Annual nesting and population trend
Leatherback nesting at Bird's Head occurs year-round in contrast to many other beaches both in the Pacific and the Atlantic where nesting is seasonal, occurring either in the austral summer as in Papua New Guinea (Benson et al. 2007b, Steckenreuter et al. 2010) and in Pacific Mexico and Costa Rica (Chaves et al. 1996) or in the boreal summer as in the Caribbean and eastern Atlantic (Godley et al. 2001, Dutton et al. 2005, Troeng et al. 2007). However, in Papua Barat, Indonesia, the temporal distribution of nesting activity differs between the beaches of Jamursba Medi and Wermon even though they are only 30 km apart. The nesting distribution at Wermon is bimodal (Fig. 2), with peaks during both the boreal and austral summer nesting seasons, whereas peak nesting only occurs at Jamursba Medi during the boreal summer. A bimodal seasonal nesting distribution has also been found for leatherbacks in French Guiana where nesting occurs primarily in the boreal summer, with a small pulse of nests laid in the austral summer (Chevalier et al. 1999). This temporal variation may be related to multiple demographic stocks using the same nesting beach at different times of the year (Dutton et al. 2007). Our PIT-tag recapture results support this hypothesis suggesting that there are two temporally distinct nesting groups in Bird's Head, with some females nesting during the boreal summer and different ones nesting during the austral summer. There are no records of ‘transitional' females whose nesting season spanned the time period between the boreal and austral nesting seasons. Satellite tracking studies also demonstrate that females nesting during the boreal summer at Jamursba Medi and Wermon tend to depart around September and migrate to foraging areas in the northern hemisphere, whereas the Wermon austral summer nesters depart by late April and migrate to forage in the southern hemisphere (Benson et al. 2007a, Benson et al. 2007b, Benson et al. 2011, Bailey et al. 2012). Further, there may be multiple foraging groups that nest within a specific season. Results from stable isotope analysis of tissues sampled from Jamursba Medi females during the boreal summer identified two groups corresponding to animals foraging in the eastern Pacific and those foraging in the western Pacific (Seminoff et al. 2012). Dutton et al. (2007) have proposed that a spatio-temporal demographic structuring occurs among Bird's Head leatherbacks and that they are members of a regional (western Pacific) metapopulation that appears to be part of a single genetic stock based on mtDNA analysis. Genetic analyses are currently underway to provide a better understanding of this metapopulation (Dutton, unpublished data).
Our results, together with those from previous studies reviewed by Hitipeuw et al. (2007) indicate a significant and continuous decline in the population of leatherbacks nesting at Jamursba Medi (1984–2011) and Wermon (2002–2011). This has significant conservation implications since 75% of leatherback nesting in the western Pacific occurs at Bird's Head (Dutton et al. 2007), and recent aerial surveys indicated that Jamursba Medi and Wermon are the two main nesting beaches in the region (Benson et al. 2009). When evaluating the magnitude of the decline, it is of particular importance to note that there were relatively high nesting numbers in 1984 and high nest count variability between 1984 and 1985. The number of nests reported by Bhaskar (1985) for 1984 (14,522; Table 1, Fig. 3) was significantly greater than for any other year reported. Interestingly, it was followed by a sharp decline in 1985 with only 3,000 nests reported (Bhaskar 1987). It is not clear how comprehensive Bhaskar's surveys were in 1984 and 1985, and the estimates should therefore be viewed with caution due to potential variation in sampling effort resulting from two primary factors: (1) their efforts were focused on night patrols to tag nesting females and there was less emphasis on day time nest counts and (2) a large proportion of the leatherback nests at that time were disturbed by fishermen who were collecting eggs. On the other hand, a sharp decline in the number of nests during the mid 1980s may be valid, and the decline may have been partially due to local harvesting of eggs and turtles. Betz and Welch (1992) indicated that large numbers of eggs were being harvested from Jamursba Medi based on their visit to that beach in 1991. Many of the local villagers have said that from 1980 through 1993, fishermen would visit the beaches seasonally (from May to August, during seasonally calm weather at Bird's Head) and ship the eggs to nearby markets (Tapilatu, personal observation). The fishermen bartered with the local villagers from Saubeba and Warmandi to obtain permission to collect leatherback eggs. Approximately 4 to 5 wooden boats were observed visiting the beach weekly and returning with 10,000–15,000 eggs per boat (Betz and Welch 1992). During the egg collection season, the beaches would become crowded with temporary dwellings that housed egg collectors, and camp fires were common every night. In addition to taking leatherback eggs, fishermen would also set gillnets for sharks, which resulted in 2–3 female leatherback deaths per week (Bakarbessy, personal communication). Thus, Bhaskar's (1985) report of relatively high nesting numbers in 1984, as well as anecdotal reports of extensive egg harvesting and female deaths in shark gillnets support the hypothesis that the nesting population was severely impacted during the 1980s. This is consistent with information from local villagers who say that fewer turtles nest now than in the 1980s (Hitipeuw et al. 2007).
