Hawksbill turtle © Robert Van Dam
Satellite tracking and flipper-tagging techniques have enabled a better understanding of marine turtle life histories. In 2017, for the first time, a Hawksbill female having been flipper tagged during her juvenile stage while foraging on a reef at the Cocos (Keeling) Islands (CKI) was recorded nesting at Frégate Island –an inner island of the Seychelles, 4,600 km to the west of CKI (van de Crommenacker et al., 2022). CKI is an external Australian Territory within the South-East Indian Ocean (SEIO) Hawksbill Regional Management Unit (RMU), whereas the Seychelles belong to the South-West Indian Ocean (SWIO) RMU (Wallace et al., 2010). RMUs have often been used in marine turtle conservation to effectively identify anthropogenic threats and impacts at the population level (Hamann et al., 2021).
As with other turtle species, there is a substantial knowledge gap as to what happens with hawksbill turtles after the hatching life stage, including their growth patterns and survivorship (Hamann et al., 2021). The general understanding is that newly hatched hawksbills enter the sea and are carried by offshore currents into major gyre systems where they drift passively with oceanic currents in the pelagic surface waters of the oceanic gyres until reaching a carapace length of some 20 to 30 cm. They thereafter shift to relatively shallower benthic habitats that may comprise coral reefs or other hard bottom habitats, sea grass, algal beds, sand, mud, or mangrove bays and creeks (Mortimer and Donnelly, 2008).
The clip below displays the wide dispersal of sea turtle hatchlings of various species from 42 nesting sites across the globe experienced during their first year of life under a model developed by Scott et al., 2014 (Figure 1). These passive drift scenarios illustrate well the important connections between far distant locations, supporting also such findings as the one described above of the hawksbill turtle tagged in CKI and years later found nesting in the Seychelles. Genetic evidence also supports this understanding (FitzSimmons, 2010).
Figure 1. 1-year trajectories from 42 sea turtle nesting sites, each of which is represented by 1000 randomized individuals. Input data comes from 7 nesting seasons recorded in the 2000-2006 period (Scott et al., 2014). Click here to see the modelled trajectories in a short animation.
The animation clearly exemplifies how, just as other long-lived marine vertebrates, marine turtles occupy broad geographic ranges, using different areas for breeding and foraging depending on their life stage or level of maturity.
Such migration across RMU boundaries (see Figure 2) have implications for effective conservation management. van de Crommenacker et al. (2022) therefore propose that the area off CKI be treated as an area of overlap between these two RMUs. The ongoing update of the RMUs presents a good opportunity to take such new evidence into account so that RMUs can continue to serve their important role of informing decision-makers about likely population boundaries.
Figure 2. Hawksbill turtle (Eretmochelys imbricata) Regional Management Units (RMU) and Nesting Sites. Red circle indicates the Seychelles rookeries located in the SWIO RMU, orange circle indicates the CKI developmental area located in the SEIO RMU. Adapted from Wallace et al., 2010.
Wide ranges pose various challenges for the conservation and management of migratory species. This example of connectivity between two RMUs shows how important – besides ever more scientific evidence – multilateral cooperation is for conservation and management of migratory species. Accordingly, the IOSEA Marine Turtle MOU brings together governments from across the IOSEA region to help tackle conservation challenges jointly, while also involving other relevant stakeholders.
FitzSimmons, N. 2010. Population genetic studies in support of conservation and management of hawksbill turtles in the Indian ocean. Final Report: Marine Turtle Conservation Fund Award 98210-70G126 US Fish and Wildlife Service. pp. 1-24.
Hamann, M., Flavell, F., Frazier, J., Limpus, C. J., Miller, J. D., Mortimer, J. A. (2021). Assessment of the Conservation Status of the Hawksbill turtle in the Indian Ocean and South-East Asia Region. CMS Secretariat. Accessed on https://www.cms.int/en/publication/assessment-conservation-status-hawksbill-turtle-indian-ocean-and-south-east-asia-region on 12 September 2022.
Mortimer, J.A., Donnelly, M. (2008) Eretmochelys imbricata. IUCN Red List of Threatened Species. Version 2008: e.T8005A12881238. www.iucnredlist.org/species/8005/12881238#assessment-information.
Scott R, Marsh R, Hays GC (2014). Ontogeny of long distance migration. Ecology 95, 2840-2850. doi: 10.1890/13-2164.1. Available e.g. from https://oceanrep.geomar.de/id/eprint/25939/1/Scott_et_al_2014.pdf accessed on 12 September 2022.
van de Crommenacker, J. Mortimer, J.A., Whiting, A., Macrae, I., Flores, T. & Whiting, T. (2022). Linkage Between Cocos (Keeling) Developmental Habitat and Hawksbill Nesting Beaches of Seychelles. Marine Turtle Newsletter, 165. Accessed on http://www.seaturtle.org/mtn/PDF/MTN165.pdf on 12 September 2022.
Wallace, B. P., DiMatteo, A. D., Hurley, B. J., Finkbeiner, E. M., Bolten, A. B., Chaloupka, M. Y., Hutchinson, B. J., Abreu-Grubois, F. A., Amorocho, D., Bjornal,K. A., Bourjea, J., Bowen, B. W., Briseño-Dueñas, R., Casale, P., Choudhury, B. C., Costa,A., Dutton, P. H., Fallabrino, A., Girard, A., Girondot, M., Godfrey, M. H., Hamann, M., López-Mendilaharsu, M., Marcovaldi, M. A., Mortimer, J.A., Musick, J. A., Nel, R., Pilcher, N. J., Seminoff, J. A., Troëng, S., Witherington, B., Mast, R. B. (2010). Regional Management Units for Marine Turtles: A Novel Framework for Prioritizing Conservation and Research across Multiple Scales. PLoS ONE, 5(12), e15465. https://doi.org/10.1371/journal.pone.0015465
Last updated on 26 September 2022