Sound pollution from offshore wind turbines poses a threat to Baltic Sea porpoises, a German marine mammal specialist said on Monday.

"It was highlighted that wind turbines are so noisy that the harbour porpoises in the Baltic Sea are threatened," Harald Benke, the director of Germany's Sea Museum, told AFP.

"There are methods to reduce the noise. They are costly but allow the considerable reduction of volume of noise produced by the wind turbines, which threaten at least that population" of marine mammals, said Benke, who speaking from a whale researchers conference in the northern German city of Straslund.