Taken out of context: the effects of shark removal in the dynamic
environment of a coral reef
Jonathan L.W. Ruppert, Mike Travers, Marie-Josée Fortin and Mark G. Meekan
Coral Reefs: Human Activity or Top–Predators?
• High numbers sharks –
high coral cover, absent
– low coral cover
• Changes in trophic
structure of fish
communities
• Confounding of human
activity (fishing down
food chain) and natural
disturbance
Sandin et al. 2008 PLoS ONE
Aims
1. Does fishing impact shark abundance?
2. Do changes in the abundance of sharks affect the structure of
the reef fish community?
3. How do benthic disturbances (cyclones, bleaching) alter fish
communities?
4. Are there synergistic effects of these top-down and bottom-up
processes?
Ashmore (Fished)
Rowley Shoals (Non-Fished)
Scott Reefs
(Fished)
Fished Reefs: Coral Bleaching
80
South Scott
Coral Cover (Mean ± 95%CI)
North Scott
Seringapatam
60
40
20
0
1994
1996
1998
2000
2002
2004
2006
2008
Non-Fished Reefs: Cyclonic Event
Coral Cover (Mean ± 95%CI)
80
Mermaid
Clerke
Imperieuse
60
40
20
0
1994
1996
1998
2000
2002
2004
2006
2008
Treatments: Fishing and Disturbance
• Fishing (Fished vs.
Non-Fished)
– Shark density
differences between
Rowley and Scott
Scott Reefs
Rowley Shoals
• Disturbance (Disturbed
vs. Non-Disturbed)
< 30% coral cover
threshold
Coral Cover (%)
80
60
40
30%
Coral
Cover
20
0
1994
2001
2008
Shark Community Data: BRUVS
• BRUVS (Baited Remote Underwater Video
Station)
– All reefs (2003/2004)
– Target apex predators & carnivores
– Depth range: 5 – 100 m
– 1 to 1.5 hrs per drop
– MaxN statistic
Trophic Structure of Coral Reef Fishes
• AIMS LTMP (Long-Term
Monitoring Program)
– Scott and Rowley (1994 –
Present)
– 6 reefs; 3 sites each (n=18)
– Northeast reef slope
– Benthic surveys alongside
– Abundance counts (18
families; non-cryptic
species; excludes recruits)
SOURCE: AIMS
Results: Shark Density (permuted ANOVA)
Sharks Hour¯¹(mean ± 95% CI)
1.6
p < 0.01
1.4
n = 94
α=9
1.2
1
0.8
n = 46
α=5
n = 28
α=5
Ashmore Reef
Scott Reef
0.6
0.4
0.2
0
Rowley Shoals
0.4
0.6
1
Differences in Trophic Structure (RDA)
Fished/Disturbance
Fished/No Disturbance
Non-Fished/Disturbance
Non-Fished/No Disturbance
Fished vs. NonFished:
Carnivores &
Herbivores
(Territorial & Non
– Territorial)
Detritivore
0
0.0
-0.2
Herbivore
Herbivore_T
-1
-0.6
-0.4
Planktivore
Corallivore
-0.8
2 (9.5 %)
RDARDA2
0.2
Carnivore
-0.5
0.0
RDA 1 (13.6 %)
RDA1
0.5
Disturbance vs.
Non-Disturbance:
Herbivores,
Planktivores,
Corallivores, and
Detritivores
Trophic Group Responses: Carnivores
Density Km-2 (mean ± 95%CI)
60
*
50
* p < 0.05
** p < 0.01
40
Fished/Disturbance
Fished/No Disturbance
30
Non-Fished/Disturbance
Non-Fished/No Disturbance
20
Trophic Group Responses: Herbivores
Density Km-2 (mean ± 95%CI)
1000
**
*
**
850
* p < 0.05
** p < 0.01
700
Fished/Disturbance
Fished/No Disturbance
550
Non-Fished/Disturbance
Non-Fished/No Disturbance
400
Trophic Group Responses
Density Km-2 (mean ± 95%CI)
80
**
**
120
50
80
20
40
Corallivore
2200
**
**
Detritivore
**
Fished/Disturbance
Fished/No Disturbance
Non-Fished/Disturbance
Non-Fished/No Disturbance
1500
800
Planktivore
Summary
1. Does fishing impact shark abundance?
– YES
2. Do changes in the abundance of sharks affect the
structure of the reef fish community?
– YES
– Meso-predator release of carnivores and subsequent
trophic cascade during disturbances with herbivores
3. Do benthic disturbances alter fish communities?
Are there synergistic effects of top-down and
bottom-up processes?
– YES
– Herbivores, planktivores and corallivores show
responses to changes in coral and algal cover
– Detritivores show synergistic responses
Significance: Resilience
• Herbivores – more abundant on unfished reefs
where more sharks are present
• Central to recovery of reefs from processes
that result in the loss of coral cover
• Implications for reef resilience
Acknowledgements
Field/lab assistance
Funding/Data
Dr. Jim Underwood
Mike Cappo
AIMS Monitoring Team
Conrad Speed
Places
Owen O’Shea
Rachelle Ninio
Helpful Insights
Dr. Donald Jackson
Dr. Brian Shuter