Throughout the Pacific basin, leatherback turtle (Dermochelys coriacea) populations have been in decline for decades, and the species is considered Critically Endangered (Spotila et al. 1996, 2000; IUCN 2004). Known factors in the decline include fishery bycatch in longline gear and gill nets, beach erosion, and harvesting of eggs and adults. Compared to the eastern Pacific, less is known about leatherbacks in the western Pacific, with populations known from Malaysia, Solomon Islands, Papua (Indonesia), Vanuatu, and Papua New Guinea (PNG). In this study, we focus on populations along the north coast of PNG.

Leatherbacks that nest on beaches in PNG have long been an important part of local cultures and traditions (Spring 1982b). The meat and eggs of leatherbacks have provided a source of protein to local villagers in many areas, and sea turtles have provided economic benefits through the selling of eggs (Spring 1982a, 1982b). Although local communities have recognized a decline in nesting populations of leatherbacks, conservation and protection of this important resource has been hindered by the lack of broad-scale knowledge of nesting beaches and leatherback movements following local nesting.

Previous attempts to document leatherback turtle nesting sites in PNG through local beach surveys and questionnaires yielded incomplete and largely qualitative information (Spring 1982a; Hirth et al. 1993). Several separate local studies were initiated and subsequently discontinued during the 1980s (Bedding and Lockhart 1989; Hirth et al. 1993), with little or no coordination between sites. The lack of a broad regional perspective has hindered population assessment and conservation efforts. Since 1998, a community-based monitoring program has been developed for one nesting beach along the Morobe coast within the Kamiali Wildlife Management Area (WMA). This monitoring program has provided in-depth, quantifiable information for this nesting beach, but a regional or population-level perspective is needed. In particular, an estimate of the total nesting population size is critical for effective conservation efforts. To achieve this objective, several information gaps must be filled, including 1) identification of nesting beaches used by individual females within a season and by the population as a whole, 2) the total number of nests at each site, and 3) data on female reproductive behavior to convert the number of nests into a total adult female population size. Furthermore, effective conservation efforts will require identifying relevant offshore internesting habitats and migratory routes to mitigate at-sea threats. While nesting beach protection alone has been highly successful for Atlantic leatherbacks (e.g., Dutton et al. 2005), eastern Pacific leatherbacks have continued to decline despite conservation efforts on nesting beaches in Costa Rica and Mexico (Spotila et al. 1996, 2000; Eckert and Sarti 1997).

In this study, we present telemetry data and results from an aerial survey designed to address the first 2 of the above 3 required steps. The telemetry data provide the first information on post-nesting movements of leatherbacks from the Kamiali WMA, including identification of additional nesting locations utilized by individual females, and key marine habitats utilized during the internesting period and post-nesting migration. The aerial survey is the first comprehensive survey of leatherback turtle nesting activity along the north coast of PNG, New Britain Island, and several smaller islands. These 2 studies complement efforts under way as part of the community-based monitoring project at Kamiali WMA, which is obtaining long-term information on female nesting behavior that will ultimately allow estimating the total population size.

METHODS

Telemetry

Transmitters were attached to 19 leatherbacks at Kamiali WMA during consecutive nesting seasons. Nine were attached during December 2001, and 10 were attached during February 2003 (Table 1). All transmitters were attached with a flexible harness constructed of soft nylon webbing with flexible polyvinyl tubing over the shoulder straps to prevent chaffing. An elastic dorsal ring connecting the shoulder straps to the belly strap contained a corrodible link designed to limit the duration of the harness attachment to about 18 months (Eckert and Eckert 1986). Transmitter types included 4 Wildlife Computer model SDR-SSC3s, 2 Telonics SD14s, 3 Telonics SD18s, and 10 Sirtrack Kiwisat 101s (Table 1). All transmitters were duty-cycled to optimize battery life and transmitted at an interval of 48 hours. Pop-up satellite archival tags (PSATs) were attached to a subset of available turtles to obtain additional independent dive, location, and mortality data. Results of these PSAT deployments will be reported elsewhere. Reported locations with quality codes of 0–3 were used to plot tracks, unless large gaps in the tracks would result. In these cases, location codes A & B were also included, if visual inspection determined them to be reasonable.

Table 1. Summary of satellite tag deployments at Kamiali Wildlife Management Area (WMA), 2001–2003.

