Recovery of the South Atlantic’s largest green turtle nesting population
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Although many species of marine mega-vertebrates are threatened as a result of human activity, some populations are showing promising signs of recovery following decades of protection. In this study, we report on the status of the South Atlantic’s largest green turtle (Chelonia mydas) nesting aggregation at Ascension Island, 70 years after legal protection and the cessation of commercial turtle harvesting that decimated the population. Using a monitoring dataset spanning 36 years, we modelled long-term trends in nesting activity at both a rookery level and across individual nesting beaches and beach clusters. Since monitoring began in 1977, the average number of green turtle clutches deposited annually at Ascension Island has increased sixfold, from approximately 3,700 to 23,700; a trend that has been accompanied by a significant decrease in the average size of nesting females. Interestingly, however, rates of increase in nesting activity have varied dramatically among nesting beaches, ranging from 0.4 to 6 % growth per annum. More than 97 % of this variation could be explained by distance from the main human settlement of Georgetown—the historic centre of turtle harvesting—with beaches closer to Georgetown experiencing the most rapid growth. More rapid population growth close to human centres seems counterintuitive, but may reflect the more intensive depletion of these accessible, local stocks during the harvesting era. Overall, the Ascension Island green turtle population appears to be recovering strongly, mirroring positive trends for this species across many parts of its geographic range. While not a cause for complacency, these trends are encouraging and demonstrate the capacity of marine megafauna to rebound when anthropogenic pressures are alleviated through conservation action.
KeywordsMarine turtle Population trend Population size Demographic unit Conservation status Female size
This work was funded by the UK Government’s Department for Environment food a Rural Affairs (DEFRA) through the Overseas Territories Environment Programme (OTEP) and the Darwin Initiative. The authors would like to thank T. Weber and A. Jackson for help with population models, and the many volunteer ‘rakers’ who have contributed to turtle monitoring over the years. We are grateful to the Associate Editor and two anonymous reviewers for comments and suggestions that substantially improved an earlier draft of the manuscript.
- Bjorndal KA, Bolten AB (2003) From ghosts to key species: restoring sea turtles to fulfil their ecological roles. Mar Turt Newsl 100:16–21Google Scholar
- Bjorndal KA, Bolten AB, Chaloupka M (2000) Green turtle somatic growth model: evidence for density dependence. Ecol Appl 10:269–282Google Scholar
- Fletemeyer J (1986) Ascension Island turtles still in trouble. Mar Turt Newsl 37:11Google Scholar
- Heppell SS, Crouse DT, Crowder LB, Epperly S, Gabriel T, Henwood T, Marquez R, Thompson N (2005) A population model to estimate recovery time, population size, and management impacts on Kemp’s ridleys. Chelonian Conserv Biol 4:767–773Google Scholar
- Huxley RC (1997) Ascension Island and turtles: a monograph. Ascension Island Heritage Society, Ascension IslandGoogle Scholar
- Huxley RC (1999) Historical overview of marine turtle harvesting, Ascension Island, South Atlantic Ocean. Mar Turt Newsl 84:7–9Google Scholar
- Jackson JBC, Kirby MX, Berger WH, Bjorndal KA, Botsford LW, Borque BJ, Bradbury RH, Cooke R, Erlandson J, Estes JA, Hughes TP, Kidwell S, Lange CB, Lenihan HS, Pandolfi JM, Peterson CH, Steneck RS, Tegner MJ, Warner RR (2001) Historical overfishing and the recent collapse of coastal ecosystems. Science 293:629–637PubMedCrossRefGoogle Scholar
- National Research Council (2010) Assessment of sea turtle status and trends: integrating demography and abundance. National Academies Press, WashingtonGoogle Scholar
- Pritchard PCH (2004) Long-term sea turtle nesting cycles in the Guianas. In: Coyne MS, Clark RD (eds) Proceedings of the 21st Annual Symposium on Sea Turtle Biology and Conservation. NOAA Technical Memorandum NMFS-SEFSC-528, pp. 66–68.Google Scholar
- Seminoff JA (2004) MTSG global assessment of green turtles (Chelonia mydas) for the IUCN Red List. IUCN Species Survival Commission, CambridgeGoogle Scholar
- Witherington BE, Koeppel CM (2000) Sea turtle nesting in Florida, USA, during the decade 1989–1998: an analysis of trends. In: Kalb H, Wibbels T (eds) Proceedings of the 19th Annual Symposium on Sea Turtle Biology and Conservation. NOAA Technical Memorandum NMFS-SEFSC-443, pp. 94–96.Google Scholar