|Scientific Name:||Dermochelys coriacea (Southwest Indian Ocean subpopulation)|
|Species Authority:||(Vandelli, 1761)|
|Red List Category & Criteria:||Critically Endangered C2a(ii) ver 3.1|
|Assessor/s:||Wallace, B.P., Tiwari, M. & Girondot, M.|
|Reviewer/s:||Bolten, A.B., Chaloupka, M.Y., Dobbs, K., Dutton, P.H., Eckert, K.L., Limpus, C., Miller, J., Mortimer, J.A., Musick, J.A., Nel, R., Pritchard, P.C.H. & van Dijk, P.P.|
|Contributor/s:||Nel, R. & Hughes, G.|
The Southwest Indian Leatherback subpopulation nests principally along the Indian Ocean coast of South Africa (KwaZulu-Natal), with some nesting in Mozambique. Marine habitats extend around the Cape of Good Hope in both the Indian and Atlantic Oceans (Figure 1 in attached PDF). Despite some areas of overlap in distribution with the Southeast Atlantic subpopulation, the Southwest Atlantic subpopulation is genetically distinct from this and all other Leatherback subpopulations (Dutton et al. 1999, 2013).
The Leatherback nesting population in South Africa has been monitored consistently for 50 years, and accounts for >90% of the total abundance of the subpopulation (Table 1 in attached PDF) (Nel et al. 2013). Based on analysis of long-term time series datasets of abundance—i.e. annual counts of nesting females and nests—this Leatherback subpopulation has declined slightly (-5.6%) during the past three generations (Table 1 and Figure 2 in attached PDF). Although the population decline does not meet any threatened category under Criterion A, the restricted range (area of occupancy=1,500 km2), single location (i.e. one genetic stock for the entire subpopulation), small population size (estimated 148 mature individuals), and continuing (although small) decline meet thresholds for the Vulnerable category under B2ab(v), the Endangered category under criteria C2a(i,ii) and D and the Critically Endangered category under Criterion C2a(ii). Because IUCN Red List Guidelines stipulate that the criterion that triggers the highest threatened category must be selected for the assessment, the Southwest Indian Ocean Leatherback subpopulation is considered Critically Endangered (C2a(ii)) based on IUCN Guidelines (IUCN 2011).
Considering the small number of mature individuals (estimated 148 adult males and females total in this subpopulation) and the evidence of a small but continuing decline, assessment of available data under Criterion C resulted in a Critically Endangered classification (C2a(ii)).
Our analyses of long-term abundance trends in this Southwest Indian subpopulation agree generally with results of the long-term monitoring project in KwaZulu-Natal, South Africa, which have reported rapid increase in the Leatherback population during the first decade of monitoring, followed by inter-annual fluctuations in abundance, but an overall stable or slightly decreasing trend (Nel et al. 2013) (Figure 2 in attached PDF). This abundance trend for Leatherbacks contrasts with the increasing trend and larger abundance of Loggerheads that nest along the same coastline, but drivers of these divergent patterns are unclear at present (Nel et al. 2013).
We assessed the status of the Southwest Indian Leatherback subpopulation by Criteria A-D; because no population viability analysis has been performed, Criterion E could not be applied.
Criterion A: We compiled time series datasets of monitored nesting activities (e.g. tracks or nests) or individual nesting females from KwaZulu-Natal, South Africa (Nel 2008, 2012; Nel et al. 2013), and Mozambique (Lombard and Kyle 2010). 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). The South Africa time series began in 1965, whereas the Mozambique time series began in 1994; the nesting abundance in South Africa is >90% of the total subpopulation abundance (Table 1 in attached PDF). 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 both the South Africa and Mozambique datasets, and then calculated the average subpopulation trend by weighting rookery population trends by historical rookery abundance relative to historical subpopulation abundance. Both time series datasets were ≥10 yr, so were both included in trend estimations (Table 1 in attached PDF). The most recent year for available abundance data across rookeries and subpopulations 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 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 (Tables 1 and 2 in attached PDF).
We also applied A4 to the Southwest Indian subpopulation, using the same overall scheme as described above. 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. This is a reasonable assumption, based on available information, because threats to Leatherbacks in this region persist (see Nel 2012 for review).
Based on our assessment of the multi-decade time series available for the Southwest Indian subpopulation, the slight decline (-5.6%) over the past three generations did not meet thresholds of any threatened criteria under Criterion A2 (Table 1 in attached PDF). Likewise, future projections of abundance of this subpopulation show abundance declining slightly (up to -8.3%) by the year 2040—i.e., within the next generation (see below)—which does not meet thresholds for threat categories under Criterion A4 (Table 2 in attached PDF).
