BSEP116B Biodiversity in the Baltic Sea - Helcom

80
Number of breeding pairs
3000
2500
2000
1500
1000
500
0
1994
1995
1996
1997
1998
1999
Figure 4.3.9. The breeding population of the sandwich tern in Denmark 1 ,
Sweden 2 , Germany, and Poland, 1994–2007. No detailed data was available
for the Estonian population, which is estimated at about 600–900 bp
(Herrmann et al. 2008).
1
Only Baltic breeding sites, i.e., colonies in the central and southern Kattegat, Belt
Sea and the Sound are considered, but not the breeding population of the northern
Kattegat and the North Sea.
2
Monitoring of the sandwich tern in Sweden was not complete in all years.
2000
Total Denmark Sweden Germany Poland
2001
2002
2003
2004
2005
2006
been in response to a deterioration of the environmental
conditions in the North Sea.
The Baltic breeding population grew constantly and
reached about 2 500 bp by the end of the 1970s.
Since then, despite some fluctuations and frequent
shifts of breeding sites, it can be considered as more
or less stable. More detailed surveillance data from
the mid-1990s until now reveal a population size
fluctuating between 2 000 and 3 500 bp (Figure
4.3.9).
The main conservation measure for the sandwich
tern is the protection of suitable breeding
sites. These are especially breeding colonies of
black-headed gulls (Larus ridibundus) on small
islands covered by low grass vegetation, without
human disturbances or the presence of predatory
mammals (Herrmann et al. 2008).
Razorbill (Alca torda)
The razorbill is a widespread breeder in coastal
areas of northwestern Europe. Its European
breeding population is large (430 000–770 000
bp, BirdLife International 2004), with Iceland
2007
hosting more than 50%. In the Baltic Sea area,
the razorbill breeds in Sweden (9 000–11 000
bp in 1999–2000), Finland (6 000–8 000 bp in
1998–2002), Russia (St. Petersburg Region, 150
bp), Denmark (maximum 965 bp in 2006), and
recently also with a few (1–10) pairs in Estonia.
The Baltic population increased during the 1990s
(Birdlife International 2004, Elts et al. 2003).
The only Danish breeding colonies are found on
Bornholm and Græsholm (a small rocky island of
the Ertholmene archipelago east of Bornholm).
This site was colonized by the species during
the 1920s. From 1983–2000, the population
increased from about 280 bp to 745 (Lyngs
2001), and reached 965 bp in 2006 (Christiansø
Feltstation).
Ringing recoveries show that razorbills from the
Baltic breeding population usually stay all year
round in the Baltic Sea. For example, out of 269
ringing recoveries of birds ringed in Denmark
(Græsholm and Bornholm), 265 derived from the
Baltic Sea and Kattegat, three from the Skagerrak,
and only one from the North Sea, close to the
entrance of the Skagerrak (Bonlokke et al. 2006).
Most birds of the Baltic breeding population
winter in the central part of the Baltic Sea, including
Irbe Strait and the Gulf of Riga (Durinck et al.
1994). However, ringing recoveries of razorbills
from Bornholm/Ertholmene show that the inner
Danish waters are also used as wintering sites
(Bonlokke et al. 2006).
Razorbills of the North Atlantic breeding population
winter in large numbers in the central and
northern Kattegat. Danish ringing recoveries give
evidence that outside the breeding season birds
from the British Isles (mainly Scotland), Norway,
and Russia visit the Kattegat and inner Danish
waters (Bonlokke et al. 2006). The number of birds
wintering in this area shows large fluctuations.
From 1988–1993, on average 13% of the North
Atlantic population was wintering in the Kattegat/inner
Danish waters, but in some years the
numbers were much higher (Durinck et al. 1994).
Ringing recovery data as well as by-catch studies
show that the gillnet fishery is an important mortality
factor for the species (Hario 1998, I.L.N. &
IfAÖ 2005).