Harbour porpoises flocking in the Fehmarnbelt

Since the start of 2009, the porpoises in the Fehmarnbelt have been closely monitored to determine their movements and any impact a fixed link above or below the sea may have on these animals. Provisionally, investigations will continue until the end of 2010, and they have cast new light on the extent of the porpoise population in the area.

"We do not have any precise numbers, but when we look at previous investigations, there are significantly more porpoises in the Fehmarnbelt than we had expected. In spring 2009, in the space of just eight hours we observed around 40 animals, and in autumn 2009 we saw more porpoises than expected," says Dr. Georg Nehls of BioConsult SH.

The new discoveries are now being included as part of the environmental investigations prior to construction of the Fehmarnbelt Fixed Link and may be of significance for planning the remaining work.

Artificial reef attracting fish
"If the bridge or tunnel is built in an appropriate way, then the impact of noise and vibration can be kept to a level which does not exceed that caused by an ordinary ship. And the porpoises appear to be managing to live with the shipping traffic in the belt today," says Georg Nehls.

He also emphasises that experience gathered from similar projects shows that underwater constructions such as bridge pylons or tunnels actually attract fish, because they function as a sort of artificial reef. And fish are followed by porpoises who feed on them.

Permanent guest or holidaymaker
The harbour porpoises are monitored from land, water and the air depending on the weather, and using special underwater microphones which work in all weathers. Together, all these methods of measurement have helped to chart the rich porpoise life in Danish, German and Swedish waters.

The investigations also point to there being fewer porpoises in the Fehmarnbelt in the summer months than in spring or autumn. That can help explain the movements of the animals in the waters, and whether there is a seasonal migration of porpoises.

"We are basically working on two theories: Either the porpoises are moving around the Fehmarnbelt all year round, or the Belt is playing a greater role as a migration route between the North Sea and the Baltic Sea. But we don’t yet know the answer," says Georg Nehls.