|Sea Turtles of South Africa|
Sea Turtles of South Africa
Of the seven sea turtle species, five occur in South African waters. The green turtle, hawksbill and occasionally the olive ridley forage on reefs but only leatherback and loggerhead turtles nest along the northern beaches along the east coast of South Africa. The nesting area completely falls within the iSimangaliso Wetland Park and is protected and managed by Ezemvelo KZN-Wildlife. South Africa has the longest continuous turtle monitoring program in the world. It was initiated in the 1963/1964 season. Since then, important research has been conducted that has contributed to the conservation of turtles globally.
The Turtle Story
Females come ashore at night, crawl to a suitable site and start digging a body pit. Then, with their hind flippers, they carefully hollow out the egg tunnel and chamber and lay a clutch of eggs. Eggs incubate for roughly 60 to 70 days. Interestingly, it is during this time in the nest that sex determination occurs. Sea turtles don’t have sex chromosomes like humans do. The temperature of the nest determines the sex: average temperatures above 28°C produce mostly females, below, males. Hatchlings emerge as a clutch to share the workload of digging to the surface. They then remain just below the sand surface until nightfall when the temperature drops before breaking the surface. They then scramble down the beach, dodging hungry ghost crabs, towards the ocean. Here a different suite of predators await; large fish, sharks and sea birds snack on defenseless hatchlings.
Hatchlings travel with the currents and feed on soft-bodied creatures such as jellyfish and blue bottles. Not much is known about hatchlings and immature turtles and this stage is known as “the lost years”. Once they have reached sexual maturity, they return to the beach where they hatched to breed.
Loggerheads are classified as Endangered on the IUCN Red list. They have a distinctly large head and are commonly seen on coral reefs feeding on sponges and crustaceans. Females nest along the coast from Cape Vidal into Mozambique, the nesting “hotspot” being at Bhanga Nek. Loggerheads reach sexual maturity at approximately 17 years. They nest 4 – 5 times in a season, every 2 to 3 years. Each clutch contains between 90 and 130 eggs. Hatchlings weigh about 20 grams while adults can weigh up to 250 kg.
Leatherbacks are listed as critically endangered on the IUCN Red list. They can be easily identified as they are the only sea turtles that have a soft shell with longitudinal ridges. Leatherback hatchlings weigh about 40 grams, while adults can weigh up to a ton. Considering they only feed on jellyfish and reach maturity in approximately 12 years, the enormous size they attain is quite remarkable.
Over the last few years students from Nelson Mandela Metropolitan University have been involved in a variety of research projects based at Bhanga Nek. The stretch of beach between Bhanga Nek and Kosi Mouth is the loggerhead nesting “hotspot” and this is where the majority of the research takes place. The beach is patrolled on foot every night and when a female is encountered, great care is taken to work quickly and efficiently without disturbing her. Once the female starts laying her eggs, she is almost in a “trance” and not easily disturbed. General data collected:
Titanium tags are attached to the back part of the front flipper of loggerheads and are used to identify individuals. Each tag has a country code (eg. South Africa is ‘ZA’), a year code (eg. ‘ST’) and a unique number. So a tag may read: ZAST 759. Hundreds of individuals have been tagged and thus their nesting patterns can be monitored over time.
Currently the reproductive biology and sex determination of both leatherbacks and loggerheads is being studied. Temperature loggers are deposited into a number of nests while the female is laying her eggs. These devices record the temperature in the nest at a set time interval and are retrieved once the nest has hatched. Knowing the temperature in the nest during the entire incubation period cab help us better understand the sex determination process. Because very little research has been done on males, the sex ratio of the populations is unknown. DNA samples from hatchlings will shed some light on the activities of males in the South African loggerhead and leatherback populations. Other research involves determining how loggerhead females find their way back to their natal beach to nest after about 17 years of roaming the seas. One of the major cues is thought to be the fresh water from the Kosi lake system seeping into the groundwater and then into the sea. The sources that threaten the survival of these species are also under investigation. There are a number of natural predators on beaches and at sea, but the man-induced sources of mortality, especially fisheries, are threatening turtle populations globally. (Publications out soon!)
Between 1996 and 2006, 11 leatherbacks were equipped with satellite transmitters. The device was first attached to a harness which was then attached to the turtle. Leatherbacks travel thousands of kilometers when migrating. Some of the tagged individuals moved westwards past Namibia while others moved eastwards up the Mozambique Channel (See Luschi et al 2006).
More recently, satellite transmitters were attached to 12 loggerhead females at the beginning of December 2010, and 6 more at the end of January 2011. Attaching the device to the turtle’s shell is quite a complicated process. The shell must first be scrubbed to remove algae before a mixture of glue is used to stick the device to the shell. To prevent growth of algae and barnacles the dried glue is then covered in anti-fouling paint. Once the paint is dry, the female is released. The data from the first set of tagging will show inter-nesting behaviour. The transmitters attached to turtles at the end of the nesting season will show post-nesting migration routes and give us an indication of their overlap with fisheries. Turtles are often caught incidentally in longline fishing gear and trawl nets. The data from the satellite transmitters will be very important in the development of bycatch mitigation measures. Look out for loggerheads with satellite transmitters while diving at Sodwana! (See Peterson 2008 for leatherback bycatch in SA longline fishery).
KZN Wildlife Annual Reports
Luschi P; Lutjeharms J.R.E.; Lambardi P; Mencaccia R; Hughes G.R and Hays G.C. 2006. A review of migratory behaviour of sea turtles off southeastern Africa. South African Journal of Science, 102.
Peterson, S. 2008. Understanding and Mitigating Vulnerable Bycatch in southern African Trawl and Longline Fisheries. University of Cape Town.
By A de Wet
|Last Updated ( Monday, 10 October 2016 14:02 )|