Big question: why are our sea turtles shrinking?

SEA TURTLES in South Africa are getting smaller – and scientists are trying to figure out why and how this will affect future populations.

“What we are finding is the size at reproduction of individual loggerhead and leatherback turtles is getting smaller over time and we are gearing our research to find out what could be the cause,” said Nelson Mandela Metropolitan University’s head of zoology, Associate Professor Ronel Nel, one of the world’s leading sea turtle researchers.

“This could also impact on the population in the future… If they are small, the threat from predators (such as sharks, dolphin fish, kingfish, ghost crabs and sea birds) is greater, as they can’t swim as fast, so can’t escape,” said Nel, who has been conducting sea turtle research for the past 15 years.

She is the regional vice-chairperson of the International Union for Conservation of Nature’s Marine Turtle Specialist Group, an advisory committee member for the Indian Ocean and South-East Asia region Marine Turtles and a member of the Western Indian Ocean Marine Turtle Task Force.

Nel said the benchmark age for loggerhead turtles to reach sexual maturity was 36 years. This is the age the adult females returned to the beaches where they were born to lay their eggs.

“Once a sea turtle reaches sexual maturity, it doesn’t grow anymore… What this means is that those turtles that show a decline in size will never catch up (to traditional size norms).”

Nel said scientists were trying to work out why the females were getting smaller.

“Some scientists think it’s driven by the state of the oceans. There is not enough food around and so it is advantageous for animals to breed at a younger age… Or because there is not enough food around, they are not growing as big as they used to.”

She said changing sea temperatures could also affect tur- tle growth, particularly loggerhead turtles, which were much smaller than the 400kg to 500kg leatherback turtles.

“If the sea is too cold, their growth will slow down, as tur- tles are cold-blooded.”

South Africa’s sea turtles spend a lot of their lives in the cold Benguela current on the west coast of the country.

“Their metabolism depends on their environment – they don’t regulate their own body temperature. Turtles tend to lose energy during the day, when their body temperature is lower, as they swallow lots of cold water when feeding.”

Scientists are also trying to determine the impact of smaller females on the reproduction output.

“Do they lay fewer eggs of the same size or the same number but much smaller eggs? Are the hatchlings smaller?”

Some of these questions are already being answered, thanks to good baseline research in South Africa, spanning the last 40 to 50 years.

“What we are finding is despite the change in the size of individuals, they are laying the same number of eggs but the eggs are getting smaller. The hatchlings are smaller – and the adults will likely be smaller,” Nel said.

“Because South Africa has such a good baseline of research, we can build a whole bunch of new research questions… It is a good springboard for future work.

“However, our research has concluded the problem is not on South African beaches or conservation areas under our jurisdiction. Sea turtles on land in South Africa are as well off as they could be.”

Researchers take the measurements of a female loggerhead turtle, which has come to the beach to lay her eggs.

A loggerhead turtle makes its way back to the sea.