#ThreatenedSpecies of the Week: Chelonia mydas — Green Turtle

Photo Credit: Department of Parks and Wildlife Western Australia.
Photo Credit: Department of Parks and Wildlife Western Australia.

Chelonia mydas (or Green Turtle) is listed as Vulnerable under the Commonwealth Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act) and as Specially Protected Fauna under the Wildlife Conservation Act 1950.

In Australia, the main current threats to Green Turtles are disturbance (e.g. light disturbance) and habitat damage due to coastal development; by-catch from fisheries and shark control measures; predation on nests; boat strikes; entanglement and ingestion of marine debris; and in some areas, indigenous harvesting. Potential threats include changes to the sea surface temperature, particularly changes to the Southern Oscillation Index, which determines breeding intervals; chance disasters (e.g. oil spills); and feral predator invasions.

The Green Turtle has an olive green, nearly circular or heart-shaped carapace (upper shell) up to 1 m in length. The carapace is usually variegated with brown, reddish-brown and black on the top and whitish or cream underneath. There are four pairs of costal shields (shell plates located on either side of the mid-line) between the centre and outer margin of the upper shell. Hatchlings are shiny black above, and white below. They weigh about 25 g and measure 5 cm (straight carapace length).

The green turtle is found in all of the world’s tropical and warm-temperate oceans. Green turtles are quite often seen in northern WA in areas such as Ningaloo Marine Park, Shark Bay Marine Park, the proposed Dampier Archipelago Marine Park, Lalang-garram / Camden Sound Marine Park and sometimes also in the waters of southern areas such as Marmion and Shoalwater Islands marine parks and even as far south-east as King George Sound in Albany.

Female turtles only breed once every six years or so. At mating time, males cluster around and compete for individual females, which inevitably breed with more than one male. Within a short time, the female lays her first parchment-shelled eggs on the beach, repeating this every fortnight up to six or even eight times. The sex of the hatchlings depends on the temperature in the nest – hatchlings on the warmer northern beaches are mostly females, but hatchlings that emerge from beaches further south are mostly male. The hatchlings dig their way out of their nests and journey to the sea from January to April. Crawling the short distance between the nest and the sea is a dangerous journey and huge numbers of the youngsters are killed and eaten by foxes, silver gulls and birds of prey on the way.

Green Turtles spend their first five to ten years drifting on ocean currents. During this pelagic (ocean-going) phase, they are often found in association with driftlines and rafts of Sargassum (a floating marine plant that is also carried by currents). Once Green Turtles reach 30 to 40 cm curved carapace length, they settle in shallow benthic foraging habitats such as tropical tidal and sub-tidal coral and rocky reef habitat or inshore seagrass beds. The shallow foraging habitat of adults contains seagrass beds or algae mats on which Green Turtles mainly feed

Adult Green Turtles eat mainly seagrass and algae, although they will occasionally eat other items including mangroves, fish-egg cases, jellyfish and sponges. Young turtles tend to be more carnivorous than adults. During their pelagic phase (while drifting on ocean currents), young Green Turtles also eat plankton.

Information Source: Government of Australia, Department of Environment and Department of Parks and Wildlife Western Australia

Related Posts

Leave a reply