Living on the edge: dugongs prefer to forage in microhabitats that allow escape from rather than avoidance of predators
Section snippets
Study Site
We undertook this study in the Eastern Gulf of Shark Bay (∼25°45′S, 113°44′E; Fig. 1). Shark Bay is home to a large population of dugongs (10 000–14 000 individuals; Marsh et al., 1994, Preen et al., 1997, Gales et al., 2004), but their use of our study area fluctuates seasonally (Wirsing et al., in press a). In 1991, Shark Bay was listed as a World Heritage Area, and anthropogenic activity in the region has always been minimal. Thus, we were able to evaluate the microhabitat use of dugongs
Results
We sighted 105 foraging dugongs while conducting transects through the seven survey zones; 64 individuals were found in edge microhabitat and 41 were found in interior microhabitat. Most encounters involved solitary animals (53/75), and mean ± SD group size was 1.40 ± 0.81, facilitating robust estimation of daily densities for survey zones and across microhabitats. The best model of foraging dugong density incorporated large tiger shark abundance (S) and the interaction between shark abundance and
Discussion
Our results are consistent with the hypothesis that microhabitat use by dugongs foraging over seagrass meadows is influenced by the danger of predation. Foraging dugong densities were not equivalent in edge and interior microhabitats throughout the year (rejecting the food quantity hypothesis), nor did foragers consistently overuse meadow interiors (rejecting the food quality hypothesis). Instead, the degree of dissimilarity between forager densities in edge and interior microhabitats was a
Acknowledgments
We thank R. Abernethy, V. Alla, T. Bujas, C. Chow, M. Davis, A. Greenley, K. Harper, L. Heithaus, M. Kerford, S. Kowalewsky, A. Krickan, J. Laski, L. Marshall, K. Martin, R. McPhie, B. Stalvey, J. Wilder and K. Wirsing for their assistance in the field. This material is based upon work supported by the National Science Foundation under Grant Number 0526065. Other grants and support were provided by Humminbird, Mercury Marine Australia, the Monkey Mia Dolphin Resort, Monkey Mia Wildsights
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