Transcript Vetemaa_et_al

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Markus Vetemaa1, Redik Eschbaum1, Toomas Saat1 & Rögnvaldur Hannesson2
1Estonian
Marine Institute, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia
([email protected])
2Norwegian School of Economics and Business Administration, Helleveien 30, 5045 Bergen, Norway.
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Impact of newly established cormorant colony to fish
of the shallow Käina Bay, eastern Baltic Sea
T A S TA R
T
Year
Introduction
Table 1.
Number of
cormorants
in Käina Bay
In Estonia, the first colony of great cormorant Phalacrocorax carbo sinensis (L.), newcomer to Estonian fauna, was
established in 1984. By 2005 already 20 colonies (including Käina Bay, Fig 1) with around 10000 nesting pairs were
established.
Small estuarine fish are often an important source of nutrition for sea birds. To the extent vibrant bird colonies are seen as
desirable, it may be necessary to leave the estuarine fish as feed for these colonies. But if the estuarine fish would
otherwise be a source of recruitment for commercial fish stocks, we have a conflict between two use values, namely how
does the forgone commercial catch measure up against the benefits of having a sufficiently vibrant bird colony?
In this study we argue that expanding bird colonies could play a role similar to an overexpanding fishing fleet and
decimate the fish stocks on which they feed, to the detriment of the commercial fishery. We report on a study of the effects
of the great cormorant in the brackish waters of Estonia. Due to the low salinity, many freshwater species are abundant
and have high commercial importance in the north-eastern Baltic Sea.
Number of
nests
0
18
0
18
150
335
500
390
611
no data
724
1401
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
Number of
birds
0
~75
0
~75
~600
~1400
~2000
~1500
~2500
~2700
~3000
~5700
Study area
Väinameri (Moonsund) is a shallow (average depth ca 5 m) archipelago area
in Western Estonia. Dominating species are percids and cyprinids, while
only two abundant commercially important marine species, herring and
garpike, enter the area for spawning in spring. Käina Bay (≈ 9 km2) is
situated in the northern Väinameri (see figure). It is a very shallow sea area
(average depth < 1 m) almost totally enclosed by the Hiiumaa and Kassari
islands. Water exchange with the rest of Väinameri is possible only through
two channels in west and east. The bay has several islets. Käina bay with its
surroundings is nature protection area, a bird sanctuary. Both commercial
and recreational fishing is prohibited. It is (or to be more exact, was...) most
important spawning area of freshwater species such as roach, perch, ide,
ruffe and crucian carp. In 1995 the state of the fish populations in Käina Bay
was much better than in most of other areas of the Väinameri, which was
related to its protection status.
Material and Methods
Fish were sampled with gill nets in Käina Bay in 1995 and 2005.
Standardised coastal fish monitoring routines for the coastal areas of the
northern Baltic Sea were followed. Seven nets (mesh sizes 17, 21.5, 25, 30, 33,
38 and 45 mm knot to knot) were used together in line. The nets were set into
the deepest area (1.0-1.5 m) of the bay, between Ristlaid and Männaklaid
islets in both study years. The nets covered all the water column from
bottom to surface; they were set 16:00 – 19:00 p.m. and lifted 8:00 – 11:00
a.m. the following day. Catch per unit of effort (CPUE) data was calculated
as the total number of fish caught per station (i.e. 7 nets). For comparison,
CPUE data from regular fish monitoring areas in the Väinameri was used,
collected from 3 monitoring areas: Sarve (15 km north-east from the Käina
Bay), Saarnaki (12 km east) and Matsalu (40 km south-east. The number of
breeding pairs of cormorants in the Käina Bay was determined by counting
nests and using the correction factor suggested by the relevant literature.
Table 2. Comparative CPUE of fish in the Käina Bay in 1995 and 2005 during five periods
in spring/summer. Each row represents the average value of CPUE of two fishing stations.