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Aerial Survey and Calibration Ground Counts

An aerial census of leatherback turtle nests was conducted along the north coast of PNG during 13–20 January 2004, just after the peak of the austral summer nesting season. The area of coverage included the north coast of PNG, from the Indonesian border eastward through the Sanduan, East Sepik, Madang, and Morobe provinces (Fig. 1). Additional surveys were conducted along the entire coast of New Britain Island and the northeast coasts of Goodenough, Fergusson, and Normanby Islands. Aerial surveys were conducted using methods developed by Sarti et al. (1998) at an altitude of 150–200 feet (46–61 m) at 90–100 knots (167–185 km/hr) air speed in a high-wing twin-engine Islander aircraft. The aircraft flew over water parallel to the coast at a distance that allowed good visibility of the beach. During each flight, 3 observers searched for nests through flat lateral windows. Two observers recorded nests using a handheld counter and cued the third observer, who recorded positions for all nests with a handheld Global Positioning Satellite (GPS) receiver. Following the flights, individual observer counts were averaged, occasionally excluding anomalously low counts when it was clear the observer had missed nests. Flights were conducted during morning or late afternoon hours when the sun angle was low to maximize visibility of nests and tracks. Concurrent nest counts were performed on foot at areas of high nest density (at Kamiali WMA and Maus Buang; Fig. 1) to estimate detection errors for the aerial survey. These 2 high-density nesting sites are situated within the Huon Gulf and differ in physical characteristics and nest detectability. Kamiali WMA is characterized by a narrow beach, and turtles often nest in and under vegetation at the upper margin. Maus Buang is a broader beach, and nests are generally located away from the vegetation line. At each site, 2 observers walked a predetermined length of beach and recorded all tracks and nests on the day of the aerial survey. This allowed estimation of the proportion of nests missed by the aerial observer team (detection error term E2 as defined by Sarti et al. 1998).

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Analysis Methods

Nesting beach aerial survey data were analyzed following the methodology of Sarti et al. (1998) to account for detection error. Sarti et al. (1998) defined 3 error terms to account for nests missed during aerial surveys: E1 = the number of nests prior to the aerial survey that were no longer visible; E2 = the number of nests present (and recorded by ground teams), but missed by the aerial survey team; and E3 = the number of nesting events that occurred after the aerial survey. Daily nesting counts were not available throughout the season to estimate errors E1 and E3; therefore, the nest counts provided in this study will underestimate total nesting activity. The degree of bias will depend on the number of prior nests no longer visible at the time of the survey, and the number of nesting events that occurred after the survey. Detection error, E2, was estimated independently from beach ground counts, G, and aerial counts, A, obtained at the 2 index beaches, denoted by subscripts KWMA and MB, as follows:

and

The average detection error for all beaches other than the index beaches, E2_Other, was estimated as:

Total nests, T, along the entire surveyed coastline was estimated as:

where

• G _KWMA  = ground count at Kamiali WMA

• G _MB  = ground count at Maus Buang

• A _other  = aerial counts at all other beaches surveyed

RESULTS

Beach Monitoring

Nesting at Kamiali WMA occurred between November and March during the monitored years. On average, the greatest number of nesting events was documented during December–January (Fig. 2). During the 2003–2004 season, however, nesting continued after the peak at a higher level than during previous years (Fig. 2). The total number of nesting females at Kamiali WMA ranged from 41 to 71 between 2000–2001 and 2003–2004, with an average internesting interval of 11 days (see Kisokau and Ambio 2005 for further details).

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Telemetry

The distance traveled by tagged turtles ranged from 123 to 9438 km before the harnesses released or transmissions stopped. Track durations ranged from 6 to 240 days (Table 1). Turtles that renested repeatedly after harness attachment had tracks of shorter duration than those individuals that left the nesting areas immediately to begin their post-nesting migration, suggesting harness failure caused by multiple renesting events.

All satellite-tagged turtles spent the most time between nesting events in the Huon Gulf (Fig. 3). Inference of renesting location was not conclusive in all cases due to the duty cycle of the transmitters, but some renesting patterns were evident. Nesting was not restricted to a single beach, although many of the tracked leatherbacks exhibited some site fidelity to the Kamiali nesting beach (Fig. 3). Renesting also appeared to occur at other nearby beaches to the south, along the Morobe coast, and at east-facing beaches of the Huon Peninsula. One leatherback turtle moved out of the Huon Gulf into waters of the Madang Province where renesting may have occurred at north-facing beaches of the Huon Peninsula (Fig. 3e). Additional renesting also may have occurred at other islands prior to post-nesting migration, including Bougainville Island and Woodlark Island (Figs. 3a and 3c).

