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22/01/2019

Have South Africa’s leatherback and loggerhead sea turtles been displaced from their optimal habitats by human activities or by changing climatic conditions?

This is the question that Professor Ronel Nel from the Department of Zoology is investigating for her Pew Fellowship, awarded for unique research that informs better management and conservation of the world’s marine life and oceans. Prof Nel is one of eight scientists and conservationists selected from seven countries for 2018.

“To inform conservation planning, my research is testing the applicability of the ‘refugee species’ concept with regards to two sea turtle species in South Africa – the South Western Indian Ocean leatherback (Dermochelys coriacea) and the loggerhead (Caretta caretta) nesting along the iSimangaliso coast of northern KwaZulu-Natal,” Prof Nel explains.

“We know from displaced migratory populations on land, also known as ‘refugee species’, that they may not be able to recover their population sizes when displaced to inappropriate or low-quality habitat, even in the presence of adequate protection.”

The refugee species concept is relatively new, starting in about 2012 with the European bison when it was found that their current distribution is in areas such as forests, where they were forced to retreat as a result of extensive hunting. These are in fact suboptimal foraging areas for these grassland animals. The same concept appears to apply to the Cape mountain zebra, which zoology professor Graham Kerley is researching.

Prof Nel has been researching sea turtles since 2002 as part of the iSimangaliso Turtle Monitoring Programme, which dates back 55 years. She is comparing the two turtle species’ habitat use and population dynamics along the protected iSimangaliso coastline in northern KwaZulu-Natal (an area of about 180km) to test whether this concept can be applied to marine environments.

“A large proportion of the work I have done, together with Ezemvelo KZN Wildlife, is to try to understand why the turtle populations in the iSimangaliso area are not recovering,” Prof Nel explains.

There are currently about 70 female leatherbacks nesting per year in iSimangaliso and about 1000 loggerheads – both are modest populations. Leatherback turtles have remained critically endangered in the region, despite being protected for 55 years. South Africa only has the occasional illegal harvesting of eggs, unlike in neighbouring Mozambique or Madagascar where poaching of eggs and turtles is more rampant. 

“The leatherback is far larger than the loggerhead; the shell length of the loggerhead is 80–90cm, while the leatherback is double the length at approximately 160cm. The literature says that both species can live to ±150 years, and I have worked with a number of turtles that are ±60 years old. Leatherbacks are very different from other sea turtles and the only one left in the family.

It is a more ancient species, and has been around for at least 100 million years, dating back to the Cretaceous era of the dinosaurs. “The low numbers for leatherbacks don’t make sense; leatherbacks produce almost twice as many eggs in a season as loggerheads. Both nest in our summer between mid-October and mid-February and leatherbacks come ashore up to eight times in a season to lay 80–100 eggs at a time, while loggerheads come ashore only four times in a season. It may be that the survival of the hatchlings which emerge from early January into April, is different between the two species.

“Our long-term monitoring suggests the incubation environment is favourable so that hatching and success is very good, but what we don’t know is the health and body condition of the females when

they come ashore. So we are going to draw blood from a sample group to see if there is an increased amount of stress hormones in leatherbacks, which could suggest the condition in their foraging grounds is not ideal. To test this further, we will compare these blood metrics to other populations where we know the species are doing well.”

Prof Nel goes on to explain that the leatherbacks’ lack of recovery can partly be attributed to their evolutionarily programmed nesting habits, where they very rarely turn back once they have exited the sea to nest, irrespective of whether there is too much light or human presence: “The hormone cascade kicks in and there is no turning back, almost like a woman in labour. The loggerheads, by comparison are skittish, scope out the high shore from the low shore and may come out later or on a different part of the beach.”

Turtles are not only threatened on the coast, however; the biggest threat offshore to leatherbacks and loggerheads is fisheries. Both species are attracted to bait or light sticks used in commercial longlining and so get hooked when they go for the bait, or they get entangled in artisanal or  commercial fishing gear including coastal gill nets. The greatest “new” threat is plastic pollution.

As airbreathing marine reptiles, sea turtles are very vulnerable,because they spend most of their time in the top 20m of ocean. Plastic floats on the surface and when the turtles come up to breathe they see plastic, often blue, white or transparent in colour and accidentally swallow it, because the bits of plastic are the same colour as bluebottles and jellyfish – a main part of the sea turtle diet for the first ten years of their life, and the exclusive diet of leatherbacks.

“These threats are certainly contributing factors, but the lack of recovery needs additional research. The largest population of loggerheads in the world is off the coast of Oman, where they have 20 000 nesting females annually, and the largest population of leatherbacks in the world is off the coast of Gabon where they have 30 000 nesting female leatherbacks annually. Both populations face the same threats as the iSimangaliso population but their numbers are staggering by comparison.”

Most turtles are highly migratory, so they leave their national waters after hatching and again after nesting to go and forage in areas off other countries. This trait makes conservation of these species far more difficult as it requires international agreements and cooperation for the protection of migratory species. 

Prof Nel is now trying to determine whether the iSimangaliso leatherbacks and loggerheads are unique founder populations or part of populations that were bigger in the past. “It could also be that they have become environmental refugees, forced to lay their eggs along a part of the coast that is not ideal, or it could be that the habitat into which we forced them is not nutritious enough for optimal growth and reproduction. We may need to apply for additional protected areas with better foraging habitat. These areas may be outside of South Africa’s exclusive economic zone and will require international cooperation. If our sea turtles are not refugee species, it means the pressure is entirely human induced – fishing pressure, plastic and other pollution, or habitat destruction.”

Contact information
Prof Ronel Nel
Associate Professor
Tel: 27 41 504 2024
ronel.nel@mandela.ac.za