Caretta caretta (South West Indian Ocean subpopulation)
|Scientific Name:||Caretta caretta (South West Indian Ocean subpopulation)|
See Caretta caretta
|Red List Category & Criteria:||Near Threatened ver 3.1|
|Assessor(s):||Nel, R. & Casale, P.|
|Reviewer(s):||Wallace, B.P. & Pilcher, N.J.|
The South West Indian Ocean Loggerhead subpopulation nests along the southeast African seaboard. Its marine distribution extends throughout a wide area including the Agulhas Current, the southern extent of the Somali Current (Indian Ocean) and possibly the south-east Atlantic Ocean. This subpopulation has been identified as one genetic stock different from other Loggerhead stocks (Shamblin et al. 2014) – although based on samples from South Africa only - supporting its designation as a distinct subpopulation, or regional management unit (RMU) (Wallace et al. 2010).
The available long-term series of nest counts (used as an index of population abundance) show an increase over the past generation. Although geographic distribution (of nesting sites ~300 km of coast) and population size (~1,300 nesting females per annum) are relatively small, the South West Indian Ocean Loggerhead subpopulation does not qualify for a threatened category under any criterion. However, this subpopulation qualifies for the Near Threatened category under criterion B2 because its area of occupancy is less than 2,000 km² and it has fewer than 10 locations.
The analysis of time series datasets with ≥10 years of data of nesting activities (nest counts) on one nesting site (of 300 km) revealed consistent trends within the southern African part of the population loggerhead subpopulation but with the smaller Madagascan component being data deficient. There is an overall increase in the subpopulation over time (monitored between 19–48 years) (criteria A1-A2; Table 1 in the Supplementary Material).
We also assessed the subpopulation under criteria B, C, and D to the South West Indian Loggerhead subpopulation, but even though the subpopulation is relatively small, it is not in decline or envisaged to decline in the near future (assuming status quo in conservation). This subpopulation does not qualify for a threatened category under any criterion using available information. However, the subpopulation meets only two out of three subcriteria needed for a threatened category under criterion B2 (area of occupancy < 2,000 km² and < 10 locations) and in such circumstances the subpopulation qualifies for the Near Threatened category.
No population viability analysis (criterion E) was available for the Southwest Indian Loggerhead subpopulation, and the assessment was thus conducted by applying criteria A-D.
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 (National Research Council 2010).
To apply criterion A, three generations (or a minimum of ten years, whichever is longer) of abundance data are required (IUCN 2014). In the case of the Loggerhead, we conservatively estimate its generation time as 45 years (see Habitats & Ecology section). For criteria A1-A2, data from three generations ago (~135 years) are necessary to estimate population declines beginning three generations ago up to the present (i.e., assessment) year. The challenges of this requirement on long-lived species like turtles—with generation lengths of 30 years or more—are obvious (see Seminoff and Shanker 2008 for a review). Abundance data from ~135 years ago are not available for Loggerheads anywhere in the world. Extrapolating backward using population trends based on current datasets was considered 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 ago (~135 years, one generation estimated 45 years; see Habitats & Ecology section) was similar to the first observed abundance rather than to assume that the population has always been in a decline (or increase) of the same magnitude as in the current generation. A similar approach was used in the Red List assessment of another sea turtle, the Leatherback Turtle Dermochelys coriacea (Wallace et al. 2013) and of another long-lived, geographically widespread taxon, the African Elephant (Blanc 2008). Thus, to apply criterion A we assumed that the abundance at the beginning of an available time series dataset had not changed significantly in the preceding three generations, and therefore used the same abundance value in trend calculations. For the Loggerhead global and subpopulation assessments we only considered time series datasets of ≥10 years.
For the South West Indian Ocean Loggerhead subpopulation, we included time series datasets of 19-48 years, from one rookery spanning two countries (South Africa and Mozambique; Table 1 in the Supplementary Material) where the great majority of clutches are laid. Other rookeries are known to be in Madagascar, however the information for Madagascar is scant with no indication of substantial (current) nesting. We evaluated the subpopulation against criteria A1-A2 (population reduction in the past), by calculating population trends for each of the two countries within the subpopulation. From one past and one recent abundance values (each representing the annual average of five year nest counts) we calculated annual and overall trends (past-present) for each index nesting sites within the subpopulation, and then we calculated the overall subpopulation past trend. Both trends of the two countries as well as the overall trend for the South West Indian Ocean Loggerhead subpopulation were positive (+319%) (Table 1 in the Supplementary Material). In conclusion, the South West Indian Ocean Loggerhead subpopulation does not qualify for a threatened category under criterion A.
