Rationale
The global population of the Loggerhead Turtle (Caretta caretta) comprises 10 subpopulations (see Figure 2 in the Supplementary Material) that vary widely in population size, geographic range, and population trends, and are the appropriate units for assessment of global conservation status for this species (Wallace et al. 2010, 2011). As such, assessments have been completed for each of the 10 subpopulations, in addition to the combined global population assessment required by the IUCN (see Table 1 in the Supplementary Material). At the global level, both geographic distribution and population size are much larger than required to qualify for a threatened category. The available long-term series of nest counts (used as an index of population abundance) show an important decrease in the past (47%). Therefore, the Loggerhead Turtle is considered as Vulnerable under current IUCN Red List Criteria (criterion A2b). The previous listing, published in 1996, was Endangered under criterion A1bd (Marine Turtle Specialist Group 1996).

Results indicate that the Loggerhead Turtle, as a single taxonomic entity, will not go extinct globally in the next generation according to any Red List criteria. However, the global listing is not an appropriate representation of the conservation status of the biologically relevant subpopulations that make up the global Loggerhead Turtle population. Subpopulation assessments demonstrated wide variation not only in status of individual subpopulations (as indicated by IUCN Red List Categories), but also in the criteria under which the individual subpopulations qualified for a threatened category (see Table 1 in the Supplementary Material). For these reasons, the subpopulation-level assessments for the Loggerhead Turtle should be given priority in evaluating the true global conservation status of this species. This conclusion follows the precedent for other long-lived, widely distributed species, including the Leatherback Turtle (Wallace et al. 2013).

Justification
The extent of occurrence and area of occupancy exceeds the thresholds for criterion B, and the population size exceeds the thresholds for criteria C and D. Regarding criterion A, trends were estimated on time series datasets with ≥10 years of data of nesting activities (nest counts) at 153 index nesting sites from six subpopulations out of 10 (North West Atlantic, Mediterranean, South West Atlantic, North West Indian, South West Indian, North Pacific). These six subpopulations comprise about 90% of the current annual nests globally (see Table 2 in the Supplementary Material). The analysis revealed different trends for different subpopulations, with an overall -47% population decrease relative to population size three generations ago, which qualifies for the Vulnerable category (under criterion A2). The species trend at a global scale is basically determined by the two most abundant subpopulations, the North West Atlantic and the North West Indian, which altogether comprise about 75% of the current annual nests. They showed positive and negative trends, respectively. The other four subpopulations for which trends could be calculated show positive trends. For the remaining four subpopulations (comprising about 10% of the current annual nests; North East Atlantic, North East Indian, South East Indian, South Pacific), current trends are unclear, however past negative trends are known or suspected at least for the South Pacific and the North East Atlantic subpopulations respectively.
 
The overall scenario suggests that, on the basis of the current knowledge, the extinction of the species at the global level is highly unlikely to occur in the short and medium term. However, the global status of the species in terms of distribution, number of subpopulations, genetic variability, regional ecological roles, and vulnerability, could change dramatically from the past and current situations. For this reason, the global assessment cannot be considered as an indicator of the true conservation status of the species, and priority should be given to the subpopulation assessments.

Assessment Procedure:
Criterion A
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 Standards and Petitions Subcommittee 2014). In the case of the Loggerhead, we conservatively estimate its generation time as 45 years (see the Habitats and Ecology section below). For criterion A, 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 and Ecology section below) 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 species, 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 global Loggerhead population, we considered time series datasets of 10-50 years, from 153 index nesting sites from six subpopulations (see the individual subpopulation assessments). The index nesting sites included in the analysis are assumed to be representative of their subpopulations, and these six subpopulations comprise about 90% of the current total annual nests (Table 2 in the Supplementary Material).

The assessment under criterion A was conducted in three steps, as follows. Please see the separate subpopulation assessments for further details.

• Step 1: We estimated past trends for each of the six individual subpopulations. Specifically, from one past and one recent abundance values (each representing the annual average of five year nest counts) we calculated overall trends (past-present) for each index nesting sites within subpopulations, and then we calculated overall subpopulation past trends for each subpopulation (criteria A1-A2).

• Step 3: The past and present total abundance values of the six subpopulations (obtained from Step 2) were summed and from these totals the overall population change (past-present) was calculated (Table 3 in the Supplementary Material).

Five of the above six subpopulations showed positive trends and one showed a negative trend. Altogether, they showed a negative trend (-47%). The overall negative trend depended on the negative trend of one subpopulation (North West Indian) where criterion A2 was appropriate – i.e. the causes of reduction may not have ceased or may not be understood or may not be reversible. Therefore, the overall trend of the six subpopulations was assessed against criterion A2. The past estimated trend was above the threshold for the Vulnerable category under criterion A2 (30% decline) but below the threshold for the Endangered category (50% decline). Therefore, the Loggerhead Turtle qualifies for the Vulnerable category under criterion A2 and the applicable subcriterion is (b), an index of abundance appropriate to the taxon (counts of nests or tracks).

