Dermochelys coriacea (West Pacific Ocean subpopulation)

Status_ne_offStatus_dd_offStatus_lc_offStatus_nt_offStatus_vu_offStatus_en_offStatus_cr_onStatus_ew_offStatus_ex_off

Taxonomy [top]

Kingdom Phylum Class Order Family
ANIMALIA CHORDATA REPTILIA TESTUDINES DERMOCHELYIDAE

Scientific Name: Dermochelys coriacea (West Pacific Ocean subpopulation)
Species Authority: (Vandelli, 1761)
Parent Species:
Common Name(s):
English Leatherback

Assessment Information [top]

Red List Category & Criteria: Critically Endangered A2bd+4bd ver 3.1
Year Published: 2013
Date Assessed: 2013-06-24
Assessor(s): Tiwari, M., Wallace, B.P. & Girondot, M.
Reviewer(s): Chaloupka, M.Y., Dobbs, K., Dutton, P.H., Eckert, K.L., Hughes, G., Limpus, C., Miller, J., Mortimer, J.A., Musick, J.A., Nel, R., Pritchard, P.C.H. & van Dijk, P.P.
Contributor(s): Pilcher, N.J., Tapilatu, R. & Tiwari, M.
Justification:

Rationale

The West Pacific Leatherback subpopulation nests primarily in Papua Barat, Indonesia, Papua New Guinea, and the Solomon Islands, and to a lesser extent in Vanuatu, all of which belong to a single regional genetic stock (Dutton et al. 2007).  The marine habitat for this subpopulation extends north into the Sea of Japan, northeast and east into the North Pacific to the west coast of North America, west to the South China Sea and Indonesian Seas, and south into the high latitude waters of the western South Pacific Ocean and Tasman Sea (Benson et al. 2011; Figure 1 in attached PDF). The once large nesting population in Terrengannu, Malaysia, is now functionally extinct (Chan and Liew 1996; Malaysian Fisheries Department unpubl. data).   Despite some areas of overlap in distribution with the east Pacific subpopulation, the West Pacific subpopulation is genetically distinct from all other Leatherback subpopulations (Dutton et al. 1999), and it occupies broad foraging and migratory habitats (Benson et al. 2011, Bailey et al. 2012). Based on analysis of long-term time series datasets of abundance—i.e. annual counts of nesting females and nests—this West Pacific Leatherback subpopulation has declined 83.0% during the past three generations. Because the threats to this subpopulation (e.g. human exploitation of females and eggs, low hatching success, fisheries bycatch) have not ceased, the West Pacific Leatherback subpopulation is considered Critically Endangered according to IUCN Red List Criterion A2, subcriteria (b) and (d). Likewise, applying Criterion A4 reveals a population decline of 96% by the year 2040, or one generation from now, which meets the requirements of Critically Endangered under A4, subcriteria (b) and (d).

 

Justification

Application of Criterion A2 is appropriate, as population reduction has been observed in the past where the causes of reduction may not have ceased OR may not be understood OR may not be reversible. Furthermore, applicable subcriteria under Criterion A2 include (b) an index of abundance appropriate to the taxon (i.e., annual counts of nesting females, nests), and d) actual or potential levels of exploitation. Based on Criterion A4, this subpopulation will be greatly diminished—population decline of 96%—in another generation (i.e., by 2040), with a remaining abundance of approximately 572 nests (~104 females) per year, or approximately 260 adult females total.

We also applied Criterion B, C, and D to the West Pacific subpopulation, but the extent of occurrence and area of occupancy exceeded the thresholds of Criterion B; the subpopulation triggered an Endangered category under Criteria C, but did not trigger Vulnerable under Criterion D. Criteria A2bd+4bd are the criteria met for this assessment because they triggered the highest threatened category.

Our analyses of long-term abundance trends in the western Pacific corroborated a recent evaluation of nesting trends in Papua Barat, Indonesia, which supports 75% of the Leatherback nesting in the western Pacific (Dutton et al. 2007). Tapilatu et al. (2013) demonstrated that nesting abundance has declined at the two Indonesian index beaches by 78.3% over 27 years at Jamursba-Medi and by 62.8 % over the past nine years at Wermon. The drivers of this decline—both anthropogenic (e.g. bycatch, egg harvest, exploitation of females) as well as environmental (e.g. lethal sand temperatures, predation, erosion)—have been described in detail (Eckert 1993, Bellagio Report 2007, Tapilatu and Tiwari 2007, Tapilatu et al. 2013). Egg harvest and exploitation of females have been minimized at the two most significant nesting beaches of Papua Barat, Indonesia, and the impact of environmental factors is being addressed through a science-based management and conservation programme. Fisheries bycatch is still considered the major obstacle to this population’s recovery (Benson et al. 2011, Bailey et al. 2012, Tapilatu et al. 2013, Wallace et al. 2013).

