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Australia's mysterious big fish |
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| Big fish? Big mystery more like it. Gentle giant or not, the job of tracking whale sharks is not one to take lightly, even if we are eating them quicker than they are eating us. |
The whale shark is one of the ocean's most placid leviathans. Big shark - yes. Big mouth - absolutely. Fiercely predatorial - no; just a modestly enormous diet of plankton, tiny fish and squid that it manages to swallow while lazily swimming along with its huge mouth open.
Although they often feed at the surface, where snorkellers frequently swim alongside them, they also dive down to 1 000m. Why? Shrug of shoulders. They also travel huge distances. More shrugs.
The list of questions about this gentle giant is nearly as huge as the fish itself. Lifespan, areas populated by the species, reproductive habits and the whale sharks' behaviour are all question marks.
As the sharks are hunted and harvested for human consumption, these questions are becoming more urgent. Popular in shark fin soup, single whale shark fins have been known to fetch more than $19 500.
The obscurity of the species and their vulnerability to hunters has led to a tagging program. At Ningaloo Reef, off the Western Australia's north-west coast, scientists from the Australian Institute of Marine Science (AIMS) are tracking and tagging whale sharks to learn more about them.
Virtually nothing is known about their reproductive habits, and we don't know where most of the females are. Most of the whale sharks around Ningaloo Reef are young males.
Fish ecologist Dr Mark Meekan is undertaking a tagging program using new 'SPLASH' tag technology. Transmitters housed in small, torpedo-shaped floats are tethered by metre long cords to special clips on the sharks' dorsal fin. Snorkellers use a special applicator to attach the transmitters.
When the shark is on or near the surface, the float breaks the water surface and locates a satellite signal. Having established communication it rapidly sends 'packets' of summarised information. Scientists are then able to download the information from the satellites.
Mark explained, "The new tags log information on location, swimming depth and water temperature at one minute intervals.
"The tags also store information just in case the tag comes adrift, or the shark is caught. The data can then be downloaded from the recovered tag."
Carried out under an Animals Ethics permit, the tagging causes little or no reaction from sharks.
This season AIMS tagged six sharks, which ranged in size from four to eight metres.
"One shark tracked northwards and is now in the Indian Ocean, at a longitude of about Sri Lanka, others have moved to the north east and are in the Timor Sea, near Indonesia" said Mark.
Improved knowledge of the whale sharks' movement patterns will form the basis of wise management and conservation plans for the species in Australia, and assessment of likely impacts from overseas fisheries.
"We want to know what drives their movements, whether they drift on ocean currents or move strategically between culinary oases. How their populations are linked across the world's tropical and warm temperate seas also remains a mystery," said Mark.
Another missing part of the whale shark story is the whereabouts of the female of the species. According to Mark, "Around 80% of the whale sharks around Ningaloo are juvenile males.
"The only pregnant female ever caught was in Taiwan. She had 300 pups in varying stages of development inside her. It could be that these fish are pupping all the time, but we don't know if they do, or where they do it.
"We just don't know."
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