Year
1995
1995
2005
2005
1995
1995
2005
2005
1995
1995
2005
2005
1995
1995
2005
2005
1995
1995
2005
2005
Date
14.IV
18.IV
13. IV
14. IV
22.IV
28.IV
25.IV
26.IV
5.V
12.V
6.V
7.V
28.V
8.VI
27.V
28.V
14.VII
8.VIII
31.VII
2.VIII
Water ºC
4.9
7.3
8.4
9.8
9.5
8.8
8.9
10.6
12.9
12.2
11.5
13.7
17.4
20.9
16.6
15.6
23.2
19.5
19.7
20.2
Perch
37.0
75.0
9.5
18.0
63.0
30.0
9.0
10.5
37.0
24.0
8.0
5.5
17.0
43.0
1.0
0.5
76.0
170.0
0.5
0
Gibel carp Crucian carp
0
0
0
0
4.0
0
2.0
0
0
0
0
0
2.0
0
2.0
0
0
2.0
0
5.0
0
0
1.5
0
0
3.0
0
0
1.0
0
0.5
0
0
0
0
2.0
11.0
0
6.0
0
CPUE
Ruffe
251.0
129.0
1199.5
1881.0
194.0
250.0
401.5
536.0
221.0
162.0
331.5
398.5
60.0
26.0
39.5
28.0
0
4.0
4.5
0.5
Rudd
0
0
1.5
3.0
0
0
0
0
0
0
6.0
1.5
0
0
2.5
0
21.0
35.0
5.0
1.0
Ide
30.0
24.0
1.0
0
18.0
3.0
2.0
2.0
0
4.0
1.0
1.0
0
0
3.0
1.0
0
4.0
3.0
1.5
Roach
79.0
230.0
1.5
2.0
463.0
353.0
2.0
3.0
636.0
574.0
19.0
0
464.0
41.0
0.5
0
34.0
122.0
0.5
0
Bleak
1.0
1.0
0
0
1.0
1.0
0
0
2.0
2.0
0
0
2.0
2.0
1.0
0.5
0
0
1.5
0.5
Table 3. Comparative CPUE of fish in regular monitoring areas in Väinameri.
Sarve, CPUE
1995 (n = 34) 2005 (n = 36)
Perch
6.1
2.2
Roach
6.1
1.2
Ruffe
2.0
1.0
%
p
35 < 0.01
20 0.09
51 0.11
Saarnaki, CPUE
1995 (n = 36) 2005 (n = 36)
12.8
8.9
20.5
4.1
0.6
2.2
% p
70 0.01
21 < 0.01
339 < 0.01
Matsalu, CPUE
1995 (n = 50) 2005 (n = 40)
20.1
14.9
54.0
55.7
1.7
0.2
% p
74 0.01
103 0.17
12 < 0.01
Results and conclusions
Gillnet sampling indicated that the abundance (CPUE) of roach, most abundant spawning fish species in
Käina Bay in 1995, declined more than 100 times in 1995 – 2005 (Table 2). The number of spawning
perch, second most important species, decreased ten-fold from 1995 to 2005 (Table 2). At the same time,
abundance of ide (too big to be predated by cormorants) remained roughly the same (Table 2).
Since coastal monitoring using standardized methods has been carried out in Väinameri since 1993 every
year, there is good background data reflecting the dynamics of fish stocks in this area in general.
Comparison of 1995 and 2005 data shows that the abundance of roach and perch in Sarve and Saarnaki
has declined around five and two times, respectively (Table 3), which is much less than the dramatic
change of more than 100 times (roach) and 5-10 times (perch) in Käina Bay. The decrease of fish also in
the Väinameri could be related to the predation by cormorants in the most important spawning ground,
the Käina Bay. In the Matsalu reference area ca 40 km from Käina Bay (too far for the cormorants) the
differences in CPUE of perch and roach between 1995 and 2005 were rather small.
Fishing effort has steadily and significantly decreased in Väinameri during last 15 years. There are no
sources of pollution nearby. So, the drastic decline in fish populations of the Käina Bay and in permanent
coastal fish monitoring areas in the archipelago sea nearby, indicate that the observed decrease in fish
abundance is most likely related to the establishment and explosive increase of the cormorant colony.
Daily consumption of cormorants was analysed on the base of pellets and related to the abundance of fish.
Our data demonstrates that the invasion of cormorants, which could be encouraged by the existence of
nature protection areas like Käina Bay, is especially dangerous in spatially restricted spawning grounds,
which have high density of fish during the reproduction period. The establishment of new cormorant
colonies could seriously damage the normal functioning of historically important spawning grounds,
preventing such areas from providing recruitment to adjacent fisheries. At the same time, the possibility
that a new natural predator could impact coastal fish stocks in a way comparable to overfishing, is not
always easily acceptable to the environmental administration, which has been the major reason why there
have been relatively slow steps to limit the quick population growth of cormorants in Europe.