seaturtle.org : MTN : ARCHIVES : Sign In

Shoalwater Bay (22°20’S, 150°12’E) in central Queensland has been the site of regular winter surveys of a large foraging C. mydas population since 1986. The bay supports extensive seagrass pastures fringed by mangrove forests as well as rocky outcrops with fringing coral reef. C. mydas ranging in size from small immature to large adult turtles (CCL = 40-120cm) forage daily within the mangrove forests at high tide. These turtles move out of the mangroves and fall back across the seagrass flats as the tide drops and return with the rising tide. While seagrass constitutes the main part of the diet of these turtles, at times, mangroves also form a significant part of their diet. For example, in July 1989 there was a very dense crop of A. marina fruit in Shoalwater Bay. In that season, a series of C. mydas encompassing all size classes, were captured by the turtle rodeo method (Limpus & Reed 1985) while foraging within the mangrove forest and included in diet studies (n=20). These turtles were examined for mouth contents on capture and for stomach contents by levage several hours later (Forbes & Limpus 1993). Although mouth contents of these C. mydas mostly consisted of seagrass blades (Halophila sp. and Halodule sp.), two mouth samples contained A. marina leaves. While all levage samples contained seagrass (mostly Halophila sp. and Halodule sp.) and small amounts of unidentified algae, most levage samples also contained substantial amounts of well chewed A. marina cotelydons. Only three of these turtles had chewed A. marina leaves in the levage samples. Necropsy of a large adult female C. mydas in July 1988 revealed alternating broad bands of chewed A. marina cotyledons and seagrass extending from the crop well down into the small intestine. This observation suggests that the turtle had been visiting the mangrove forest over the course of many high tides and feeding on seagrass during the intervening periods.

We were unsuccessful in directly observing whether the turtles were feeding on the cotelydons of mangrove propagules while they were still fruit on the trees or as dispersing fruit that had dropped from the trees into the water or whether they were feeding on the cotyledons of the growing seedlings on the bottom. During July in Shoalwater Bay, the highest tides occur at night. During the night high tides in July 1989, C. mydas were observed by spotlight within the mangrove forest and apparently feeding within the partly submerged canopy of the trees. In contrast, by daylight when the mangrove canopy was not submerged by the high tide, C. mydas were observed foraging across the submerged forest floor beneath the mangrove forest canopy. However, the local fishermen reported seeing C. mydas foraging by day among the leafy branches of mangroves submerged at high tide (W. Chippendale, pers. comm.). Day time high tides are the highest during the summer months in this region. Further evidence of these turtles moving within the canopy of the forest at the higher levels of the tidal cycle comes from two records of live and otherwise healthy C. mydas found hanging from trees within the mangrove forest with their flippers wedged in the forks of the branches. During 1989, the vast C. mydas herd (probably tens of thousands of turtles) in Shoalwater Bay would have been a significant consumer of the A. marina fruit and/or seedlings. In contrast to the regular occurrence of A. marina in diet samples at Shoalwater Bay, we have only a single record of the fruit of a second species of mangrove (Rhizophora sp.) from a levage sample.

At Shoalwater Bay in July 1994 when there had been a very poor fruiting by A. marina, relatively few C. mydas were found feeding within the mangrove forest. Little or no A. marina fruit occurred in the gut contents from three C. mydas captured foraging within the mangrove forest (Brand 1995) or in the levage samples (n>20) examined from C. mydas captured while feeding within or adjacent to the mangrove forest.

In subtropical (south) Queensland, Read (1991) studied the diet of immature C. mydas that foraged in Flathead Gutter in eastern Moreton Bay (27°20.5’S, 153°24.8’E; Limpus et al. 1994). As in Shoalwater Bay, these turtles foraged across the seagrass flats up to the mangroves with the rising tide and fell back to the gutter at low tide. Read (1991) recorded that 21% of the C. mydas levage samples contained A. marina fruit. In contrast, Brand (1995) using the same study site in 1994, reported no A. marina in the entire gut samples of 3 individuals or in the 20 levage samples from immature C. mydas.

In south and central Queensland, mangrove leaves appear to constitute only a trivial part of the diet of C. mydas. However when they are available to C. mydas feeding in inshore bays, the potentially nutritionally rich and readily digestible A. marina cotyledons appear to be a major food item taken in preference to seagrass. This would be a “fruitful” topic for further study.

Acknowledgements: This study was conducted as part of the Queensland Turtle Research Project of the Queensland Parks and Wildlife Service. That part of the study within commonwealth waters was conducted under a research permit from the Great Barrier Reef Marine Park Authority. Access to the study site was permitted by the Australian Army. Dr. C. J. Parmenter, the late D. Reimer and numerous volunteers provided assistance during the study. Bill Chippendale shared his wide experience gained from decades of commercial fishing in the bay. This assistance is gratefully acknowledged.

BRAND, S. J. 1995. Diet selection by juvenile green turtles, Chelonia mydas, in sub-tropical flathead Gutter, Moreton Bay. B.Sc. (Hons.) Thesis, Department of Zoology, University of Queensland, Brisbane.

FORBES, G. & C. J. LIMPUS. 1993. A non-lethal method for retrieving stomach contents from sea turtles. Wildlife Research 20:339-343.

LIMPUS, C. J. 1998. Overview of marine turtle conservation and management in Australia. In: R. Kennett, A. Webb, G. Duff, M. Guinea and G. Hill (Eds.). Marine Turtle Conservation and Management in Northern Australia. Northern Territory University, Darwin. pp. 1-8.

LIMPUS, C. J., P. J. COUPER & M. A. READ. 1994. The green turtle, Chelonia mydas, in Queensland: population structure in a warm temperate area. Memoirs of the Queensland Museum 35:139-154.

LIMPUS, C. J. & P. C. REED. 1985. The green turtle, Chelonia mydas, in Queensland: population structure in a coral reef feeding area. In: G. Grigg, R. Shine, and H. Ehmann (Eds.). Biology of Australian Frogs and Reptiles. Surrey Beatty and Sons, Sydney. pp.47-52.

PENDOLEY, K. & J. FITZPATRICK. 1999. Browsing on mangroves by green turtles in Western Australia. Marine Turtle Newsletter 84:10.

PRITCHARD, P. C. H. 1971. Sea turtles in the Galapagos Islands. IUCN Publications New Series, Supplementary Papers 31:34-37.

READ, M. A. 1991. Observations on the feeding ecology of immature green turtles, Chelonia mydas, in the Moreton Banks region of Moreton Bay, South East Queensland. B.Sc. (Hons.) Thesis, Department of Zoology, University of Queensland, Brisbane.