1. Using Tiger Sharks To Restore Coastal Seagrass Ecosystems
ENV 100 – Winter 2012
Using Tiger Sharks To Restore Coastal Seagrass Ecosystems
Aaron J. Wirsing

2. The Oceans
Photo by William Curtsinger (Nat Geo)

3. Coastal Seagrass Ecosystems
Photo from WCMC Global Seagrass Database

4. The Importance of Seagrass Ecosystems
Fish nurseries
Shelter for fishes, invertebrates
Food for grazers small and large
Nutrient cycling (trap nutrients, promote high productivity)

5. How Do We Protect Seagrass Ecosystems?
Establish preserves, no boating zones
Minimize pollution (run-off)
Protect, restore top predators (sharks)

6. Shark Bay, Western Australia

7. Shallow Banks (lush seagrass beds)

8. The Green Turtle
Chelonia mydas

9. The Tiger Shark
Galeocerdo cuvier
Photo by Neil Hammerschlag

10. The effect of shark as a predator on prey is
Only directly via mortality
Only indirectly via changing prey behavior
A combination of 1 & 2, but 1 the most important
A combination of 1 & 2, but 2 being more important

11. Shark Fishing

12. Shark Tracking

13. Use of Shallow and Deep Waters
Preferred
Preference (observed – expected)
No Preference
Avoided
Shallow Banks
Deep Waters

14. How Might Turtles Negotiate This Problem?
Feed in the deep?
Not really an option – too little seagrass
Feed on the edge of seagrass meadows?
Closer, but seagrass is better near the middle
Feed in interior when in poor condition?
Yes, a condition dependent trade-off between food and safety!

15. Turtle Habitat Use 15

16. Turtle Condition
Body condition index based on visual inspection of physical characteristics

17. Results Condition Category Increasing seagrass quality
Increasing predation risk
Very good
Good
Fair
Poor
Condition Category
Heithaus et al. (2007) J Anim Ecol

18. Discussion
Green turtles only use profitable interior seagrass when desperate (emaciated)
Most turtles in good/very good condition (~ 70%), feed along edges
~ 20% of available seagrass along edges
Sharks prevent lots of seagrass from getting eaten!

19. + Tiger shark Green Turtle (herbivore) Seagrass (increased biomass)
Avoid interior
when in good
condition (70%)
Green Turtle
(herbivore)
Trophic cascade +
Transmitted by
prey behavior
Reduced herbivory
in interior portions
of meadows
Seagrass
(increased biomass)
SharkBay.org

20. Without Sharks… What is likely to occur?
Turtle population growth (more seagrass “available”, higher K)
Reduced seagrass biomass (overgrazing)

21. Do We See This? Yes, in the Caribbean
Large sharks on the decline (historical overfishing, continued by-catch)
Huge, growing green turtle populations, evidence of heavy grazing
In some areas, seagrass meadows eliminated altogether

22. Implications for Seagrass Conservation
How can we manage for healthy seagrass meadows in Shark Bay?
Maintain large numbers of tiger sharks (“Seagrass shepherds”)

23. Implications for Seagrass Conservation
How can we help restore degraded (overgrazed) seagrass ecosystems?
Recover shark populations
That’s right, we can use sharks to manage for healthy coastal seagrass systems!

24. The effect of wolves on elk
Is very similar to that of sharks on their prey
Is very different
I do not remember

25. Take Home Message
Like wolves in Yellowstone*, tiger sharks not just a source of mortality and fear but also a tool for ecosystem restoration
*Wirsing and Ripple (2011)

27. Oh no!!!!!

28. Thanks!