Newly Discovered migratory corridor and foraging ground for Atlantic green turtles, Chelonia mydas, nesting on Bioko Island, Equatorial Guinea

This study is the first to use satellite telemetry to track post-nesting movements of endangered green turtles (Chelonia mydas) in the Gulf of Guinea. Satellite transmitters were attached in 2018 to six Atlantic green turtles nesting on Bioko Island, Equatorial Guinea, to track their post-nesting movements and locate their foraging grounds. Track lengths of 20-198 days were analyzed, for a total of 536 movement days for the six turtles. Migratory pathways and foraging grounds were identified by applying a switching state space model to locational data, which provides daily position estimates to identify shifts between migrating and foraging behavior. Turtles exhibited a combination of coastal and oceanic migrations pathways that ranged from 957 km to 1,131 km. Of the six turtles, five completed their migration and maintained residency at the same foraging ground near the coastal waters of Accra, Ghana until transmission was lost. These five resident turtles inhabit heavily fished and polluted waters and are vulnerable to a variety of anthropogenic threats. The identification of these foraging grounds highlights the importance of these coastal waters for the protection of the endangered Atlantic green turtle.


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After entering the ocean as hatchlings, sea turtles spend the majority of their lives in the water, 35 only emerging to lay eggs and or in some cases to bask [1] [2] [3]. Because of this, research on sea turtles 36 has been largely restricted to nesting females, which has led to conservation efforts primarily focused on 37 nesting beaches, rather than in-water habitat. Since in water habitats come with a variety of unique

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In West Africa, the Gulf of Guinea has experienced an increase in anthropogenic disturbances,

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putting the turtle population at risk from many threats including ship traffic, pollution, commercial and 46 small-scale fishing operations [33], [5], and oil and gas development [7], [34]. Marine and coastal pollution in the waters of the Gulf of Guinea has caused a host of environmental threats, including oxygen 48 depletion, faunal die-offs, and heavy metal and hydrocarbon accumulation in marine consumers [6], [35].

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While sea turtles are protected under international and national law in most West African nations, 50 incidental bycatch in fisheries operations is a major threat in the Gulf of Guinea. Green turtles are 51 common bycatch in both gillnet and pelagic longline fishing operations [36], [37], [5]. Oil and gas 52 development has also rapidly intensified in the Gulf of Guinea in recent years [34], and poses diverse, but 53 difficult to measure, threats to sea turtle populations, with an increase in channel dredging, ship traffic, oil 54 leaks, and chemical pollution, which can affect adult turtles that forage or travel close to offshore 55 platforms [7]. These threats highlight the need to study migration patterns and foraging ground locations 56 of sea turtles to better understand their vulnerabilities.

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Little is known about the in-water movements and behavior of green turtles in the Gulf of Guinea.

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Green turtles that were flipper tagged on Bioko Island, Equatorial Guinea, in 1996   Nigeria, and then maintained a coastal route (Fig. 2). Both oceanic and coastal migration routes remained and red circles represent foraging behavior identified by the state-space model. Arrows represent current 162 direction.

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All five turtles ultimately began extended periods (>30 days) of residency and foraging behavior 172 off the coast of Ghana, in a 50 km stretch east of Accra and west of the Volta River delta, after migration 173 periods of 14-28 days (Fig. 4). Three turtles exhibited migrations interspersed with short (<3 days)

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periods of foraging off the coasts of Lagos, Nigeria, and Togo and Benin (Fig. 4). While the turtles 175 exhibited both oceanic and coastal migrating behaviors, all exhibited near-shore foraging activity.

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All six turtles migrated westward from Bioko Island, and five turtles completed their migration,

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ending at a previously undocumented foraging ground in the coastal waters of Ghana (Fig. 1) (Fig. 3). Given the low speeds of the currents, it is unlikely that countercurrent migrations greatly 214 increased energy cost in this case.

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Given the existence of nesting populations of green turtles on the beaches nearby this foraging 216 ground in Ghana, and the apparent habitat suitability, it is likely that this foraging ground is used by

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Genetic similarities between populations suggests there are multiple populations using the same foraging 221 and breeding grounds as those that nest on Bioko Island [58].

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The discovery of this foraging ground is of particular importance, as only one other foraging