 

Assessment Procedure

We assessed the status of the West Pacific Leatherback subpopulation by Criteria A-D; as no population viability analysis has been performed, Criterion E could not be applied.

 

Criterion A: We compiled time series datasets of abundance of nesting females or their nesting activities from all index beaches for the West Pacific subpopulation, including two beaches in Indonesia, seven beaches in Papua New Guinea, eight beaches in the Solomon Islands, and one beach in Malaysia, which together comprise the vast majority of abundance for this population (Table 1 in attached PDF). Time series were between 5 and 20+ years, and included monitored nesting activities (e.g. tracks or nests) or individual nesting females. For marine turtles, annual counts of nesting females and their nesting activities (more often the latter) are the most frequently recorded and reported abundance metric across index monitoring sites, species, and geographic regions (NRC 2010). We presented and analysed all abundance data in numbers of nests yr-1, as this metric was the most commonly available (Table 1 in attached PDF).

We calculated annual and overall population trends for each rookery within the subpopulation, and then calculated the average subpopulation trend by weighting rookery population trends by historical rookery abundance relative to historical subpopulation abundance. We only included time series datasets of ≥10 yr in trend estimations (Fig. 2 in attached PDF), although we included all rookeries for which we obtained abundance values in the overall summary tables (see Table 1 in attached PDF). The most recent year for available abundance data across all rookeries and subpopulations, except the Solomon islands for which there are sporadic nest counts, was 2010.      

To apply Criterion A, three generations (or a minimum of ten years, whichever is longer) of abundance data are required (IUCN 2011). For A2, data from three generations ago (~100 yr) are necessary to estimate population declines beginning three generations ago through the present (i.e. assessment) year. The challenges of this requirement on long-lived species like marine turtles—with generation lengths of 30 yr or more—are obvious (see Seminoff and Shanker 2008 for review). Abundance data from ~100 yr ago are not available for Leatherbacks anywhere in the world. We considered extrapolating backward using population trends based on current datasets inappropriate because estimates produced would be biologically unrealistic and unsubstantiated, given what is currently known about sea turtle nesting densities on beaches and other factors (Mrosovsky 2003). In the absence of better information, we assumed that population abundance three generations (~100 years, one generation estimated at 30 yr; see below) ago was similar to the first observed abundance than to assume that the population has always been in a decline (or increase) of the same magnitude as in the current generation (Table 1 in attached PDF). A similar approach was used in the Red List assessment of another long-lived, geographically widespread taxon, the African Elephant (Blanc 2008). Thus, to apply Criterion A to this subpopulation, we assumed that the abundance at the beginning of an available time series dataset had not changed significantly in three generations, and therefore used the same abundance value in trend calculations. 

We also applied A4 to the West Pacific subpopulation, using the same overall scheme as described above (Table 2 in attached PDF). Criterion A4 permits for analysis of population trend during a “moving window” of time, i.e. over three generations, but where the time window must include the past, present, and future. Therefore, we made the same assumption about earliest available historical abundance being equivalent to the subpopulation abundance for generations past, and estimated future population abundance in 2020, 2030, and 2040, i.e. within one generation. This future projection assumes that the derived population trend will continue without deviation during the next generation. Implicit in this assumption is that no changes to degree of threats impacting rookeries or the subpopulation will occur during that time. Although some threats (e.g. environmental factors on the nesting beach, egg harvest, exploitation of females) that have caused observed declines would hopefully be minimized in the future through effective conservation and management plans, the threat of fisheries in the future remains uncertain. Based on application of Criterion A4 to the available data, this subpopulation will have declined by 96% by the year 2040, or one generation from now. This would correspond to a total subpopulation abundance of approximately 572 nests per year—roughly 104 females per year—at the index sites, or 260 adult females total, and would represent an extremely diminished population.

