Green turtle population recovery at Aldabra Atoll continues after 50 yr of protection

Abstract

Green turtles Chelonia mydas have been subject to high levels of anthropogenic exploitation, with harvesting at their nesting sites especially pronounced throughout the late 19th and early 20th centuries, leading to worldwide declines. Due to their delayed sexual maturity, long-term protection and monitoring is crucial to allow and accurately demonstrate population recovery. Subsequent to their exploitation, Aldabra Atoll (Republic of Seychelles) has offered the longest continuous protection for nesting green turtles anywhere in the Western Indian Ocean, beginning in 1968. Here, we document the continuing recovery of that population by estimating clutch production within 12 mo nesting seasons over 50 yr of monitoring. An estimated mean of 15297 clutches were laid annually between December 2014 and November 2019. This represents an increase of 173% since Aldabra’s intensive monitoring programme was initiated in 1980, and 410-�665% since 1968. Clutch number increases were recorded at all but 1 of 6 monitored beach groups around the atoll but were most pronounced at Settlement Beach, where exploitation of nesting females was historically most intense. Seasonality data since 2000 showed a year-round nesting season, with elevated activity in April-June peaking on average in May, and a potential shift to later in the year over time. This study highlights the considerable contribution of Aldabra Atoll to regional green turtle numbers and the benefit of long-term protection and monitoring at what can be considered a global reference site for this species.

Long-term changes in adult size of green turtles at Aldabra Atoll and implications for clutch size, sexual dimorphism and growth rates

“…At Ascension Island where overall nesting activity and numbers of egg clutches laid annually had increased by almost sixfold since 1977, the mean annual CCL of green turtle nesters declined from about 116 cm in 1970 to about 112 cm in 2010, i.e. a decrease of about 1 cm per decade (Weber et al 2014), compared to the decline in size of about 0.6 cm per decade that we recorded on Aldabra, where nesting activity has increased by more than threefold between 1980 and 2016 (Pritchard et al 2022). Mean size of nesters may be influenced by an increase in the number of first-time nesters (neophytes or recruits) that are presumably smaller than established nesters (remigrants) (Hays et al 2022).…”

“…In fact, our present study supports this theory, given that across atolls at the start of our data collection (early 1980s) there was no significant difference in the mean size of nesters at protected and unprotected sites. During the 1990s, our dataset does show a clear reduction in average size of nesters, which also corresponded with an increase in the absolute size of the nesting population (Pritchard et al 2022) and most likely an increasing proportion of neophyte (recruit) nesters.…”

“…There was a clear decline in the mean size of nesting turtles at Aldabra from 1996 to 2016 that co-occurred with an increase in nesting numbers (Pritchard et al 2022), as has been reported in some other populations around the world, for example green turtles at Ascension Island (Weber et al 2014) and loggerhead turtles, Caretta caretta, in South Africa and the Cape Verde islands, respectively (Le Gouvello et al 2020;Hays et al 2022). At Ascension Island where overall nesting activity and numbers of egg clutches laid annually had increased by almost sixfold since 1977, the mean annual CCL of green turtle nesters declined from about 116 cm in 1970 to about 112 cm in 2010, i.e.…”

“…How hatchling success varies with clutch size is not well known. Between 1996 and 2016, the estimated number of green turtle clutches each year at Aldabra increased from around 10,000 to 16,000 (Pritchard et al 2022). Therefore, even though the reduction in mean turtle size is predicted to have reduced the mean clutch size from 128.0 and 124.7 eggs per clutch, the total number of eggs laid per year between 1996 and 2016 is still estimated to have increased hugely, from 1.3 million to 2.0 million per year.…”

“…Other important green turtle nesting sites of the southern islands of Seychelles (Mortimer 1984) received legal protection 23 years later in 1994 with passage of national legislation protecting all sea turtles in Seychelles, the Wild Animals (Turtles) Protection Regulations (Mortimer et al 2011). This protection of green turtles at Aldabra has led to a huge increase in nesting numbers over the last 30 years (Pritchard et al 2022) but the relationship between increase in nesting numbers and mean turtle size has so far not been explored.…”