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The transmitter (Kiwisat 101 PTT with built-in VHF transmitter) was bolted to a 250 mm x 155 mm x 3mm polycarbonate plate, which was placed behind the median anterior nuchal scute. To construct a well fitting plate previous design work had been carried out. A slightly bigger mould of sheet aluminum shaped to conform the profile of a flatback turtle carapace was made, using a flatback turtle skeleton (J14463), and a plastic casting of this specimen, both from the Queensland Museum, Brisbane, Australia. The polycarbonate plate was then placed on top of the moulded aluminum sheet and placed in a commercial oven at 60°C. After 15-20 minutes it had partially melted and required the preferred shape. The gap between the moulded plate and the transmitter was filled with a sealer to prevent possible drag effects. Six slits in the plate were made for the attachment of the harness straps, and a piece of 3 mm thick neoprene wet suit material was used between the plate and the carapace as padding. The built-in VHF transmitter proved useful when retrieving the unit on the beach and was also intended as a way to retrieve the unit should it detach from the turtle while at sea.

Nylon seat-belt webbing (2.5 cm in width) was used to make six straps radiating from a central ring, with two running cranially over the shoulders, two running caudally, one on either side of the tail, and two running laterally around the middle of the turtle. A strap holding the two caudal-straps together prevented the harness from sliding forward (see front cover). Each strap was fastened to the ring by fashioning attachment loops which where manually sewn on for the first deployment, and secured with multiple cable zip ties for subsequent deployments. The ring, destined to be in a central location on the underside of the turtle with the six straps running dorsally towards the attachment package, was made of a magnesium alloy and was intended to function as the corrosive link for the harness:
<https://seaturtle.org/cgi-bin/imagelib/index.pl?photo=471>. The dimensions of the rings were based upon those previously used in a study of green turtles (Papi et al. 2000). The ring is rectangular in section (8 x 5 mm) with an outside dimension of 80 mm. The aim was to allow the harness stay on for at least the maximum inter-nesting interval (ca. 20 days), but not much longer, so that the harness would detach in the event that a turtle was not recaptured to remove equipment. Given nest site fidelity, high patrol effort and monitoring for radio transmissions from the VHF unit, this was unlikely.

Several experiments with the rings were carried out in the water off Bare Sand Island before deployment on the turtles, and these showed a corrosion rate that was greater than predicted. Two rings glued together with a silicone sealer appeared that they would last longer, but results were equivocal. Because of this ambiguity, small self tapping screws was used to attach the webbing to the edges of the carapace when deployed on turtles for the first time, in addition to the glued double rings holding the straps together. Four deployments were performed and when all the turtles returned 17 - 19 days later the state of the rings ranged from only demonstrating surface corrosion to almost full-thickness corrosion. As the rings were placed underneath the turtles they could easily have been subject to varying degrees of wear and tear when in contact with different substrata. Differences in the turtles preferred inter-nesting location may have played a role. For the subsequent four deployments at Curtis Island, the magnesium rings were exchanged with stainless steel rings and no screws were used. Here only the Zinc staples together with the Velcro fastenings, as described below, would be the weak points of the harness allowing eventual detachment after a period of time at large. It was felt that this was acceptable because the very high fidelity and well known breeding history of the specimens chosen meant it was very unlikely that the turtles would not return for a subsequent nesting or fail to be detected for any other reason.

Different types of flotation material on the plate were trialed in the water off Bare Sand Island before deployment, in order to allow the whole harness to be slightly positively buoyant if detached, preferably with the two antennae protruding above the water's surface maximizing chances of retrieval should harnesses detach at sea. Adding steel plates of different thicknesses as a means of trimming the ballast was trialed, resulting in the need of adding even more flotation. High density foam was cut to fit around the transmitter and glued to the underside of the plate using gasket sealer (Loctites Blue Maxx) for the deployments at Bare Sand Island. The foam was highly compressed on return and time-depth recorder data show that the transmitters had been down to a maximum depth of 40 m (Sperling, unpublished data) resulting in higher pressure than the foam could withstand. At Curtis Island neoprene wet suit material was added for buoyancy instead, but should detachment at sea have occurred would have yielded insufficient buoyancy to allow surfacing of the antennae. Neoprene material seemed to withstand the depth and showed no sign of abrasion could be seen on the carapace where the plate had been after detachment. More work needs to be done to make any unit float with the antennae clear of the surface.

Attachment was undertaken by placing the magnesium ring and six straps on top of an overturned 60 cm long 40 cm wide 39 cm high square plastic bin (Nally Limited, Australia). The turtle was then placed on top of the bin, with the ring in the middle of its plastron. While the turtle was held in position the six straps were attached over the carapace and through the slits in the plate. Each strap was tightened by pulling and secured on itself with Velcro <https://seaturtle.org/cgi-bin/imagelib/index.pl?photo=470>, pre-sewn onto the straps beforehand with Poly A/bond thread (V138, Barbour Campbell Drybond). Zinc-covered staples were used to further secure the straps. These did not affect the integrity of the straps, and they were easy to pull out with a pair of pliers when removing the harness.

The strap system on the harness seemed to work well, as all three attachments that came back using the stainless steel ring and no self-tapping screws, were very firm and secure when returning. One turtle did lose its harness only a few days after deployment, and the reason for this is not known. Some abrasion of the skin occurred at the back of the turtles close to the hind flippers, but there was no sign of bleeding or broken skin.

In conclusion, although the basic 6-strap design seems to be well suited to the size and shape of the flatback turtle, more experimentation is needed in order to make the flatback harness more advantageous. However, the actual method of attachment worked well, and could easily be performed in 30-40 minutes by two people. It did not require the overturning of sea turtles, and being carried out at the end of the laying sequence, caused minimum disturbance to the natural nesting behaviour. Likewise, detachment of the harness was carried our after laying of a subsequent clutch. This required only the removal of a few staples before it could be loosened, allowing the turtle to simply walk out of the harness. It did not require moving the turtle and was accomplished in less than 2 min.

Acknowledgements: The project was funded by Coast and Clean Seas as part of the ongoing nesting beach monitoring at Bare Sand Island, Northern Territory. Tracking time at Curtis Island was paid for by the Gladstone Port Authority. The magnesium rings were made and paid for by the Australian Magnesium Corporation. Support from Sea World Research and Rescue Foundation, the University of Queensland and the Northern Territory University is appreciated. Ideas and suggestions from Col Limpus, Queensland Parks and Wildlife Service, Patrick Couper, Queensland Museum, Brisbane, and Scott Whiting, Biomarine International, proved very useful. Gordon Grigg gave useful comments on an earlier draft. The help of numerous field volunteers is also greatly appreciated. The manuscript benefited from input of one anonymous referee and the editors.

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