Estonian researchers study potential impacts of wind farms on valuable fish stocks
This article was featured in Eurofish Magazine 4 2025.
As efforts to increase energy security in the Baltic gather strength, offshore wind farms will play an important role. The impact of noise from wind turbines on fish stocks is a little-studied subject, which researchers in Estonia seek to remedy.
The growing demand for renewable energy in the Baltic region has prompted a surge in plans for offshore wind development across the region. Polish researchers estimate the potential for offshore energy in the Baltic Sea at 85 GW. But as this push accelerates, so too does concern over the potential ecological impacts. The construction and operation phases of offshore wind farms result in geophysical, geochemical, and hydrological, among other, influences on the environment. One of the key questions under investigation is how underwater noise from offshore wind turbines could affect marine life, particularly Baltic herring, a commercially and culturally significant species.
Dr Mehis Rohtla, Estonian Marine Institute at the University of Tartu, is studying
the impact of underwater noise from offshore wind turbines on Baltic herring stocks.
Studies based on the artificial reproduction of sound
At the forefront of this research is Dr Mehis Rohtla from the Estonian Marine Institute at the University of Tartu. Since 2022, his team has been conducting a government-funded study to simulate and assess the behavioural responses of Baltic herring to underwater noise resembling that of offshore wind turbines. Notably, since there are currently no offshore wind farms operating in Estonian waters (though several are planned), the researchers have had to recreate the noise environment artificially. Using underwater loudspeakers capable of emitting low-frequency sounds, the researchers are simulating the kinds of acoustic disturbances expected from future high-capacity turbines—those rated at 15 to 20 megawatts. These devices, when deployed, are expected to emit predominantly low-frequency noise. -Bandwidths -chosen by Dr Rohtla’s team for their experiments are 100 Hz and 500 Hz.
Studies on the impact of noise from wind turbines on fish stocks are few and far
between partly because of the expense involved in terms of equipment and man-hours.
The project is multidisciplinary, bringing together marine biologists, physicists, and chemists, with collaboration between the University of Tartu and Tallinn University of Technology. Chemists played a critical role in developing a novel hydrogen-powered system for generating electricity offshore, which allows the noise equipment to operate continuously at sea, independent of battery limitations. Fieldwork has been conducted across multiple locations, including the Gulf of Riga, Pärnu Bay, and off the western island of Hiiumaa. Each site hosts a different set of experiments and/or herring behaviour: spawning migration, spawning, and feeding. By studying these phases separately, the researchers aim to understand the full scope of the herring’s behavioural responses to sound at different points in their yearly life cycle. Experiments involve both free-swimming herring in open water and controlled tests in net cages. The open-sea studies are especially notable for their scale and complexity. Once a herring shoal is located using sonar, researchers anchor the sound source and monitor the fish’s movements by conducting -transects—systematic paths taken by a survey vessel around the sound source. These allow them to map the distribution and density of fish before and after the introduction of artificial turbine noise. In the net cage experiments off Hiiumaa, a location selected for the clarity of its water, GoPro cameras and live-scope sonar provide visual and acoustic data on individual fish reactions within a fixed space.
Responses to noise depend on several factors
The core objective of the research is to identify a reaction threshold—the point at which sound intensity triggers a measurable behavioural response in the fish. Establishing this threshold, expressed in decibels at a specific frequency (primarily 100 Hz), would allow developers to model the potential impact zones of their proposed wind farms using local acoustic propagation models. Such data would be crucial for environmental impact assessments and could inform more responsible site selection and turbine spacing. So far, the experiments have yielded nuanced findings. During the spawning period, which occurs in shallow waters nearshore, herring appear relatively unaffected by turbine-like noise. Despite the presence of sound levels reaching 150–160 decibels near the source, herring continued to gather in large numbers in spawning zones, often within a few metres of the speaker. Dr Rohtla hypothesises that the drive to spawn may override the fish’s sensitivity to external stressors. A similar tolerance was observed during spawning migration through deeper waters, suggesting that fish remain largely undisturbed during this phase. However, a different picture emerged during the feeding period, which occurs in deeper offshore zones. Here, some herring were observed avoiding areas within a 500-metre radius of the noise source, although the fish did not abandon the broader area entirely. According to Dr Rohtla, the feeding phase is probably more sensitive to disturbance, since the fish are not engaged in an urgent biological function such as reproduction.
The team also noted that sound propagation characteristics vary with water depth. In shallow waters, low-frequency noise scatters and is also absorbed by the seabed, limiting its range. In deeper water, however, such noise can travel further, potentially broadening the impact zone. These physical factors mean that each prospective wind farm site would require its own tailored acoustic modelling. In the controlled cage experiments, fish began exhibiting flight responses—such as erratic swimming and depth changes—at distances of around 100 metres from the sound source. These reactions occurred when exposed to sound levels of around 130–140 decibels. However, Dr Rohtla cautions that the confined conditions of cage experiments may influence fish sensitivity to sounds, as the fish are under stress and unable to flee entirely, which may alter their reaction threshold. By triangulating findings from free-swimming and cage-based experiments, the researchers hope to define a reliable behavioural threshold for noise exposure. This will allow regulators and developers to more accurately predict the size and scale of potential disturbance zones in different underwater environments.
Herring is a commercially critical species in the Baltic
Herring was chosen for this research not only because it is Estonia’s national fish and a cornerstone of its commercial fishery, but also because of its sensitivity to sound. Compared to many other fish species, herring have well-developed hearing across a wide range of -frequencies while also being very sensitive to the intensity of sound. Dr Rohtla points out that this makes them an ideal indicator species for acoustic impacts in marine ecosystems. This line of research is particularly urgent in Estonia, where up to ten offshore wind farm developments are in planning stages. Each could significantly affect coastal marine habitats. The Gulf of Riga and Western-Estonia, both important herring habitats, are among the regions earmarked for development. While current environmental assessments often rely on literature from other regions or on theoretical assumptions, Dr Rohtla argues that real-world experiments in Baltic conditions are essential.
Once a herring shoal is located using sonar, researchers anchor the sound source and monitor the
fish’s movements by conducting transects—systematic paths taken by a survey vessel around the sound source.
He also stresses that offshore wind infrastructure decisions must be based on data from the same ecosystem in which they are to be deployed. The Baltic Sea, with its shallow depth, unique salinity profile, and distinct seasonal cycles, is unlike the North Sea or Atlantic. As a result, extrapolating from studies conducted elsewhere may not provide an accurate picture of local impacts. The project has also catalysed the revival of underwater acoustics research in Estonia. According to one of the team members, the study of underwater acoustics in Estonia has been largely dormant since the Soviet period. The acquisition of hydrophones, underwater speakers, and new measurement protocols has re-established a foundation for future marine noise studies in the region. So far, no peer-reviewed publications have emerged from the project, though Dr Rohtla notes that a master’s student is preparing a thesis based on the results. After the termination of the project in this September, the results will be communicated through peer-reviewed publications and conference presentations.
Results should inform planning of wind farms in the Baltic
For now, Dr Rohtla and his colleagues remain focused on finishing field experiments, analysing results, and refining their threshold estimates. With offshore wind projects advancing rapidly, the team hopes their findings will contribute to more sustainable planning and a clearer understanding of how energy transitions intersect with marine ecology. While offshore wind remains essential for reducing carbon emissions, Dr Rohtla stresses the importance of ensuring that its deployment does not come at the expense of critical marine species like the Baltic herring. As usual with scientific studies they generate at least as many questions as they answer. He would therefore recommend establishing one wind farm first and studying the impacts before proceeding with more.
