There are seven species of sea turtle around the world. While they each occupy different parts of the ocean, their territories sometimes overlap and they all exhibit migratory behaviour.
Green turtles (Chelonia mydas), shown in Figure 1, are listed as Endangered by the International Union for the Conservation of Nature. They are found throughout the Caribbean, yet relatively little is known about the behaviour of individual groups of animals that populate distinct island groups, which distribution and genetic studies suggest are distinct sub-populations.
They have an extraordinarily accurate ability to travel long distances, sometimes through unknown waters, in search of their preferred areas for breeding and feeding. Their navigation system principally depends on the Earth’s magnetic field (which we discuss in Chapter 30), coupled with olfactory signals and information from wind and water currents.
Green turtles in tropical and sub-tropical zones have land-based nesting sites. They migrate between these and their preferred foraging sites offshore where they graze on sea grass and algae. In the Caribbean, nesting takes place on a two-year cycle, with eggs laid between July and November.
To manage and protect green turtles, conservationists would like to understand more about their movements between islands, especially between the sites where they forage and nest. Green turtles appear to favour specific locations for egg laying. Their main food is sea grass, and this too needs to be managed if the green turtles’ habitats are to be protected.
One group of green turtles is found at the Buck Island Reef National Monument in the St. Croix region of the south Caribbean, which is part of the U.S. Virgin Islands. Buck Island itself is a small, uninhabited island surrounded by waters 15–20m deep. Its sea grass beds support loggerhead, leatherback, and hawksbill turtles, as well as green turtles. Starting in 2011, Kristen Hart and colleagues studied this group of green turtles, using satellite tracking to follow their movements and foraging behaviour in the non-breeding season.
Ten adult female green turtles from Buck Island were fitted with tracking devices. They were intercepted on the beach shortly after nesting, individually identified with a device attached harmlessly to the carapace (upper shell) with epoxy resin. Tracking continued for four years; some of these results can be seen in Figure 2.
As anticipated, the turtles fed in shallow, near-shore waters. Individual turtles showed great consistency in the locations they chose for foraging, and also in their more distant migrations, providing evidence of their skilful, accurate navigational abilities.
Of the ten turtles, seven remained locally resident after the nesting season (which you can see in Area A on the top left map in Figure 2): five foraged around Buck Island (Map A, blue striped area) and two used regions near Buck Island and at the other end of St. Croix (Map A, red spotted area).
The other three turtles made longer distance migrations (Map B and yellow triangle track symbols on top left map in Figure 2): one migrated to St. Kitts & Nevis (200 km) and one travelled to Antigua (300 km), while the third went to an island north off the coast of Venezuela (700 km; Map C and green circle track symbols on top left map).
These studies reveal the foraging behaviour of the green turtle and highlight the need to protect crucial sea grass beds in areas close to the shorelines of the islands, which make up the animals’ habitat. They also show that some green turtles migrate greater distances than others. This means that habitat management strategies need to include areas further away from the expected home range.
Observations of highly consistent breeding behaviour, centred on a single location but showing a mixture of local and distant foraging preferences, show that individual movement patterns can vary within small populations of animals. Other turtle species in different parts of the world, such as loggerhead turtles (Caretta caretta) in the Mediterranean and Kemp’s ridley turtles (Lepidochelys kempii) in the Gulf of Mexico, have similarly well-defined breeding sites but individually diverse migration routes in the non-breeding season.
Overall, the conclusion from satellite-based tracking studies is that individual animals travel repeatedly and reliably to the same sites for feeding and breeding. They are excellent navigators and find their preferred destinations with great accuracy. This kind of information is of great importance in understanding animal lifestyles. It is especially valuable to biologists interested in habitat conservation and the protection of endangered species.