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Experiment 1: Overall performance of sonic vs. VHF transmitters. For this experiment, we hung one Sonotronics (www. sonotronics.com) acoustic tag (IBT 96-2-E, w = 4.9 g out of water, transmitting at 68 KHz) from a mooring buoy at a depth z = 0.8 m. We attached an ATS (www.atstrack.com) VHF tag (R1655, w = 1.1 g out of water, 149.102 MHz) to the upper (dry) part of the buoy, so that the antenna was at ~20-25 cm above sea surface, the same height as when affixed to a fishing bobber tethered to a hatchling (following the method of Okuyama et al. (2009)). We stopped the boat at distances of 100, 200, 500, 800, 1200 and 1500 m from the buoy to measure the maximum strength and directionality of the signals emitted by both transmitters. We used a 3-element VHF Yagi antenna and scanning receiver (ATS R410) to detect radio signals from the ATS tag, and a directional hydrophone (Sonotronics DH-4) with an ultrasonic receiver (Sonotronics USR-08) to detect “pings” from the sonic tag. We evaluated two parameters. The “maximum signal strength” received at each station and given on a qualitative scale of 1 to 5 (1 being weakest and 5 strongest) with the reference maximum strength (5) measured at 1 m from the transmitter. The second parameter we evaluated at each listening station was the “directionality”, defined here as the arc length (in degrees) obtained by rotating the hydrophone or Yagi antenna while receiving signals of maximum strength. We measured the arc length using a digital compass (Garmin Oregon 450t) affixed to either the hydrophone pole or the handle of the Yagi antenna. We carried out this experiment during “calm (glassy)” sea state, following the World Meteorological Organization’s Douglas sea scale (<http://www.wmo.int/pages/prog/ amp/mmop/faq.html>).

Table 1. Results of transmitter range, directionality and optimum depth tests (Experiments 1 and 2). †: IBT = Sonotronics IBT96 acoustic tag, VHF = ATS R1655 radio tag, EMT = Sonotronics EMT01-3 acoustic tag. * In Experiment 2, receivers were set at maximum gain from d = 100-1500m. ** In experiment 1, maximum gain was used for both the acoustic and VHF receivers at d ≥ 800 m.

Experiment 2: Optimum transmitting depth of sonic tag. The aim of the second experiment was to determine the optimal depth of the sonic tag when attached to the fishing bobber. It also provided an opportunity to repeat the radio tracking trial in order to see whether or not the results yielded during Experiment 1 were due to a malfunctioning VHF tag (Table 1). We used two buoys in an area where the water depth was 4 m, one floating at the surface and one consisting in a polystyrene disc floating in the water column at 2 m from the sea bottom. We tethered an IBT 96 tag (transmitting at 72 KHz) to the surface buoy so that it hung 1 m below the water surface (z=1m). We attached a new VHF tag (149.280 MHz) to the buoy as in Experiment 1. We hung an IBT 96 tag (78 KHz) 1m underneath the polystyrene disc (z=3m). We also attached a more powerful Sonotronics acoustic tag (Equipment Marking Transmitter EMT 01-3, transmitting at 75 Khz) to the disc at the same depth to assess the effect of higher transmission power on directionality and tracking range (signal strength). We used the same detection equipment and distances as in Experiment 1 and carried out the tracking during “calm (rippled)” sea conditions, with wavelets in the 0 to 0.1 m range.

Figure 1. Leatherback nesting sites of Jamursba Medi and Wermon, on West-Papua’s Bird’s Head Peninsula (BHP).

Experiment 3: live trials with VHF tag. We tethered the VHF transmitter and bobber unit with a 2.5 m long strand of fishing line (0.13 mm, 2.7 kg strength) attached with a small hook to a pygal scute of a hatchling (Okuyama et al. 2009, see Fig. 2). A 1.9 cm plastic bobber (6.49 cm3, weight out of water: 2.5 g) was tethered at the other end of the line. We glued a VHF tag onto the bobber so its antenna would rise 20 cm (its outstretched length) above the water line (Fig. 2). To contrast the dimensions of the tracking unit with the turtles, the reported average weights of Pacific leatherback hatchlings range from 40.5 g (East-Pacific: Jones et al. 2007) to 44.4 g (West- Pacific: Simkiss 1962). We painted the upper half of the bobber with fluorescent orange paint to facilitate spotting. We released a hatchling fitted with the bobber and VHF tag 250 m from shore during “smooth” sea state (wavelets in the 0.1-0.5m range) and tracked as follows: we recorded its initial position using a hand-held GPS unit (Garmin Oregon 450t) and then let it swim away for 10 min. The position of the hatchling was then tracked back using the Yagi antenna. After its new position was recorded we stopped the boat’s engine and gave the hatchling a 20 min. head start before attempting to relocate it. Each subsequent lap was 10 min. longer than the previous one. We recorded 3 different laps, with the final one lasting 30 min. We repeated the experiment a second time with another hatchling and transmitter.
Experiment 4: live trials with acoustic tag.