In response to the continued decline, the Sorong government enacted laws in 1993 that banned egg collection and protected nesting females (Bupati Sorong 1994). Additionally, regular nesting surveys began at Jamursba Medi in 1993. Although annual fluctuations were evident, there was an overall decline in nesting from 1996 to 2004. Our results indicate that this decline has continued through to 2011 (Table 1, Fig. 3). Our analysis of all the available datasets indicates an overall population decline of 78.3% at Jamursba Medi from a high of 14,455 nests in 1984 to a low of 1,532 nests in 2011. Nesting numbers at Wermon were unknown until recently; surveys in 2002 to 2004 indicated that there were relatively large numbers of nests on Wermon (World Wildlife Fund 2003, Hitipeuw et al. 2007). The first comprehensive surveys carried out by the World Wildlife Fund at Wermon in 2004 found almost as many nests laid on Wermon as on Jamursba Medi (Table 1, Fig. 3). It was hypothesized that the decline at Jamursba Medi may have been offset by an increase at Wermon (Hitipeuw et al. 2007). However, our findings are not consistent with this hypothesis and indicate that nesting at both beaches has declined significantly, and at the same approximate rate. However, the movement by nesters between beaches that we documented might contribute to the small peak in nesting during the boreal nesting season at Wermon. Indeed, the boreal nesting season at Wermon most likely represents “spill-over” from neighboring Jamursba Medi. If this is the case, the spill-over appears to be restricted to Wermon, since aerial survey showed no evidence of significant nesting at other beaches in the region (Benson et al. 2009). The extent of exchange between beaches during the austral summer season is unknown, since PIT-tag data on nesting females were not collected at Jamursba Medi during this time period.
Estimated clutch frequency and females nesting annually
Our estimated clutch frequency (ECF) of 5.5 ± 1.6 (mean ± SD) clutches per female per season is similar to other published estimates of ECF from leatherbacks in the eastern Pacific including Mexico (5.5 ± 1.9; Sarti et al. 2007), Playa Grande, Costa Rica (5.6 ± 1.2; Reina et al. 2002), and estimates reported for Atlantic leatherbacks in St. Croix (5.8; reviewed in Hitipeuw et al. ) and Culebra, Puerto Rico (6.7 ± 2.7; Tucker and Frazer 1991). The maximum observed clutch frequency at Jamursba Medi and Wermon was 10 clutches compared to 12 clutches for Mexican beaches (Sarti et al. 2007) and similarly 12 clutches for St. Croix (Dutton et al. 2005). Considering that not all nesting events may have been observed along the entire distribution range or even within Jamursba Medi and Wermon, and that females may move between beaches, the actual ECF could be slightly higher than the current estimate and the estimated of number of nesting females would consequently be lower. Regardless, the results indicate a continual declining trend in the number of leatherbacks nesting annually at both Jamursba Medi and Wermon and that the overall Bird's Head leatherback population has been reduced to approximately 500 females nesting annually (FNA).
Despite the decline in nesting numbers, Bird's Head remains the largest leatherback nesting aggregation in the Pacific, if only because the other populations are severely depleted or extinct. There may be a variety of reasons why this population has not collapsed in comparison to other Pacific rookeries, which may be facing potential extinction as suggested by Spotila et al. (1996, 2000). The nesting beaches on Bird's Head are in remote locations with a limited number of local residents in the area, thereby (1) limiting the historic levels of direct exploitation of eggs and females which resulted in the collapse of other rookeries in Malaysia (Chan and Liew 1996, Chua 1988, Liew 2011) and Pacific Mexico (Marquez et al. 1981, Sarti et al. 2007, Sarti et al. 1996), and (2) preventing the development of areas adjacent to the nesting beach, which has impacted nesting habitats in Malaysia, Costa Rica, and Mexico (Chan and Liew 1996, Spotila et al. 1996, Sarti et al. 2007). However, the occasional harvesting of eggs at Jamursba Medi and Wermon, and the occasional harvesting of eggs and adults females on minor nesting areas on Bird's Head still need to be evaluated and addressed (Tapilatu, unpublished data).
