Offshore wind turbine construction could be putting seals' hearing at risk

There are currently 1,184 offshore wind turbines around the coast of the UK, between them generating around 4GW of power. The next round of construction, which began in 2014, will see hundreds more turbines installed to generate a further 31GW, yet we know little about the impact of construction noise on sea mammals' hearing.

The team of ecologists from the University of St Andrews attached GPS data loggers to 24 harbour seals while offshore wind turbines were being installed in the Wash in 2012. The data loggers collected information on the seals' locations and their diving behaviour, relaying the information onshore via the mobile phone network.

They then combined this data with information from the wind farm developers on when pile driving was taking place to produce models which predicted the noise each seal was exposed to, and compared this with noise levels that other studies show cause auditory damage. The model revealed that half of the tagged seals were exposed to noise levels that exceeded hearing damage thresholds.

Offshore wind turbines are installed using pile drivers - essentially large hammers that drive the foundation posts into the sea bed - which produce short pulsed sounds every few seconds. According to lead author of the study Dr Gordon Hastie of the university's Sea Mammal Research Unit: "These are some of the most powerful man-made sounds produced underwater, noise capable of travelling large distances underwater."

Although we have some information on the effects of noise on harbour seals' hearing, very little is known about the impact of the pulsed sounds produced by pile driving. However, a wealth of data exists on the effect of noise on humans and other terrestrial species, data which shows that powerful pulsed sounds can damage mammals' hearing.

"Like most marine mammals, harbour seals have very sensitive underwater hearing at a much broader range of frequencies than humans," he says. "Seals probably use underwater hearing during the mating season and to detect and avoid predators. They may also rely on their hearing for navigation and finding prey."

The team's results are important because seals are protected under European law and any impacts that might affect their conservation status need to be assessed prior to the construction of wind farms.

"Our predictions highlight that seals may routinely be exposed to potentially hazardous levels of underwater noise during pile driving, with potential implications for the conservation status of some populations. To reduce these potential impacts, regulators and industry are currently investigating engineering solutions to reduce sound levels at source, and methods to deter animals from damage risk zones in order to potentially reduce auditory damage risk," Hastie adds.

The team now hopes to validate their predictions by making hearing measurements on seals using special 'seal headphones', monitoring individual seals' movements at sea, and collecting long-term data on their growth, reproduction and survival.

Harbour seals live around the coasts of the North Atlantic and North Pacific from the subtropics to the Arctic. Around 30% of European harbour seals are found in the UK. An epidemic of phocine distemper virus cut numbers along the east coast of England by half in 1988, and although a second outbreak in the Wash in 2002 resulted in a decline of 22%, the population has increased dramatically since 2009.

Harbour seals weigh in at around 80-100kg and are long lived, living up to 30 years. They dive from the surface to depth to catch a variety of prey, from sandeels and herring to octopus and squid. They haul out in sheltered waters, usually on intertidal sand banks and estuaries, pups are born in June and July and the seals moult in August.