April, 2008
Whale sharks & life on the reef
Whale sharks visit the waters beyond Ningaloo Reef
The whale shark (Rhincodon typus) is the largest living shark and one of only three filter-feeding shark species. It has a broad, flattened head and minute teeth as well as a distinctive patterning of light spots and stripes over a dark background, fading to a light colour on the underside. This natural camouflage allows it to 'blend' into its surroundings when viewed from any angle.
Whale sharks are found in all tropical and warm temperate seas except the Mediterranean. They are known to inhabit both deep and shallow coastal waters and the lagoons of coral atolls and reefs. Whale sharks visit the Exmouth and Ningaloo area from April to June.
Whale sharks are not usually sighted in large numbers and are regarded as fairly solitary. It is believed that they sieve zooplankton as small as 1mm in diameter through the fine mesh of their gill-rakers. However, unlike the megamouth and basking sharks, the whale shark does not rely on forward motion but can hang vertically in the water and 'suck' food. Whale sharks are ovoviviparous - they produce live young (eggs hatch inside their twin uteri). Little more is known about Whale Sharks and further study is needed to conserve them, particularly with regards growth rates, age at maturity, maximum age, the length of gestation, localities of birth, frequency of reproduction and of course threats.
While few natural predators of the whale shark are known, juvenile specimens have been reported from the stomachs of a blue marlin and a blue shark. Additionally whale sharks possess scars that may be the result of shark attack at an early age. Whale sharks seem to be vulnerable at an early age and it appears that their rapid growth after birth is their main defence later in life. It is possible that pressure from fishing and accidental vessel contact may be their greatest threat.
Humpback Whales and other cetaceans
The Ningaloo area plays host to six species of toothed whales and, usually in the deeper waters, eight species of baleen whales. Five of the eight species of baleen whales found in the region are listed as rare or likely to become extinct. Humpback, Minke, Blue and occasionally Killer whales are all seen in the area around Ningaloo Reef.
Humpback whales visit the Ningaloo Reef between July to November each year migrating twice annually through the Ningaloo Reef Marine Park into the Exmouth Gulf. The first visit occurs in autumn during their northern migration, and they return in their southern migration each spring Humpback whales are the fifth largest of the great whales; adult females grow to 19 metres, slightly longer than adult males. A mature humpback may weigh 40 tonnes.
Dolphins are also found in the Ningaloo Marine Park, the majority are bottlenose dolphins but the Indo-Pacific humpback dolphin has also been sighted in the area.
Turtles
There are four turtle species found at Ningaloo, the loggerhead, green, hawkesbill and flatback turtles. The loggerhead and the green turtles are particularly endangered species. Turtles feed on molluscs, crustaceans, fish, jelly-fish and other small to medium-size marine animals, all of which are plentiful on at Ningaloo. Areas of the Ningaloo coast are known breeding grounds for loggerhead and hawkesbill turtles. As turtles are marine reptiles that come to shore only to lay their eggs, they are very sensitive to changes to the coastal environment.
Dugongs
The dugong (Dugong dugon) is one of four living species of the order Sirenia and the only sirenian to span the waters of at least 37 countries throughout the Indo-Pacific. Like all modern sirenians, the dugong has no dorsal fin or hindlimbs, instead it uses its paddle-like forelimbs to manoeuvre itself. Despite being legally protected in many countries the main causes of population decline remain hunting, habitat degradation, and fishing-related fatalities. With its long lifespan and slow rate of reproduction, the dugong is especially vulnerable to these types of exploitation. In addition, dugongs are threatened by storms, parasites, and their natural predators - sharks, killer whales, and crocodiles.
Australia provides one of the last strongholds for the worldwide dugong population and Ningaloo Reef and Exmouth Gulf support a significant dugong population of approximately 2000 individuals.
Manta Rays
Manta birostris is the only species of its genus. It can grow up to 4.8 metres in length and 6.8 metres across with a weight of up to 2000 kg. Its food is mainly plankton, shrimp and small fish ingested by filtering with its gills. Manta rays bear live young of more than 5 kg and 1.2 m across, there is no regular breeding season. Little more is known about these spectacular animals and only a small number of scientific studies have been conducted on this species. Almost nothing is known about their population ecology, use of critical habitats, movements or reproduction.
Damselfish (Pomacentridae)
Brightly coloured small fish – some species hide in the coral, others form into large plankton-eating schools and still others cultivate a patch of algae, which they guard zealously. They will even "attacked" divers and snorkellers.
Wrasses (Labridae)
Another group of colourful fish that have an almost birdlike way of swimming, interestingly most wrasses are capable of female to male sex change.
Butterflyfish(Chaetodontidae) & Angelfish(Pomacanthidae)
Exquisite colour patterns and a graceful appearance are the hallmarks of Butterfly and Angel fish. They feed on live coral and are often seen in pairs as they form permanent life time bonds.
Cardinalfish (Apogonidae)
Seen mainly at night when they feed on small shrimps and crabs. Males incubate the female's egg masses in their mouth, every so often spitting them out to aerate them and then slurping them back in again.
Groupers and Basslets (Serrnidae)
Some of the largest reef fishes, the Potato cod is always a favourite with divers as they are very large, have unusual markings and are very friendly. They can grow up to 100 kgs and the average adult is 1.5mtrs in length.