What is the impact of ocean noise on cetaceans?

The ocean is readily imagined as a world of silence. The opposite is true. Below the surface, sound travels five times further and five times faster than in air. Water is in fact a remarkably noisy medium: waves, rain, earthquakes, cracking ice, thousands of species vocalising. But this natural soundscape, which shaped the evolution of cetaceans over fifty million years, has been profoundly transformed in the space of a century by human activity. In some areas, underwater noise levels have at least doubled every ten years since 1960. For animals whose survival depends on sound, this is a silent revolution with massive consequences.

The anthropogenic sources

Anthropogenic ocean noise comes mainly from four sources. The first, and by far the most widespread, is shipping traffic. More than fifty thousand large vessels are permanently at sea, producing low-frequency noise (10 Hz to 1 kHz) at between 180 and 195 decibels at the source. Commercial routes are particularly noisy: the English Channel, the Strait of Singapore, the Persian Gulf, the Mediterranean Sea. This chronic noise is everywhere, twenty-four hours a day.

The second source is seismic surveying for oil exploration. Specialised vessels tow compressed-air guns that fire in sequence, producing acoustic shocks of 220 to 260 decibels peak. These signals are designed to travel through kilometres of rock beneath the seabed and reveal its geological structure. They are perceptible hundreds of kilometres away.

The third category covers military sonars, among which MFAS have become notorious for their effects on beaked whales (see our dedicated page). Their impact is concentrated in time and space but can be lethal.

The fourth and most recent source is pile driving for the installation of offshore wind farms. A pile several tens of metres long, driven into the seabed with a hammer weighing several tonnes, produces peak acoustic waves of 245 to 260 decibels, enough to drive every porpoise away within a radius of several tens of kilometres for weeks.

Masking: the main blow

The most widespread and probably the most serious impact is acoustic masking: the covering by anthropogenic noise of the signals cetaceans use to communicate with one another. Baleen whales vocalise at low frequencies (10 to 500 Hz), exactly the band in which ship noise dominates. The consequence: their calls carry far shorter distances than before.

A study by Christopher Clark and colleagues published in Marine Ecology Progress Series in 2009 quantified this effect. The upcall of the North Atlantic right whale, the signature call that maintains group cohesion, carried about 10 km in the pre-industrial ocean. Today, in high-traffic areas such as the Bay of Fundy, it carries no more than 1 to 2 km. For a species whose world population is below 400 individuals and which has to breed and coordinate its movements, this is a critical loss. The chances of finding a mate, of finding a calf again, of coordinating a migration, are divided by five to ten.

Measured behavioural disturbance

Beyond masking, several behavioural effects have been quantified. Fin whales and humpback whales stop their feeding dives during noisy events, seismic surveys in particular. Porpoises actively avoid wind farm pile-driving sites at distances of up to 25 km. Pacific blue whales have lowered the central frequency of their songs by about 30% over recent decades, a phenomenon documented by McDonald and colleagues in 2009. The leading hypothesis is that this drop in frequency compensates for rising ambient noise in the higher frequencies, but other explanations remain possible (demographic change, mean size of individuals, and so on).

Chronic stress: an elegant piece of evidence

One of the most convincing demonstrations of the impact of ocean noise comes from a tragic event turned into a natural scientific experiment: the attacks of 11 September 2001. In the days that followed, commercial shipping traffic was drastically reduced in the North Atlantic, in particular in the Bay of Fundy in Canada, an area frequented by right whales.

Since 1999 Rosalind Rolland and colleagues had been regularly collecting faeces from these whales in order to measure their stress hormone (glucocorticoid) levels. The September and October 2001 samples showed a significant drop in stress levels compared with the preceding and following years. Published in Proceedings of the Royal Society B in 2012, this study established a direct quantitative link between ambient noise level and physiological stress in a large wild marine mammal. Chronic stress has lasting consequences: reduced fertility, vulnerability to disease, increased mortality.

What can be done?

Several levers exist. Reducing the speed of vessels in sensitive areas is extremely effective: a 10% drop in speed reduces radiated noise by about 40%. Several ports have adopted voluntary programmes (Vancouver’s “Echo Program“, Marseille, Le Havre). In 2014 the International Maritime Organization published recommendations for the design of quieter hulls and propellers, which are gradually being incorporated into new vessels.

At offshore wind farm sites, bubble curtains around the piles can attenuate pile-driving noise by 10 to 20 decibels. Seismic surveys can avoid critical periods (breeding, migration) and use quieter alternative techniques (marine vibroseis) that are under development.

What is at stake is immense. Without a reduction in ocean noise, several already critical species (North Atlantic right whale, vaquita, remnant populations of Mediterranean sperm whales) might not recover. Noise is not a visible form of pollution, but it is probably one of the most insidious for cetaceans.

Sources

• Clark, C.W. et al. (2009). “Acoustic masking in marine ecosystems: intuitions, analysis, and implication”. Marine Ecology Progress Series, 395, 201-222.
• McDonald, M.A., Hildebrand, J.A. & Mesnick, S.L. (2009). “Worldwide decline in tonal frequencies of blue whale songs”. Endangered Species Research, 9, 13-21.
• Rolland, R.M. et al. (2012). “Evidence that ship noise increases stress in right whales”. Proceedings of the Royal Society B, 279, 2363-2368.
• Erbe, C., Marley, S.A. et al. (2019). “The effects of ship noise on marine mammals, a review”. Frontiers in Marine Science, 6, 606.
• OSPAR (2014). “Underwater noise indicator assessment”.

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Further reading