Whale Shark (Rhincodon typus)

Legislative protection

Environment Protection and Biodiversity Conservation Act 1999

The Whale Shark is listed as vulnerable and migratory under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act).
EPBC Act Status and Documents - Rhincodon typus — Whale Shark

A recovery plan for the whale shark was released in 2005. This recovery plan is still in place.

Australian Government Action

Domestic

A number of recovery-targeted projects for the Whale Shark have been funded - Whale shark reports.

History

The whale shark (Rhincodon typus) is a relatively recent addition to the human record of the ocean and its inhabitants. However, the ancestry of this shark goes back to the Jurassic and Cretaceous periods 245-65 million years ago, when the present groups of sharks began to appear.

It was not until 1828 when the first whale shark specimen known to science was discovered off the South African coast. Dr Andrew Smith formally described this species later that year as the largest living shark in the ocean.

This species is rare. Prior to the mid-1980's, there had been less than 350 confirmed reports of whale sharks worldwide. Since this time, consistent sightings have been recorded in Australia. A lucrative ecotourism industry revolving around their annual appearance at Ningaloo Marine Park on the Western Australian northwest coast is now well established.

Biology and ecology

This species is closely related to the bottom-dwelling sharks (Orectolobiformes), which include the wobbegong. There is a pattern of lines and spots on the skin of each shark which enables them to 'blend' into their surroundings. This 'camouflage' makes the sharks less conspicuous in their oceanic environment. The unique patterning does not appear to change over time and can be used to identify individual sharks.

One of only three filter-feeding sharks (the other two being the basking and megamouth sharks), the whale shark feeds on minute organisms including krill, crab larvae, jellyfish etc. Although they have approximately 3000 tiny teeth (each less than 6 millimetres in length), these teeth are not used while feeding. Instead, the whale shark can sieve prey items as small as 1 millimetre through the fine mesh of the gill-rakers. They are able to open their mouth to a great width (greater than 1 metre) to optimise feeding.

Whale sharks can also feed via 'suction' while vertical in the water. Information on feeding behaviour, when combined with sighting data, may help researchers understand how shark appearance is related to natural events in the marine environment.

Breeding

Whale sharks have internal fertilisation and produce live young. Males can be distinguished by the presence of two claspers near the pelvic fin. These organs are absent in females.

A long-term study has been undertaken at Ningaloo Marine Park since 1995 by Brad Norman. This has established that male whale sharks do not usually mature before they reach a length of around 8-9 metres. The size at maturity of female whale sharks cannot, however, be determined through similar external observation.

It is, at present, not known where whale sharks breed. Only one pregnant whale shark has ever been recorded. There have been very few juvenile whale sharks seen at any location throughout their range.

Distribution

Whale sharks have a broad distribution in tropical and warm temperate seas, usually between latitudes 30°N and 35°S.

They are known to inhabit both deep and shallow coastal waters and the lagoons of coral atolls and reefs.

Australia is one of the most reliable locations to find whale sharks. Regular sightings have also been recorded from many other regions including India, the Maldives, South Africa, Belize, Mexico, the Galapagos Islands, Southeast Asia and Indonesia.

This species is widely distributed in Australian waters. Although most common at Ningaloo Marine Park (and to a lesser extent at Christmas Island and in the Coral Sea), sightings have been confirmed further south than Kalbarri (on the mid-west coast of Western Australia) and Eden (on the New South Wales south coast). Whale sharks have also been recorded from Commonwealth waters between Australia and Indonesia.

This species is thought to prefer surface sea-water temperatures between 21 - 25°C. Sightings at Ningaloo Marine Park, however, are most common in water temperatures around 27°C.

The sharks (regularly) appear at locations where seasonal food 'pulses' are known to occur. The predictable annual whale shark aggregation at Ningaloo Marine Park is closely linked with an increase in productivity of the region. This is associated with a mass coral spawn which occurs around March/April each year.

Habits

Whale sharks are fish, and therefore obtain oxygen via their gills. They have no physiological requirement to swim at the surface - unlike air-breathing whales and dolphins. Although they are most often observed swimming at the surface during 'seasonal' aggregations, evidence from tracking studies undertaken at Ningaloo Marine Park and at other international locations indicate that whale sharks can dive to great depths (~700 metres). They can remain away from the surface for long periods.

Although whale sharks are often sighted with numerous other fish - these gaining some protection from the larger whale shark - they invariably react when subject to physical contact. It is important for boats and swimmers not to impede the path of the sharks in any way.

Radio-tracking studies at Ningaloo Marine Park indicate that individual whale sharks may stay close to Ningaloo Reef over day/night periods. In addition, using the Whale Shark Photo-identification Library, it was possible to show that one identified shark had been resighted at Ningaloo Marine Park on 14 separate days over a 28 day period - within a very restricted area. Some sharks appear to show a level of 'site-attachment' when returning to the Australian northwest coastline.

Migration

Whale sharks are regarded as highly migratory - although these 'migration patterns' are poorly understood. Previous research at Ningaloo Marine Park suggests the sharks may undertake a northerly migration when leaving the area. Their seasonal appearance at Christmas Island and sightings near Ashmore Reef provide support for this theory. Sadly, it is when the sharks leave Australian waters that they are potentially at risk of 'unsustainable hunting pressure'.

Satellite tracking of whale sharks in US waters and also in the South China Sea reveal that whale sharks can travel great distances (1000's of kilometres). These migrations may take years to complete. A far greater understanding of whale shark movements will be possible with the continuation of tagging and tracking studies throughout the world. To date, short-term movements and behaviour of whale sharks at Ningaloo Marine Park have been successfully investigated using acoustic tracking.

It will be interesting to determine the preferred habitat of whale sharks visiting the Australian coastline. Further information on the ecology and oceanography from locations where shark sightings are common will provide a better understanding of the reasons for whale shark movements. In addition, satellite technology will enable researchers to map the movements of tagged sharks and broaden our knowledge of this species.

Natural events (e.g. weather patterns) and the particular physical geography of a region can influence productivity. Warm tropical surface-waters are often nutrient-poor, in contrast to areas of cold-water (nutrient-rich) upwellings. Some long-distance migrators travel to and from areas of increased food abundance e.g. another filter-feeder - the humpback whale.

Additional information on the biology and ecology of whale sharks is needed to help with conservation and management.

Information provided by Brad Norman (ecocean@ozemail.com.au)

Resources