle motion and vibration, sediment disturbance and rapid changes to localised habitats (Honkanen et al., 2024; Svendsen et al., 2022). Chronic impacts persist in the medium to long-term while the wind farm is operational and, based on our current understanding, may derive principally from: 1) exposure to electromagnetic fields (EMFs) in the vicinity of cables; 2) flicker and shadow effects from rotating turbine blades, and 3) on-going effects of habitat modification associated with the turbine and control centre structures. The latter may change within the site’s operational life as the ecosystem that colonises infrastructure matures (McLean et al., 2018).

Rotating turbine blades and the dynamic shadowing that results from interruption of direct sunlight potentially influence fish behaviour, reducing their ability to detect prey and predators (Williamson et al., 2024). A recent review by Williamson et al. (2024) concluded that aquatic organisms in proximity of the turbines and those uppermost in the water column would be most susceptible to flicker effects, which is highly relevant to juvenile Atlantic salmon which migrate predominantly near the surface (Davidsen et al., 2008; Newton et al., 2021).

There is high uncertainty about the effects of EMFs on fish. While there is theoretically potential for EMF to disrupt some fishes’ ability to migrate (Verhelst et al., 2025), studies looking at the impact of EMF emanating from subsea power cables on free swimming salmonids are lacking. In notable exceptions, acoustic telemetry was used to track Chinook salmon (Oncorhynchus tshawytscha) post-smolt movements within San Francisco Bay (Klimley et al., 2017; Wyman et al., 2018). Results were mixed with indications of both avoidance and attraction to energised cables, but overall it was concluded there was no strong barrier effect to migration. Current research suggests close field (<20 m) effects, if any, have the most potential to impact salmonids (Verhelst et al., 2025). Offshore wind energy export cables from fixed foundation turbine arrays are predominantly located in waters deeper than 20 m, and typically buried, so the relevance of EMF effects to surface-oriented salmon and sea trout post-smolts are likely to be limited to shallow areas i.e. particularly near the coast or in estuaries. However, greater focus on floating turbine installations may increase the prevalence of EMF effects on fish generally, and requires better understanding (Honkanen et al., 2024; Verhelst et al., 2025). Salmon transit relatively quickly through coastal waters during both their juvenile feeding emigration (Gilbey et al., 2021; Lilly et al., 2024; Rodger et al., 2025) and adult return spawning migration (Hansen et al., 1993). Conversely, sea trout predominantly remain in coastal waters and often make multiple movements between freshwaters and the marine environment (Thorstad et al., 2016). Further, adult sea trout make greater use of deep water than previously thought (Artero et al., 2025). These behaviours mean sea trout are likely to be more temporally exposed than salmon to the export cable corridors for OWDs (Honkanen et al., 2024).

The potential consequences of habitat modification caused by the presence of turbines and other marine infrastructure likely represent the most far-reaching and multi-faceted effects of OWDs on fish. The localised increases in biodiversity and abundance of marine biota resulting from the artificial reefs potentially created by wind farm piles and scour protection has been well documented (Bicknell, Gierhart, & Witt, 2025; Svendsen et al., 2022). The potential cumulative effect of introducing multiple widely distributed engineered hard structures to the seascape may also help some species spread beyond their natural range by creating a “stepping stone effect” (Coolen et al., 2020, 2022; Dannheim et al., 2020; Langhamer, 2012). This aggregation of biomass and prey species elicits a response among higher trophic levels, including piscivorous fish species, marine mammals, and seabirds, with the potential to create predation ‘hotspots’ that impose an artificially elevated loss rate on prey species (Bicknell, Gierhart, Newton, et al., 2025). Seals and porpoises have been shown to alter foraging behaviour in response to large offshore structures, with seal even adopting a grid-like hunting strategy (Russell et al., 2014). Salmon and sea trout are prey items for sea mammals such as seals and porpoises (Middlemas et al., 2007), so the aggregation of such predators within OWDs could increase marine loss rates. Conversely, heightened predation of piscivorous fish, which are in turn predators of juvenile salmon, may, along with the potential shelter provided by OWD structures, actually enhance salmonid survival rates during their migration.

Salmon populations are declining globally and in 2023 the International Union for the Conservation of Nature (IUCN) classification for Atlantic salmon (Salmo salar) moved from Least Concern to Near Threatened globally, and to Endangered in Great Britain (Nunn et al. 2023). Declines are likely driven by the complex interplay of multiple pressures, particularly habitat degradation and fragmentation in freshwaters and overexploitation, aquaculture, and broadscale changes in oceanographic conditions and productivity in the marine environment (Dadswell et al., 2022; Gillson et al., 2022). Similarly, these pressures have contributed to local collapses of the sea trout populations (Butler & Walker, 2007; Gargan et al., 2007; Skaala et al., 2014). When juvenile salmon and sea trout emigrate from freshwater to undertake their marine feeding migration, and when the adult component of the population returns to freshwater to spawn, there is the potential for migration routes to intersect with marine development sites. Understanding the spatial and temporal distribution of anadromous salmonids in the marine environment in general, and their overlap with OWDs in particular, has been highlighted as a ScotMER priority evidence gap (Diadromous Fish ScotMER Receptor Group, 2024) and is crucial for robust Environmental Impact Assessment of proposed developments. Around the Scottish coast, the paradigm of juvenile Atlantic salmon, on leaving their natal river (during a life phase generally termed ‘post-smolts’), making rapid directed migrations to feeding grounds is being increasingly challenged through detailed study of their migration routes. Acoustic tracking can generate spatially and temporally detailed information on the migration of individual fish. It has provided evidence of indirect migration paths and delay (e.g. Main, 2021; Newton et al., 2021; Rodger et al., 2025) that would appear, at a coarse scale at least, to be inefficient if assuming a strategy to minimise delay and energy expenditure to reach optimal feeding grounds in the Norwegian sea and West Greenland (Holm et al., 2000; Thorstad et al., 2012).

This study had 3 principal aims 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 salmon post-smolt swim paths within two operational OWDs in the Moray Firth (Beatrice and Moray East); and

3) Investigate patterns of residency and behaviour of adult sea trout within an operational OWD in the Moray Firth (Beatrice)