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

5. Conclusions

Given the current and projected rapid growth of offshore wind developments around the coast of Scotland, predicting possible encounter rates and understanding the potential effects of wind turbines on salmonids as they undertake their post-smolt and adult migrations is imperative to inform Environmental Impact Assessments and contribute to the conservation of these imperilled species.

By employing acoustic telemetry to track the movements and behaviours of Atlantic salmon post-smolts and adult sea trout in the Moray Firth, this study showed that almost half (44%) of salmon post-smolts emigrating from Wick Bay did not follow a northerly direction that would represent the logical ‘shortest route’ route to their northerly feeding grounds but instead initially travelled east, south-east or south. A total of 19% of salmon post-smolts undertook routes that passed within 500 m of turbines. This represents a minimum passage rate through the OWDs studied because only 48% of turbines were associated with a receiver station. Most salmon post-smolts exhibited fairly direct movement paths through the OWDs, on average residing for less than 1 h. However, a component of study group showed migration paths contrary to the hypothesis of rapid transit through the Moray Firth during emigration, thus increasing potential exposure to turbine areas.

Adult sea trout translocated into an operational OWD and nearby “Control Area” outside of the OWD remained close to the turbines for a minimum average duration of 4.2 h, with 40% making multiple re-entries to the area over a period of one week or more. Fish exhibited frequent diving behaviour and extensive use of deep waters, with over 50% of detections recorded at 40 m depth or greater. No differences were detected in adult sea trout behaviour and residency while in the OWD compared to the “Control Area”.

While it is acknowledged that inherent limitations of the detection array and tracking technology reduce certainty about the fate of a small number of individuals, 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.

Given the extended and complex association with turbine areas observed for the first time in salmon post-smolts and adult sea trout, greater consideration needs to be given to the possible postulated negative effects that may arise in wind energy generation sites, including predation hotspots, sound and vibration, visual cues, and electromagnetic zones. The findings of this study highlight the potential for multiple and extended exposure periods. This needs to be considered in future research in the context of possible cumulative effects from exposure among fish populations encountering multiple OWDs; as may result from the realisation of consented and in construction wind farm developments, in addition to those already in operation. Further, dense grid arrays of acoustic receivers that permit continuous tracking of individuals within OWDs would provide the opportunity to expand on the possible drivers of complex migration paths, such as those observed both in salmon post-smolts and adult sea trout in the current study.