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Raine Island (11° 35’ S, 144° 02’ E) is an elongate sand cay approximately 830m long and 430m wide at its widest point, and is the primary site for the largest nesting cohort of C. mydas in the world (Limpus 2003). Nightly nesting densities reach a peak in December, with several hundred to several thousand turtles coming ashore nightly (Limpus et al. 1993).

Devices were attached to one turtle that had successfully nested, and to five turtles that had ‘false crawled’ (attempted & failed nesting). Turtle selection commenced when sufficient tidal depth covered the reef flat to allow turtles beach access. Because C. Mydas on Raine Island typically take three hours (Limpus et al. 2003) to complete the nesting process, turtles that were returning to the sea with their carapaces still damp with sea water were assumed to have been unsuccessful in their nesting attempt.

The TDRs (8 Bit Minilog, Vemco Pty Ltd., Nova Scotia, Canada) were pre-programmed to record depth (0.5 m - 900 m ± 0.4% of selected depth range). TDR attachment commenced immediately following oviposition or as the turtle was heading back to the sea. Two small holes (≈1.5 mm) were drilled in each supracaudal scute and the TDR was secured in place using thin (0.5 mm) stainless steel wire.

The nesting beach was subsequently searched nightly for turtles which had been fitted with a TDR, and were attempting to nest. When a TDR-carrying turtle was recaptured, the data logger was removed entirely and the turtle was allowed to continue looking for a nest site or return to the sea.

To mitigate recording false dives due to wave action or submergence during repetitive breathing while at the surface, time spent above a threshold of one metre below the sea surface was classified as a surface interval. The six recovered TDRs yielded continuous data for between 17.2 and 174.4 h (Table 1).

Table 1. Tag numbers, curved carapace lengths, TDR deployment times for green turtles at Raine Island.

Table 2. Mean, ±standard deviation and range (in parentheses) for dive duration (minutes), surface interval (minutes) and depth (meters) of TDR-fitted green turtles at Raine Island.

While individual dive behaviour variations existed between turtles, the six turtles displayed generally similar dive patterns. Average submergence times of 13.5 min (R: 0.3-68.5) were accompanied by unexpectedly long mean surface intervals of 2.5 min (R: 0.3 – 329.3). Extended surface intervals (392.3 min) might be explained by the fact that five of the six turtles returned to the sea without having successfully laid a clutch of eggs. These turtles may have attempted to re-nest on the same night, spending extended time on the beach and therefore skewing the surface interval. While individual dive behaviour variations existed between turtles, the six turtles displayed generally similar dive patterns (Table 2). Average submergence times of 13.5 min (R: 0.3-68.5) were accompanied by unexpectedly long mean surface intervals of 2.5 min (R: 0.3 – 329.3). Extended surface intervals (392.3 min) might be explained by the fact that five of the six turtles returned to the sea without having successfully laid a clutch of eggs. These turtles may have attempted to re-nest on the same night, spending extended time on the beach and therefore skewing the surface interval. Unlike other several other studies (Hays et al. 2001; Hochscheid et al. 1999; Starbird 1993; van Dam & Diez 1997) that reported five or six distinct dive profile types, Raine Island inter-nesting turtles typically only displayed three general dive profiles. At night turtles exhibited a flat-bottomed “U-shaped” dive profile (Fig. 1). At approximately 06:00 turtles began swimming to a depth (usually the maximum for that dive), then slowly ascended over the remainder of the dive time until finally ascending directly to the surface (Fig. 1). We called these “slowly ascending” dive types. The third dive type was a near vertical descent and ascent “V” shaped dive (Fig. 2).

Figure 1. Dive profile for K56504 showing early morning shift from “U-shaped” dives to “slowly ascending” dive types.

Figure 2. Dive profile for turtle K 56502 showing “V-shaped” dive types.

All turtles made occasional dives to depths greater than two standard deviations from their mean dive depth. Few (5%) of these deeper dives were undertaken during the daytime (08:00-15:00) with most (54%) occurring at night (18:00-06:00). The remainder took place at dawn (19%) and dusk (22%). The benefits of deep diving as an energy conservation strategy during inter-nesting periods are described elsewhere (see Hays et al. 1999; Hays et al. 2004). However this behaviour may also be related to predator avoidance, with shark attacks on nesting turtles being common at Raine Island, (Limpus et al. 2003) and documented at other nesting rookeries (Stancyk 1982; Fergusson et al. 2000). These preliminary data suggest that at least some of Raine Island’s inter-nesting turtles do not dive to the great depths available to them adjacent to the reef, but use shallow water habitat adjacent to the reef edge then return to refugia within reefal structure at night.

Acknowledgements: We acknowledge Natural History New Zealand for providing financial support in purchasing the telemetry equipment used in this study. We would thank the team of research assistants including C. Copeman, H. Smith and T. Shultz.

AUS Chart 836 (Cape Weymouth to Olinda Entrance). Royal Australian Navy Hydrographic Service.

FERGUSSON, I.K. 2000. Predation by white sharks, Carcharodon carcharias, upon chelonians, with new records from the Mediterranean Sea and a first record of the ocean sunfish, Mola mola, as stomach contents. Environmental Biology of Fishes 58: 447-453.

HAYS, G.C., P. LUSCHI, F. PAPI, C. DEL SEPPIA & R. MARSH. 1999. Changes in behaviour during the inter-nesting period and post-nesting migration for Ascension Island green turtles. Marine Ecology Progress Series 189: 263-273.

HAYS, G.C., A.C. BRODERICK, F. GLEN, B.J. GODLEY & W.J. NICHOLS. 2001. The movements and submergence behaviour of male green turtles at Ascension Island. Marine Biology 139: 395-399.

HAYS, G.C., J.D. METCALFE & A.W. WALNE. 2004. The implications of lung-regulated buoyancy control for dive depth and duration. Ecology 85: 1135-1145.

HOCHSCHEID, S., B.J. GODLEY, A.C. BRODERICK, & R.P. WILSON. 1999. Reptilian diving: highly variable dive patterns in the green turtle Chelonia mydas. Marine Ecology Progress Series 185: 101-112.

LIMPUS, C.J., J.D. MILLER & C.J. PARMENTER. 1993. The Northern Great Barrier Reef green turtle, Chelonia mydas, breeding population. In: Smyth, A.K., K.H. Zevering & C.E. Evering (Eds.). Raine Island and Environs, Great Barrier Reef: Quest to preserve a fragile outpost of nature. Raine Island Corporation in cooperation with Great Barrier Reef Marine Park Authority, Townsville, Queensland, p. 47-50.

LIMPUS, C. J., J.D. MILLER, C.J. PARMENTER & D.J. LIMPUS. 2003. The green turtle, Chelonia mydas, population of Raine Island and the northern Great Barrier Reef: 1843-2001. Memoirs of the Queensland Museum 49: 349-440.

STANCYK, S.E. 1982. Non-human predators of sea turtles and their control. In: Bjorndal, K.A. (Ed.). Biology and Conservation of Sea Turtles. Smithsonian Institution Press, Washington D.C. pp. 139-152.

STARBIRD, C.H. 1993. Telemetry studies of the internesting movements and behavior of hawksbill sea turtles (Eretmochelys imbricata) around Buck Island Reef National Monument, St. Croix, U.S. Virgin Islands. MSc. Thesis, San Jose State University, San Jose.

VAN DAM, R.P. & C.E. DIEZ. 1997. Diving behavior of immature hawksbill turtles (Eretmochelys imbricata) in a Caribbean reef habitat. Coral Reefs 16: 133-138.