On the Move: West Africa’s Green Turtles Connect Five Nations


Yashi Dwivedi

One of the largest sea turtles and a herbivore, the green turtle is widespread across the globe. Green turtles primarily feed on seagrass and algae that give their cartilage and fat the green colour. Historically, green turtles were exploited for their fat, meat, and eggs, causing global population decline. Now, many countries prohibit killing of sea turtles and collection of their eggs and have established protected areas along their coasts to protect the feeding and nesting grounds of the turtles and other marine life.

Green sea turtle. Photo credit: Bernard Dupont, via Wikimedia Commons

Similar to national parks and wildlife sanctuaries on land, Marine Protected Areas (MPAs) are nature reserves for marine biodiversity. They play a crucial role in the long-term survival of species and their habitats.

Traditionally, MPAs were not specifically planned with migration routes and connectivity in mind. However, several marine species, including green turtles migrate long distances between their feeding grounds and nesting sites. Boat strikes and entanglement in fishing gear are some of the threats that the sea turtles face while migrating long distances. Hence, assessing the connectivity across existing MPAs and identification of migration corridors are essential for ensuring the effectiveness of the protected area networks for conservation of marine species.

A recent study, published in the Frontiers in Marine Science journal, investigated the connectivity between marine protected areas in West Africa. This region, along the coasts of Guinea-Bissau, Senegal, the Gambia, and Mauritania, hosts rich coastal ecosystems including seagrass meadows, mangroves, and tidal flats, and is home to the largest population of green turtles in the eastern Atlantic Ocean. One of the core nesting sites for the green turtles in this region is the island of Poilão which lies in the Bijagós archipelago off the coast of Guinea-Bissau, where thousands of turtles nest every year.

During the nesting season in 2018, the researchers attached satellite tracking devices on 45 green turtles in Bijagós archipelago, Guinea-Bissau, and tracked their movements over two years. They divided the movement behavior into three distinct periods: inter-nesting (the duration between consecutive nesting events in a season – females lay eggs 3 to 6 times in a single season!), migration (the movement after nesting to foraging grounds or post-foraging to nesting grounds) , and post-nesting foraging (when they are primarily foraging and not breeding). 

The researchers found that during the inter-nesting period, the females spent most of their time around the nesting sites and 95% of their movement was restricted to  the MPA that covered the nesting sites. Post the nesting season, some turtles migrated hundreds of kilometers away from the nesting sites, while some others remained around Guinea-Bissau throughout the year. Thirteen of the tagged turtles traveled as far as 1,000 km. north of the Bay of Arguin, in Mauritania. During migration, most turtles followed the shoreline, while some of them traveled several hundred kilometers offshore. None of the migratory turtles stopped within the archipelago for foraging during their migration. Only 21% of the areas used by migrating turtles overlapped with the MPAs in the region.

Feeding grounds of the West African green turtles

The study also investigated the movement patterns of turtles as they searched for food after nesting. Most of the turtles that migrated to Senegal, the Gambia, and Mauritania spent most of their time (78%) in the MPAs of those countries while foraging.

The study found that the migrant green turtles from Guinea-Bissau connect at least five West African nations. Overall, the nesting and foraging grounds of the turtles are well-covered by the existing protected areas. However, most areas used for migration, the migratory corridors, are not a part of the network. Since migration is seasonal, the authors of the study suggest implementing seasonal, region-specific bans on fishing practices that often cause turtle bycatch mortality.

Beyond green turtles, the findings of the study have important implications for marine spatial planning and the conservation of marine biodiversity in West Africa. While the study highlights the importance of MPAs for the conservation of marine species, it also emphasizes the need for international collaboration and information exchange for the effective conservation of migratory species. The findings of this study can be relevant and informative for policymakers, researchers, and conservationists in other regions facing similar conservation challenges.

Technical summary of the analysis methodology

For the identification of the migratory corridors, the authors used the data from the tracking devices and a movement modeling-based approach called the dynamic Brownian bridge movement model (dBBMM). This modeling approach incorporates temporal and behavioral characteristics of movement paths for estimating migratory corridors and home ranges. The movements of the 45 turtles tracked during the inter-nesting period from August to early December were used to analyze their home ranges and core-use areas using a Kernel density estimation (KDE) technique which plots the extent of the locations where the turtles were found to create a boundary. The KDE’s maps were used to identify the areas that the turtles use the most. This was combined with the dBBMM outputs showing the important pathways between protected areas.

Figure (from the paper): The migratory routes of female green turtles traveling from the nesting island (Poilão) in Guinea-Bissau to foraging areas. (A) Percentage of overlapping migration routes, defined as 95% probability areas estimated using dynamic Brownian bridge movement models. (B) Identified high-passage corridors, defined as grid cells used by at least 25% of migrating turtles. Black solid and dotted lines represent existing conservation areas in the region.

This piece is based on the following study:

A. R. Patrício, M. Beal, C. Barbosa, D. Diouck, B. J. Godley, F. M. Madeira, A. Regalla, M. S. Traoré, C. Senhoury, E. Sidina and P. Catry. (2022, March 31). Green turtles highlight connectivity across a regional marine protected area network in West Africa. Frontiers in Marine Science, Volume 9. https://doi.org/10.3389/fmars.2022.812144