The Odyssey of the Sea Turtle

36-the-odyssey-of-the-sea-turtle_1577084173.jpg

Hawksbill looking into the camera. Photo: - naturepl.com, Inaki Relanzon, WWF.

By Dee Harris

The ancient sea turtle evolved some 300 million years ago in the Triassic era well before the age of dinosaurs. It survived as dinosaurs became extinct, living on through climatic warmings and severe environmental disruption well before the age of modern man. But today we must wonder if the challenges facing the turtle will be too great for its continued survival.

Nearly all species of sea turtle are now classified as Endangered; of the five species that voyage to New Zealand waters the Leatherback and Hawksbill are Critically Endangered while the Green, Loggerhead, and Olive Ridley are Endangered.

Sea turtles have proved difficult to study for, among other things, they travel extraordinarily long distances, and it’s difficult to determine which sex they are. Nonetheless lately several different research projects are piecing together a picture of them which reveals both promise, and a deep concern for their future.

Up against it

Sea turtles confront huge barriers getting their lives started. Before and after they hatch they’re up against a long list of predators, on land and sea, including birds, crabs, small mammals, and fish. After hatching they race to the sea to begin a swimming frenzy to get away from the seashore where their predators are the more prevalent. A small fraction, way less than one percent, will survive to adulthood under natural conditions.

Photo: Andy Belcher, Legend Photography

Then bring homo sapiens into the reckoning. Sea turtles have been a source of trade and culture, livelihood and spiritual supposition entwined in many nations’ cultures since forever. Turtles have been slaughtered for eggs, meat, skin and their shells. And they are still being poached and over-exploited.

One of the biggest threats to sea turtles’ future is demand for turtle meat, primarily in China which is being met from Vietnam, Bangladesh, Indonesia, North America and elsewhere. Aside from the meat, turtle soup and turtle eggs, turtle bone is ground up for Chinese medicine supposedly for longevity. Poachers have been going further afield too; in 2017 federal wildlife inspectors in the US confiscated 170 native species turtles headed for China.

Black markets

For centuries the Hawksbill turtle has been hunted for its shell. (See accompanying item) A recent report studiously derived from Japanese Customs archives estimated that between 1844 and 1992, 19 million hawksbill turtles were killed for their shells. Today official estimates put the remaining Hawksbills to be only 25,000.

Since 1977, CITES (the Convention on International Trade in Endangered Species) has imposed a trade ban designed to protect them but it hasn’t stopped black market demand in China and Japan. Recently confiscated specimens show smaller, more juvenile turtles being harvested. It seems the beliefs in some cultures cannot be overcome in a couple of generations though a species may well be sacrificed in less than that time.

Man takes over habitat

The sea turtle shares with most wild species a loss of habitat brought about by human activity. Coastal development, erosion, pesticides, sewage discharge, oil spill; all have played a role in reducing nesting sites. A study of over three generations of Hawksbill turtles, around 105 years, showed a decline of 90% in the number of females nesting annually. The decline has been going on for decades though higher awareness recently is helping save some prime nesting sites.

Thoughtless human habits have brought newer menace: plastic and other debris ingestion, entanglements, and boat strikes. Over a third of rescued sea turtles are found with plastics in their digestive tracts and of those, half die from the effects. They die painfully. Plastic bags mistaken for jellyfish, one of their favourite foods, are a major threat.

In New Zealand injured turtles are often brought to places like Kelly Tarltons for recovery and rehabilitation. The stomach of a green sea turtle named Nebs, rescued in the Far North in 2011, was full of plastic. The recent decision to abolish plastic bags is certainly a step in the right direction for sea turtles. Nebs spent two years recovering from several surgeries and was then released in 2013 back to the sea with a transponder on board. But his trail soon went cold. Then, in March 2018 he was pulled up by a fisherman near Mangere Bridge, Auckland and after a short period to determine he was still healthy, a group of school children in Kaitaia released him again into the wild.

Photo: Andy Belcher, Legend Photography

Bycatch

The single largest threat to extant pbycatch of commercial fishing. While difficult to quantify because most is never reported, estimates are that 150,000 sea turtles are being killed each year in shrimp trawls while 250,000 are captured, injured or killed by longliners. Many more drown each year in gill nets. (see table)

Combatting this are improvements in fishing gear, and better communication between fishermen where sea turtle beds are known to exist, better and compulsory reporting systems, more education of fishermen, and the continued advocacy and diplomacy work between nations: sea turtles inhabit most of the world’s tropical and temperate oceans, Carlos Drews a regional coordinator for WWF’s expressed it this way:

“The good-will and expertise of fishermen are part of the solution to the bycatch problem. These gentle giants need fishermen to be part of the collective effort to save them for our grandchildren to see.”

