Sea turtles have been roaming the Earths oceans for over 100 million years and currently have been limited to a mere seven species (14). All of these species are currently labelled as vulnerable or endangered and are the focus of many conservation management programs (6). The green turtle, Chelonia mydas is one such endangered species, found not only in Australia but world-wide.
The green turtles declining numbers have been a concern for some time and this paper not only provides an overview of the turtle but also addresses the issues surrounding the reasons for decline in population numbers and the current attempts to reverse the effects relating to its endangered status.
The green turtle is listed relating to its taxonomy as Animalia, Chordata, Reptilia, Testudinata, Cheloniidae, Chelonia mydas (15) as per Linnaeus in 1758. The C. Mydas has only the one common name of green turtle. There are two taxonomic families relating to marine turtles. These include the Cheliniidae and the Dermochelyiidae, with six and one species belong to each respectively (14).
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Despite the common misconception that the green turtle is named after it shell-colour, its name originates from the green fat deposits found surrounding their internal organs (8).
Green turtles can grow up to 1.5 metres long and can weigh up to 200 kg however these sizes vary due to their different habitat and environmental pressures (8). The green turtle is one of the biggest marine turtles despite its smaller head (REF). Its carapace is usually a circular or heart shaped and can be up to 1m in length (15). Shell colours can vary between green, brown, reddish-brown and or black on top; with usually a white or cream hue underneath (15). It has also be described by (15) that there are four pairs of shell plates located on either side of the middle of the upper shell, called costal shields.
Eggs when examined in their nest, located on specific beaches, have a mean diameter of 4.6 cm (5).
Hatchlings have a carapace of black and pure white below (15). They have been measured to be approximately 5cm in length and weigh 25g (15).
The difference in sex can be identified by the curve of the carapace length (MORE ) (8).
In a controlled experiment it was shown by (3) that the growth rates of the green turtle can be as varied from 0.1 to 10.cm/yr, with a mean average of 4.2cm/year, this being due to difference environmental pressures and other contributing factors, such as diet.
The green turtle is an herbivorous reptile, with its diet mostly consisting of seagrasses and algae (5). However juveniles are carnivorous, their diets not only consisting of plants but jellyfish and small invertebrates (5). The main plant consumed by the C. mydas is the Thalassia grass, which they maintain by continuously re-cropping (3). Most of their forage fodder is found at least 2m below the ocean's surface, thus they must dive for their food (1). Researchers Hadjichristophorou and Grove found that on a daily basis the turtles only ate 0.24 and 0.33 % of their body weight (9). The green turtles digestion, as it consists mainly of leafy greens, is aided by cellulose based microflora present in the gut (9). The green turtle has found to be as efficient as an ruminant with respect to fibre digestion (10) with the cecum as the initial site of cellulose based activity. The gut fermentation that occurs in the turtles provides a large amount of energy with the colon as the main active site of fermentation (10).
As well as travelling long distances it has been measured that green turtles can reach speeds up to 24 kph. They have a very efficient respiration system which is essential as turtles swim for long periods of time and are subject to various pressures as they dive. The turtle enables a rapid exchange of oxygen to all tissues at extreme rate despite the external pressure. The turtle's ability to hold its breath is compromised by strenuous activity and stress. Thus when turtles are caught in fishing nets they can drown in a relatively short amount of time.
A female green turtle will reach sexual maturity at approximately 8.95 years old (4). Their trend follows such that at intervals of 2 to 10 years, the female will migrate from their designated feeding ground to their nesting rookeries (2). The travelling between these two areas can be anywhere between 100 to 1000 km (2). The females will utilize many beaches but their choices are specific and they can leave many suitable beaches desolate (2). In any one nesting season a female has the potential to lay two to seven clutches of eggs, with each clutch consisting of roughly 100 eggs, after which they will return to the feeding grounds (2). It is unknown why but the females will return faithfully to the same nesting sites, or rookeries, the following breeding season (2).
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In the breeding season the female will undergo a certain 'heat' period, which lasts between two to four days, where she will become sexually receptive to male advances (4). The green turtle also follows a form of 'courtship' prior to mating. The courtship consists of many actions and expressions including floating high in the water, females will circle to face the male head-on, participation in active pursuit and the pressing of hind flippers over the cloacal region (4).
