1. Harbour Seals
The harbour seal
hoca
vitulina,is
the animal most commonly associated with the Wadden Sea. They
haul-out on sand banks and catch fish in the tidal channels and the
open sea. The population has recovered well from the last distemper
epizootic in 2002. Recent investigation have revealed that harbour
seals are using the Wadden Sea but also the neighboring North Sea to
a much greater degree than was previously known.
Result of
the aerial survey in 2007
2. Grey Seals
The grey seal Halichoerus grypus was a common species along
mainland Europe during the Neolithic and early Bronze Age (4000 -
1200 BC). Most of the sub-fossil seals remain found in Dutch
deposits dated between 2000 BC and 1000 AD, which is comparable to
finds in other parts of the Wadden Sea. By the end of the Middle
Ages (1400 -1500 AD), grey seals virtually disappeared from the
Wadden Sea area. Up until the mid-20th century, only straggling
animals were reported on the Dutch, German and Danish North Sea
coasts.
In recent times, about three decades ago, grey seals started to
re-establish themselves in the Wadden Sea, first a haul/out rookery
off the German Island of Amrum, followed later by a few
rookeries in the western part of the Dutch Wadden Sea. More regular
surveys from boats have been carried out in the Netherlands since
1980 and from 1988 onwards off Amrum. Maybe the re-colonisation has
started sometime earlier, but grey seals present were just taken as
harbour seals.
Present occurrence
The grey seal rookeries in the Netherlands are mainly found in the
western part of the Wadden Sea, although gradually more single
animals and small haul-out groups are observed in the middle and
eastern part. Numbers at the main haul/outs have increased
exponentially since 1980, on aver age by 20% annually.
In the westernmost part of the German Wadden Sea (Niedersachsen),
small groups have recently been observed near the islands of Borkum,
Norderney and Juist, and single ones in the Weser-Elbe estuary.
The haul/out rookery off Amrum (Schleswig-Holstein) is growing and
moult counts (carried out from boats) indicate an annual increase of
about 4-5%.
Another stronghold of grey seals in the German Bight is located on
the island of Helgoland. Regular occurrence has been known there
since
1989. Only straggling seals are found in the Danish Wadden Sea.
Aerial surveys of Grey Seals in 2006
Since the grey seal evidently belongs to the indigenous fauna of the
Wadden Sea, its monitoring is part of the operative trilateral Seal
Management Plan. It has been agreed to carry out trilaterally co-ordinated
and synchronised aerial surveys for grey seals from 2006 onwards.
For the time being, at least two surveys should be carried out
during the moult (March-April). These moult counts are supplemented
by either multiple aerial or boat counts, during the pupping season
(December-January). In the case of Helgoland, these surveys are
carried out from land.
The maximum numbers of grey seals
counted in
the Dutch Wadden Sea during the moult in 2006
are 1,786, 42 in Niedersachsen, 117 in Schleswig-Holstein, and 194
at Helgoland. That brings the total number of grey seals nearly
simultaneously observed in the Wadden Sea and at Helgoland, to 2,139
animals. As with harbour seals, this number is an index only,
because a certain fraction of the seals does not haul out during
surveys. It is yet unknown how the counted numbers relate to the
actual number present at that time of the year.
The reported maximum number of newly born pups observed were 200,
19, 24 in respectively the Wadden Sea of the Netherlands,
Niedersachsen, and Schleswig-Holstein, and 23 at Helgoland
(Schleswig-Holstein) in the breeding period Dec 2005/Jan 2006.
However, we emphasize that these pup production figures were
obtained in different ways and under different conditions.
Thus the relation of newly born pups counted to the total number
born varies between sub-areas. In the Netherlands, the quoted figure
is the sum of all newly born pups observed during different surveys
in the whelping season. On Helgoland, presumably all pups actually
born are observed. Off Amrum, newly born pups are registered during
the birth season in weekly intervals, so number of newly born pups
observed are believed to represent the vast majority of pups
actually born there. In Niedersachsen, the quoted figure is the
highest number observed in one of the two surveys, and the relation
to total number born is unknown.
We do not yet know whether there is a genetic
connection between the different colonies in the
Wadden Sea. From sightings of marked animals
we know that there is contact between animals
from rookeries within the Wadden Sea and moreover with haul-out
groups on the UK east coast, namely the Farne Islands and Orkney.
Results from a few recently satellite-tagged animals confirm
migration of grey seals from the Wadden Sea to the UK east coast and
vice-versa. Therefore, we consider the grey seals in the Wadden Sea
as part of an open population, meaning that the occurrence of grey
seals in the Wadden Sea area is significantly influenced by
movements of animals within the North Sea. Changes in numbers and
pup production should therefore be interpreted in a meta-population
concept, preferably in a North Sea-wide geographic context.
Trilateral Seal Expert Group (TSEG)
Peter J.H. Reijnders and Sophie M.J.M. Brasseur, IMARES, Texel, The
Netherlands,
Kai Abt, Wildlife Consulting, Schleswig-Holstein, Germany
Ursula Siebert, FTZ-Büsum der Universität Kiel, Schleswig-Holstein,
Germany
Michael Stede, LAVES Cuxhaven, Niedersachsen, Germany
Svend Tougaard, Fiskeri- og Søfartmuseet, Esbjerg, Denmark
