BSEP116B Biodiversity in the Baltic Sea - Helcom

There have been no fully compatible surveys of
grey seals until very recently. Survey methods have
changed over time and gradually improved, which
precludes an overall assessment of the population
growth rate of the entire Baltic grey seal population.
The only longer time series providing compatannual
increase up to 2007 (Figure 4.2.1) is about
half of what can be expected in a depleted healthy
ringed seal population. The number counted on ice
in the Bothnian Bay was 4 800 in 2007.
Data from the Gulf of Finland and Estonia are
scarce because only three full surveys from the air
have been carried out in these areas. However, the
data suggest a growth rate close to zero in both
regions at least since 1996. Counted numbers in
the Gulf of Finland are about 300 animals, whereas
about 1 500 are estimated to haul out during
moult in Estonia (Härkönen et al. 1998, Ivar Jüssi
pers. comm.). About 150 ringed seals have been
counted in the Archipelago Sea (Antti Halkka pers.
comm.), which totals to a counted population size
of close to 7 000 ringed seals.
Numbers hauled out
7000
6000
5000
4000
3000
2000
1000
y = 5E-33e 0,0413x
0
1985 1990 1995 2000 2005 2010
Figure 4.2.1. Numbers of ringed seals counted on ice in the Bothnian
Bay, 1988–2007. The mean annual rate of increase was 4.3%
for the study period.
Grey seals (Halichoerus grypus)
The main concentrations of grey seals are found
in the northern part of the Baltic Proper, although
single individuals or scattered groups can be seen
throughout the Baltic. In wintertime, the northern
distribution is limited to areas with open waters.
Baltic grey seals give birth in February–March and
alternate between breeding on land sites and
on open ice. However, when available, ice is preferred
because pup mortality is higher and quality
is much poorer when pups are born on land (Jüssi
et al. 2008).
Grey seals were hunted heavily in the beginning
of the 20th century and the previously abundant
grey seals in the Kattegat and the southern
Baltic were extirpated by hunting in the 1930s.
The estimated total population of greys seals
dropped from 90 000 to about 20 000 before
1940. A further decline occurred in the 1960s, as
for ringed seals, and perhaps only 2 500–3 000
remained at the end of the 1970s (Harding &
Härkönen 1999). Several lines of evidence indicate
that environmental pollution also severely
affected this population. Uterine disorders (occlusions
or stenosis), probably as a result of fetal
death, uterine tumours (leiomyomas), hormonal
disturbances (adrenocortical hyperplasia), arteriosclerosis,
and the occurrence of skull, renal, intestinal
and integumental lesions were suggested to
be caused by organochlorines (Bergman & Olsson
1986, Bergman et al. 1992, Bergman 1999,
Bäcklin et al. 2003, Bredhult et al. 2008).
The prevalence of uterine disorders has dropped
considerably during the past two decades and other
lesions have also decreased somewhat, but the
prevalence of colonic ulcers has increased since the
mid-1980s and the nutritional condition of grey seals
appears to have diminished recently (e.g., Bergman
1999, Bäcklin et al. 2007, Routti et al. 2008).
Grey seal (Halichoerus grypus)
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