A Comparison of the Seasonal Movements of Tiger Sharks and Green Turtles Provides Insight into Their Predator-Prey Relationship

Richard Fitzpatrick,John D Stevens,Adam Barnett,Ian Bell,Mark G Meekan,Michele Thums,Graeme Clive Hays

doi:10.1371/journal.pone.0051927

Abstract

During the reproductive season, sea turtles use a restricted area in the vicinity of their nesting beaches, making them vulnerable to predation. At Raine Island (Australia), the highest density green turtle Chelonia mydas rookery in the world, tiger sharks Galeocerdo cuvier have been observed to feed on green turtles, and it has been suggested that they may specialise on such air-breathing prey. However there is little information with which to examine this hypothesis. We compared the spatial and temporal components of movement behaviour of these two potentially interacting species in order to provide insight into the predator-prey relationship. Specifically, we tested the hypothesis that tiger shark movements are more concentrated at Raine Island during the green turtle nesting season than outside the turtle nesting season when turtles are not concentrated at Raine Island. Turtles showed area-restricted search behaviour around Raine Island for ∼3–4 months during the nesting period (November–February). This was followed by direct movement (transit) to putative foraging grounds mostly in the Torres Straight where they switched to area-restricted search mode again, and remained resident for the remainder of the deployment (53–304 days). In contrast, tiger sharks displayed high spatial and temporal variation in movement behaviour which was not closely linked to the movement behaviour of green turtles or recognised turtle foraging grounds. On average, tiger sharks were concentrated around Raine Island throughout the year. While information on diet is required to determine whether tiger sharks are turtle specialists our results support the hypothesis that they target this predictable and plentiful prey during turtle nesting season, but they might not focus on this less predictable food source outside the nesting season.

Highlights

Predators play important roles in ecosystems by influencing the distribution, behaviour and abundance of their prey and predatorprey interactions have long been recognised as important in ecosystem dynamics [1,2,3]
The number of locations obtained per day was 0.9960.82 for tiger sharks and 2.9461.77 for green turtles (Table 1)
Our results suggest that while tiger sharks might target green turtles when they are concentrated at Raine Island during the nesting season, they might not focus principally on this prey source outside the nesting season when turtles are widely dispersed

Summary

Introduction

Predators play important roles in ecosystems by influencing the distribution, behaviour and abundance of their prey and predatorprey interactions have long been recognised as important in ecosystem dynamics [1,2,3]. By comparing movement patterns of predator and prey we can gain some understanding of when, where and how often predator and prey overlap spatially and gauge the chance of interactions [4,5,6,7]. Such studies can vastly improve our understanding of large mobile predators’ spatial use in relation to their prey and shed light on their foraging strategies. Is the predators’ foraging strategy to move directly between distinct habitats exploiting seasonally abundant prey, or do they concentrate on targeting a specific prey and their movements are dictated by the prey’s movements?

Methods

Results

Discussion

Conclusion