By Nicole Esteban, Swansea University,
Jeanne Mortimer, Turtle Action Group of Seychelles,
and Graeme Hays, Deakin University

The IUCN Red List of Threatened species relies on national or regional groups such as IOSEA Task Forces to guide collection of systematic metrics on population size and trends in abundance over time. An important goal of the IOSEA Conservation and Management Plan is to increase understanding of sea turtle population status by countries across the region.

Population status of hawksbill and green turtles nesting in the Chagos Archipelago in British Indian Ocean Territory (BIOT) was reported in a recent study by Mortimer & Esteban et al. (2020). To assess seasonality and egg clutch production, standardised 2-weekly surveys of nesting tracks have been conducted on a representative 2.8 km ‘Index’ beach in Diego Garcia since 2006. Additionally, available nesting habitat and spatial distribution of nesting activity (nesting track and body pit counts) have been recorded along 90% (212 km) of the BIOT oceanic coastline during five expeditions between 1996-2016. At a regional scale, the contribution of Chagos Archipelago egg clutch production to the southwestern Indian Ocean (SWIO) region was estimated for each country using information available in the literature including 50 publications from SWIO countries (see citations in Mortimer and Esteban et al. 2020) and SWOT 12 (2017).

The Chagos Archipelago provides an estimated 132 km of suitable nesting habitat across 58 islands, most of which is found in two atolls (Diego Garcia and Peros Banhos) that accounted for 90% of hawksbill and 70% of green turtle nesting activities. Similar to other rookeries in the SWIO, hawksbills show a distinct seasonal peak (October-February). Green turtles nest year-round with around 50% of nests between June-October. Importantly, the nesting population size has increased to around 6300 hawksbill and 20,500 green turtle clutches laid annually (2011-2018) with an increase in abundance by 2-5 times for hawksbills and 4-9 times for green turtles since 1996. Satellite tracking has shown that green turtles that nest in the Chagos Archipelago migrate to foraging grounds across the SWIO (Hays et al. 2020), i.e., the Chagos Archipelago provides a nesting sanctuary for green turtles from across the region.

SWIO regional estimates indicate that although green turtles produce ten times more egg clutches than hawksbill, individual green turtles may produce almost twice as many egg clutches as individual hawksbills (Mortimer & Bresson, 1999; Esteban et al. 2017).  The Chagos Archipelago nesting population accounts for 39-51% of estimated 12,500-16,000 hawksbill (see Figure 1a) and 14-20% of estimated 104,000-143,500 green turtle clutches (Figure 1b) laid annually. The improved status of sea turtles in the Chagos Archipelago may reflect 50 years of protection.

Figure 1. Important turtle nesting populations in Southwest Indian Ocean region for: (a) hawksbills and (b) green turtles. Circle sizes indicate estimated mean annual egg clutch production. ? indicates poorly surveyed sites. Abbreviations: Cha, Chagos Archipelago; Com, Comores; Eur, Europa; Glo, Glorieuse; Jua, Juan de Nova; Ken, Kenya; Mad, Madagascar; Mau, Mauritius; May, Mayotte; Moz, Mozambique; Reu, Réunion; Sey, Seychelles; Som, Somalia; Tan, Tanzania; Tro, Tromelin. Adapted from Mortimer & Esteban et al. 2020.

References

Esteban N, Mortimer JA, Hays GC (2017) How numbers of nesting sea turtles can be overestimated by nearly a factor of two. Proc. R. Soc. B 284: 20162581. doi:10.1098/rspb.2016.2581

Hays GC, Cerritelli G, Esteban N, Rattray A, Luschi P (2020) Open ocean reorientation and challenges of island finding by sea turtles during long-distance migration. Curr. Biol. 30:3236-3242. doi:10.1016/j.cub.2020.05.086

Mortimer JA, Bresson R (1999) Temporal distribution and periodicity in hawksbill turtles (Eretmochelys imbricata) nesting at Cousin Island, Republic of Seychelles, 1971-1997.  Chel. Conserv. Biol. 3: 292-298.

Mortimer, J., Esteban, N., Guzman, A., Hays, G. (2020) Estimates of marine turtle nesting populations in the south-west Indian Ocean indicate the importance of the Chagos Archipelago. Oryx 1-12. doi: 10.1017/s0030605319001108.

SWOT 12 (2017) State of the World’s Sea Turtles. XII. Downloaded from http://seaturtlestatus.org/sites/swot/files/report/SWOT%20Report_12.pdf on 9 September 2018.