Since the subpopulation area is the entire Agulhas Current, the southern extent of the Somali Current (Indian Ocean) and possibly the South East Atlantic Ocean, the extent of occurrence (EOO) exceeds the threat category threshold (20,000 km²). The area of occupancy (AOO) for sea turtles is identified with the nesting beach habitat, which represents the smallest habitat for a critic life stage. The total length of known loggerhead nesting beaches in the South West Indian Ocean is estimated at a 300 km linear extent (Nel et al. 2013, Videira et al. 2011). Since the appropriate scale for AOO is a grid 2x2 km, the above linear measure was converted to 600 km². This value meets the vulnerable category threshold (2,000 km²). Regarding the number of locations, the nesting area includes one continuous strip of coast of ca. 300 km in South Africa and Mozambique, plus possible nesting sites in Madagascar. In such a situation, the identification and quantification of locations is difficult. The term ‘location’ defines a geographically or ecologically distinct area in which a single threatening event can rapidly affect all individuals (IUCN 2014). The current protection status in the nesting area in South Africa and Mozambique has eliminated threats such as coastal development or direct harvest and depends on management authorities in two different countries. Therefore, a minimum of two locations (on a jurisdictional base) can be identified along the mainland African coast. Even considering further fragmentation of threats and the nesting sites in Madagascar, it is unlikely that the number of locations is >10, therefore the subpopulation can be assumed to meet the vulnerable category threshold (<10 locations). However, there is no evidence of continuing decline nor of extreme fluctuations, the other subcriteria required for a threatened category. In conclusion, the South West Indian Ocean Loggerhead subpopulation would meet only two out of three requirements for a threatened category and so does not qualify for a threatened category under criterion B, but does qualify as Near Threatened under this criterion.
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 = 4,600) (see the Population section) by the average number of clutches per female (3.7) (Nel et al. 2013) to obtain an average annual number of nesting females (1240). Next, we multiplied this value by the average remigration interval, i.e. years between consecutive nesting seasons (3.0) (Nel et al. 2013). The resulting total number of adult females, i.e. including nesting as well as non-nesting turtles, was 3,730. Finally, we divided this value by the average proportion of adult females of the total adults (0.42) calculated from the adult sex ratio of turtles caught in the shark nets (Brazier et al. 2012). This calculation provided an estimated mature adult population of 8,860 individuals, which triggers the vulnerable category threshold (10,000). However, the subpopulation does not meet any other parameter thresholds, i.e. continuing decline, % of mature individuals in one subpopulation, or extreme fluctuations. In conclusion, the South West Indian Ocean Loggerhead subpopulation does not qualify for a threatened category under criterion C.
The number of mature individuals (see criterion C) and the AOO value exceed the thresholds for a threatened category (1,000 in individuals and 20 km2 respectively). Regarding the number of locations, the nesting area includes one continuous strip of coast of ca. 300 km in South Africa and Mozambique, plus nesting sites in Madagascar. In such a situation, the identification and quantification of locations, as defined for this criterion (IUCN 2014), is difficult. Moreover, a future threat that could drive the subpopulation to CR or EX in a very short time has not been identified. In conclusion, the South West Indian Ocean Loggerhead subpopulation does not qualify for a threatened category under criterion D.
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 (National Research Council 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 (National Research Council 2010). However, there remains great uncertainty about most of these fundamental demographic parameters for Loggerheads, including age at maturity, generation length, survivorship across life stages, adult and hatchling sex ratios, and conversion factors among reproductive parameters (clutch frequency, nesting success, remigration 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.
The data used in this analysis comes from a strong trend analysis from the South African component of the population (which is ~75% of the population). See Nel et al. (2013) for details. Further, all the reports for Mozambique indicate that the Mozambique component follow the same pattern as the South African loggerheads (Fernandes et al. 2014, Videira et al. 2010, Videira et al. 2011). However, the uncertainty is introduced from the lack of available data from Madagascar. There are no indications as to the size of this sub-population component but is expected to be small. For further reading on other sources of uncertainty in marine turtle Red List assessments, see Seminoff and Shanker (2008).
