Wind farms threaten seal hearing

In recent years research has sounded an ever-louder alarm about the effects of noise from industry, boating, and military activities on marine mammals. One source of this noise is pile driving during construction of offshore wind farms. Scientists have feared that this activity could particularly affect seals which, like wind farms, tend to cluster around offshore sand banks.

The first study to track seals’ movements during wind farm construction bears out such fears, finding that half of seals were exposed to noise levels above the threshold believed to cause permanent damage to pinniped hearing.

In the study, researchers from the University of St. Andrews in Scotland glued tracking tags to the backs of 24 harbor seals at haul-out sites in the vicinity of a wind farm under construction 8 kilometers off the southeast coast of England.

The researchers tracked the seals’ movements from early January to mid-May 2012, while wind farm workers installed 31 steel pilings as wind turbine foundations. The tags provided each seal’s location every 15 minutes to an accuracy of within 50 meters. They also recorded the depths of seal dives, enabling the researchers to reconstruct the animals’ ups and downs on a second-by-second basis.

The researchers also assembled data on the time, location, and force of nearly 78,000 individual pile driving blows, which produce a pulse of underwater sound as loud as 250 decibels every 1 to 2 seconds.

Finally, they fed all these data into a computer model to calculate each seal’s noise exposure for the duration of the study.

All of the seals were exposed to noise levels thought sufficient to cause temporary hearing impairment, the researchers reported May 19 in the Journal of Applied Ecology. Twelve of the 24 seals were exposed to noise levels above the threshold for permanent hearing damage at least once and, in some cases, up to 9 times over the course of the study period.

These conclusions depend on certain assumptions about how pile driving sounds travel through shallow water and how pulsed noises affect the animals’ hearing, the researchers caution. Both of these topics require further study, they say.

Still, the results are worrisome because underwater hearing likely plays a big role in helping seals find and compete for mates, follow the movements of prey species, and avoid predators.

Moreover, the risks to seal hearing are likely to increase in the coming years. Several thousand wind turbines are proposed for installation off the coast of Europe, many of them in seal habitat.

To minimize these risks, the wind farm industry should engineer strategies to reduce noise from pile driving, or perhaps find ways to keep seals out of the area when turbines are being installed, the researchers suggest.

Even so, a hard truth to confront is that renewable energy development is still, well, development. Whether it’s changes in prairie-chicken mating behavior from wind farms on land or damage to seal hearing from those at sea, even green industry has an environmental impact. – Sarah DeWeerdt | 26 May 2015

Source: Hastie G.D. et al. 2015 Sound exposure in harbour seals during the installation of an offshore wind farm: predictions of auditory damage. Journal of Applied Ecology DOI: 10.1111/1365-2664.12403

Header image: A harbor seal (Phoca vitulina) swims near Dover, on the southeast coast of England. Credit: Tony Morris via Flickr.