|
As
in the foregoing years, in 2006 the surveys to monitor the harbour
seal Phoca vitulina population in the entire Wadden Sea were
coordinated and carried out trilaterally according to the Seal
Management Plan. The counts, performed almost simultaneously in the
different sub-regions, provided the following results: the maximum
number observed in the moulting period (August) amounted to 4,065 in
the Netherlands, 3,820 in Niedersachsen/Hamburg, 5,543 in
Schleswig-Holstein, and 1,998 in Denmark, bringing the grand total
to 15,426 seals. The maximum number of pups observed during the
whelping period (June) was 850 in the Netherlands, 1,173 in
Niedersachsen/Hamburg, 2,085 in Schleswig-Holstein and 411 in
Denmark, bringing the grand total of pups to 4,519. The percentage
pups-per-total-number is 29.3%.
The
maximum number counted for the entire Wadden Sea is just over 8%
higher than counted in 2005, the maximum number of pups observed
equals the figure for 2005. Compared to the previous years since the
2002 epizootic, the annual increase continued to slow down from
18.5% in 2003/2004 to 8.1% in 2005/2006.
This is similar to the pattern observed in the first years following
the 1988 epizootic. The initial increase was again higher than the
average for the entire period 1989-2001. This underlines our
previous postulation (Reijnders et al., 2003, Abt et al.,
2005) that the post-epizootic age structure has changed in favor of
adult (reproducing) females. The overrepresentation of adult males
found dead in 2002 corroborates that assumption. Additional support
for that postulation can be deducted from the fact that the ratio of
observed numbers in the pupping season to numbers observed in the
moult are higher in the post-2002 epizootic years compared to the
pre-epizootic period.
|
The
results of the pup counts of this year were puzzling. With an
increasing population, it was expected that more pups would be
counted than last year. However, the maximum number of pups observed
in 2006 was almost equal to the number observed in 2005. We
postulate that pup counts in 2006 encompassed a lower fraction of
pups actually born than counts in 2003–2005. Unusually low count
results in the early pupping season and the relatively late dates of
first weaned pups observed, suggested that the pupping season was
about one week later in 2006 than in previous years. As a
consequence peak numbers would have occurred in the end of June,
instead of around 20 June when the surveys were made. This means
that counts from both years are not entirely comparable and numbers
obtained for this year are more likely an underestimate. The reason
for the delay in the 2006 pupping season is presumably the extremely
cold spring this year.
Nevertheless, with the presently observed trend in growth, the
population may have recovered to the pre-epizootic level by 2008.
References
Abt,
Kai, Svend Tougaard, Sophie M.J.M. Brasseur, Peter J.H. Reijnders,
Ursula Siebert & Michael Stede 2005. Counting Harbour Seals in the
Wadden Sea in 2004 and 2005 – Expected and unexpected results.
Wadden Sea Newsletter 2005 (1): 26-27.
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.
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 |