Another bigger threat

With all these challenges heaping up against the sea turtle’s ability to survive, the most recent, climate change, is introducing major new issues.

The heat of the sand where eggs hatch determines a sea turtle’s sex. Simplistically put, males hatch at lower temperatures: below 27C with females hatching above 31C. A study led by Dr Camryn Allen looking at green sea turtles in the Great Barrier Reef used genetic markers and ‘mixed-stock analysis’ to determine Green sea turtles’ sex through a combination of laparoscopy and endocrinology. In turtles originating from the cooler southern Great Barrier Reef nesting beaches the results showed a moderate female sex bias (65% to 69% female), But for turtles originating from the warmer, northern part of the reef’s nesting beaches the female bias was extreme (99.1% of juveniles, 99.8% of sub adult, and 86.8% of adult-sized turtles). The results were then matched against temperature data to show the northern reef green turtle rookeries have been producing primarily females for more than two decades, with a near complete feminization of this population upon us. [1] Whereas a shift towards an excess population of females was expected, the extremeness of the findings was not.

“I can’t deny it. Seeing those results scared the crap out of me,” Allen said in the April 2019 edition of National Geographic.

Will there be enough males around to take the species forward, and will there be sufficient genetic diversity for them to survive?

On the bounce back?

If there is a bright side, despite the array of challenges, sea turtleopulations of sea turtles is as numbers do appear to be bouncing back, according to a recent study based at Aristotle University in Greece. That study shows that, among other things, even small populations of sea turtles have the capacity to recover, and that their abundance or absence is linked strongly to the effective protection of eggs, and the nesting females, and reducing the numbers caught as bycatch. The importance of ongoing conservation and monitoring efforts hold the key for their numbers to trend up. [2]

Long distance travellers in the South Pacific In New Zealand waters sea turtles appear to be visiting more frequently and staying longer. Four of the five species found in New Zealand: the Leatherback, Green, Loggerhead, and Hawksbill are sighted regularly, mainly on New Zealand’s eastern coasts between January and April. Part of this is likely due to El Nino weather patterns swinging warmer ocean currents from the tropics down to New Zealand.

But for the most part sea turtles are global travellers. Some species travel much further and for longer periods than others. They appear not to confine their territory but may visit a series of foraging sites long distances apart. The Leatherback and Olive Ridley can travel in excess of 16,000 kilometres a year. Massey University Coastal Marine Research Group researcher Daniel Godoy’s has suggested Green sea turtles are making New Zealand waters a place of permanent residence whereas it was previously thought they ranged here only as part of their outer territory.

“While I accept that New Zealand is on the cusp of Green sea turtles’ preferred range, my research is showing a very different scenario to the waifs-and-strays theory, “ Daniel said. “My data suggest they’re settling here – New Zealand is part of their natural habitat.” The effect of climate change, with an increase of one or two degrees in the water temperature, may be making New Zealand waters a more comfortable place for sea turtles to linger.

In 2018 a group of three, usually solitary, Leatherback turtles was spotted off the coast of Tauranga. The owner of Bay Explorer Island and Wildlife Cruises, Brandon Stone, told the Bay of Plenty Times Weekend that seeing them feeding and travelling together blew apart the theory that turtles came here haphazardly by accidental drift.

What we can do

Sea turtles have survived due to their ability to adapt but today their survival is largely dependent on humankind.

We can choose to interfere for better or worse.

Evidence from researchers with the US Geological Survey has shown that sea turtles survive better when there is less beach debris. In one study for areas where debris was cleaned away sea turtle numbers increased by 200%. Let’s keep plastics off our beaches and waterways, and yes regular beach clean-ups do help.

Any sightings of sea turtles should be reported – the information builds a better picture of the numbers, habitats, and dangers they face in New Zealand. Call the Department of Conservation’s hotline if you see a beached sea turtle. And keep dogs away.

As divers its awesome to spot a sea turtle; without our active care for them our children and grandchildren may not get the same opportunity.

For a list of the references numbered throughout this feature please email us at DiveNZ@divenewzealand.co.nz

scroll to top