tore johannessen, einar dahl, tone falkenhaug and lars natustvoll institute of marine research

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

Outline1. Historical evidence of abrupt recruitment

collapses in gadoids2. Recent recruitment patterns

3. Trends in oxygen4. Trends in phytoplankton

5. Trends in copepods6. Summary and conclusion

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

Relationship between 0- and I-group cod(same year-class)

R2=0.72

1. 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.

Områder 1

The Grenlandfjords

The Grelandfjords

0-group cod (6 – 8 months old)

92% reduction

97% reduction

The Grelandfjords

0-group pollack

94% reduction

The Grelandfjords

0-group whiting

Comprehensive testing in the field using cod as a model species:1.July to mid-August has been identified as the critical period for recruitment in cod in areas with and without recruitment collapses.2.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.3.Evidence suggests that there has been a switch towards smaller pelagic prey in areas with recruitment collapse as a result of gradual eutrophication.

Områder 2

The Grenlandfjords

Collapse in the mid-1960

Inner Oslofjord

Collapse in the early 1930s

The Skagerrak coast

Recruitment failure in the mid-1970s

Holmestrandfjord

Collapse in the mid-1960

Eelgras

Abrupt increase in sedimentation indicates change in the planktonic community

Reversal of the oxygen trend indicates change in the planktonic community

Chlorophyll a measured three times a week 1990-1999

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)

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,

Summary1. Repeated incidents of abrupt and persistent recruitment

collapses in gadoids (historically)2. Very poor recruitment in gadoids after 2002

3. Reversed trend in oxygen at intermediate depths >20024. Reduced phytoplankton biomass in summer ≥2002

5. Disappearance of the autumn bloom of red tide species ≥2002

6. Reduced abundance of Oithona spp. and Pseudocalanus/Paracalanus >2003

7. Generally poor recruitment in North Sea >2000

ConclusionEvidence 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!