From 13 July to 3 September 2018, Hans Slabbekoorn of the University of Leiden and his collaborators deployed hydrophones and accelerometers 50 km off the coast of Belgium. The researchers’ goal was to investigate the underwater sound conditions that prevail in waters that are home to fish and busy with ships.

Their results appear in the September volume of the Journal of the Acoustical Society of America.

The site they chose had been used the year before to investigate the effectiveness of a technique, acoustic positional telemetry, applied to tracking fish tagged with tiny transmitters. Led by Slabbekoorn’s PhD student Inge van der Knaap (working from the Universities of Ghent and Leiden), the study followed Atlantic cod that spend most of the summer and fall among the monopile foundations of the Belwind offshore wind farm (shown here). Excluded from the wind farm’s vicinity, fishing boats couldn’t reach and harvest the study’s subjects, the cod.

Falling rain, ships’ propellers, and seafloor dredging were among the ambient sounds that collaborator Peter Rogers, from the Georgia Institute of Technology and first author on the paper, expected to detect. They also had a Norwegian research vessel, MV GEO Caribbean, conduct a seismic survey near the wind farm for 3.5 days. Such surveys are used to prospect for oil and other resources beneath the seabed. They entail towing an array of air guns behind the ship. Reflections from guns’ acoustic pulses are detected by an array of microphones towed behind the guns.

Fish and marine invertebrates are sensitive to the particle motion associated with sound waves rather than to the pressure changes. Cod are presumed to hear in the range 40–500 Hz. In that range, the hydrophones that Slabbekoorn’s team deployed could detect the survey’s pulses up to a distance of 31 km from the array. The accelerometers, which measure particle motion, could detect the pulses up to 10 km away. The directionality of the pulse signal, determined by both devices together, could be traced up to about 18 km away.

During the study’s 52-day span, the most consistently strong ambient signal in both acoustic pressure and particle motion followed a temporal pattern of tides, which is logical as their ebb and flood are known to generate turbulent flow. Because the detectors were sensitive to direction, Slabbekoorn and his collaborators could identify the geographical origins of the tidal signal. This led to the most surprising finding because of the distance of the most likely sources (tens of kilometers away)- the estuaries of the Schelde and the Thames, the Rhine delta, the canal that connects Amsterdam to the North Sea, and the 20-km-long beach at Zeebrugge, Belgium.

If the sound associated with tides affected fish, they can be presumed to have adapted to it after millions of years of living with it. What about seismic surveys? The acoustic pulses, even when overhead, are typically not strong enough to harm fish. But the pulses do have the potential to interfere with their ability to sense their soundscape over a large volume. (P. Rogers et al., J. Acoust. Soc. Am. 150, 2174, 2021.)