 

Criterion B: We defined extent of occurrence (EOO) as the total area included within the geo-referenced boundaries of the West Pacific Leatherback subpopulation (Fig. 1), which we calculated to be >134 million km2. We defined area of occupancy (AOO) as the linear distribution of nesting sites within the EOO, multiplied by 2 km to account for the IUCN Guidelines for calculating linear AOOs using minimum grid cell size of 2 km x 2 km. The AOO for this subpopulation was calculated in excess of 2,000 km2. We defined “locations” as biological rookeries, i.e. genetic stocks, within the EOO (n=1; Dutton et al. 2007). Due to the broad distribution of the West Pacific Leatherback subpopulation, Criterion B did not trigger a threatened category.

 

Criterion C: To apply Criterion C, we first calculated the number of mature individuals in each subpopulation, i.e., the total number of adult females and males. First, we divided the current average annual number of nests (n=2,379, Table 1 in attached PDF) by the estimated clutch frequency (i.e. average number of clutches per female; n=5.5, Tapilatu et al. 2013) to obtain an average annual number of nesting females. Next, we multiplied this value by the average re-migration interval (i.e. years between consecutive nesting seasons; n=2.5, Tapilatu et al. unpubl. data) to obtain a total number of adult females that included nesting as well as non-nesting turtles. Finally, to account for adult males, we assumed that the overall sex ratio of hatchlings produced on nesting beaches in the West Pacific is approximately 75% female, or 3:1 female:male ratio, because beach temperature measurements in Papua (Tapilatu and Tiwari 2007) suggest largely female bias  and that this reflected the natural adult sex ratio. This calculation provided an estimated mature adult population of 1,438 individuals, which meets the threshold for Endangered under Criterion C, in addition given the continuing decline of 96% under Criterion A4, this easily meets the requirements under Criterion C1. Hence the subpopulation qualifies as Endangered under Criterion C1, but as it already meets Critically Endangered under Criterion A, the more threatened listing applies.

 

Criterion D: The West Pacific subpopulation has 1,438 mature individuals hence it does not qualify as Vulnerable under Criterion D.


Estimating Generation Length:

Leatherback age at maturity is uncertain, and estimates range widely (see Jones et al. 2011 for review). Reported estimates fall between 9-15 yr, based on skeletochronology (Zug and Parham 1996), and inferences from mark-recapture studies (Dutton et al. 2005). Furthermore, updated skeletochronological analyses estimated Leatherback age at maturity to be between 26-32 yr (mean 29 yr) (Avens et al. 2009). Extrapolations of captive growth curves under controlled thermal and trophic conditions suggested that size at maturity could be reached in 7-16 yr (Jones et al. 2011). Thus, a high degree of variance and uncertainty remains about Leatherback age at maturity in the wild. Likewise, Leatherback lifespan is unknown. Long-term monitoring studies of Leatherback nesting populations have tracked individual adult females over multiple decades (e.g. Santidrián Tomillo et al. unpublished data, Nel and Hughes unpublished data), but precise estimates of reproductive lifespan and longevity for Leatherbacks are currently unavailable.

The IUCN Red List Criteria define generation length to be the average age of parents in a population; older than the age at maturity and younger than the oldest mature individual (IUCN 2011). Thus, for the purposes of this assessment, we estimated generation length to be 30 yr, or equal to the age at maturity (estimated to be 20 yr on average), plus a conservative estimate of reproductive half-life of 10 yr, as assumed by Spotila et al. (1996).

 

