Over Ten Years of Sea Turtle Nest Monitoring on Klein Bonaire
– by Dr. Sue Willis (Sea Turtle Conservation Bonaire)
At about the same time as Willem Alexander was being crowned King of the Netherlands, a female loggerhead sea turtle (Caretta caretta), weighing around 150 kilogrammes and measuring over one metre in length, laboured onto the shoreline of Klein Bonaire to lay her first nest, kick starting the 2013 nesting season on Bonaire. Thus began the eleventh consecutive year that Sea Turtle Conservation Bonaire (STCB) staff has monitored nesting activities on Klein Bonaire, Bonaire’s index beach.
Sea Turtle Conservation Bonaire’s mission is to ensure the protection and recovery of Bonaire’s sea turtle populations throughout their range. Index beach monitoring is an important STCB activity as, although Klein Bonaire holds a relatively small nesting colony, its protection is critical in maintaining hatchling production and genetic diversity of the species. Additionally, monitoring provides important information over time about population status, allowing inter-annual comparisons. Annual fluctuation in nesting activity is normal in marine turtles, as each female typically nests every two to three years, so this long-term data collection is the only true indication of the health of Klein Bonaire’s nesting turtle populations.
Visitors to Klein Bonaire may notice that at every 50 metres along No Name Beach, are transect posts, placed by STCB to record the exact position of each sea turtle nest found during their thrice weekly patrols of this nature reserve. Starting very early in the morning, when possible turtle tracks are still fresh in the sand, staff and volunteers walk along the two kilometre index beach and record every turtle activity seen. This includes:
- “false crawls” — where a female turtle has simply walked onto the beach
- “attempts” — where she has begun the nesting process by making a body pit or begun to dig but encountered stones or sand too soft to lay her eggs in, and
- “nests” — where 100 to 200 eggs, dependent on species, have been laid.
An important aspect of the monitoring and conservation work of STCB is to confirm whether the turtle activity is a definite nest and exactly where the eggs have been laid. This is time consuming, skilled work, as sea turtles have survived for 20 million years due in part to their ability to perfectly conceal the location of their eggs. Trained STCB staff will study the sea turtle tracks or “crawl” looking for clues, working out which turtle species has come on shore and where the eggs are most likely to be located, based on where the cover sand is piled, marks on vegetation and even the colour of the sand. Having established the likely position, STCB staff, often with the help of trained volunteers/interns, will carefully dig down around 50 centimetres into the sand to confirm the nest, until one of the eggs is found. At that point digging stops, details of the nest location are recorded and marked, both manually and with GPS, and sand is quickly replaced to allow the eggs to incubate naturally for 52-60 days until they hatch.
Once a nest has hatched, STCB staff dig up and investigate the remaining empty nest, noting numbers of empty egg shells, dead eggs, and remaining dead (or live) hatchlings. From this data the nest survival statistics can be calculated.
Frequently, particularly in hawksbill nests, hatchlings are found trapped in the nest after their siblings have left. They may be perfectly healthy but have become tangled in tree roots as they made their way to the top of the nest and so release of these critically endangered hatchlings is a valuable conservation activity, and a very rewarding one for staff and volunteers. For example, in 2012, 1,473 hatchlings were released during nest excavations on the index beach at Klein Bonaire alone. Research indicates that of every 1000 eggs laid, only one will continue to become an adult breeding turtle so conservation measures at every stage of the sea turtle’s life history are vital to the survival of the species as a whole.
Since 2002, when nest monitoring began on Klein Bonaire, STCB has encountered three sea turtle species nesting, the Hawksbill (Eretmochelys imbricata), the Loggerhead and, in 2012, for the first time a Green Turtle (Chelonia mydas) nested on the index beach. Data collected over the past ten years of nest monitoring indicates that 48,304 sea turtles have hatched from nests laid on Klein Bonaire
– by Sea Turtle Conservation Bonaire
In an attempt to learn more about the behavioural patterns and habitat use of Hawksbill turtles (Eretmochelys imbricata) at Lac Bay, Bonaire, Sea Turtle Conservation Bonaire (STCB) performed abundance surveys and, with funding from Wageningen IMARES UR, deployed dataloggers on the carapace of four Hawksbill turtles in 2012.
Three of these dataloggers were retrieved by the end of 2012 and showed some valuable results. In January 2013, one more Hawksbill was fitted with a datalogger and that one has been retrieved in July and it is being analysed now. A preliminary assessment of the data seems to confirm the patterns found with the other three turtles.
The dataloggers are programmed to record depth every 5 seconds and obtain GPS coordinates whenever the animal comes up to the surface to breathe. The three dataloggers already recovered in 2012 were analysed and yielded detailed data on Hawksbill behaviour in and around Lac Bay, revealing that these turtles regularly move in and out of the bay. When outside the bay, the animals adhere to a diurnal pattern of resting at night and activity during the day. When inside Lac, such a diurnal pattern is more difficult to perceive due to the shallow waters and limitations concerning depth resolution of the dataloggers, but it appears that a similar pattern is maintained.
Of particular interest to STCB researchers is this new evidence that Hawksbills appear to reside inside Lac Bay and feed where dense seagrass beds are and near the mangroves, presumably eating organisms, such as sponges, associated with the seagrass stands and the mangrove roots. It remains unclear whether Hawksbills actually enter the mangroves to any extent.
Conserving Sea Turtles
Sharing Sea Turtle Knowledge
Season of the Sea Turtle