Dermochelys coriacea (Southwest Indian Ocean subpopulation)
|Scientific Name:||Dermochelys coriacea (Southwest Indian Ocean subpopulation)|
|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 Supplementary Material). 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).
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)).
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.
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 unpublished data, Nel and Hughes unpublished data), but precise estimates of reproductive lifespan and longevity for Leatherbacks are currently unavailable.
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 Supplementary Material). 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 Supplementary Material).
Native:Angola; Comoros; French Southern Territories (Mozambique Channel Is.); 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).
|Current Population Trend:||Decreasing|
|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).
|Generation Length (years):||30|
|Movement patterns:||Full Migrant|
|Congregatory:||Congregatory (and dispersive)|
|Use and Trade:||Harvest of eggs and of Leatherbacks at-sea and on nesting beaches persists in Mozambique (Nel 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).
|Citation:||Wallace, B.P., Tiwari, M. & Girondot, M. 2013. Dermochelys coriacea (Southwest Indian Ocean subpopulation). The IUCN Red List of Threatened Species 2013: e.T46967863A46967866.Downloaded on 21 March 2018.|
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