Tore Johannessen, Einar Dahl, Tone Falkenhaug and Lars Natustvoll Institute of Marine Research

1. Concurrent recruitment failure in gadoids and changes in the planktonic community along the Norwegian Skagerrak coast after 2002 Tore Johannessen, Einar Dahl, Tone Falkenhaug and Lars Natustvoll Institute of Marine Research Flødevigen Santander 10-12 May 2011

2. Outline1. Historical evidence of abrupt recruitment collapses in gadoids2. Recent recruitment patterns3. Trends in oxygen4. Trends in phytoplankton 5. Trends in copepods6. Summary and conclusion

3. Strandnotbilde Annual beach seine survey along the south coast of Norway since 1919 No methodological changes 0-group gadoids Oxygen measurements at 30 stations since 1927

4. Relationship between 0- and I-group cod(same year-class) R2=0.72 • The year-class strength in cod seems mainly determined at the 0-group stage (September). • 2. The beach seine sample 0-group cod in relatively precise numbers.

5. The Grenlandfjords Områder 1

6. 92% reduction The Grelandfjords 0-group cod (6 – 8 months old) TorskGrenland

7. 97% reduction The Grelandfjords 0-group pollack LyrGrenland

8. 94% reduction The Grelandfjords 0-group whiting HvitingGrenland • Comprehensive testing in the field using cod as a model species: • July to mid-August has been identified as the critical period for recruitment in cod in areas with and without recruitment collapses. • The number of cod that survived through the critical period (3 -5 months old) was related to food conditions: High survival was observed in relation to a diet of large herbivorous copepods, low survival was observed when the diet was dominated by hyperbentic prey (e.g. fish, mysids) or small copepods. • Evidence suggests that there has been a switch towards smaller pelagic prey in areas with recruitment collapse as a result of gradual eutrophication.

9. Inner Oslofjord Collapse in the early 1930s Holmestrandfjord Collapse in the mid-1960 The Grenlandfjords Collapse in the mid-1960 The Skagerrak coast Recruitment failure in the mid-1970s Områder 2

14. Eelgras

17. Abrupt increase in sedimentation indicates change in the planktonic community

18. Reversal of the oxygen trend indicates change in the planktonic community

19. Chlorophyll a measured three times a week 1990-1999

20. Chlorophyll a measured three times a week 1990-1999 Chlorophyll a measured three times a week 2002-2010 The autumn bloom of red tide species has disappeared (Ceratium spp. and Karenia mikimotoi)

22. Rothschild, B. J. 1998. Year class strength of zooplankton in the North Sea and their relation to cod and herring abundance. J. Plankton Res. 20: 1721-1741,

23. Summary • Repeated incidents of abrupt and persistent recruitment collapses in gadoids (historically) • Very poor recruitment in gadoids after 2002 • Reversed trend in oxygen at intermediate depths >2002 • Reduced phytoplankton biomass in summer ≥2002 • Disappearance of the autumn bloom of red tide species ≥2002 • Reduced abundance of Oithona spp. and Pseudocalanus/Paracalanus >2003 • Generally poor recruitment in North Sea >2000

24. Conclusion Evidence of changes in the planktonic community and poor recruitment in gadoids along the south coast of Norway is worrying seen in the light of historical incidents of abrupt and persistent recruitment collapses. However, it is to early to conclude that there has been a (non-reversable) regime shift. I have proposed a biological mechanism for resilience in the planktonic community which I am happy to discuss over a cold beer. Thank you very much!