Marine mammals

Key research findings:

Analysis of aerial bird surveys and Seabirds at Sea data shows that harbour porpoise have major distribution centres in the North Sea and Baltic Sea, including in the German EEZs.

Harbour porpoise (Phocoena phocoena) in the North Sea

There are three particularly important harbour porpoise sites in the North Sea EEZ:

Sylt Outer Reef
(North Sea, west of the islands of Sylt and Amrum)

• Area with the highest density of harbour porpoise in German waters; ‘core area’ for harbour porpoise in the German EEZ
• Continuous presence, including in winter
• Large proportion of calves in May and June
• Harbour porpoise display a heightened frequency of ‘vulnerable’ behaviour in the area (for example resting)

Borkum Reef Ground
(North Sea, north-west of the East Frisian Islands)

• Area of medium density but continuous presence
• High density in spring since 2004
• Calves sighted in June and July
• May be part of the southern North Sea subpopulation

Dogger Bank
(North Sea, in the ‘duckbill’ part of the German EEZ)

• Unexpected, relatively large number of sightings in the area
• Calves also sighted
• Regular surveys carried out so far despite remoteness of Dogger Bank from coast and unpredictable weather conditions

Harbour porpoise (Phocoena phocoena) in the Baltic Sea

There are three important harbour porpoise sites in the German Baltic Sea EEZ:

Fehmarn Belt
(Baltic Sea, north of Fehmarn)

• Harbour porpoise regularly recorded over course of year
• ‘Vulnerable’ behaviour common in summer (for example resting)

Western Rønne Bank and Odra Bank
(Baltic Sea, eastern Pomeranian Bay)

• Probably part of the severely endangered eastern harbour porpoise population (western range limit roughly marked by Darss Sill)
• Notable gathering first observed in May and July 2002; may be a seasonal or population ecology phenomenon
• Very low density in all survey years (2003-2005)

Porpoise click detector surveys have indicated the presence of harbour porpoise in parts of the Baltic Sea where they were previously unknown from aerial or vessel-based surveys. Such areas include the following:

Kadet Trench
(Baltic Sea, main shipping lane north-west of the Fischland-Darss peninsula)

• Harbour porpoise present throughout the survey period
• The trench is presumably used as a migration corridor

Seals

Common seals are found in high densities on sandbanks off the German North Sea coast, while grey seal populations are increasing from a low level in the Wadden Sea and around Heligoland. No permanent common seal resting sites are currently known on the German Baltic Sea coast. A small group of grey seals appears to be establishing itself on the Stubber bank in the waters off the island of Rügen.

Seals are very hard to count in open seas such as the German EEZ: From seal dive profiles traced with transmitters on common seals, we know that seals mostly just briefly break the surface with their snouts to breathe. As a result, they can only be spotted from an aircraft or vessel when they are near the surface in a calm sea and very clear water. This apart, given their very brief surfacings, the chances of an airborne survey sighting a seal in any one survey transect are very slim. Despite the shortcomings of survey methods, seals have been recorded in relatively high densities on the Sylt Outer Reef around the Elbe glacial valley and Amrum Bank, on Borkum Reef Ground, and notably around Heligoland (which serves as a migration hub).

To find out more about their habits, common seals have been equipped with satellite-linked time-depth recorders and loggers in a research project lasting several years. The findings show that the seals spend most of their time in offshore areas 30 km to 60 km from land, but also forage in areas up to 150 km away. Common seals from Heligoland mainly stay in a 25 km radius of the island. In German waters, common seals from the Wadden Sea dive to depths of between 12 m and 25 m, while common seals from Heligoland have a far larger depth range of up to 60 m.

Marine mammal species

Harbour porpoise are a species of toothed whale. They are not a dolphin species. They are one of the smallest whales, with a maximum length of less than 2 m. The other two native marine mammal species regularly found in the North Sea and Baltic Sea are the common seal and the grey seal. These are both members of the Phocidae family, known as true seals or earless seals.
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Marine mammals: Research methods

Fitting common seals with loggers and transmitters that become detached after a time for retrieval by researchers is only one part of marine mammal research in the German EEZ. Another method that is very useful in terms of the information provided, but is also expensive and effort-intensive, consists of regular aerial surveys of harbour porpoise (the surveys also count seals). Harbour porpoise are also counted hydroacoustically using porpoise click detectors.
More about research methods ...