April 2009 – New Horizon Sail & Dive

The Green Turtle – Species of the Reef

The green turtle is a common sight around the reefs of Cairns, lazily eating and swimming around the dive sites they make a peaceful presence to the fantastic underwater landscape.

Quick Facts
Breeding Season – Late October to February
Years between breeding – Two to Eight Years
Nesting female carapace length – 107 cm (42 inches) (range from 91-124 cm)
Nesting weight – 130 kg (286lbs) (range from 98-184 kg)
Clutch size – 115 eggs (range from 62-153 eggs)
Hatchling emergence season – December to May
Hatching success – 84%
Hatchling size – 4.97cm (2 inches) (range from 4.02-5.19 cm)
Hatchling weight – 24.83g (0.87 oz) (range from 19.8-28.4 g)
Predators of hatchlings – Crabs, herons, dingoes and fish such as trevally and sharks

Conservation Status – Internationally Endangered — World Conservation Union
Description – Adult green turtles have a smooth, high-domed carapace, are olive green in colour, with occasional brown, reddish-brown or black highlights. Hatchlings have a black carapace with white margins around the carapace, flippers and on the plastron. Green turtles have one pair of prefrontal scales between their eyes.
Threats to Survival – Threats to survival include incidental capture in fishing and shark control program gear, ingestion of synthetic materials, boat strike, predation of eggs or hatchlings at rookeries, incidental capture in dredges, Indigenous harvest of adults and eggs within Australia and overseas and increased incidence of disease (high incidence of fibropapillomas).
Distribution and Habitat – Green turtles are found in tropical, subtropical and temperate waters around the world and appear to be the most abundant of the six species of marine turtle found in the Great Barrier Reef World Heritage Area. They are found in subtidal and intertidal coral and rocky reefs and seagrass meadows of the continental shelf. Green turtles are principally herbivorous as adults, eating mostly algae, seagrass, mangrove fruit and jellyfish.
The proportion of a green turtle population that nests each year is highly variable (up to an order of magnitude difference) and is influenced by variations in the El Nino Southern Oscillation (ENSO) Index. Green turtles are the species of marine turtle for which this correlation has been shown and it may be based upon nutrition.
For green turtles nesting and foraging in the Great Barrier Reef World Heritage Area, tag returns indicate migration to Indonesia, Gulf of Carpentaria, Arnhem Land, Torres Strait, Papua New Guinea, the Solomon Islands, Vanuatu and New Caledonia.
Breeding Areas
There is low density nesting on many islands and along the Queensland coastline. Although genetically distinct nesting aggregations are known, the stocks often occur in the same foraging habitat.

Giant Clams – Species of the Reef

One of our guests’ favourite sights on our Great Barrier Reef Tours are the Giant Clams. They come in various brilliant colours and impressive sizes, colours include bright purples, reds and blues. Divers get to experience the larger specimens up close, but if you are not a certified diver don’t worry you can probably take an introductory dive (FAQ#5).

The giant clam, Tridacna gigas, is the largest living bivalve mollusk. They can weigh more than 200 kilograms (440 pounds), measure as much as 1.2 metres (4 feet) across, and have an average lifespan in the wild of 100 years or more. By day, the clam opens its shell and extends its mantle tissue so that the algae receive the sunlight they need to photosynthesize.

History and Legend
As is often the case with uncharacteristically large species, the giant clam has been historically misunderstood. It was known in times past as the killer clam or man-eating clam, and reputable scientific and technical manuals once claimed that the great mollusk had caused deaths; versions of the U.S. Navy Diving Manual even gave detailed instructions for releasing oneself from its grasp by severing the adductor muscles used to close its shell.
Today the giant clam is considered neither aggressive nor particularly dangerous. The process of closing the shell valves is slow enough not to pose serious threat. Furthermore, many large individuals are unable to close their shells completely.

Conservation Status
The IUCN lists the giant clams as vulnerable. There is concern among conservationists for the sustainability of practices among those who use the animal as a source of livelihood. The numbers in the wild have been greatly reduced by extensive overharvesting for food and the aquarium trade. On the black market, giant clam shells are sold as decorative accoutrements, and the meat, called Himejako in Japan, is prized as an honor.

Giant clams are found in abundance on our dive sites, and make brilliant photos that you are sure to treasure for years. If you don’t have a underwater digital camera, you can hire them through us as well. Just let us know at time of booking or the day before the trip.

Birds of the Great Barrier Reef – Red-footed Booby

A common sight on the reef are the Booby Birds scanning the oceans in search of food. The Booby, a type of seabird, is part of the family Sulidae and the genus Sula. It is closely related to the gannets (Morus), which were often included in Sula in former times.
Boobies are large birds, with long pointed wings and long bills. They hunt fish, by diving from a height into the sea and pursuing their prey underwater. Facial air sacs, under their skin, cushion the impact with the water. Boobies are colonial breeders on islands and coasts. They normally lay one or more chalky-blue eggs, on the ground or sometimes in a tree nest.
Their name is possibly based on the Spanish slang term bobo, meaning “dunce”, as these tame birds had a habit of landing on-board sailing ships, where they were easily captured and eaten. Owing to this, Boobies are often mentioned as having been caught and eaten by shipwrecked sailors, notably Captain Bligh of the Bounty and his loyalists, during their famous voyage after being set adrift by Fletcher Christian and his mutineers.

Birds of the Great Barrier Reef – The Nightjar

Not only do you get to see some great reefs, dolphins, and fish on our trips you also get to experience the birds that inhabit the Great Barrier Reef. One of these seen in the reef is the Night Jar.
Nightjars are medium-sized nocturnal or crepuscular birds with long wings, short legs and very short bills. They are sometimes referred to as goatsuckers from the mistaken belief that they suck milk from goats (the Latin for goatsucker is Caprimulgus). Some North American species are named as nighthawks. Nightjars usually nest on the ground.
Nightjars are found around the world. They are mostly active in the late evening and early morning or at night, and feed predominantly on moths and other large flying insects.
Most have small feet, of little use for walking, and long pointed wings. Their soft plumage is cryptically coloured to resemble bark or leaves. Some species, unusual for birds, perch along a branch, rather than across it. This helps to conceal them during the day. Bracken is their preferred habitat.
Nightjars lay one or two patterned eggs directly onto bare ground. It has been suggested that nightjars will move their eggs and chicks from the nesting site in the event of danger by carrying them in their mouths. This suggestion has been repeated many times in ornithology books, but while this may accidentally happen, surveys of nightjar research have found very little evidence to support this idea.