Although these trends consider only 6 of 10 subpopulations, these subpopulations with sufficient available data account for about 90% of the current global population abundance (Table 2 in the Supplementary Material). Therefore, these trends likely reflect the complete global trend and represent the best information available about the global population trend. Similarly, the global Leatherback Turtle assessment used only five of seven subpopulations to estimate global trends due to lack of sufficient data for two subpopulations (Wallace et al. 2013).

Criterion B
Since the population area extends over entire oceans around the world, the extent of occurrence (EOO) exceeds the threatened 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. Since the appropriate scale for AOO is a grid 2x2 km, the threshold of 2,000 km² corresponds to 1,000 km of linear coastal tract, which is easily exceeded (by orders of magnitude) by the total length of nesting beaches globally. In conclusion, the global population does not trigger any of the thresholds for a threatened category under criterion B.

Criterion C
To apply criterion C, the number of adults is needed and can be derived from the number of nests per year with the following formula:  Adults = Nests * Nests per female^-1 * Remigration interval * Female proportion^-1. With a current estimate of annual number of nests of about 200,000, for any reasonable value of the other parameters the population would easily exceed the threshold of 10,000 adults required to qualify for a threatened category under criterion C.

Criterion D
The number of mature individuals (see criterion C) and AOO value (see criterion B) exceeded the respective thresholds. In conclusion, the population does not trigger any of the thresholds for a threatened category under criterion D.

Criterion E
Although population viability analyses (PVA) were attempted at subpopulation level (Conant et al. 2009, Van Houtan 2011), in most cases they were not suitable for criterion E under this assessment. No PVA has been attempted at species level and such an approach would also be questionable because the subpopulations are independent units by definition.

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 (e.g., clutch frequency, nesting success, remigration intervals, etc.). These values can vary within and 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. However, we have reduced this source of uncertainty by using in the analyses those data sets obtained though standardized monitoring.

For the trend analyses (criterion A) we used data from index rookeries from six populations out of 10. Possible negative past trends associated to high past abundances in the other four subpopulations not included in the analysis could have changed the final result of the assessment (past trends, criterion A2), if such information was available.

For further reading on sources of uncertainty in marine turtle Red List assessments, see Seminoff and Shanker (2008).

The Loggerhead Turtle is globally distributed throughout the subtropical and temperate regions of the Mediterranean Sea and Pacific, Indian, and Atlantic Oceans (Wallace et al. 2010) (see Figure 1 in the Supplementary Material).

Loggerheads are a single species globally comprising 10 biologically described regional management units (RMUs; Wallace et al. 2010) – hereafter subpopulations - 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 subpopulations: NorthWest 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 (see 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).

Total population size is unknown. The most common proxy for population abundance in sea turtles is the annual number of nests. A total of about 200,000 clutches are laid annually by the 10 subpopulations altogether (see Table 2 in the Supplementary Material). Considering a range of 3 to 5.5 clutches per female, the above value would correspond to approximately 36,000-67,000 nesting females annually.

The Loggerhead Turtle nests on insular and mainland sandy beaches throughout the temperate and subtropical regions worldwide. 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 in major current systems (gyres) that serve as open-ocean developmental grounds (Bolten and Witherington 2003, Putman and Mansfield 2015). 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).

Generation length
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 Standards and Petitions Subcommittee 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 (IUCN Standards and Petitions Subcommittee 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).

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:

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 affecting critical turtle habitat: 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 (for example fibropapilloma virus) on turtle health;
5. Climate change: current and future impacts from climate change on marine turtles and their habitats (increasing sand temperatures on nesting beaches affecting hatchling sex ratios, sea level rise, storm frequency and intensity affecting nesting habitats, etc.).
6. The relative impacts of individual threats to all Loggerhead subpopulations were assessed by by Wallace et al. (2011). Fisheries bycatch was classified as the highest threat to Loggerheads globally, followed by coastal development and human consumption of eggs, meat, or other products. Due to lack of information, pollution and pathogens was only scored as affecting three subpopulations and climate change was only scored for two subpopulations. Enhanced efforts to assess and reduce the impacts of these threats on Loggerheads—and other marine turtle species—should be a high priority for future conservation efforts.

More detailed information at regional level can found in the specific subpopulation assessments.

Loggerhead Turtles and their eggs are taken for human use (i.e., consumption and commercial products).

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. In most areas, 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.

More detailed information at regional level can be found in the specific subpopulation assessments.