Migratory behavior and management implications of green sea turtles in the Western Indian Ocean
With the largest marine turtle distribution in the Western Indian Ocean, green sea turtles perform seasonal long-range migrations between nesting and foraging sites throughout the region.
The Western Indian Ocean is home to a variety of marine animals that spend great amounts of time in both coastal and oceanic habitats throughout the region. Green sea turtles (Chelonia mydas) are one of these species, using these regions as critical nesting grounds, productive foraging sites, and migratory corridors. When nesting females lay eggs on beaches, they offer scientists the opportunity to easily approach them and apply satellite tags to track their use of the ocean. With the largest marine turtle distribution in the Western Indian Ocean, green sea turtles have nesting sites scattered over many of the small islands and along the coasts of East Africa and Madagascar (Figure 1). Nesting among these many sites has been shown to occur year-round, making the protection of these beaches vital to the breeding success and conservation of the species.
Green sea turtles perform seasonal long-range migrations between nesting and foraging sites. Post-nesting movements of green sea turtles have been shown to involve both oceanic and coastal migration routes in the Western Indian Ocean, with some individuals migrating extensively through both before reaching their foraging grounds. Individuals tagged at nesting beaches along the coast of East Africa have undertaken migrations of hundreds of kilometers, passing through several different jurisdictions along the way (Figures 2 & 3). Conversely, individuals tagged on the beaches of small islands in the Western Indian Ocean have shown wide dispersal through the high-seas en-route to coastal foraging grounds, including some of the longest known post-nesting migrations of hard shelled turtles (Figure 4). They remain at these shallow foraging sites for up to 7 months at a time, exhibiting high site-fidelity as they feed on the abundant seagrass within these coastal areas.
During all stages of their life history and migration cycles, green sea turtles experience numerous stressors with population level impacts. Incidental bycatch in gillnets and pelagic longlines, has a high impact on marine turtles globally. In the Western Indian Ocean specifically, bycatch is often identified as one of the largest threats to green sea turtles in addition to harvesting of turtles and loss of on-land nesting. As a result, green turtles are currently categorized globally as “Endangered” based on IUCN Red List reporting.
The potential cumulative impact of these regional-scale stressors, along with the migratory movement patterns of green sea turtles within West Indian Ocean waters, results in the potential for impacts of stressors in one part of a region being felt strongly in ecosystems in another part of the region. Conservation of green sea turtle populations in the Western Indian Ocean presents a key example of the need for coordinated management across sectors and jurisdictions. The importance of developing regional, transboundary conservation strategies (including areas beyond national jurisdiction) is fundamental to ensuring the continued delivery of ecosystem services provided by green sea turtles including climate regulation, nutrient cycling, food provisioning, and ecotourism.
Green turtle nesting sites
Figure 1 reprinted from Bourjea et al. 2015: Geographic locations of 15 different green turtle nesting sites sampled throughout the West Indian Ocean in Bourjea et al. (2015) and Bourjea et al. (2007). Numbers of samples (nesting females) per locality are shown in brackets.
(Left) Figure 2 reprinted from Garnier et al. 2012: Coastal (V1, V2, V4) and oceanic (V3) migration routes of four tagged adult female green turtles from nesting grounds on Vamizi Island, Mozambique to their distinct foraging grounds.
(Right) Figure 3 reprinted from Sea Sense Annual Report 2014: Coastal migrations of 11 satellite tagged green turtles after nesting on Northern Tanzanian beaches between 2012 and 2014, showing tendency to stay in inshore waters rather than crossing open seas.
Migratory movement and kernel density results
Figure 4 reprinted from Hays et al. 2014: (Left) Migratory movements of eight adult female green turtles from their nesting beach on Diego Garcia (in red circle) to identified foraging grounds for each individual (red star).
(Right) Results of kernel density analyses (red, 50% kernel home-range; orange, 90% kernel home-range use; blue, 95% kernel home-range use) for 3 green turtles at their foraging grounds a) Desroches Island (Amirantes, Seychelles) b) Fonadhoo Island (Maldives) and c) coast of Somalia.
Bourjea, J., S. Lapegue, L. Gagnevin, D. Broderick, J. A. Mortimer, S. Ciccione, D. Roos, C. Taquet, and H. Grizel. 2007. Phylogeography of the Green Turtle, Chelonia Mydas, in the Southwest Indian Ocean. Molecular Ecology 16: 175–86.
Bourjea, J., J. A. Mortimer, J. Garnier, G. Okemwa, B. J. Godley, G. Hughes, M. Dalleau, C. Jean, S. Ciccione, and D. Muths. 2015. Population Structure Enhances Perspectives on Regional Management of the Western Indian Ocean Green Turtle. Conservation Genetics 16: 1069–83.
Garnier, J., N. Hill, A. Guissamulo, I. Silva, M. Witt, and B. Godley. 2012. Status and Community-Based Conservation of Marine Turtles in the Northern Querimbas Islands (Mozambique). Oryx 46: 359–67.
Hays, G. C., J. A. Mortimer, D. Ierodiaconou, and N. Esteban. 2014. Use of Long-Distance Migration Patterns of an Endangered Species to Inform Conservation Planning for the World’s Largest Marine Protected Area. Conservation Biology 28: 1636–44.
Sea Sense Annual Report 2014. http://www.seasense.org/uploads/media/Sea_Sense_2014__Annual_Report.pdf
Data available on SWOT:
Kot, C.Y., E. Fujioka, A. DiMatteo, B. Wallace, B. Hutchinson, J. Cleary, P. Halpin and R. Mast. 2018. The State of the World's Sea Turtles Online Database: Data provided by the SWOT Team and hosted on OBIS-SEAMAP. Oceanic Society, IUCN Marine Turtle Specialist Group (MTSG), and Marine Geospatial Ecology Lab, Duke University. http://seamap.env.duke.edu/swot.
Halpin, P.N., A.J. Read, E. Fujioka, B.D. Best, B. Donnelly, L.J. Hazen, C. Kot, K. Urian, E. LaBrecque, A. Dimatteo, J. Cleary, C. Good, L.B. Crowder, and K.D. Hyrenbach. 2009. OBIS-SEAMAP: The world data center for marine mammal, sea bird, and sea turtle distributions. Oceanography 22(2):104-115
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