Sharks play an essential rol in the health of our oceans


Size does matter! So… how do you measure a shark??

Size matters. When it comes to understanding the health and growth rate of an animal, that is. Normally, sharks were measured with either guesstimates based on the distance between their dorsal and tail fins, or via catch, measure, and release. Knowing body size is a key component of understanding just how healthy a population of sharks is since the bigger (and therefore older and more well-fed) the better.

But taking a whaleshark or other shark out of the water to measure it can be difficult! So how do you measure sharks when they are swimming in the water?

Peter Verhoog of the Dutch Shark Society visited two projects, and documented the use of two different measuring systems.

A volunteer measures a whaleshark, the green laserbeams are visible

In Djibouti, whalesharks are measured during a three week expedition to the Gulf of Tadjoura, where juveniles aggregate. The project is conducted by the Marine Conservation Society of the Seychelles, and here laser photogrammetry is used: two parallel lasers that project dots a known distance apart onto the object being photographed. A digital photograph is taken and the laser projections on the image provide a scale bar that can be used to measure the size of physical traits in the image. This study measures the length and other parameters to determine growth rates of the animals, that are all identified by a unique spot pattern.

Dr. Mark Meekhan of the Australian Institute of Marine Science and Gabriel Vianna of the University of Western Australia have created a new tool that measures sharks with great accuracy while maintaining a reasonable distance from them. This tool allows researchers to get the information they need while giving the sharks plenty of room to swim hassle-free.

Gabriel Vianna measuring a grey reef shark

Using two cameras mounted in a rig, a diode used to sync video frames, and the geometry between them to measure the sharks from afar. The technique is highly accurate and has been described before in a 2009 paper entitled “A Review of Underwater Stereo-Image Measurement For marine Biology and Ecology Applications”.

Peter Verhoog documented this in Palau, where grey reef sharks were measured and in the Maldives, where manta rays were measured.

mark Meekan waiting for a manta ray

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