Ecosystem regime shifts triggered by overfishing21/12/2011 1

Christian MöllmannInstitute of Hydrobiology and Fisheries

Science, University of HamburgGermany

ECOSYSTEM REGIME SHIFTSTRIGGERED BY OVERFISHING

Ecosystem regime shifts triggered by overfishing21/12/2011 2

Background

• The general theme of this briefing note is the importance of overfishing in triggering ecosystem regime shifts !

• Ecosystem Regime Shifts are defined as infrequent and abrupt changes in ecosystem structure and function, occurring at multiple trophic levels and on large geographic scales !

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This study ...• reviews prominent examples of ecosystem regime shifts; • discusses trophic cascades as the phenomenon by

which overfishing can cause ecosystem regime shifts;• reviews alternative stable state theory and evidence for

stable states in the reported cases of marine ecosystem regime shifts;

• presents conclusions on the causes of ecosystem regime shifts with special reference to overfishing;

• makes recommendations for future research and ecosystem-based management strategies.

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Why are Ecosystem Regime Shifts of concern for management ? – Alternative Stable States Theory !

Ecosystem)state)

Pressure)

State)B)

State)A)

Ecosystem)state)

Pressure)

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Review of Ecosystem Regime Shifts

1"

2"

3"4"

5"

1 – North Pacific, 2 – Eastern Scotian Shelf, 3 – North Sea, 4 – Baltic Sea, 5 – Black Sea.

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Selection of studies

• covered all trophic levels, i.e. phyto- and zooplankton, meso-predatory and top-predatory fish

• used multivariate methods (e.g. Principal Component Analysis – PCA – to summarize and integrate the changes into a few variables

• used a regime shift detection method• for Northern Pacific, Eastern Scotian Shelf, North Sea

and Baltic Sea• exception Black Sea, where changes over several

trophic levels are reported separately

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Example of Ecosystem Regime Shift – the North Pacific (Hare and Mantua, 2000)

a)

Year1965 1970 1975 1980 1985 1990 1995

Ecos

yste

mSt

ate

(PC

1&

PC2)

-2

-1

0

1

2

PC1-2 -1 0 1 2

PC

2

-2

-1

0

1

2

65

66 6768

69

70

7172

737475

76

77

78

79 80

81

82

8384

8586

87

88

8990

91

92

93

9495

96

97

b)

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Comparison of Regime Shift timing –common climate driver ?

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Comparison of Regime Shift timing –common climate driver ?

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Review of Ecosystem Regime ShiftsECOSYSTEM STUDY TIMING MAIN DRIVER

North Pacific Hare and Mantua, 2000 1977 and 1989 Climate

Eastern Scotian Shelf Choi et al., 2005 1976 and 1990Climate,

overfishing

North Sea

Beaugrand, 20041982-88; two periods:1982-85 and 1988/89

Climate

Weijerman et al., 2005 1979 and 1988 Climate

Kenny et al., 2009 1993 Climate, overfishing

Baltic Sea Möllmann et al., 2009 1988-93Climate,

overfishing,eutrophication

The Sound Lindegren et al., 2010a 1987/88 Climate

Black Sea Oguz and Gilbert, 2007 1973/4 and 1989 Overfishing, species invasion

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Fishing as a driver – Trophic Cascades

Phytoplankton

Zooplankton

Planktivores

Piscivores

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Review of trophic cascadesECOSYSTEM STUDY SPECIES/TROPHIC LEVELS

INVOLVED

Eastern Scotian Shelf Frank et al. 2005Demersal fish, pelagic forage fish, large

crustaceens, large zooplankton,phytoplankton

Baltic SeaCasini et al., 2008, Casini et al., 2009 ; Möllmann et al., 2008

Cod, sprat, zooplankton, phytoplankton

Black SeaDaskalov et al., 2007; Oguz andGilbert, 2007

Pelagic predatory fish, smallplanktivorous fish, gelatinous plankton,

zooplankton, phytoplankton

Trophic cascade at the Eastern Scotian Shelf (from Scheffer et al., 2005, after Frank et al., 2005)

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Drivers of Ecosystem Regime Shifts

• Climate (atmospheric forcing)• (Over-) Fishing• Eutrophication• Species invasion („jellification“)

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How do multiple drivers interact ? – Resilience !

Resilient ecosystem – deep basin of attraction !•External driver (black arrows) cannot change the ecosystem into a new state!

Degraded ecosystem – shallow basin of attraction !•External driver 1 (e.g. FISHING) has degraded the ecosystem•External driver 2 (e.g. CLIMATE -black arrows) can change the ecosystem into a new state!

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Positive feedbacks lead to Alternative Stable States !

Top-Pre-dator

Meso-predator

Decreased predation !

Increased predationon fish eggs and larvae !

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How to avoid unwanted ecosystem regime shifts –early warning systems !

Time series of spatial variance as an early warning indicator and mean ecosystem state in Pavlof Bay and Eastern Scotian Shelf (Litzow and Ciannelli, 2008)

Critical slowing down as an early warning indicator – increase in lag-1 autocorrelation in climate dynamics; the period preceding thetransition from a greenhouse state to an icehouse state on the Earth 34 Myrago (Scheffer et al., 2009)

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Key findings• The synchronicity in timing of ecosystem regime shifts over the Northern

hemisphere during the late 1980s/early 1990s suggests a common climate driver.

• Trophic cascades triggered by overfishing are regularly observed, indicating that multiple external drivers are usually needed to cause ecosystem regime shifts.

• Multiple external drivers interact in a way that one undermines resilience (e.g. fishing) and another one (e.g. abrupt climate change) gives the final impulse for an ecosystem regime shift.

• Ecosystem regime shifts can be difficult to reverse when alternative stable states are involved. Indications for the latter exist from multiple ecosystems.

• Ecosystem-based management strategies are needed to avoid ecosystem regime shifts and early warning systems need to be developed.