Kirke Paris examined in a master's thesis defended at the University of Tartu how low-frequency noise typical of offshore wind farms affects the behavior of Baltic herring. A field experiment showed that herring schools were less dense near the noise source. However, the preliminary result does not mean wind farms necessarily cause significant harm to fish.

The research was driven by a practical need, as large offshore wind farms are planned for Estonia's coastal waters. At the same time, human-generated underwater noise is increasing due to growing shipping traffic and other marine infrastructure. Paris said understanding how animals perceive their surroundings is essential to understanding how fish use their habitats and move through them.

Researchers decided to observe the fish in the wild because sound waves may not propagate under laboratory conditions in the same way they do in the open sea. An artificial environment can also affect animal behavior. "A fish in captivity certainly won't respond to a stressor in the same way it would in the wild," Paris said, explaining the choice of experimental method on Vikerraadio.

The experiment took place in the Gulf of Riga, in an area of sea south of Kihnu. A special speaker simulating the operating noise of an offshore wind turbine was installed in the center of the roughly 16-square-kilometer study area. Researchers traveled along predetermined routes aboard a research vessel and monitored the density of fish schools in the water column.

The sound played underwater had a frequency of 100 hertz, fairly typical of offshore wind turbines. The speaker's source level reached 170 decibels during the experiment. "In the open air, that could be very loud to a person, but underwater it's more of a low, continuous hum," Paris said.

When researchers compared conditions with and without the noise, a clear pattern emerged from the data. Under noisy conditions, the density of herring schools decreased near the sound source. Although not all the fish left the area affected by noise pollution entirely, the herring schools became considerably less dense.

The experimental data showed that the noise had a statistically significant effect within about 350 meters of the source. Paris said a radius of less than half a kilometer may not pose a major obstacle to an animal in the context of a single wind turbine. "The fish can swim away if it finds the noise unpleasant," she said.

The situation changes significantly, however, when a wind farm consists of dozens or even hundreds of turbines. In that case, the affected area expands and the combined impact of multiple turbines becomes important. Location becomes particularly critical if large development areas impede fish from reaching their spawning grounds. Paris emphasized that the current study does not establish that such a disruptive effect will necessarily occur in the Baltic Sea.

In addition to noise pollution, wind farms can affect fish in other ways. Sediment stirred up by construction work on the seabed, for example, can reduce underwater visibility. Electromagnetic fields generated by cables connecting wind farms to shore can also interfere with fish's natural ability to navigate.

Not all changes are necessarily harmful to marine life, however. The massive foundations of wind turbines often attract fish, increasing local biomass and the amount of available food. Fishing exclusion zones often established around wind farms can also provide fish with a safe refuge.

Herring have excellent hearing

Paris said fish hearing is considerably more complex than it might initially appear. Although aquatic animals have no externally visible ears, their inner ears detect changes in sound pressure in their surroundings. Many species can also detect the movement of water particles.

Because fish are an extremely diverse group, hearing ability varies greatly between species. Some fish can detect ultrasound, while others can perceive infrasound. Most species, however, hear best at low frequencies, ranging from below 100 hertz to a few hundred hertz. "Sound helps fish navigate their environment, for example when searching for food and avoiding predators," Paris said.

Baltic herring are likely among the fish with the best hearing in the Baltic Sea. Although their tolerance of noise has not previously been studied directly, there is extensive data on Atlantic herring. Baltic herring are a closely related subspecies of Atlantic herring. "Because their hearing anatomy is very similar, we can assume that Baltic herring have quite good hearing," she said.

Field experiments for the research were conducted from 2023 to 2025 and the master's thesis formed part of a broader research project. Paris focused specifically on the response of feeding Baltic herring. The experiments were conducted in the fall when the fish feed actively and range widely across the Gulf of Riga.

Researchers later tested the behavior of Baltic herring during the spring spawning season in the shallower waters of Pärnu Bay. They found no significant effect from noise at the spawning grounds, a positive finding for offshore wind farm planning.

Paris plans to examine the specific mechanisms behind fish perception in greater detail as she begins her doctoral studies. She is particularly interested in how aquatic animals perceive the movement of water particles caused by sound. The field remains relatively understudied worldwide and is technologically novel.

Paris said assessing the impact of wind farms requires looking at more than a single factor. The cumulative effects of different factors, including noise, sediment, shipping and other variables, must all be taken into account. "Ultimately, what we're interested in is how this noise affects the entire ecosystem, not just one species," Paris said.

She said the study's findings therefore do not mean wind farms can be built at sea without restrictions. At the same time, there is no reason to fear that noise from a single wind farm would immediately destroy the Baltic herring population. "It's certainly a very bad idea to fill the entire Gulf of Riga with wind turbines, but we need to look for solutions intelligently," Paris said.

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