Criterion B: We defined extent of occurrence (EOO) as the total area included within the geo-referenced boundaries of the Southwest Indian Leatherback subpopulation (Fig. 1), which we calculated to be >19 million km2. We defined area of occupancy (AOO) as the linear distribution of nesting sites within the EOO, which we estimated based on the linear extent of nesting sites in the State of the World’s Sea Turtles—SWOT database (http://seamap.env.duke.edu/swot) to be approximately 750 km. We then multiplied this linear extent 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 thus estimated to be 1,500 km2. There is a single “location” (defined as biological rookeries, i.e. genetic stocks; Dutton et al. 1999, 2013) within the EOO for this subpopulation. Taking into account the continuing and projected negative population trend over three generations (-5.6% through 2010, -8.3% through 2040; Tables 1 and 2 in attached PDF), this subpopulation meets thresholds for Vulnerable under Criterion B2, subcriteria a (1 location) and b (v; continuing decline in number of mature individuals).
Criterion C: To apply Criterion C, we first calculated the number of mature individuals in the subpopulation, i.e. the total number of adult females and males. First, we divided the current average annual number of nests (n=259, Table 1 in attached PDF) by the estimated clutch frequency (i.e. average number of clutches per female; n=7; Nel 2008, Nel 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=3 yr; Nel et al. 2013) 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 sex ratio of hatchlings produced on nesting beaches in the Southwest Indian was similar to sex ratios elsewhere (i.e. East Pacific; approximately 75% female, or 3:1 female:male ratio), and that this reflected the natural adult sex ratio. This calculation provided an estimated mature adult population of 148 mature individuals, which met thresholds for threatened categories under Criterion C. Taking into account the continuing and projected negative population trend (-5.6% through 2010, -8.3% through 2040; Tables 1 and 2 in attached PDF), this subpopulation meets thresholds for Critically Endangered under Criterion C2, subcriterion a(ii) (i.e. 90-100% of individuals in one location).
Criterion D: The Southwest Indian subpoplation has <250 mature individuals (estimated 148), a single location (Dutton et al. 1999, 2013), and a continuing and projected population decline. Considered together, this subpopulation meets thresholds for Endangered 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 Nel et al. (2013).
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).
Leatherbacks are distributed circumglobally, with nesting sites on tropical sandy beaches and migratory and foraging ranges that extend into temperate and sub-polar latitudes; see Eckert et al. (2012) for review. The Southwest Indian subpopulation nests along the Indian Ocean coast of South Africa and Mozambique, and marine habitats extend through the Agulhas Current around the Cape of Good Hope in the Indian and Atlantic Oceans (see Figure 1 in attached PDF).
Native:Angola (Angola); Comoros; Kenya; Madagascar; Mauritius; Mayotte; Mozambique; Namibia; Seychelles; South Africa; Tanzania, United Republic of
|FAO Marine Fishing Areas:||
Indian Ocean – western
|Range Map:||Click here to open the map viewer and explore range.|
Leatherbacks are a single species globally comprising 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 Southwest Indian Ocean, Northeast Indian Ocean, East Pacific Ocean, West Pacific Ocean, Northwest Atlantic Ocean, Southeast Atlantic Ocean, and Southwest Atlantic 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).
|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).
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.
For the Southwest Indian subpopulation, fisheries bycatch has been considered to be the biggest threat (Wallace et al. 2011, Nel 2012). Rigorous estimates of Leatherback bycatch in fishing gear throughout the region are necessary to adequately quantify the relative impacts on this subpopulation. Threats to nesting females and their eggs and hatchlings have been addressed by the ongoing monitoring and conservation efforts in South Africa, while harvest of eggs and of Leatherbacks at-sea and on nesting beaches persists in Mozambique (Nel 2012).
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, Santidrián Tomillo et al. 2007, Sarti Martínez et al. 2007, Nel 2012). 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, in particular—persist, and in some places, continue to hinder population recovery (Alfaro-Shigueto et al. 2011, 2012; 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 (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:||Wallace, B.P., Tiwari, M. & Girondot, M. 2013. Dermochelys coriacea (Southwest Indian Ocean subpopulation). In: IUCN 2013. IUCN Red List of Threatened Species. Version 2013.2. <www.iucnredlist.org>. Downloaded on 08 March 2014.|
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