Growth rates and movement patterns of immature turtles foraging in Turtle Cove at Diego Garcia

by ZSL on

At the end of the November/December 2018 sea turtle expedition to Diego Garcia, British Indian Ocean Territory (BIOT), Jeanne Mortimer reports on some of the work achieved with immature foraging turtles.

We have continued our long-term study of the large aggregations of juvenile sea turtles at Turtle Cove. These mostly comprise Critically Endangered hawksbill turtles along with small numbers of Endangered green turtles. Turtle Cove provides a unique feeding habitat that includes three rocky tidal creeks located at the extreme south end of the inner lagoon of Diego Garcia island, approximately 24 km (15 miles) from the northern entrance to the lagoon. The turtles apparently feed on algae and small invertebrates living on the floor of the Cove and on rock surfaces. To protect this food source and minimize stress to the turtles, no one is allowed into the water at Turtle Cove except by special permission to conduct scientific research on the turtles. Base personnel can visit Turtle Cove at any time, however, to watch the turtles from viewing platforms constructed in 1999 specifically for this purpose.

Turtle cove on Diego Garcia
Location of Turtle Cove at Diego Garcia Atoll in Google Earth view (left); One of the two viewing platforms constructed to enable turtle to be viewed without disturbance (right).

In 1996 I began studying the Turtle Cove turtles by catching individual animals, weighing them, measuring the dimensions of their shells, and applying numbered metal tags to their front flippers so they could be recognized again if re-captured. Surveys of the Turtle Cove turtles, each lasting from four to ten days were conducted in 1996, 1999, 2006, 2012, and twice in 2018 (in June-July; and November-December).

Jeanne measuring and weighing turtles. Turtle Cove, Diego Garcia, BIOT
Jeanne measuring and weighing turtles captured in Turtle Cove

To maximize the numbers of turtles captured during the survey period, we engage the assistance of volunteers stationed at Diego Garcia. During our most recent survey conducted during 23-26 November, each day we were assisted by up to 11 volunteers and engaged 42 individual volunteers. This enabled us to capture a total of 49 turtles (30 of which had previously been flipper tagged, measured and weighed during earlier surveys) and to assess the growth rates of the turtles living in Turtle Cove.

2018 Nov. Volunteer briefing. Turtle Cove, Diego Garcia, BIOT
Nicole briefing volunteers prior to heading off to catch turtles in Turtle Cove (left); volunteers searching for turtles in Turtle Cove (right)

Amongst our most interesting captures in November 2018 was a previously tagged adult male hawksbill with a shell length of 66 cm (26 inches) and a tail length of 26 cm (10 inches).  All immature sea turtles and all adult females have short tails; and only adult males have long tails.  It turns out that this turtle was originally tagged in 1999 (almost 20 years ago!), when he was a small turtle with a shell length of only 41 cm and a tail only 7 cm long. Our long-term studies have shown that individual turtles may reside at Turtle Cove for more than 20 years, and during that time grow slowly—averaging only about 1.35 cm (0.5 inches) per year. While limited food availability at Turtle Cove may result in slower growth amongst the turtles, they benefit from the absence of large predatory sharks.

2018 Nov. Turtle tags and tail length. Turtle Cove, Diego Garcia, BIOT
(clockwise from top left): Comparison of long-tail male (left) and short-tailed animal (right). Turtle intercepted in November 2018 with flipper tag that had been applied in 1999 (bottom)

In November 2018, Dr. Nicole Esteban attached satellite tags to 10 juvenile hawksbill turtles—bringing the number of satellite-tagged Turtle Cove turtles to 20 (including the 10 to which Prof. Graeme Hays and Nicole applied satellite tags during our expedition in June/July 2018). Satellite telemetry has shown that although most turtles spend their time in the vicinity of Turtle Cove, a few of the tracked turtles departed the inner lagoon and travelled along the outer coastline of Diego Garcia and even beyond.

2018 Nov. Releasing tagged turtles. Turtle Cove, Diego Garcia, BIOT
Volunteers preparing to release flipper-tagged turtles (top) and satellite tagged turtle (bottom)

How do the diets and growth rates of turtles living inside and outside the lagoon differ?  Do the Turtle Cove turtles eventually migrate away from Turtle Cove permanently?  What about the long-tailed males? At what point will they venture out to find female turtles, and where will they go to find them? Will they ever return to Turtle Cove? For every question about the turtles that we answer, new ones arise in need of answers!

This research was kindly funded by the Bertarelli Foundation as part of the Bertarelli Programme in Marine Science.

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