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Results
Again in 2007 the surveys to monitor developments in the harbour
seal Phoca vitulina population in the entire Wadden Sea were
trilaterally co-ordinated and synchronously carried out according to
the Seal mangement Plan. The nearly simultaneous surveys in the four
subregions revealed the following results: the maximum number
counted in the moulting period (August) was 4,159 in The
Netherlands, 4,561 in Niedersachsen/Hamburg, 6,386 in
Schleswig-Holstein, and 2,499 in Denmark. This brings the grand
total to 17,605 harbour seals. The maximum number of pups observed
during the whelping/lactation period (June) was 978 in The
Netherlands, 821 in Niedersachsen/Hamburg, 2,095 in
Schleswig-Holstein, and 341 in Denmark, bringing the grand total for
pups in the international Wadden Sea at 4,235.
The total number of harbour seals counted in 2007 is just over 14%
higher than counted in 2006, and the maximum number of pups counted
is 284 less than in 2006 (Reijnders et al. 2006). The average annual
increase in seal numbers between 2004 and 2007 is 10.9%.
Interpretation
The total number of harbour seals observed in 2007 indicates that
the population has returned to its pre 2002-epizootic level of
17,900 animals. This is one year earlier than predicted.
Growing 10.9% per year in average, the average increase in the post
2002-epizootic period is slightly lower than the 12.4% average
increase in interim epizootic period 1988-2002.
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Despite the population increase, last year’s pup production (number
of pups per total number) did not increase compared to the pupping
result in 2005. This year, pup production even decreased by around
5.5%. Even if we take into account that part of a pup survey in
Niedersachsen had to be cancelled and if we correct for the assumed
missed number of pups, pup production only increased marginally.
Contrary to the initial post 1988- epizootic years, the percentage
of pups per total numbers counted shows a decreasing trend. However,
this is not necessarily an alarm. This percentage (24.1%) is still
higher than the average (21.6%) for the period between 1988 and
2002.
This corroborates our previous postulation (Reijnders et al. 2003)
that the post-epizootic age structure has changed in favour of adult
(reproducing) females, and may now gradually returning to a more
stable one. Besides the to be expected usual annual fluctuations in
population performance and slight survey errors, it can not
completely be ruled out that fecundity is gradually decreasing.
Surveys in the coming year will show whether or not the decrease in
pup percentage will level off. The level at which that will happen,
will define the further population developments. Currently, relative
short time series do not allow more elaborate conclusions on the
present developments in the population.
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, Kai F.Abt, Ursula
Siebert, Michael Stede & Svend Tougaard 2006. Aerial surveys of
harbour and grey seals in the Wadden Sea in 2006. Wadden Sea
Newsletter 32: 9-11.
Trilateral Seal Expert Group (TSEG)
Peter J.H. Reijnders and Sophie M.J.M. Brasseur, 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, LAVES Cuxhaven, Niedersachsen, Germany
Svend Tougaard, Fiskeri- og Søfartmuseet, Esbjerg, Denmark |
