Harbour porpoises are small toothed whales widespread in the cool, coastal waters of the Northern Hemisphere. They are identified as a protected species within the European Union. They echolocate by sending out “clicks” at a frequency of around 130 kHz. Photo by Solvin Zankl, Fjord&Belt.
Eigil has spent 12 years of his porpoise life in captivity and knows every inch of his home so he approaches the object that has appeared out of nowhere with a comfortable curiosity. It seems harmless. Soon, Freja, Sif and Frigg join him.
“Usually they get nervous when you put new objects into their pool, but they don’t seem to mind the hydrophones,” said Magnus Wahlberg, Chief Scientist at the Fjord&Belt marine research centre on the island of Funen in Denmark.
Maersk Oil has been monitoring harbour porpoises around oil platforms in the North Sea since 2007. The results are being used to assess the impact of underwater noise from offshore oil and gas activities on marine mammals.
Fjord&Belt helped Maersk Oil deploy a number of hydrophones around Maersk Oil’s platforms in the Danish part of the North Sea. These underwater microphones were first calibrated at the Fjord&Belt centre to better record and estimate the intensity of the high frequency ‘clicks’ made by porpoises.
The clicks are part of the porpoises’ process of echolocation in which, as in bats, they emit ultrasonic sounds and listen to the returning echoes to be able to locate objects around them and navigate through the waters.
By recording these sounds, Maersk Oil can estimate the number of porpoises around platforms and determine whether noise from its activities adversely affects the animals.
“Our internal standards require us to conduct environmental impact assessments of our activities to identify and document any environmental risk they pose,” explained Steffen Bach, a marine biologist with Maersk Oil for a decade.
“Underwater noise and potential impact on marine life is a topic that has received a lot of attention lately. Both we and our external stakeholders need to know more about this topic in order to make the right decisions,” he said.
Initial results reveal that harbour porpoises are present around platforms, sometimes in relatively high numbers, suggesting that offshore platforms might function as artificial reefs attracting fish and other sorts of prey.
“So far, the results indicate that we do not have an overall negative impact on their presence in the area. Nevertheless, we want to make some changes such as initiating “slow-start procedures” for loud activities which help bring the noise levels down,” Bach said.
“Through our studies, we noticed a brief reduction in the number of recorded ‘clicks’ during the few occasions when activities have created loud noise, but we also saw that porpoises return within a day after high noise activities have ceased. So one thing we can do is to keep these activities infrequent and ensure they are completed quickly. This way, we can minimise our disturbance.”