Whale sharks: Biggest fish could be even bigger

Ella Davies BBC News 7 Feb 11;

Whale sharks, the world's biggest fish, could be even bigger than previously recorded, a new study reveals.

Scientists working in Mozambique have developed a new method of measurement using a camera mounted with lasers.

Although previously estimated at up to 20 metres in length, accurate details of the giant fish have been difficult to obtain in the past.

Researchers believe regular measurements will reveal more about the lifecycles of these sea giants.

Scientists studying whale sharks (Rhincodon typus) describe a technological breakthrough in understanding the plankton-eating giants.

Working with the University of Queensland, the Marine Megafauna Foundation and CSIRO Marine and Atmospheric Research, the team's findings are published in the Journal of Fish Biology.

"Our paper is the first to publish accurate measurements for whale sharks in the field," says PhD candidate Christoph Rohner.

"Other researchers have previously tried to measure the sharks with a tape measure, or by visually estimating size, which is obviously difficult to do accurately," he explains.

Many previous size records were based on the photogrammetery method: estimating measurements from photographs.

Researchers claim they have dramatically improved the precision of this method with the addition of two laser pointers.

By positioning the lasers 50cm apart on either side of the camera, the distance between the projected points provides a fixed scale so that photographs can be analysed with greater accuracy.

"The laser system will allow us to reliably obtain accurate measurements from free-swimming sharks, so we may well find out that the world's largest fish is even larger than presently recorded," says Mr Rohner.

Researchers say their new method has already seen the recorded size of some individuals increase by up to 50cm.

In addition to recording the length of whale sharks, researchers say the photographs could reveal more about the enigmatic species.

"Whale sharks can be individually-identified using the distinctive pattern of spots on their flanks. We project laser spots onto this region, allowing us to obtain both the identity and length of the shark with a single photograph," says Mr Rohner.

"At present we have no clear idea about how long whale sharks live, but it may be for over one hundred years. By repeatedly measuring the same individuals over time, we hope to be able to eventually find out how old a twenty metre shark might be," he adds.

Conservationists are concerned that the giant fish are under threat from commercial fishing including harpoon fisheries and incidental capture.

Researchers claim that by understanding whale sharks' lifecycles, they can more accurately predict how populations are impacted by these activities.

Mr Rohner emphasises the importance of measuring individuals over time to learn more about their development and confirm their growth rate.

"Whale sharks are globally threatened, and this kind of basic but hard to get information is vital for effectively conserving the species," he says.

Researchers estimate that 19% of the global population of whale sharks has been recorded off the coast of Mozambique.

Recent satellite tagging experiments have shown the fish migrating extremely long distances of up to 13,000 km.