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Post-nesting leatherbacks initially traveled east and southeast as they moved away from Huon Gulf nesting beaches and into the Solomon Sea (Figs. 4a and 4b). Most moved in a route over the New Britain Trench (Fig. 5). Turtles moved southeast from the Solomon Sea and traversed the eastern edge of the Coral Sea as they continued to move in a southeastern direction. Only 6 of the original 19 transmitters remained active to 20 degrees south latitude. Four of the 6 continued to move southeast between the islands of New Caledonia and Vanuatu, over the New Hebrides Trench, while 2 others moved in a southerly direction through the Coral Sea. Only 2 turtles carried transmitters that remained active south of 20 degrees south latitude. Both accessed the South Pacific Ocean over the New Hebrides Trench between the islands of New Caledonia and Vanuatu. A single turtle continued into the high latitudes of the productive Southern Transition waters, where she remained for over 5 weeks before moving northward back into tropical latitudes. The track ended near the islands of Tonga (Fig. 5).

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Aerial Survey of Nesting Beaches

Nearly 2800 km of coastline was surveyed during 40.8 hours of flight time (including transits). Four hundred and fifteen nests were counted during the flights, confined to a subset of beaches of ca. 363 km combined length; > 71% of nests (295) were found along the Huon Gulf coast (Table 2, Fig. 1). Within the Huon Gulf region, only 29% of nests were recorded outside of the 2 index beaches at Kamiali WMA and Maus Buang. Nest counts were also performed by foot at selected areas of high nest density to establish coefficients of error for the aerial survey. Results of aerial counts combined with the derived coefficients resulted in an estimate of 559 nests for the entire area surveyed (Table 2); however, considerable uncertainty exists in this estimate because the E2 correction factors for Kamiali WMA and Maus Buang were very different (0.509 and 0.938, respectively), and the average (0.724) may not adequately represent other beaches. Therefore, the individual E2 estimates from Kamiali WMA and Maus Buang were applied separately to the entire area surveyed, yielding an estimated range of 495–679 nest pits (Table 2).

Table 2. Beach characteristics, leatherback (Dermochelys coriacea) nest counts, correction factors, and corrected estimates of the total number of nests and nests per km within the Papua New Guinea aerial survey study area during Jan 2004. The corrected number of nests at “Other beaches” had a greater degree of uncertainty and was estimated 1) based on the average correction factor for Kamiali WMA and Maus Buang and 2) as a range (in parentheses) derived from each of the 2 separate correction factors (see Table 1 for definition of abbreviation).

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DISCUSSION

The number of leatherbacks nesting on the north coast of PNG, including New Britain Island, remains unknown. However, the total is likely significantly less than on Wermon and Jamursba Medi beaches, located on the north coast of Birds Head Peninsula, Papua, Indonesia (Hitipeuw et al. 2007). Nesting in PNG occurs primarily on Huon Gulf beaches, and little scattered nesting activity occurs on beaches adjacent to the Bismarck Sea, south of Kamiali WMA, or on New Britain Island during the austral summer. The leatherback turtle nesting beach near the Buang River (Huon Gulf) was identified previously by Quinn and Kojis (1985), Bedding and Lockhart (1989), and Hirth et al. (1993). Previous estimates of turtles using that beach ranged from 10 turtles each night from November to January (Quinn and Kojis 1985) to 300 annually (Bedding and Lockhart 1989). These estimates are greater than the results from the current monitoring effort at Kamiali WMA and the aerial survey reported in this study. Although it is unknown whether the difference is attributable to sampling inconsistencies or a decline of leatherbacks at Huon Gulf beaches during the past 15 years, the villagers of Lababia (located within the Kamiali WMA) have indicated that leatherback nesting was significantly greater in decades past. More systematic, long-term monitoring will be necessary to accurately determine trends in leatherback abundance at Huon Gulf beaches. The ongoing monitoring at Kamiali WMA, and a new monitoring program at Maus Buang that began in November 2004, will provide these data if they can be fully supported for multiple decades.

