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The coordinated surveys of
common seals in the international Wadden Sea in 1999 revealed
the following results. The rounded off total number of seals
counted in 1999 amounted to 15,200 (2,150 in DK, 5,850 in SH,
4,800 in Nds and 2,400 in NL), and of those 3,110 were pups.
Though the percentage of pups per total number is of the same
order as the years before, the increase of 5.5% of the total
population is lower.
It would be premature to
conclude anything about a change in population trend from this,
because more years' data is needed to establish whether the 1999
result is the start of a downward trend or just a one-off event.
Nevertheless, it is worthwhile to speculate about possible reasons
for the lower population growth between 1998-1999.
A lower annual increase could
have been caused, in principle, by either a lower recruitment
between 1998-1999 (lower pup production in 1998 and/or higher
mortality), or by less optimal survey conditions in 1999, or
even a combination of both. Because the survey conditions in
1999 are considered to be similar to other years, this factor
cannot explain the lower population growth.
Concerning a possible lower
pup production in 1998: the total pup count in 1998 was lower
than in 1997, despite an increase in total population between
1997-1998. Survey conditions were indeed sub-optimal (bad weather)
in most areas in the Wadden Sea and could have influenced the
low pup count in that year. Still, though unlikely, these observations
could be a true reflection of a lower pup production. |
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Pup survival is difficult
to measure. There seems to be an increase in sick and animals
found dead (pups born in 1998), in the season 1998-1999. The
lacking of data expressing both the searching effort for dead
or sick animals and the environmental circumstances, which could
influence the observed increase, hampers the assessment of the
meaning of this increase. However, this change in animals found
dead could be in fact a reflection of an increased mortality
potentially leading to a lowered recruitment. The increase in
animals found dead or moribund should therefore be investigated
as it could help in assessing a lower pup survival and the consequences
thereof for the population growth.
In conclusion: it is too
early to conclude about a possible change in population trend.
Further modeling studies are ongoing to predict population trends
assuming fluctuations in pup production as well as varying mortality
rates. Surveys in the years to come will show whether the strong
population growth in the last decennium is retarded, and if so,
which population parameter is affected.
Trilateral Seal Expert Group:
DK: Svend Tougaard, Fiskeri-og Søfartsmuseet, Esbjerg
SH: Ursula Siebert, FTZ Büsum der Univ. Kiel
Nds: Ekkehard Vareschi, Universität Oldenburg
NL: Peter H. Reijnders, IBN-DLO Inst. for Forestry and Nature
Research, Texel |
