Proceedings of the 19th Annual ISTS: tracking leatherback turtles

Techniques for applying biotelemetry devices to leatherback sea turtles are limited to onshore application and do not allow researchers the opportunity to track males or females beyond one year post nesting. A dart and pole spear were developed to apply devices to in-water leatherbacks in areas where they aggregate or to incidentally captured turtles that can be released alive from commercial fisheries. The dart was tested on nesting leatherbacks and found to remain secure for periods of 9-13 days. A satellite transmitter was fitted to a buoy made of syntactic foam and applied to an in-water leatherback using the dart and pole arrangement. The transmitter remained attached to the turtle for six days before being removed when it was snagged on floating debris or pulled away by a copulating male.

Introduction

Leatherback turtles (Dermochelys coriacea) are the most widely distributed of reptiles occurring throughout tropical and temperate oceans of the world (Pritchard, 1980; Carr 1952). Movements of leatherbacks are poorly known as

researchers have relied on flipper tag recoveries to describe migratory destinations and philopatry of free-ranging individuals. Recently, satellite telemetry has been success- ful at monitoring leatherback movements and gathering behavioral data (Eckert 1997, Keinath and Musick, 1993). The advantages of satellite telemetry over other tracking techniques include long-term data acquisition without the need for vessel support. Movements of leatherbacks within one year of nesting have been successfully monitored (Eckert, 1997) although there is no information on movements of male, subadults or those of females during subsequent years at sea. This is due to limitation in attachment including longevity and researchers inability to attach tracking instrumentation once the turtle has left the nesting beach.

Methods of attaching biotelemetry equipment to turtles include adhesive (Beavers et al., 1992), harnesses (Keinath and Musick, 1993; Eckert, 1997) or through a hole drilled in the carapace (Dizon and Balazs, 1982). The method chosen for attaching devices depends on the size, behavior, potential future growth and catchability of the species, as well as the characteristics of the environment and the principal study objectives (Boarman et. al., 1998). Anchor holes and harnesses have been used most frequently with leatherbacks as most adhesives do not adhere well to their carapace. These techniques require that the leatherback is stationary and on land for application to be completed.

A leatherback turtle was located on the surface near Kei Kecil Island, Indonesia. The turtle was approached to within 2 m and the pole spear was used to apply a dart and satellite package. The transmitter provided information on position only and data were accessed via computer and modem daily.

Results

Four darts and two mock buoys were retrieved from nesting leatherbacks as they returned to the nesting beach on days nine, 11, 12 and 13 of their internesting periods respectively. Two buoys had been broken off tethers although darts remained as they had been originally attached. Darts were found to be in good condition and firmly anchored. Exit wounds created by the removal of darts (diameter=0.5 cm) were no larger than the original entry wound. Penetration (2.5-3.0cm) remained in the connective tissue of the carapace and each dart protruded approximately one cm from the surface of the carapace. Fourteen-day magnesium links holding barbs in place were nearly gone upon retrieval.

A dart and buoy containing a ST-10 satellite transmitter were successfully attached to the rear third of the carapace of a female leatherback turtle around Kei Island, Indonesia. Following application the turtle was tracked for five days. On the sixth day the buoy and transmitter disengaged from the turtle and floated to the surface. It was retrieved ten km from where the package had been applied. The line leading to the barbs of the dart had been broken although the buoy and tether were in their original condition.

Discussion

Mock buoys that broke free from turtles may have been snagged on debris, pulled by the rear flipper of the turtle they were attached to or by a copulating male. Tethers were one meter long and allowed buoys to trail 0.75 m behind the turtle where their rear flipper could have come in contact with the buoy. Breaks in the tether arrangement occurred where a ferrule was used to secure the tether to the darts. This attachment point proved weaker than the line running to the barbs of the dart and two or three ferrules should be used in future studies to insure that tethers do not break and darts disengage when force is applied to the buoy. The length of tethers should allow the buoys to rise to the surface when the turtles surfaces and short enough to avoid entanglement with the rear flippers or floating debris (i.e. <1.0 m). Length of a tether will also depend on the point of attachment of the dart since leatherbacks rear carapace is partially submerged upon surfacing.

Wounds caused by darts did not increase in size despite 9-13 days at sea and darts remained within the cartilaginous layer of the carapace (<3 cm depth). Leatherbacks have been encountered with penetrating billfish wounds to a depth of six cm in their carapace and despite this they have nested and showed sign of healing following removal (Eckert, 1994; P. Pritchard, pers. comm., 1999). All four leatherbacks darted with mock buoys returned to successfully nest after their internesting periods and showed no sign of altered behavior. Although the effect of secondary infection could not be evaluated, wounds caused by darts did not compromise leatherback function or behavior during this study.

A satellite transmitter applied using a dart stayed attached for six days and disengaged when it became snagged on floating debris or was pulled by a copulating male turtle. Leatherbacks have been seen copulating in the area in which the transmitter was applied and 26% of those taken around the Kei Islands are male (Suarez and Starbird, 1996). Male leatherbacks use their front flippers to hold themselves in place often grappling at the carapace of the female (Godfrey and Barreto, 1998). This movement could have resulted in the removal of the dart and buoy from the female they were attached too.

Development of a system to apply tracking equipment to leatherbacks while in water may provide opportunity to study open-water movements including those of subadults and males. It may have special application for tracking incidentally captured leatherbacks or for those that can be approached in oceanic areas where they aggregate. Buoys can be designed to house an assortment of biotelemetry equipment including time-depth-recording devices, satel- lite, radio or sonic transmitters. Information on diving behavior could be acquired by applying buoys equipped with time-depth-recording devices, radio transmitters and timed releases for retrieval at a predetermined time. This technique has been successful in studies of marine mammals (Croll et. al., in press). Pop-up satellite transmitters (Microwave Telemetry, Inc., Columbia, Maryland) have been used successfully on pelagic animals (Block et. al., 1998) and may provide another opportunity to researchers interested in the behavior of sea turtles.

Commercial fisheries such as drift gill, set net and longlines have been found to incidentally take leatherback sea turtles (Balazs and Pooley ,1994; Wetherall et al., 1993) and have been implicated in the decline of major leatherback nesting beaches around the world (Eckert, 1997; Chan et al., 1988). Leatherbacks are commonly found alive when entangled in longlines and may be set free although subsequent mortality may result from hook wounds, internal bleeding or entanglement. Applying a dart with tracking equipment to turtles following entanglement could lead to information on mortality or migration following encounters with commercial fisheries.

Aggregations of leatherbacks around Kei Island and in the coastal areas of the United States (Starbird et al., 1993; Lazell, 1980) and Europe (Pritchard pers. comm., 1999) may represent groups of individuals using migratory corridors or foraging areas. Aggregations such as these could greatly increase the vulnerability of the entire population since it is more susceptible to localized threats. Conservation efforts should seek to learn as much as possible about the habits of leatherbacks in areas that are found to be focal points of populations. Application of biotelemetry equipment using darts would increase our understanding of leatherback behavior in these areas. These types of studies along with those focusing on movements in areas of fisheries conflict should enhance the prospects for effective international conservation of this species.

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Acknowledgments

This study would not have been possible without the support of S. Ohiorat, J. deGroot, H. Audel, S. Hayes, B. Hool, the Matiputty family and the Teniwut family. Valuable feedback was provided by G. Roemer, B. Tershy, H. Paige and P. Pritchard. Funding and support provided by the Condor Manufacturing, Chelonia Research Foundation, Stan’s Skin & SCUBA Diving, Telonics, Megellan Products and HUALOPU.