Conservation of nesting habitat at Huon Gulf beaches will be important to maintain the leatherback turtle population that nests in PNG. Quinn and Kojis (1985) recognized the importance of these beaches to leatherbacks and described risks to their long-term viability. Leatherbacks are not eaten by villagers of the Huon Gulf, although eggs are harvested outside of Kamiali WMA. Conservation initiatives at Maus Buang have been attempted previously but lacked sufficient funding and commitment to maintain their objectives for more than a few years (Quinn and Kojis 1985; Bedding and Lockhart 1989). Although the importance of leatherback turtle conservation is understood by inhabitants of these coastal villages, financial incentives will be necessary to substitute income from the sale of leatherback turtle eggs. Tourism is a developing industry in PNG and typically emphasizes areas outside of the Huon Gulf region. Transportation and facilities are inadequate to support tourism on a scale that might bring economic benefits to Huon Gulf communities. Nonetheless, the creation of a WMA was advocated by Hirth et al. (1993) to address leatherback turtle conservation at Maus Buang. Although tourism does not always directly benefit local communities (e.g., Lindberg et al. 1996; Ross and Wall 1999) and cannot be considered a panacea for conservation efforts, the work of the Lababia villagers to conserve leatherbacks, intact lowland forests, and the exceptional reef habitats within the Kamiali WMA has been recognized internationally, and the area is being considered for Ramsar designation. Thus, Kamiali WMA provides a model and incentive to other local communities considering a WMA designation.

The peak of nesting activity at Huon Gulf beaches occurs during that regions' dry season when the sea is relatively calm and most beaches are replete with sand. Conversely, little sand remains at Huon Gulf beaches during the wet season, due to the effects of wave-induced erosion, and no nesting activity occurs at that time. The dry season along the segment of PNG coastline between the Indonesian border and the Huon Peninsula is not synchronous with the dry season of the Huon Gulf region. Most sandy beaches along that coastline were narrow, underwater, or fronted by extensive coral reef during the period of the aerial survey, however, we were informed by local villagers of infrequent leatherback turtle nesting activity at beaches adjacent to the Bismarck Sea during the austral winter (June–August), suggesting that some leatherback nesting may occur year-round depending on local dry seasons.

Aerial survey methodology was effective for identifying broad-scale nesting patterns. The location of nesting beaches encountered during the aerial survey were similar to the results of village surveys conducted by Spring et al. (1982a), although that study did not conduct surveys within Morobe Province and consequently did not identify leatherback nesting beaches in the Huon Gulf. Additional data from daily ground counts at monitored beaches will be necessary to estimate correction factors accurately and obtain an annual estimate of nests at beaches outside of monitored areas. Furthermore, it is unknown if the index beaches at Kamiali WMA and Maus Buang are characteristic of all beaches surveyed along the entire coastline. The locations of some nests encountered outside of Kamiali WMA during the aerial survey were consistent with telemetry results of internesting behavior of leatherbacks from Kamiali WMA. Turtles that nest at Kamiali WMA likely also nest on adjacent beaches within the Huon Gulf, on the Huon Peninsula, and south along the Morobe Province. Huon Gulf waters appear to be important habitat for leatherbacks from Kamiali WMA during the internesting period.

Post-nesting leatherbacks from Kamiali WMA displayed little variation in their initial movement away from the nesting beach. All animals transited east over the deep waters of the New Britain Trench as they moved into the Solomon Sea. The longest tracks exhibited remarkable consistency as the turtles moved southeast, through the Solomon Sea, maintaining a course over deep bathymetry. The lack of significant variation in the post-nesting movements of Kamiali turtles suggests that this route to the South Pacific over deep bathymetry is a major migratory pathway. A relationship between bathymetric features and leatherback migration patterns was also suggested by Morreale et al. (1996) for post-nesting animals from Playa Grande, Costa Rica. The consistency of post-nesting migrations of these PNG nesters contrasts with diverse patterns documented for Atlantic leatherbacks (Ferraroli et al. 2004; Hays et al. 2004; Eckert 2006), and for other western Pacific leatherbacks that nest on the northwest side of the island of New Guinea, in Papua, Indonesia (Benson et al. 2007).

Genetic analysis of skin samples indicates that PNG leatherbacks are part of a Western Pacific metapopulation that includes leatherbacks nesting in the Solomon Islands and Papua, Indonesia (Dutton et al. 1999, 2007). Similar haplotypes were found from leatherbacks that stranded on North American beaches or were caught incidentally in fishing operations in the eastern North Pacific Ocean (Dutton et al. 2000), suggesting that PNG leatherbacks may migrate to Northern Hemisphere foraging areas following nesting. Telemetry results presented here, however, suggest that the leatherbacks nesting in PNG during the austral summer are not linked to Northern Hemisphere foraging areas, but rather utilize productive waters in the high latitudes of the South Pacific. Further telemetry studies of leatherbacks nesting in PNG will be required to confirm this apparent pattern with greater sample sizes and longer track durations.