|Range Description:||Loggerhead turtles are distributed across the subtropical to temperate regions of the globe in the Indian, Atlantic and Pacific Oceans including the Mediterranean Sea (Wallace et al. 2010) (Figure 1 in the Supplementary Material). Loggerhead Turtles have been divided into 10 subpopulations (or regional management units) of which the South West Indian Ocean subpopulation is distributed along the southern tip of the African continent with nesting limited to the beaches around the border between South Africa (Nel et al. 2013) and Mozambique (Videira et al. 2011), along one continuous ~300 km strip of coast in the iSimangaliso Wetland Park, South Africa to Maputo Bay, Mozambique, and two rookeries on the south coast of Madagascar (Rakotonirina and Cooke 1994). Its marine habitats extend throughout a wide area including the Agulhas Current, the southern extent of the Somali Current (Indian Ocean) and possibly the south-east Atlantic Ocean (Figure 2 in the Supplementary Material).|
Native:Comoros; Kenya; Madagascar; Mauritius; Mozambique; Somalia; South Africa; Tanzania, United Republic of
|FAO Marine Fishing Areas:|
Atlantic – southeast; Indian Ocean – western
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||Loggerheads are a single species globally comprising 10 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. Regional management units are functionally equivalent to IUCN subpopulations, thus providing the appropriate demographic unit for Red List assessments. There are 10 Loggerhead RMUs (hereafter subpopulations): North West Atlantic Ocean, North East Atlantic Ocean, South West Atlantic Ocean, Mediterranean Sea, North East Indian Ocean, North West Indian Ocean, South East Indian Ocean, South West Indian Ocean, North Pacific Ocean, and South Pacific Ocean (Figure 2 in the Supplementary Material). Multiple genetic stocks have been defined according to geographically disparate nesting areas around the world and are included within RMU delineations (Wallace et al. 2010) (shapefiles can be viewed and downloaded at: http://seamap.env.duke.edu/swot).|
The South West Indian Ocean Loggerhead subpopulation nests along the southeast African sea board with about 80% of the nests in South Africa, and 20% of the nests in Mozambique. A historical, minor nesting site has been reported in Southern Madagascar (Hughes 1974), but recent nesting data i.e., the size of the rookery or quantitative nesting trends are unknown for Madagascar. In total, about 4,600 clutches are laid annually in recent years: ca. 3,900/year in South Africa (R. Nel, unpubl. data) and ca. 700/yr in Mozambique (Videira et al. 2010, Videira et al. 2011). Genetic analysis is limited with comparisons of the SWIO population (sampled only from South Africa) and tested against other populations in the Atlantic and Northern Indian Ocean (Shamblin et al. 2014). The SWIO stock has some alleles in common with the Atlantic population (which indicates historic connectivity) but none with Oman (Shamblin et al. 2014). The SWIO population is therefore identified as a single subpopulation, or regional management unit (Wallace et al. 2010). The continuous observation of tagged female with both a South African and Mozambican flipper tags confirms the single nesting unit between South Africa and Mozambique but the genetic interaction with Madagascan turtles is currently uncertain.
|Current Population Trend:||Increasing|
|Habitat and Ecology:||The Loggerhead Turtle nests on insular and mainland sandy beaches throughout the temperate and subtropical regions. Like most sea turtles, Loggerhead Turtles are highly migratory and use a wide range of broadly separated localities and habitats during their lifetimes (Bolten and Witherington 2003). Upon leaving the nesting beach, hatchlings begin an oceanic phase, perhaps floating passively in major current systems (gyres) that serve as open-ocean developmental grounds (Bolten and Witherington 2003). After 4-19 years in the oceanic zone, Loggerheads recruit to neritic developmental areas rich in benthic prey or epipelagic prey where they forage and grow until maturity at 10-39 years (Avens and Snover 2013). Upon attaining sexual maturity Loggerhead Turtles undertake breeding migrations between foraging grounds and nesting areas at remigration intervals of one to several years with a mean of 2.5-3 years for females (Schroeder et al. 2003) while males would have a shorter remigration interval (e.g., Hays et al. 2010, Wibbels et al. 1990). Migrations are carried out by both males and females and may traverse oceanic zones spanning hundreds to thousands of kilometres (Plotkin 2003). During non-breeding periods adults reside at coastal neritic feeding areas that sometimes coincide with juvenile developmental habitats (Bolten and Witherington 2003).|
The IUCN Red List Criteria define generation length to be the average age of parents in a population (i.e., older than the age at maturity and younger than the oldest mature individual) and care should be taken to avoid underestimation (IUCN 2014). Although different subpopulations may have different generation length, since this information is limited we adopted the same value for all the subpopulations, taking care to avoid underestimation as recommended by IUCN (2014).
