Abstract
Green turtles (Chelonia mydas) are key herbivores of tropical and subtropical neritic habitats and play a major role in structuring seagrass meadows. We present the first detailed assessment of green turtle diet in the Western Indian Ocean using the gut contents of salvaged animals from three atolls in the Republic of Seychelles separated from each other by 400–825 km: Cosmoledo (adults, n = 12), Farquhar (adults, n = 33; immature, n = 1) collected in 1982–1983; and Desroches (immatures, n = 8) in 2016–2018. We report the first comparison of the diets of gravid females (n = 17), males (n = 26) and non-breeding females (n = 2) at sites providing both foraging and breeding habitat. Seagrass (mostly Thalassodendron ciliatum) dominated the diet, accounting for 95% of the mean gut content biomass for males and non-breeding females but only 58% for gravid females, alongside relatively large amounts of substrate (14%) and macroalgae (13%). Satellite tracking of post-nesting green turtles from Chagos Archipelago in 2016 located foraging sites at Farquhar Atoll that coincided with capture locations of 26 of the 33 adult turtles sampled there in 1983. In situ surveys of those sites in 2018 revealed extensive nearly monospecific beds of T. ciliatum. The prominence of seagrass in the diet of green turtles and connectivity between foraging and nesting habitats throughout the region illustrate the need to conserve and monitor seagrass habitats of the Western Indian Ocean especially in the context of changing green turtle population densities.
Highlights
Green turtles (Chelonia mydas) are marine ecosystem engineers that structure seagrass meadows through their foraging activities, often increasing its complexity (Coleman and Williams 2002) or, reducing species composition (Kelkar et al 2013a; Lal et al 2010) and production ratesResponsible Editor: P
At sites in the Republic of Seychelles, we report an almost exclusive diet of seagrass for green turtles
Seagrass biomass represented 95% of the diet of the adults sampled in our study (n = 28), excluding the gravid females (n = 17)
Summary
Green turtles (Chelonia mydas) are marine ecosystem engineers that structure seagrass meadows through their foraging activities, often increasing its complexity (Coleman and Williams 2002) or, reducing species composition (Kelkar et al 2013a; Lal et al 2010) and production ratesResponsible Editor: P. Due to numerous anthropogenic threats and population declines, the green turtle had been listed as globally endangered on the IUCN Red List (Seminoff 2004), but long-term population recovery, thanks to successful conservation strategies such as protection of turtles and their habitats at nesting sites, has led to long-term population recovery. This is reflected in recent and ongoing IUCN downlisting of various green turtle subpopulations around the world (e.g., to “least concern” for the Hawaiian and South Atlantic subpopulations (Chaloupka and Pilcher 2019; Broderick and Patricio 2019) and to Vulnerable for the North Indian Ocean subpopulation (Mancini et al 2019) and, in literature, documenting of increasing population trends globally (Chaloupka et al 2008; Seminoff et al 2015; Mazaris et al 2017), including upward trends in the Western Indian Ocean (Bourjea et al 2007; Lauret-Stepler et al 2007; Mortimer et al 2011)
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