The long-distance migratory habits of Bird's Head leatherbacks increase the probability of fisheries interactions as they travel through multiple fishing zones and encounter multiple types of fisheries (Benson et al. 2011, Bailey et al. 2012). The impact of commercial fisheries (Frazier and Brito Montero 1990, Nishimura and Nakahigashi 1990, Eckert 1993, Wetherall et al. 1993) and artisanal fisheries (Chu-Chen 1982, De Silva 1982, Polunin and Nuitja 1982, Siow and Moll 1982, Suwelo et al. 1982, Eckert 1993, Starbird and Suarez 1994, Suarez and Starbird 1996, Alfaro-Shigueto et al. 2007, Finkbeiner et al. 2011) on Pacific leatherbacks has been well-documented. Furthermore, some females nesting during the austral summer season migrate to waters off eastern Indonesia (Benson et al. 2011) where they may be subjected to relatively high mortality by traditional leatherback hunters from the Kei Kecil Islands (Starbird and Suarez 1994, Suarez and Starbird 1996). It was reported that the Kei islanders successfully hunted and killed 135 leatherbacks between November 2003 and December 2007 (SIRAN and World Wildlife Fund, unpublished reports). However, the use of multiple foraging grounds by the Bird's Head leatherbacks may also buffer the population to some extent against potentially severe impacts of fisheries, directed take, and environmental perturbation in the marine environment (Dutton 2006, Benson et al. 2011), and protection of key foraging areas could further mitigate these threats. For example, the Maluku Tenggara government recently designated a Marine Protected Area (MPA, ∼150,000 ha) in waters surrounding the Kei Kecil Islands (WWF Indonesia 2012) in response to proposed temporal closures of a marine foraging ground for leatherbacks (Benson et al. 2011). Elsewhere, a large area (10,877,950 ha) in US waters in the northeastern Pacific has recently been designated critical habitat due to its importance as a distant foraging area for western Pacific leatherbacks (Wallace 2012).
Finally, the extent to which environmental factors have contributed to the decline of Pacific leatherback populations is unknown but could be significant. Multi-decadal climate oscillations and global climate change could have a variety of impacts including (1) increased nesting beach temperatures resulting in reduced hatching success and highly biased sex ratios, and (2) rising sea level and alteration of ocean current circulation that would affect beach erosion, beach topography, and hatchling and juvenile dispersal patterns and juvenile recruitment (Saba et al. 2008, Van Houtan and Halley 2011, Gaspar et al. 2012, Saba et al. 2012).
Regardless of the underlying causes, the current number of nesting females offers a glimmer of hope that this declining population is still large enough that it could recover if effective management measures can be implemented to address the major threats immediately. The management program at Jamursba Medi and Wermon has included experimental evaluation of small-scale hatcheries since 2006 (Tapilatu and Tiwari 2007, Tiwari et al. 2011) and relocation of doomed nests (i.e., nests that would be lost to erosion) to stable sections of beach. The hatcheries are used to mitigate low hatching success due to (1) predation of eggs and hatchlings by introduced pigs and dogs (Bhaskar 1987, Maturbongs 2000, Hitipeuw and Maturbongs 2002, Suganuma et al. 2005, Suganuma 2006), (2) beach erosion (Bhaskar 1987, Hitipeuw et al. 2007), and (3) elevated sand temperatures (Tapilatu and Tiwari 2007). Preliminary results indicate that nest relocation significantly enhances hatching success (Tapilatu and Tiwari 2007). Maximizing hatchling production at Bird's Head nesting beaches is paramount for the recovery of western Pacific leatherbacks (Tiwari et al. 2011).
Involvement of the local community members is a pre-requisite for the development of a stable long-term conservation program on these beaches. Through an intensive and collaborative effort between conservation organizations and the communities adjacent to Jamursba Medi and Wermon, villagers now participate side-by-side with biologists to patrol the beaches, maximize hatchling production, and protect in situ nests and nesting females (Hitipeuw et al. 2007). As evidence of their dedication to leatherback conservation, the local communities are actively addressing the threat of coastal development by asking the Papua Barat Provincial government to re-route the proposed trans-Papua Barat road 5–10 km inland from Jamursba Medi. Future efforts should focus on maintaining and enhancing co-management approaches and a traditional conservation ethic within the local communities and the new regency government to ensure the sustainability of a long-term conservation program.
However, beach conservation alone, while a necessary pre-requisite, is unlikely to tip the scales in favor of the recovery of this population. Under some scenarios complete protection at the nesting beach would compensate for fisheries-related mortality (e.g., 5% per year) with the population growing rapidly (Spotila et al. 1996). However, it has been suggested that annual fisheries-related mortality of leatherbacks in the Pacific has been significantly higher than 5% per year (Spotila et al. 2000, Alfaro-Shigueto et al. 2011). Furthermore, climate-driven oceanographic processes may further influence survival and population recruitment (Van Houtan and Halley 2011). Therefore, it is imperative to adopt a holistic approach (Dutton and Squires 2011) that addresses threats at all life history stages. A broad suite of management and conservation measures should be implemented at the nesting beaches and in national and international waters, to help reverse the decline of what might be the last remaining stronghold for leatherbacks in the Pacific, in a renewed effort to defy predictions of their extinction.