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Results
In 2008 the surveys
to monitor developments in the harbour seal Phoca vitulina
population in the entire Wadden Sea were trilaterally co-ordinated
according to the Seal Management Plan. In Denmark, a new survey team
was installed under careful instructions from the former survey
team. In addition, a member of the former team took part of the
first aerial count in 2008 to secure consistency. Like in other
areas, counts in Denmark are done using a digital camera, allowing
post hoc quality assurance.
For 2008, the maximum number of harbour seals counted during moult
in August was 2,483 seals in Denmark, 5844 in Schleswig-Holstein,
2,407 in Niedersachsen and 5,972 counted in the Netherlands.
However, these numbers cannot be summed as done in former years, to
obtain an index for the Wadden Sea seal population. For the German
areas Niedersachsen and Schleswig-Holstein the moult surveys could
not be completed as planned. This is due to bad weather conditions
and technical problems.
In Niedersachsen only part of the survey could be carried out.
Assuming that the distribution of the seals in this area was similar
to that of earlier years, the number of seals in the missed area was
estimated and that brought the total estimate for Niedersachsen at
4,912. In Schleswig-Holstein, the survey team was forced to carry
out the survey later than planned (end of August instead of mid
August). To correct for that, the mean ratio between the counts
during the pupping season and the moult obtained in former years,
was used to estimate the number of seals expected during the moult
peak. This amounted to 6887 seals.
Using the corrected counts for Niedersachsen and Schleswig-Holstein
and the counts in the Netherlands and Denmark would bring the
estimated maximum number of harbour seals that would have been
counted in the entire Wadden Sea at 20,250 seals.
Surveys in the coming years could help to evaluate the given
estimates.
The maximum number of pups counted during the breeding period in
June was 4,632 in the international Wadden Sea. This comprises 976
in the Netherlands, 1,076 in Niedersachsen/Hamburg, 2,096 in
Schleswig-Holstein, and 484 in Denmark.
If the calculated estimate for the number of harbour seals in 2008
is used, the increase compared to last year would be 15%. The number
of counted pups grew by slightly more than 9%.
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Interpretation
The total number of harbour seals in 2008 presented here would
actually be the highest number ever counted in the international
Wadden Sea during the moult. It indicates that the population is
still growing prosperously and has evidently passed its pre
2002-epizootic level of 17,700 animals (Reijnders et al. 2003). As
expected (Reijnders et al. 2007), the percentage of pups has
slightly decreased but is still 22.9%. This could indicate that the
age-structure of the population is gradually stabilising, less
dominated by adult females as the young born after the epizootic
slowly recruit into the breeding population.
Estimating when, or even if a stable age-structured population will
be seen, might not be appropriate at the moment. However, one should
concentrate in the coming years to monitor changes in population
parameters occur. These may indicate whether or not the population
is approaching the carrying capacity of the Wadden Sea and the
adjacent North Sea for this species. Investigating the changes in
the population towards approaching carrying capacity, and the time
scale at which this may happen, is evidently needed for adequate
management of this population in the future. This research should
take into account the growing populations of other top predators and
co-consumers such as grey seals, harbour porpoises and also marine
birds, in the region.
References
Reijnders, Peter J.H.,
Sophie M.J.M. Brasseur, Kai Abt, Ursula Siebert, Michael Stede &
Svend Tougaard 2003. The harbour seal population in the Wadden Sea
as revealed by the aerial surveys in 2003. Wadden Sea Newsletter
2003 (2): 11-12.
Reijnders, Peter J.H., Sophie M.J.M.
Brasseur, Thomas Borchardt, Ursula Siebert, Michael Stede & Svend
Tougaard 2007. Aerial Surveys of Harbour Seals in the Wadden Sea in
2007: Population age-composition returning to a stable
age-structure? Wadden Sea Newsletter 1, 2008.
http://www.waddensea-secretariat.org/news/news/Seals/Annual-reports/seals2007.html.
Trilateral Seal Expert Group (TSEG)
Sophie
M.J.M. Brasseur and Peter J.H. Reijnders, IMARES, Texel, The
Netherlands
Thomas Borchardt, National Park Schleswig-Holstein,
Schleswig-Holstein, Germany
Ursula Siebert, FTZ-Büsum der Universität Kiel, Schleswig-Holstein,
Germany
Michael Stede and Sven Ramdohr, LAVES Cuxhaven, Niedersachsen,
Germany
Lasse Fast Jensen, Fiskeri- og Søfartmuseet, Esbjerg, Denmark
Jonas
Teilman and Jacob Tougaard National Environmental Research
Institute, Aarhus University, Denmark |