Figure 2. Bobber and VHF tag attached to a leatherback hatchling.

For this experiment, we fitted a hatchling with a 2.5 m strand of fishing line and one bobber (following the method employed in Experiment 3) to which we attached an IBT 96 tag (72 KHz) at z=0.8 m. We tethered another IBT 96 tag (78 KHz) to a second hatchling, using the same methods, but adding a bobber 2 m from the hook. We attached the tag to the second (distal) bobber, at 2.5 m from the hook and at z=0.8m (Fig. 3). We used two bobbers in order to facilitate spotting the hatchling, as previous experiments with the VHF tags showed a hatchling easily drags down one 6.49 cm3 bobber during its frequent dives. The other advantage was that the alignment of the bobbers indicates the heading taken by the hatchling. We tracked both hatchlings simultaneously, in “smooth” sea conditions, and using the lap system of Experiment 3.

Figure 3. Acoustic tag with two bobbers tethered to a leatherback hatchling.

Superiority of acoustic tracking. The results given in Table 1 show that up to 200 m from the surface buoys (Experiments 1 & 2) the maximum signal strength of both the acoustic (sonic) and VHF tags was similar for up to 200 m from the surface buoys (Experiments 1 & 2). However, we found that the directionality of the VHF transmitter was 50 ̊, versus 8 ̊ for the sonic tag. At subsequent distances we found that the directionality of VHF was never less than 65 ̊ arc length whereas we picked up the signal of the sonic tags within an arc length of 10-12 ̊ at all listening stations. At the 1200 m and 1500 m listening stations the directionality of the VHF tag was inconsistent: repeated sweeps with the Yagi antenna would each yield different arc length readings (Fig. 4). Both the IBT and VHF tags had similar signal strength decay throughout the testing range (Fig. 5). By enhancing the gain of the receivers, signals were still audible up to a distance of 1,500 m. There was no apparent difference in directionality and signal strength between IBT tags placed at z=0.8 m (Experiment 1), z=1 m and z=3m (Experiment 2). However, the more powerful EMT transmitter (which weighs 223 g and can by no means be used to track hatchlings) outperformed the smaller IBTs in signal strength, but had the same directionality (Experiment 2). The two live trials with VHF tags both failed within the first hour. The first two tracking laps (10 and 20 min) where successful with hatchlings traveling a total distance of 395 and 420 m. At the end of the third lap (30 min interval) we were unable to relocate the turtles. We interrupted the simultaneous tracking of two hatchlings using Sonotronics IBT tags after 60 min., since we were able to seamlessly relocate the hatchlings at the end of the first 3 laps using on average 3 listening stations.

Figure 4. Transmitter directionality (Experiments 1&2). The NaN value represents the inconsistent arc length readings at d=1200 and 1500m.

First tracks of leatherback hatchlings. To validate the acoustic method, 20 hatchlings were tracked
in July-August 2010. The main results of this
preliminary study (to be published in the near
future), were: (1) none of the tracked turtles
were predated, (2) the presence of a near-shore
tidal current deflecting hatchlings towards the
West, (3) all turtles swam North to Northeast, (4)
the effect of hydrodynamic drag of the tracking
unit on the turtles’ swimming behavior was less
important than a) the effect of this West-flowing
surface current, b) the level of fitness of the
hatchlings and c) the state of the tide.

Figure 5. Transmitting range of acoustic and VHF tags (Experiments 1&2).