Sources of Uncertainty

Although monitoring of nesting activities by adult female sea turtles is the most common metric recorded and reported across sites and species, globally, there are several disadvantages to using it as a proxy for overall population dynamics, some methodological, some interpretive (NRC 2010). First, because nesting females are a very small proportion of a sea turtle population, using abundance of nesting females and their activities as proxies for overall population abundance and trends requires knowledge of other key demographic parameters (several mentioned below) to allow proper interpretation of cryptic trends in nesting abundance (NRC 2010). However, there remains great uncertainty about most of these fundamental demographic parameters for Leatherbacks, including age at maturity (see Jones et al. 2011 for review), generation length, survivorship across life stages, adult and hatchling sex ratios, and conversion factors among reproductive parameters (e.g., clutch frequency, nesting success, re-migration intervals, etc.). These values can vary among subpopulations, further complicating the process of combining subpopulation abundance and trend estimates to obtain global population abundance and trend estimates, and contributing to the uncertainty in these estimates. Second, despite the prevalence of nesting abundance data for marine turtles, monitoring effort and methodologies can vary widely within and across study sites, complicating comparison of nesting count data across years within sites and across different sites as well as robust estimation of population size and trends (SWOT Scientific Advisory Board 2011). For example, monitoring effort on Matura beach, Trinidad, has changed multiple times since the early 1990s, which necessitated a modelling exercise to estimate a complete time series for years with reliable monitoring levels (Table 2 in attached PDF). Furthermore, there was a general lack of measures of variance around annual counts provided for the assessment, which could be erroneously interpreted as equally high confidence in all estimates. Measures of variance around annual counts would provide information about relative levels of monitoring effort within and among rookeries, and thus reliability of resulting estimates. For all of these reasons, results of this assessment of global population decline should be considered with caution. For further reading on sources of uncertainty in marine turtle Red List assessments, see Seminoff and Shanker (2008).
For further information about this species, see 46967817_Dermochelys_coriacea_West_Pacific_Ocean_subpopulation.pdf.
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Geographic Range [top]

Range Description:

Leatherbacks are distributed circumglobally, with nesting sites on tropical sandy beaches and foraging ranges that extend into temperate and sub-polar latitudes. See Eckert et al. (2012) for review of Leatherback geographic range. The West Pacific Leatherback subpopulation nests primarily in Papua Barat, Indonesia, Papua New Guinea, and the Solomon Islands, and to a lesser extent in Vanuatu. The marine habitat for this subpopulation extends north into the Sea of Japan, northeast and east into the North Pacific to the west coast of North America, west to the South China Sea and Indonesian Seas, and south into the high latitude waters of the western South Pacific Ocean and Tasman Sea (Benson et al. 2011; see Figure 1 in attached PDF).

For further information about this species, see 46967817_Dermochelys_coriacea_West_Pacific_Ocean_subpopulation.pdf.
A PDF viewer such as Adobe Reader is required.
Countries:
Native:
American Samoa (American Samoa); Australia; Brunei Darussalam; Cambodia; Canada; China; Fiji; French Polynesia; Guam; Indonesia; Japan; Kiribati; Korea, Democratic People's Republic of; Korea, Republic of; Malaysia; Marshall Islands; Micronesia, Federated States of ; New Caledonia; New Zealand; Northern Mariana Islands; Palau; Papua New Guinea; Philippines; Russian Federation; Samoa; Solomon Islands; Taiwan, Province of China; Thailand; Tonga; Tuvalu; United States
FAO Marine Fishing Areas:
Native:
Pacific – eastern central; Pacific – northeast; Pacific – northwest; Pacific – southeast; Pacific – southwest; Pacific – western central
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:

Leatherbacks are a single species globally comprised of biologically described regional management units (RMUs; Wallace et al. 2010), which describe biologically and geographically explicit population segments by integrating information from nesting sites, mitochondrial and nuclear DNA studies, movements and habitat use by all life stages. RMUs are functionally equivalent to IUCN subpopulations, thus providing the appropriate demographic unit for Red List assessments. There are seven Leatherback subpopulations, including the West Pacific Ocean, East Pacific Ocean, Northwest Atlantic Ocean, Southeast Atlantic Ocean, Southwest Atlantic Ocean, Northeast Indian Ocean, and Southwest Indian Ocean. Multiple genetic stocks have been defined according to geographically disparate nesting areas around the world (Dutton et al. 1999, 2013), and are included within RMU delineations (Wallace et al. 2010; shapefiles can be viewed and downloaded at: http://seamap.env.duke.edu/swot).

For further information about this species, see 46967817_Dermochelys_coriacea_West_Pacific_Ocean_subpopulation.pdf.
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Population Trend: Decreasing

Habitat and Ecology [top]

Habitat and Ecology:

See the species account for a summary of the details. For a thorough review of Leatherback biology, please see Eckert et al. (2012).

Systems: Terrestrial; Marine

Use and Trade [top]

Use and Trade: Egg and females are collected for human consumption and eggs are also eaten by feral pigs and dogs.

