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.