Regionally important developmental foraging grounds for green turtles and hawksbills in the Southwest Indian Ocean

Abstract South Africa hosts a major portion (~ 75–80%) of the Southwest Indian Ocean (SWIO) loggerhead Caretta caretta and leatherback Dermochelys coriacea turtle nesting populations, supported by a beach conservation programme exceeding six decades. However, the in-water life stages of these species, and of non-nesting green turtles Chelonia mydas and hawksbills Eretmochelys imbricata which frequent adjacent neritic habitats year-round, remain poorly understood. Here, we provide the first estimates of in-water relative abundance, demographics, and distribution of turtles in two Marine Protected Areas, iSimangaliso and Aliwal Shoal, on South Africa’s east coast. Between September 2020 and August 2022, we recorded 1024 sightings (52% green turtles, 20% hawksbills, 22% loggerheads, 6% unidentified hard-shelled turtles, and one leatherback) from timed research surveys (264.3 h), citizen science (146 dive trips), and opportunistic sightings across six sites (~ 7.4 km²). In total, 222 individuals were photo-identified. Relative abundance of species was proportional to the nesting female abundance in the SWIO, with green turtles most abundant, followed by hawksbills, loggerheads, and leatherbacks; however, loggerheads were more abundant than hawksbills in iSimangaliso. Reefs supported resident aggregations of mixed size classes: green turtles ranged from juvenile to adult-sized (44.9–99.2 cm straight carapace length), hawksbills were mostly juvenile and subadult-sized (37.4–73.4 cm), and loggerheads were subadult and adult-sized (66.9–81 cm). This first in-water study of sea turtles in South Africa highlights the east coast’s importance as a developmental foraging area for regional green turtle and hawksbill populations and provides a foundation for future in-water research to build on.