Threats [top]

Major Threat(s):

Threats to Leatherbacks (and other marine turtle species), vary in time and space, and in relative impact to populations. Threat categories were defined by Wallace et al. (2011) as the following:

1) Fisheries bycatch: incidental capture of marine turtles in fishing gear targeting other species;

2) Take: direct utilization of turtles or eggs for human use (i.e. consumption, commercial products);

3) Coastal Development: human-induced alteration of coastal environments due to construction, dredging, beach modification, etc.;

4) Pollution and Pathogens: marine pollution and debris that affect marine turtles (i.e. through ingestion or entanglement, disorientation caused by artificial lights), as well as impacts of pervasive pathogens (e.g. fibropapilloma virus) on turtle health;

5) Climate change: current and future impacts from climate change on marine turtles and their habitats (e.g. increasing sand temperatures on nesting beaches affecting hatchling sex ratios, sea level rise, storm frequency and intensity affecting nesting habitats, etc.).

The relative impacts of individual threats to all Leatherback subpopulations were assessed by Wallace et al. (2011). Fisheries bycatch was classified as the highest threat to Leatherbacks globally, followed by human consumption of Leatherback eggs, meat, or other products, and coastal development. Due to lack of information, pollution and pathogens was only scored in three subpopulations and climate change was only scored in two subpopulations. Enhanced efforts to assess the impacts of these threats on Leatherbacks—and other marine turtle species—should be a high priority for future research monitoring efforts.

The greatest threats to the recovery of Leatherbacks in the Western Pacific are bycatch, egg collection and exploitation of females, and low hatching success due to high sand temperatures, erosion, feral pig and dog predation (Bellagio Report 2007, Tapilatu et al. 2013). The multi-directional and long distance migratory trajectories of the western Pacific Leatherbacks increases the probability of fisheries bycatch as they encounter multiple fishing zones and fishing gear (Benson et al. 2011, Bailey et al. 2012, Wallace et al. 2013). Rigorous estimates of Leatherback bycatch in fishing gear throughout the region are necessary to adequately quantify the relative impacts on this subpopulation. Most importantly, Leatherback bycatch in fishing gears throughout the region, especially those with the largest population-level impacts, must be reduced as soon as possible to avoid extinction of this subpopulation.

Conservation Actions [top]

Conservation Actions:

Leatherbacks are protected under various national and international laws, treaties, agreements, and memoranda of understanding. A partial list of international conservation instruments that provide legislative protection for Leatherbacks are: Annex II of the SPAW Protocol to the Cartagena Convention (a protocol concerning specially protected areas and wildlife); Appendix I of CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora); and Appendices I and II of the Convention on Migratory Species (CMS); the Inter-American Convention for the Protection and Conservation of Sea Turtles (IAC), the Memorandum of Understanding on the Conservation and Management of Marine Turtles and their Habitats of the Indian Ocean and South-East Asia (IOSEA), the Memorandum of Understanding on ASEAN Sea Turtle Conservation and Protection, and the Memorandum of Understanding Concerning Conservation Measures for Marine Turtles of the Atlantic Coast of Africa.

Long-term efforts to reduce or eliminate threats to leatherbacks on nesting beaches have been successful (e.g. Dutton et al. 2005, Chacón-Chaverri and Eckert 2007, Santidrián Tomillo et al. 2007, Sarti Martínez et al. 2007). Reducing Leatherback bycatch has become a primary focus for many conservation projects around the world, and some mitigation efforts are showing promise (Watson et al. 2005; Gilman et al. 2006, 2011). However, threats to Leatherbacks—bycatch and egg consumption and female exploitation, in particular, persist, and in some places, continue to hinder population recovery (Fretey et al. 2007, Bellagio report 2007, Alfaro-Shigueto et al. 2011, Tapilatu et al. 2013, Wallace et al. 2013). For depleted Leatherback populations to recover, the most prevalent and impactful threats must be reduced wherever they occur, whether on nesting beaches or in feeding, migratory, or other habitats (Steering Committee, Bellagio Conference on Sea Turtles 2004; Bellagio report 2007; Wallace et al. 2011, 2013); a holistic approach that addresses threats at all life history stages needs to be implemented (Dutton and Squires 2011). 

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Citation: Tiwari, M., Wallace, B.P. & Girondot, M. 2013. Dermochelys coriacea (West Pacific Ocean subpopulation). In: The IUCN Red List of Threatened Species. Version 2014.2. <www.iucnredlist.org>. Downloaded on 29 July 2014.
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