Well, it’s tricky.
First, we tried guessing their length. That didn’t seem super sciencey though. No fun.
I heard friends at the Maldives Whale Shark Research Programme were using a 10m tape measure. That sounded more fun. Then I was chatting to my friend Carley Kilpatrick, who was studying sharks in Oz, and she mentioned laser photogrammetry. You aim parallel laser spots onto the side of the shark, take a photo, and can work out the length from that. Lasers! Awesome.
Andrea Marshall and I built a frame, and being clever we made it out of stainless steel so it wouldn’t rust. Turned out it weighed about 10kg, even underwater. Oops. So, after almost drowning Andrea, we rebuilt it out of marine aluminium.
That worked, yay! Here’s Chris Rohner modelling that system.
We measured and re-measured a bunch of sharks here at Mafia Island over periods of 1-3 yrs. What I really wanted to find out was how fast whale sharks grow. That can then tell us a bunch of other interesting things, like their age at adulthood and maximum age. So cool.
Then, we analysed the data. Half of the sharks had… shrunk?
Turns out that whale sharks either grow really, really slowly, so we have to wait a loooong time to get accurate growth information, or the lasers just aren’t very precise. Or both. Most probably both.
We even published it, because science: “Repeat measurements of individual whale sharks measured over periods from 347–1,068 days yielded… implausible… growth rates.”
So yeah. That’s my story for today.
HOWEVER.
We’ve got a stereo-video setup here in Mafia this year, designed by Jesse Cochran at KAUST. That’s a great way to get accurate size estimates. And those sharks we measured with the lasers? That was way back in 2012/13.
A lot of the same sharks are still around, so I’m hoping – once again, though I’ve been hurt before – that we can get some solid wild growth rates after this season. Fingers crossed!
Simon.