World's largest fish may be bigger than we think!

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The world's largest fish – the Whale shark (Rhincodon typus) — may be larger than we currently think.

In a paper published in a recent issue of the Journal of Fish Biology, Christoph Rohner and coauthors utilise a technique known as laser photogrammetry, in which a scale bar of a known length is projected onto the whale sharks; the sharks are then photographed and the length of the shark calculated with reference to the projected scale from the photograph.

The authors used a relatively simple setup consisting of two green laser pointers mounted 50cm apart, with a digital camera mounted midway between the laser pointers to photograph the whale sharks. 

The authors then snorkeled alongside whale sharks at an aggregation site off southern Mozambique, photographing 23 individual sharks. At the same time, visual estimates of the lengths of the whale sharks were also made by trained observers.

Comparing the results from their methods and the visual estimates, the authors found that the visual estimates incorrectly reported the length of the shark by an average of 8–11%, with a slight tendency to underestimate the size.

According to lead author Chris Rohner: "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."

The new method still relies heavily on a visual element, and so can only be used in close proximity to the fish in turbid water conditions. However, this limitation also applies to earlier methods of visually estimating the size of the fish.

For more information, see the paper: Rohner, CA, AJ Richardson, AD Marshall, SJ Weeks and SJ Pierce (2011) How large is the world's largest fish? Measuring whale sharks Rhincodon typus with laser photogrammetry. Journal of Fish Biology 78, pp. 378–385.