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!