Loggerheads attain maturity at 10-39 years (Avens and Snover 2013), and we considered here 30 years to be equal or greater than the average age at maturity. Data on reproductive longevity in Loggerheads are limited, but are becoming available with increasing numbers of intensively monitored, long-term projects on protected beaches. Tagging studies have documented reproductive histories up to 28 years in the North Western Atlantic Ocean (Mote Marine Laboratory, unpubl. data), up to 18 years in the South Western Indian Ocean (Nel et al. 2013), up to 32 years in the South Western Atlantic Ocean (Projeto Tamar unpubl. data), and up to 37 years in the South Western Pacific Ocean, where females nesting for 20-25 years are common (C. Limpus, pers. comm). We considered 15 years to be equal or greater than the average reproductive longevity. Therefore, we considered here 45 years to be equal or greater than the average generation length, therefore avoiding underestimation as recommended by IUCN (IUCN Standards and Petitions Subcommittee 2014).
|Generation Length (years):||45|
|Use and Trade:||Direct harvesting direct utilization of this species for human use (i.e., harvesting of turtles and eggs for consumption, commercial products) occurs in the South West Indian Ocean region.|
Threats to Loggerheads vary in time and space, and in relative impact to populations. Threat categories affecting marine turtles, including Loggerheads, were described by Wallace et al. (2011) as:
The main threats to the South West Indian Ocean subpopulation are fishery bycatch (particularly longline and gill netting) and direct harvesting (Bourjea et al. 2008, Brazier et al. 2012, De Wet 2013, Humber et al. 2011, Petersen et al. 2009). Nesting habitat degradation due to coastal development is largely eliminated for this population as two thirds of the nesting takes place in well-managed protected areas or underdeveloped areas. This also means an overall reduction in threats such as direct harvesting or pollution. Climate change (particularly sea level rise and inundation) will have little long-term effect on the nesting habitat as the dune/beach interface is intact and can respond to seasonal/storm perturbations (Harris 2009). However, it is expected that turtle nests within specific seasons may be affected by storm inundation along the extended coast. Anthropogenic debris ingestion has also been documented in the area (Hoarau et al. 2014).
Loggerhead Turtles are afforded legislative protection under a number of treaties and laws (Wold 2002). 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). A partial list of the International Instruments that benefit Loggerhead Turtles includes the Inter-American Convention for the Protection and Conservation of Sea Turtles, 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, the Memorandum of Agreement on the Turtle Islands Heritage Protected Area (TIHPA), and the Memorandum of Understanding Concerning Conservation Measures for Marine Turtles of the Atlantic Coast of Africa.
As a result of these designations and agreements, many of the intentional impacts directed at sea turtles have been lessened: harvest of eggs and adults has been slowed at several nesting areas through nesting beach conservation efforts and an increasing number of community-based initiatives are in place to slow the take of turtles in foraging areas. In regard to incidental take, the implementation of Turtle Excluder Devices has proved to be beneficial in some areas, primarily in the United States and South and Central America (National Research Council 1990). Guidelines are available to reduce sea turtle mortality in fishing operations in coastal and high seas fisheries (FAO 2009). However, despite these advances, human impacts continue throughout the world. The lack of effective monitoring in pelagic and near-shore fisheries operations still allows substantial direct and indirect mortality, and the uncontrolled development of coastal and marine habitats threatens to destroy the supporting ecosystems of long-lived Loggerhead Turtles.
Most of the conservation protection for sea turtles along the east African sea board is spatially based i.e., using protected areas during nesting seasons or protecting foraging areas. A long-term monitoring and conservation programme has been in place to monitor loggerhead nesting in South Africa. This programme started in 1963 and provide physical protection for nesting turtles/nests for five months of the year thus for both nesting and hatching periods (Nel et al. 2013). Similar conservation measures have been in place in Mozambique since 1996 and are expanding annually. The effect is that the South Western Indian ocean loggerhead population has been maintained and is now increasing (exponentially) in the last decade. Traditional harvesting (particularly using gillnets) and trawl still pose a potential problem. Trawling effort seems to fluctuate with markets and available stocks and it is difficult to predict what the long-term future impacts will be (Fennesey and Izsaksen 2007, Fennesey et al. 2008).
|Citation:||Nel, R. & Casale, P. 2015. Caretta caretta (South West Indian Ocean subpopulation). The IUCN Red List of Threatened Species 2015: e.T84199475A84199755.Downloaded on 20 September 2018.|
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