Conclusions and future directions. Tracking of VHF radio signals proved difficult even in calm sea conditions. The directionality was insufficient to easily find the correct bearing of the signal’s source. A good level of directionality (small arc length) is especially important as the hatchlings’ small size make them hard to spot at distances of over 40 m, even when dragging an orange bobber. At the distances of 1200 and 1500 m, the irregular directionality is likely caused by the signal’s range limit. The limitations of VHF tags were further illustrated during the two live trials, which we carried out in slightly rougher sea conditions. Failure to locate the hatchlings was likely the result of the compounded effect of poor directionality, intermittent diving and wave height possibly shielding VHF signals (waves occasionally taller than antenna). The outcomes of Experiments 1-3 show the inadequacy of using VHF signals as primary cue when tracking hatchlings. Conversely, the directionality of the sonic tags remained more than sufficient to move the boat to a closer listening station and consistently obtain a stronger and more spatially accurate signal. During the live trial (simultaneous tracking of 2 hatchlings) we only needed an average of 3 listening stops to move the boat close enough to sight the hatchling and record its exact position. The small arc length of the signal’s reception area therefore reduces the chance of the tracker moving out of range of the signal, an important feature when tracking small organisms at sea, and even more so when taller waves make it difficult to spot the hatchling and/or the bobber. An additional advantage of acoustic telemetry is that the ultrasonic receiver is tuned to the specific frequency of the tag. The hydrophone picks up a limited amount of background noise, enabling to track without turning off the boat’s engine. The more powerful EMT only surpasses the miniature IBTs in transmitting range, further supporting the suitability of the IBTs. The results of the four experiments enabled us to determine the type of tag and the basic setup to track Papuan leatherback hatchlings. Future improvements include reducing drag by using one larger bobber instead of two and fitting a small LED inside the bobber, allowing to track at least two hatchlings simultaneously at night. The first series of live trials using acoustic tags suggests that in the specific case of the Bird’s Head Peninsula (Fig.1), predation at sea is limited. The presence of a surface current deflecting hatchlings towards the West shows the importance of resolving the oceanography on the near-shore scale in order to determine how and where hatchlings get entrained in larger scale features such as the New Guinea Coastal Current (NGCC), which reverses its direction seasonally (Ueki et al. 2003). The NGCC might therefore act as a “conveyor belt” distributing hatchlings either into the North or the South Pacific. Future work will focus on connecting the different spatial and temporal scales through a dispersal model that merges in-situ tracking data, Lagrangian drifters and remote-sensing data. This will provide a useful tool to validate existing “passive drift” models for hatchlings such as the one developed by Hamann et al. (2011).

Acknowledgments. We thank Dr. Mark V. Erdmann for his unwavering support of GG over the years. Thanks to Pak Ishak, Pak David, all UNIPA and WWF staff and students at Jamursba Medi for their day-to-day assistance during field work, as well as to Barakhiel Heri and Deasy Lontoh for providing hatchlings from Warmamedi. This work was carried out under permit through UNIPA with funding from Conservation International and Scripps Institution Of Oceanography.

BLUMENTHAL, J.M., F.A. ABREU-GROBOIS, T.J. AUSTIN, A.C. BRODERICK, M.W. BRUFORD, M.S. COYNE, G. EBANKS-PETRIE, A. FORMIA, P.A. MEYLAN, A.B. MEYLAN & B.J. GODLEY. 2009. Turtle groups or turtle soup: dispersal patterns of hawksbill turtles in the Caribbean. Molecular Ecology 18: 4841-4853.

CARR, A. 1987. New perspectives on the pelagic stage of sea turtle development. Conservation Biology 1:103-121.

HAMANN, M., A. GRECH, E. WOLANSKI & J. LAMBRECHTS. 2011. Modelling the fate of marine turtle hatchlings. Ecological Modelling 222: 1515–1521.

JONES, T.T., D.R. REINA, C.A. DARVEAU & P.L. LUTZ. 2007. Comparative Biochemistry and Physiology, Part A 147: 313–322.

LIEW, H.C. & E.H CHAN. 1995. Radio-tracking leatherback hatchlings during their swimming frenzy. In: J.L. Richardson & T.H. Richardson (Comps). Proceedings of the 12th Annual Symposium on Sea Turtle Biology & Conservation. NOAA Technical Memorandum NMFS-SEFSC-361, pp. 67-68.

OKUYAMA, J., O. ABE, H. NISHIZAWA, M. KOBAYASHI, K. YOSEDA & N. ARAI. 2009. Ontogeny of the dispersal migration of green turtle (Chelonia mydas) hatchlings. Journal of Experimental Marine Biology and Ecology 379:43-50.

SIMKISS, K., 1962. The source of calcium for the ossification of the embryos of the giant leathery turtle. Comparative Biochemistry & Physiology 7: 71–79.

UEKI, I., Y. KASHINO & Y. KURODA.2003. Observation of current variations off the New Guinea coast including the 1997-1998 El Niño period and their relationship with Sverdrup transport. Journal of Geophysical Research-Oceans 108(C7), 3243, doi:10.1029/2002JC001611.