Offshore wind - diadromous fish: review - January 2024

Executive Summary

• With the ongoing climate change crisis, there is an increase in renewable energy proposals, particularly offshore wind. Scotland’s goal is to reach net-zero carbon emissions by 2045, and development of offshore wind will play a large role in this. While offshore wind has many benefits, there are also potential negative impacts on surrounding habitats and fauna, which may also include diadromous fish of conservation concern.
• The purpose of this review is to identify evidence gaps, through a review of the current literature and expert panel workshops, relating to potential impacts of offshore wind development on diadromous fish at a strategic level. This report highlights further strategic research opportunities and areas for consideration.
• The focus of this review is on 10 diadromous UK fish species. Spatial focus is on Scottish waters, but evidence is also collated from the rest of the UK. Where no UK studies exist, research from elsewhere is included for additional detail and particularly for the species that have limited Scottish or UK data available.
• This review has synthesised the currently available information on the marine habitat use of diadromous fish and highlighted the existing knowledge gaps. None of the species have comprehensive, detailed information available. The best quality information is available for salmonids.
  • Atlantic salmon The most information available for marine habitat use in Scottish waters is for Atlantic salmon, however current published studies are still limited. This evidence suggests that Atlantic salmon are very likely to utilise areas with offshore wind developments. Additionally, overlap is very likely in inshore and estuarine areas where export cables run to the mainland.
  • Brown trout Globally, the marine habitat use of anadromous brown trout is quite well known, especially in near-shore areas. There have been some Scottish and UK studies, however these have mostly focused on estuaries and sea lochs. Very limited information is available for migration routes in the open sea. However, it is very likely that anadromous brown trout will overlap with most, if not all, of the Sectoral Marine Plan option areas. Additionally, overlap is very likely in inshore and estuarine areas where export cables run to the mainland – this may be the more likely location of overlap for anadromous brown trout as they generally spend more time in coastal areas.
  • European eel Marine migration routes of European eel adults are well known in coastal waters; however, these studies have not included UK eel populations therefore it is not possible to assess the potential overlap with offshore wind developments in Scottish waters with confidence. Juvenile migration data are based on trawl captures as it is not possible to track juvenile eels. Where inshore export cables enter estuaries where eel populations can be found, overlap is very likely as eels migrate in and out of freshwater habitats.
  • Three-spined stickleback No tracking studies exist, or are technically feasible, for stickleback in the marine environment. Capture records of the species in Scottish waters are limited and as a result, the distribution and ecology of three-spined stickleback is relatively unknown. Whilst seemingly common in many UK estuaries, stickleback have also been found in the open ocean and have been captured in locations overlapping with planned offshore wind developments. Overlap with export cables in inshore areas and estuaries is also very likely.
  • River lamprey Very little information is available for the marine life stage of river lamprey; all tracking work is focused on freshwater habitats and limited to returning adult fish. Only occasional opportunistic captures at sea exist. Overlap with export cables in inshore areas and estuaries where populations exist is more likely as these are habitats that lamprey must migrate through.
  • Sea lamprey Very little information is available for the marine life stage of sea lamprey; all telemetry work is focused on freshwater habitats. The marine records in Scotland are mainly from estuaries and few of the records are from the open sea, making it very difficult to assess the potential overlap with offshore wind. Overlap with export cables in inshore areas and estuaries where populations exist is more likely as these are habitats that lamprey must migrate through.
  • European flounder Despite its near ubiquitous distribution in UK waters, information available for the marine habitat use of flounder is mostly limited to estuaries. Most flounder seem to remain close to the coast, however some evidence exists for wider ranging movements. Flounder are very likely to overlap with offshore wind developments, especially those that are closer to the coast. Overlap with export cables in inshore areas and estuaries where populations exist is more likely as these are known habitats utilised by flounder.
  • European smelt/sparling, allis shad, twaite shad These species are very understudied and published work, especially in Scotland and the UK, is focused on freshwater distribution of spawning adults. Only a few tracking studies have been identified, but none in Scotland. These species are likely to have the potential for overlap with offshore wind, however with the current lack of data, it is not possible to estimate this with confidence. Overlap with export cables in inshore areas and estuaries where populations exist is more likely as these are habitats that these species must move through.
• The lack of information for populations contributing to Special Areas of Conservation designation inhibits any informed assessment of the potential impacts of offshore development on SAC site integrity.
• Four key topics were identified for potential impacts of offshore wind farms on diadromous fish:
  • Sound and vibration;
  • electromagnetic fields;
  • changes in light patterns;
  • issues associated with novel habitat creation (including community change, predation risk, increases in suspended sediment, increased vessel activity, disease risk).
• While potential impacts of sound on fish behaviour and physiology have been studied, most of the work has focused on certain taxa, studies have been primarily laboratory-based, and the majority of measurements reported from field and laboratory studies have related to sound pressure. For the fish species in this review, particle motion is likely to be a very important component of sensing sound and therefore it is highly desirable that it is measured.
• The potential for negative impacts from sound associated with offshore wind farms are most likely during construction, with pile driving creating particularly loud sound pulses. This disturbance will be relatively short term but intense. Potential impacts include behavioural changes such as avoidance, physiological changes and in some cases, physical injury. Conversely, operational sound exposure will be long term but lower intensity. It will also not be consistent over time and therefore exposure may be variable. Data suggests that in some cases operational noise may be similar in intensity to the surrounding ambient noise, however as the particle motion has not been measured and there are still limited measurements available, the potential effects are unclear.
• Electromagnetic fields (EMFs) will be emitted from subsea power cables transmitting energy from offshore wind farms to onshore grids or substations. These EMFs have the potential to cause behavioural and physiological effects on diadromous fish.
• Most research on the impacts of electromagnetic fields has been done on elasmobranchs due to their sensitivity to electric fields, but research also exists on a number of other taxa. Very little research has been done on diadromous species and in field settings which makes assessing potential impacts difficult.
• Changes in light patterns can include shadow flicker from the moving turbine blades and light reflection off the surface. There was no direct research on these topics that could be identified through the review and therefore the potential impacts on diadromous fish remain unknown. Other possible sources of light at offshore wind farm developments are those associated with navigation and construction platforms, however these were not covered as part of this review.
• Construction of offshore wind farms will create new vertical habitat and hard substrate in areas where they did not exist previously. This will lead to colonisation by species and an ‘artificial reef’ effect whereby an increase in both prey and predator species may also be found. The potential impacts associated with the ‘artificial reef’ effect on diadromous fish are complex and may include both negative and positive effects such as novel habitat creation, community change, increased predation risk and increased risk of disease. Construction work and novel habitat creation may also lead to increased amounts of suspended sediments. Very little in situ research has been done on this topic and therefore assessing impacts is difficult.
• A series of five expert panel workshops were held as part of this review to obtain the opinion of experts and gather additional knowledge on the following topics: the marine distribution of UK diadromous fish species and the potential impacts on these diadromous fish associated with sound, electromagnetic fields, changes in light patterns and issues associated with novel habitat creation.
• A review of the literature and subsequent expert panel workshops highlighted gaps in the scientific knowledge in areas related to the potential impacts on diadromous fish associated with sound and vibration, changes to light patterns, electromagnetic fields, creation of novel habitat and the associated changes to predator and prey distribution and disease profiles.
• To inform best practice for the further development of offshore wind, information relating to the spatio-temporal distribution of the diadromous fish species should be collected in concert with information addressing the four key topic areas of potential impact. A combination of laboratory and field research studies are recommended. The report provides a list of key evidence gaps, however there was no consensus, at a strategic level, of a priority order associated with any specific potential impact source.