When you imagine an image of a sea turtle, the odds are it would be a Green sea turtle (Chelonia mydas). Perhaps the most elegant swimmer of the seven sea turtles, the green has an inquisitive face with endearing eyes. It’s well rounded shell sheens in the water as greens glide through under water meadows. Despite the flippers looking slightly disproportionate the green fly’s through the water with grace.
Green sea turtles feed and nest throughout the tropics and into subtropic oceans including the North and South Atlantic, Mediterranean, much of the Pacific and the Indian Ocean. They swim and feed in the waters of 140 countries, nesting in around 80 of those. Once plentiful and easy to catch both on land and in the water, green sea turtles supported colonisation of much of the new world through sustenance.
For some time now scientists have argued whether or not we can actually separate the greens into two different sub-species. Greens in the Atlantic are genetically and morphologically different from those in the Pacific. In the past these populations could genetically mix, swimming through North and South America. However, 5 million years ago the Isthmus of Panama was created isolating these populations. The southern ends of Africa and South America are far too cold for the turtles to migrate through, and so these populations remained isolated. Further genetic study shows that the genetic difference is minor, but as we continue through time these populations could slowly diverge into differing species.
Moving on, the greens life history has been well documented through years of rigorous study. Greens migrate to and from nesting beaches, but individuals spend most of their time in small areas feeding on seagrass and algae. They are the only species that eat such a large amount of vegetation. This low protein diet does mean that the greens have very slow growth rates. Maturity can range from 20 years to as late as 40 years old. This life strategy depends on very complex age class survival rates to allow turtles to reach maturity at such a late stage.
Female turtles have to perform one of the most incredible migrations in the animal kingdom, they must leave their feeding grounds and navigate across oceans to their natal beaches to nest. Mature females will lay between 1-7 clutches in a single nesting season. Each clutch averaging 110 eggs, this is very energy consuming and so greens rarely re-nest the following year, sometimes waiting 4-6 years before nesting again. On an undisturbed nest, greens can have a hatchling success of +90%, however these hatchlings face about a 1-1000 chance of reaching adulthood.
Furthermore, greens can live about 19 years beyond maturity (45-59yr), given a female nests 6/7 of those years laying an average 330 eggs a season, she could produce 1900-2300 eggs in her lifetime. Factoring in hatchling success (considering the issues of predation and infection etc) healthy females can produce around 1000-1900 hatchlings in her lifetime. This life strategy of growing slow, maturing late, is what allows turtles to get to a certain size and health in order to produce such an amount of offspring.
In the not so recent past millions of green sea turtles filled the oceans. Reports by early European sailors indicate that, even as late as the eighteenth century, ships that had lost their way to the Cayman Islands could steer entirely by the noise generated by green turtles swimming there to lay their eggs. Columbus discovered the Cayman Islands in 1503 and named them Las Tortugas because the ocean was wall-to-wall with green turtles, ships were constantly bumping into them.
Unfortunately, over the period of just a few hundred years, populations of all sea turtles have crashed dramatically. The green sea turtle gets its name from its green fat and muscles, delicious in soup or as a steak, and this has been the cause for the drastic reduction in green turtle populations. Countless civilisations feasted on the meat of greens. The famous London aldermans soup, a highlight of official banquets until late 20th century had green’s as its main ingredients, was one of Churchill’s favourite dishes.
The decline of the green by 1900 saw populations being a fraction of the historic abundance. Even since this time the population has declined by 50-70%. For example, Michoacan in Mexico had nesting colonies of 25,000 females in 1970s, now there are just 850. The major issue for the greens is the unsustainable pressure of hunting. A single village in Baja California took 150-200 turtles a week. Egg collection still occurs in 45% of all nesting beaches. In Terengganu, Malaysia egg harvest was around 100% in the 1970’s and today is still around 58%. Due to the life history of the green, over harvesting can have long term effects on populations. If adults are harvested, then there are no juveniles to replace them until around 20 years of growth. The effects of large scale egg collection may not be seen for 30-40 years, a lost generation could lead to future population crashes.
The drastic overharvesting of the greens along with the general conservation concerns of sea turtles (feminisation, habitat degradation, plastic pollution and fishing by-catch) leads to real concern over the future of this species. Greens are recognized internationally as endangered (IUCN) and are protected in Appendix 1 of CITES. However, a long list of countries are still legally and illegally taking eggs and adults. Fighting for full protection of nesting beaches country by country and putting pressure on fisheries to adopt the use of turtle friendly practices (the use of turtle excluder devices in trawl nets) must be seen as priorities.
We will likely never see sailboats bumping into herds of migrating green sea turtles again, but we can stop the species disappearing all together. There are hundreds of NGO’s and government run programmes that are doing their best to protect nesting beaches and give baby turtle’s the protection they need to get a head start in life. However, due to the migratory nature of the species, countries must work together to protect the species at all stages of life. Every baby turtle that goes into the ocean counts, but we must work country by country, village by village, beach by beach in order to fully protect the green sea turtle.