Harbor porpoises are among the smallest whales and the only whale that is known to breed in Danish waters. The harbor porpoise was protected in 1967 in Danish waters, and researchers from Aarhus University, Denmark, have previously shown that underwater noise from ships and seismic surveys of the seabed scare the porpoises away.

A new study now shows that in some parts of the year, there are actually more porpoises searching for food around the largest Danish oil platform, Dan F, in the North Sea than just 3 to 10 kilometers away from the platform.

21 listening stations in the North Sea

A team of researchers deployed 21 listening stations at the bottom of the North Sea. The innermost listening station was deployed immediately under the very large Dan F-oil and gas platform, while the outermost station was located 25.6 km away from the platform. One of the many listening stations was placed halfway between the inner- and outermost stations near the old oil well, Ragnar, which no longer produces oil.

The listening stations were left in the North Sea for two years to record all sounds below the surface of the sea, including the noise from ships and platforms, but also the sounds produced by porpoises for orientation and the so-called "buzz sounds" by which researchers can detect that whales are actively hunting for food.