There are two species of marine turtle that can be found around on Ascension – the green turtle (left) and the hawksbill turtle (right)
Ascension Island has the second largest nesting population of the green turtle in the entire Atlantic Ocean and the largest nesting population of any marine turtle species in all of the UK Overseas Territories.The population has been the subject of a long-term population monitoring programme spanning 35 years, through the collaborative efforts of many researchers, island volunteers and funding bodies. During the nesting season, the number of tracks and successful nests on the beach from the previous night are counted and recorded. This is done every year for the two major nesting beaches – Long Beach (in Georgetown) and Pan Am (below the US Air Force Base) – with a full census of the Island’s 32 beaches being undertaken every few years. The most recent census was carried out during the 2011/2012 nesting season supported by funding from the Overseas Territories Environment Programme (OTEP).
Turtle tracks on Long Beach during the height of the nesting season.
Although counts of tracks and nests give us a good indication of how the green turtle population is faring overall, they do not give us an absolute measure of population size because individual females nest more than once in a season. To estimate how many turtles nest at Ascension we therefore need to divide our track counts by the number of clutches laid by each female, often known as the ‘clutch frequency’. Previous work using flipper tags suggested that each turtle nests 3 times on average. However, a project in 2012 supported by a JNCC Research Contribution Grant deployed radio-transmitters on 40 turtles, which allowed them to be easily relocated each time that they nested. It was found that on average, each female lays 6 clutches of 120-150 eggs per season – double the previous estimate. Thus, whilst there are fewer turtles here than previously thought, this highlights just how productive and important each individual is to the population.
A radio-telemetry tag attached to the carapace of a female green turtle. When the turtle comes ashore to lay her eggs the transmitter emits a unique radio signal that can be detected with a hand-held receiver. This allowed researchers to relocate individual females each time that they returned to nest during the season.
We are currently analysing the 35 year dataset on green turtle nesting to see how numbers have changed over time. Since 1977, numbers of nests on Long Beach have increased exponentially from around 1,000 to almost 10,000. This is an encouraging trend indeed, and charts the recovery of the green turtle population following the cessation of harvesting for meat in the 1930s.
The use of satellite-telemetry tags has allowed us to investigate the movements of green turtles in real-time. They operate by broadcasting a GPS signal of their location to a satellite each time they surface to breathe, which can then be downloaded to a computer. It has been well established that the green turtles nesting on Ascension migrate from their feeding-grounds off the coast of Brazil every 3/4years to lay their eggs – a journey that takes approximately 5/6 weeks.
Satellite map showing Destiny’s (named by the children of Two Boats School) route across the Atlantic from Ascension to Brazil (from www.seaturtle.org).
Recently we deployed satellite tags on the females at the start of the nesting season to investigate where they go in the two weeks between laying their successive clutches. We found that they did not venture very far away from the nesting beach. Green turtles do not eat while they are at Ascension as their regular food sources, seaweeds and sea grasses, are not found here. Thus, by resting close to shore in between their nesting events it is likely that they are conserving energy for hauling themselves up onto the beach at night to lay their eggs, and also for their migration back to Brazil at the end of their nesting season.
Left: A satellite-tagged turtle nesting on Long Beach. Right: The GPS locations given by the tags showed that the turtles stayed very close to shore with little movement.
In addition to Ascension Island’s famous green turtles, smaller numbers of critically endangered hawksbill turtles are also found in the nearshore habitats of the Island. Hawksbills have never been recorded nesting on Ascension, and from their sizes it seems likely that the individuals found here are sexually immature juveniles. Since 2003, a sightings programme has been in place, enabling members of the public to report sightings of hawksbill turtles. Although biased by observations from those sites heavily used by humans, the map below illustrates the presence of hawksbills around most of the coastline of Ascension Island.
Location of hawksbilll turtle sightings at Ascension Island, 2003-2012. Dots scaled according to number of sightings as per the key.
From 2003-2009 staff from AI Conservation Office and recreational divers captured and tagged around 30 hawksbill turtles to study how long they spend around the Island. So far four of these turtles have been recaptured, with an average time between tagging and recapture of over four years. During this time the turtles had grown an average of about 3cm per year suggesting that Ascension may serve as developmental habitat for juvenile hawksbills until they move on to their adult foraging and/or nesting sites, likely to be in Brazil or West Africa.