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

The Oceans Photo by William Curtsinger (Nat Geo)

Coastal Seagrass Ecosystems Photo from WCMC Global Seagrass Database

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

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

Shark Bay, Western Australia

Shallow Banks (lush seagrass beds)

The Green Turtle Chelonia mydas

The Tiger Shark Galeocerdo cuvier Photo by Neil Hammerschlag

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

Shark Fishing

Shark Tracking

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

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!

Turtle Habitat Use 15

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

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

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!

+ 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

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

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

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

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!

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

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)

Oh no!!!!!

Thanks!