Effects of grey seals on the herring population in the Baltic Sea area.
Teija Aho, Anna Gårdmark, Karl Lundström and Jukka Pönni Swedish Board of Fisheries, Finnish Game and Fisheries Research Institute
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Bothnian Sea
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Suuret petokalat vaikuttavat koko ekosysteemiin
Eläinplankton
Rihmalevät
Petokalat
Pohjaeläimiä syövät kalat, esim. piikkikalat
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Muutos Itämeren ekosysteemissäVertailu 1980-luku – 2000-luku
Seal population
0
3000
6000
9000
1975 1980 1985 1990 1995 2000 2005 2010
Est. p
op. siz
e
Both. Bay
Both. Sea
What are the ecosystem effects?Annual population increase ca 7%
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Aim: To compare effects of seal predation with the effects of fisheries
–Trawl fishery for the herring is the main fishery in Bothnian Sea–Herring stock is assessed yearly (single stock assessment, vpa)–Grey seal diet has been studied 2001-2005
• Stomach analysis of 280 seals
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Seal diet in the Bothnian Sea
Gulf of BothniaJuveniles and adults
0
0,2
0,4
0,6
0,8
Herring Common
whitefish
Salmon& trout
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Predation by seals
One ”average” seal eat 3,1 kg herring/day
In 2007 there were 8000 seals in BS
In total, seals consumed over 8900 tonnes herring in BS in 2007 (>220 million inds)• 12 % of the fisheries landings• Three times the Swedish
landings
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Herring catches
0
10000
20000
30000
40000
50000
60000
70000
80000
90000
19
73
19
75
19
77
19
79
19
81
19
83
19
85
19
87
19
89
19
91
19
93
19
95
19
97
19
99
20
01
20
03
20
05
20
07
Ca
tch
(to
ns
)
Grey seals
Fisheries
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Length distribution of herring in survey, landings and seal diet
0
0,02
0,04
0,06
0,08
0,1
0,12
0,14
45 70 95 120 145 170 195 220 245 270 295
Length class (cm)
Pro
port
ion
with
in le
ngth
cla
ss
Survey 2007
Landings 2007
Grey seal
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Age distribution of herring in catches
0
0,25
0,5
0 1 2 3 4 5 6 7 8 9+
Age
Pro
port
ion
with
in a
ge
Fishery Grey Seals
86% of the herring in seal diet consist of mature herring
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Assessment method
–Single species xsa (extended survivor analysis)• Years 1974-2007
–Comparison to the ordinary vpa without seals–”Seal fleet” as one tuning fleet comparison of the importance between the fleets
• Seal fleet since 1989• Pelagic and bottom trawls, trapnets
–We assumed an average seal individual to have same prey preferences over the years
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Results: SSB
0
100000
200000
300000
400000
500000
600000
1970 1975 1980 1985 1990 1995 2000 2005 2010
SS
B (
ton
s)
Ordinary vpa
Seal vpa
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Results: Fishing mortality
0
0,05
0,1
0,15
0,2
0,25
1970 1980 1990 2000 2010
Fis
hin
g m
ort
alit
y o
n a
ge
s 2
-7Ordinary vpa Seal-vpa
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Comparison of the fleets(tuning weights in the assessment)
0
0,25
0,5
1 2 3 4 5 6 7 8
Age
Sc
ale
d w
eig
hts
in X
SA
Trapnets
Pelagic trawls
Bottom trawls
Grey seals
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Conclusions
–Grey seals prefer large (mature) herring largest effects on older ages–Seal predation does affect the estimates for SSB and F
• On average 20% yearly differences in SSB (up to 37%)
–Seal predation should be taken into account in future assessments!–Seal predation may have contributed to the diminished catches of large herring in the coastal areas during the recent years
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Thank you for your attention!