Tuesday, October 27, 2009

Of whale sharks and lasers....

A contribution from intern Gareth Jeffreys, known on the team as 'Lazer Boy', who has agreed to share with our readers some of the hi-tec work he has been battling with for the last nine weeks...

We are now fast approaching the very end of the 2009 whale shark programme, and with the onset of some very turbulent weather it looks like the last remaining sharks may be spared having laser beams fired at them... these laser beams are in fact green laser pointers mounted 50cm apart parallel to each other on an underwater camera, also known to very few people as laser photogrammetry.

Why? Well, as an in water method of measuring whale sharks it has a certain more elegance to it than two willing volunteers trying to swim to the front and back of a shark with a tape measure in tow!
Gareth on a 'dry run' lasering the whale shark mural at Dive Seychelles

How it works is by fixing two laser dots at exactly 50cm apart on to the side of a shark (when at a right angle to it) and taking a photo, and another and another as the shark moves past you. The resulting images are then fed through Photoshop, the pixels counted, and lo and behold you have an accurate measurement of the total length of a whale shark! Sounds simple enough... However, after three incarnations of the laser set up, days searching for that elusive extra millimetre of accuracy and a number of laser failures later, I can safely say that simple ideas don't necessarily make for simple projects.

Gareth making sure his lasers are millimetre perfect...

So with the lasers finally set up, it was time to persuade the sharks to start posing nicely for the cameras. Which surprisingly they did, until you reach their tails that is…. To get an accurate length of any shark you need it to be straight, but if a shark is going to swim it is going to bend it's tail, quite considerably in fact. Even worse, you may have an inquisitive shark that is going to keep swimming in circles trying to follow you. Where in lies the trickiest in water technique for a laserer to master, the 'Straight Tail Shot'.

Fortunately for me, and my results, it was soon apparent that whale sharks do not swim continuously, but have periods where they 'glide' through the water with what turns out to be a straight tail! But again life was never going to be that simple. It turns out a great time to catch the shark gliding is when it drops it's nose and begins to slowly dive. So the end of many an encounter was accompanied by a lonely snorkeller furiously trying to out dive the whale shark (14 metres is the current record) and get the holy grail of laser shots, the head, body and tail in three sequenced photos.

How deep was that Gareth... you look really small!

All this work was obviously done for a reason, and a season's worth of data has come up with some interesting results. Most notably being that team leader Luke Riley has won the coveted contest for closest size estimation of the year at just 1mm out, but before he accepts this most prized award it should also be noted that he also takes home the 'not even close' award for an almighty 2.15 metres difference on a single estimate!... Sorry Luke!

And so for some results for the season:

Largest shark measured: 9.54m (a beautiful female encountered on a day with at least three sharks over 8 metres in the near vicinity)

Smallest shark measured: 4.00m (last shark to be measured, and Luke's saviour for the closest estimate of the year award)

Number of different individuals measured: 32

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