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. 2023 Jun 14;18(6):e0286545.
doi: 10.1371/journal.pone.0286545. eCollection 2023.

Spatial ecology and conservation of leatherback turtles (Dermochelys coriacea) nesting in Bioko, Equatorial Guinea

Affiliations

Spatial ecology and conservation of leatherback turtles (Dermochelys coriacea) nesting in Bioko, Equatorial Guinea

Francesco Garzon et al. PLoS One. .

Abstract

Bioko Island (Equatorial Guinea) hosts important nesting habitat for leatherback sea turtles, with the main nesting beaches found on the island's southern end. Nest monitoring and protection have been ongoing for more than two decades, although distribution and habitat range at sea remains to be determined. This study uses satellite telemetry to describe the movements of female leatherback turtles (n = 10) during and following the breeding season, tracking them to presumed offshore foraging habitats in the south Atlantic Ocean. Leatherback turtles spent 100% of their time during the breeding period within the Exclusive Economic Zone (EEZ) of Equatorial Guinea, with a core distribution focused on the south of Bioko Island extending up to 10 km from the coast. During this period, turtles spent less than 10% of time within the existing protected area. Extending the border of this area by 3 km offshore would lead to a greater than threefold increase in coverage of turtle distribution (29.8 ± 19.0% of time), while an expansion to 15 km offshore would provide spatial coverage for more than 50% of tracking time. Post-nesting movements traversed the territorial waters of Sao Tome and Principe (6.4%of tracking time), Brazil (0.85%), Ascension (1.8%), and Saint Helena (0.75%). The majority (70%) of tracking time was spent in areas beyond national jurisdiction (i.e. the High Seas). This study reveals that conservation benefits could be achieved by expanding existing protected areas stretching from the Bioko coastal zone, and suggests shared migratory routes and foraging space between the Bioko population and other leatherback turtle rookeries in this region.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Tagging location of female leatherback turtles (Dermochelys coriacea) from Bioko Island, Equatorial Guinea.
Moaba beach is indicated by yellow dot on the right map panel. Left panel shows the location of Bioko Island within the Gulf of Guinea. GHA = Ghana, BEN = Benin, NGA = Nigeria, GNQ = Equatorial Guinea, GAB = Gabon, COG = Republic of Congo. Basemap created in QGIS using freely available data from Natural Earth (www.naturalearthdata.com) and the General Bathymetric Chart of the Ocean (www.gebco.net).
Fig 2
Fig 2. Inter-nesting displacement and nesting events of leatherback turtles (Dermochelys coriacea) from Bioko Island, Equatorial Guinea.
Displacement was calculated as the minimum straight-line distance (km) from the tagging location. Grey and red shaded vertical bars indicate putative nesting attempts and assumed successful nesting events, respectively. Displacement series in the figure were terminated before the beginning of migratory movements away from the breeding area. Order as in Table 1.
Fig 3
Fig 3. Utilisation distribution contours of female leatherback turtles (Dermochelys coriacea) during their reproductive season in Bioko Island, Equatorial Guinea.
Filtered turtle locations obtained from satellite tracking are indicated by black dots. Core areas and home ranges of individuals obtained from ctmm modelling are displayed in red and grey polygons, respectively. Turtle 8 is not included as the individual left the nesting grounds less than 24 hours following tagging. GNQ = Equatorial Guinea, STP = São Tomé and Príncipe, CMR = Cameroon. Basemap created in QGIS using freely available data from Natural Earth (www.naturalearthdata.com) and the General Bathymetric Chart of the Ocean (www.gebco.net).
Fig 4
Fig 4. Protection offered by existing and proposed marine boundaries of the Caldera de Luba Scientific Reserve to nesting leatherback turtles (Dermochelys coriacea) in Bioko Island, Equatorial Guinea.
a) The current boundaries of the Caldera de Luba Scientific Reserve are indicated in blue, while proposed potential expansions of its marine component are represented by black dotted lines placed at 1km intervals from the current boundary up to 10 km from the shore, and also at 15 km and 20 km from the shore. Red polygons represent the core utilisation areas for all tagged individuals, yellow polygon indicates the intersection of all core utilisation areas. b) Protection offered to turtles by increasing marine buffer radii for the Caldera de Luba Scientific Reserve, quantified as the average percentage of time tracked turtles spent within them (black dots, error bars represent SD), percentage of the intersection of core areas covered by the buffer (yellow dots), and percentage of the union of core areas covered by the buffer (red dots). The buffer size currently in existence (i.e. 2 km buffer from shore) is highlighted in the blue rectangle. Basemap created in QGIS using freely available data from Natural Earth (www.naturalearthdata.com) and the General Bathymetric Chart of the Ocean (www.gebco.net).
Fig 5
Fig 5. Post-nesting migratory strategies of leatherback turtles (Dermochelys coriacea) satellite tagged in Bioko, Equatorial Guinea.
(a, c) Satellite-tracked movements and (b, d) daily displacement from the time turtles left their breeding grounds showing 2 dispersal groups: (a, b) offshore waters in equatorial Atlantic, and (b, c) direct dispersal towards South America. Colour codes for individuals are conserved among panels. BRZ = Brazil, ASC = Ascension, STH = St. Helena, STP = São Tomé and Príncipe. Basemap created in QGIS using freely available data from Natural Earth (www.naturalearthdata.com) and the General Bathymetric Chart of the Ocean (www.gebco.net).
Fig 6
Fig 6. Overlap between leatherback turtle (Dermochelys coriacea) tracked from their nesting grounds in Bioko Island and longline and seine fisheries in the Central Atlantic Ocean.
Cumulative fishing effort (top panel) for longline and seine fishing vessels for the tracking period aggregated and shown in 0.5x0.5° cells (bottom-left and bottom-right show effort separately for longline and seine, respectively). Please note fishing effort is depicted on a logarithmic scale. Original fishing dataset from Global Fishing Watch www.globalfishingwatch.org. Black dashed lines represent limits of Exclusive Economic Zones (EEZ) of the bordering countries. Regularised daily locations of tracked turtles are indicated as white dots. BRZ = Brazil, ASC = Ascension, STH = St. Helena, STP = São Tomé and Príncipe, GAB = Gabon. Basemap created in QGIS using freely available data from Natural Earth (www.naturalearthdata.com) and the General Bathymetric Chart of the Ocean (www.gebco.net).

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