1. 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

2. Om PowerPoint mallar Bothnian Sea

3. Om PowerPoint mallar Suuret petokalat vaikuttavat koko ekosysteemiin Eläinplankton Rihmalevät Petokalat Pohjaeläimiä syövät kalat, esim. piikkikalat

4. Om PowerPoint mallar Muutos Itämeren ekosysteemissä Vertailu 1980-luku – 2000-luku

5. Seal population What are the ecosystem effects? Annual population increase ca 7%

6. Om PowerPoint mallar 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

7. Om PowerPoint mallar Seal diet in the Bothnian Sea Gulf of Bothnia Juveniles and adults 0 0,2 0,4 0,6 0,8 Herring Common whitefish Salmon & trout

8. Om PowerPoint mallar 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

9. Om PowerPoint mallar Herring catches

10. Om PowerPoint mallar Length distribution of herring in survey, landings and seal diet

11. Om PowerPoint mallar Age distribution of herring in catches 86% of the herring in seal diet consist of mature herring

12. Om PowerPoint mallar 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

13. Om PowerPoint mallar Results: SSB

14. Om PowerPoint mallar Results: Fishing mortality

15. Om PowerPoint mallar Comparison of the fleets (tuning weights in the assessment)

16. Om PowerPoint mallar 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

17. Om PowerPoint mallar Thank you for your attention!