Big ques­tion: why are our sea tur­tles shrink­ing?

Weekend Argus (Saturday Edition) - - FRONT PAGE - NICKY WILLEMSE

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.

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