BSEP116B Biodiversity in the Baltic Sea - Helcom

Number of individuals
12000
10000
8000
6000
4000
2000
Ground
Air
Expon. (Ground)
y = 3E-64e 0,0773x
0
1990 1992 1994 1996 1998 2000 2002 2004 2006 2008
Figure 4.2.2. Numbers of grey seals counted from ground level along
the Swedish coast. The annual rate of increase was 8% up to 2005.
Surveys from air, started in 2006, give higher point estimates. Equation
given for exponential (Expon.) curve fit.
the 1750s. Harbour seals reproduce in June, mate
in July, and moult in July–August.
Harbour seals in the Kalmarsund region were close
to extinction in the 1970s. Hunting, possibly in
combination with effects of contaminants, resulted
in a severe population bottle-neck and only some
tens of pups were observed in the beginning of
the 1970s (Härkönen et al. 2005). After protective
measures were taken and seal sanctuaries were
established in the 1980s and 1990s, the population
increased by 7% per year up to 2007, when
630 seals were counted during moult. The pup
production also showed a similar positive trend
and close to 100 pups were born in 2007. Because
harbour seals in this area spend about 65% of their
time on land, the true population size is close to
1 000 seals. There is virtually no information on the
health status of this population.
68
ible data is for the Swedish coastal waters. The
mean annual rate of increase in the period 1990–
2005 was about 8% (Figure 4.2.2), which is somewhat
less than the theoretical maximum growth
rate of 10% for the species (Harding et al. 2007).
Surveys from air carried out in 2006 and 2007
in Sweden give higher estimates of abundance,
but it will take at least six years until such data
can provide useful information for trend analyses
(Harding et al. 2007).
The recovery of grey seals south of 59º N, where
they had been regularly present before they
were hunted to extinction in the beginning of
the 20th century, is very slow (Herrmann et al.
2007). Although grey seals are observed more
often than 10 years ago, no resident colonies have
been reported on the southern Baltic coast (from
Germany to Latvia).
Harbour seals (Phoca vitulina)
Harbour seals occur only in the southern part
of the Baltic and there are no historical records
of harbour seals north of a line from Västervik
(Sweden) to Hiiumaa in Estonia. The harbour seals
in the Kalmarsund region are genetically distinct
from adjacent populations, with gene sequences
present in only this population. The current
harbour seal population in the southwestern Baltic
(Scania and Denmark) seems to have colonized the
area after the grey seals were severely depleted in
Harbour seals in the southern Baltic, the Kattegat,
and the Skagerrak also declined steeply in the
beginning of the 20th century as a consequence
of the coordinated Nordic extermination effort.
Bounty statistics suggest that more than 17 000
harbour seals were present in the area in 1890,
but only about 2 000 seals remained at the end
of the 1930s (Heide-Jørgensen & Härkönen
1988). Heavy hunting pressure kept the population
at this level until the beginning of the 1970s,
when hunting was prohibited and seal conservation
areas were established. Annual surveys of
the population started in 1979 and the population
increased at 12% per year until 1988 when
it was hit by a phocine distemper virus (PDV)
epidemic that killed about half the population.
Subsequently, the population increased exponentially
at 13% per year until a new PDV epidemic in
2002 killed 66% of the seals in the Skagerrak and
30% in the Kattegat (Härkönen et al. 2006). A
third epidemic caused by an unknown pathogen
appeared in 2007 and killed some thousands of
seals, but the total impact cannot be evaluated
until survey results from 2008 become available.
The total ‘true’ population size was about 10 100
in 2007, but this number should be regarded as
uncertain because seals may have died in great
numbers after the surveys in 2007 were conducted.
The reproductive status of this population
appears to be normal because 95% of mature