A magnetic mystery
Dr Simon J Pierce is a marine conservation biologist and underwater photographer from New Zealand. He is a co-founder and Principal Scientist at the Marine Megafauna Foundation, where he leads the global whale shark research programme, and a regional Co-Chair for the IUCN Shark Specialist Group.
Hi. I’m Simon. I have a whale shark problem. You’d think a giant shark would be easy to find. Whale sharks are the largest of all fish, growing to about 20m in length and 42 tonnes in weight. Large adults are longer than a school bus, and twice as heavy.
That’s a big hunk of fish, which means people are usually surprised to learn that whale sharks have been rather good at eluding scientists since their discovery in 1828. Apparently even Jacques Cousteau only saw two whale sharks during a lifetime of ocean exploration.
It wasn’t until the 1980s that scientists became aware that whale sharks can be reliably seen, at least seasonally, at Ningaloo Reef in Australia. In the years since, whale shark feeding areas have been identified at a number of other places, such as Isla Mujeres in Mexico and Mafia Island in Tanzania.
What’s my problem, then?
All of these sites are dominated by juvenile, or just-adult male sharks. That also holds true for other areas where whale sharks are seen, for instance in Madagascar, Mozambique, the Philippines, the Seychelles, and Thailand.
We’re missing the babies, the larger adults, and the female sharks.
It’s a fascinating mystery, but – given that whale sharks are now a globally endangered species – I’m not just inspired by curiosity. This conundrum is a huge challenge to effective conservation. Fortunately, there are a small number of exceptions to this general rule. These sites are generally offshore seamounts or volcanic islands.
Another problem: these places are hard to get to.
I started writing this column while moored at one of these outliers: Isla Darwin, the northernmost island of the Galapagos archipelago. Despite being over 300km by boat from the nearest town, this is the best place in the world to reliably see adult female whale sharks. What’s particularly special about Darwin is that most of the sharks that pass by here are huge, and almost all of them are pregnant.
Hardly anything is known about whale shark reproduction. Only one pregnant female has ever been examined by scientists, after she was caught in a fishery in Taiwan way back in 1995. That singular female had more pups than has ever been seen in any other shark: 304 tiny little whale sharks, ranging in size to 64cm long.
We really need to know more. That’s why we’re in the Galapagos.
Whale sharks normally spend less than a day at Darwin. It’s not a place they come to feed. To be honest, we’re not quite sure why they’re here, but the most likely explanation is that they’re calibrating their built-in GPS.
Some sharks can detect the Earth’s magnetic field. Historical volcanic eruptions at Darwin have created concentric rings of magnetically polarised rock on the seafloor, providing a detailed relief map for animals – if they have the right equipment to read it. Their attraction to this area indicates that whale sharks do, and this is backed up by their incredible diving behaviour.
Last year, sadly, none of our satellite tags worked. The crush depth of these tags is around 2km, and the most likely explanation for their failure is that big whale sharks exceed these depths.
These extreme dives are also probably related to navigation. The Earth’s magnetic intensity varies on a regular daily cycle, peaking around dawn and dusk. That’s when the sharks usually dive deep. We’re assuming the diving helps the sharks get a more accurate ‘fix’ on their current position.
I think that’s very cool but, anyway, back to Galapagos. Every whale shark has a unique pattern of spots, meaning that each individual is photo- identifiable. We saw (and satellite-tagged) seven new sharks this year, raising the total number of whale sharks identified from the Galapagos to 180. None of these sharks have been re-sighted outside the Galapagos.
Where are the sharks going? They generally swim out into the Pacific Ocean, far from any landmass. There’s a long productive zone where cooler waters from the Peruvian coast meet warm tropical waters above the equator and, based on our tracking data, the whale sharks are feeding out there. They’re also, most likely, giving birth in that area too.
What’s next? We’d love to get more people submitting their whale shark photos from the Galapagos to the global database at www. whaleshark.org. It’s hard for us to visit Darwin for more than a couple of weeks per year, so photos from visiting divers (from years past, too) are a huge boost to our research. We still don’t know how often the sharks breed, but it’s likely to only be once every few years. The more photos we have, the faster we can work that out.
Thanks for helping us to solve the mystery.
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