It has been found through extensive research by (12) that turtle egg sex is temperature dependant. It was shown that higher incubation temperatures, above 30'C, produce more females and lower temperatures, between 24-28'C, will result in more males (12).
After the hatchlings emerge from their nest, they make their way to the ocean and proceed to migrate offshore in what is called a 'swimming frenzy,' which lasts between 24 to 36 hours (1). These juveniles will then spend their first few years in an oceanic habitat before proceeding to a sub-adult stage then they migrate to lower ocean depths where they stay for approximately a decade before reaching maturity (14).
The green turtle inhabits both tropical and subtropical areas in every major ocean (2). It prefers to remain in 20'C isotherms but will venture into deeper water (2). The types of areas the turtle inhabits include the feeding ground, mating areas, inter-nesting habitats and pelagic waters (6). Through tracking experiments it has been shown by (2) that hatchlings and juveniles travel and inhabit several different habitats before reaching maturity, these movements are extensive and random and can be difficult to track.
The Chelonia mydas has a complex life path which is difficult to follow as they involve large spatial and temporal scales (2). It is much easier to keep track of female green turtles as they come ashore during nesting season, males however rarely leave the ocean, thus most of the available research has been done on nesting females (2). It would be beneficial to attempt to research more on the males, especially their fertility as it would be a serious contributing factor to any conservation management plans.
It has been shown by (10) that green turtles, and other species of marine turtles, use the Earth's magnetic field as a source of directional information when travelling extreme distances. They learn the magnetic topography of their feeding grounds and maintain a 'magnetic map' allowing them to return to certain places visited before, it is not however the only directional aid used, they also use unknown non-magnetic cues (10).
In Australia, the green turtles most frequented breeding sites are the Great Barrier Reef, the north-west side of Western Australia and the Wellsley Islands (5). The turtles and their close relatives have been utilized for many purposes, including as a meat source, by costal cultures over thousands of years, however recent research by (2) has shown that due to over harvesting in the last four centuries, several populations have been driven to extinction and greatly reduced others.
Before the arrival of Europeans, it has been shown that green turtle populations were controlled by food abundance levels (3), this density dependent effect would have naturally regulated the populations. The decrease in turtle numbers since the Europeans arrival has been estimated to have reached approximately 99% (3). The dramatic declines and extinctions are a result of serious over exploitation (3).
REASONS FOR DECLINE
Every species of marine turtle are classed to varying degrees as endangered, due to a broad range of reasons. They include: by-catch of commercial fisheries, nest disturbance, marine pollution, Indigenous harvest, predation and damage of eggs by wild animals such as dogs, raccoons, pigs and goannas (5).
There is also a certain degree of harvest, for both their eggs and meat, occurring by neighbouring countries; other contributing factors include disease, coastal development and loss of habitat (6). Marine turtles are prone to become inflicted with a disease known as fibropapillomatosis, which results in multiple tumours on the skin and also on internal organs (6). Such tumours inhibit the turtle's ability to swim, forage and breathe and are a contributor to the mortality rate (6).
There are many current management plans in place to protect all Australian marine turtles. However in the case of the C. mydas there is lacking research done on the male green turtles, especially in terms of their fertility which would be a major contributing factor to increasing the green turtle populations.
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In 2003 a national recovery plan was implemented to reduce the mortality of all marine turtles (5). These recovery strategies aim to address the issues surrounding the many causes for decline in the green turtles. These problems include maintenance of sustainable harvest, implementing monitoring programs, improving reproductive success, protecting their habitat and increasing international communication with countries of the same mind (5).
These plans are executed though education of the public and children to not disturb nesting areas, the use of turtle excluder devices, laws and legislation implementation to reduce the amount of land clearing, urban and industrial development in critical areas (6). The estimated cost of recovery for all marine turtles is $5.64m over a five year period (6).
The green turtle has been described as one of the most economically important reptiles, as its meat and eggs are one of the main sources of protein for third world countries. The methods currently in place are proving to be successful. Of the current research done on the green turtle it shows that although the populations are improving there are still areas of research that need to be conducted to improve these management successes and awareness needs to the encouraged so numbers do not decrease again.