Roll off the boat, swim frantically to the bottom, cling to a rock. Welcome to diving at Darwin Arch, Galapagos. There’s a lot of water moving past this place. I know that the conditions I had on the last dive, just a couple of hours ago, are no preparation for what Iíll see now.
What Iím hoping to see, though, is a super-sized whale shark emerging from the deep blue water. And hey, I’m an optimistic person. Maybe itíll be accompanied by a few hundred hammerhead sharks.
At Darwin, it may actually happen.
Darwin Arch at sunset, with Darwin Island in the background
Welcome to the Worldís Sharkiest Ocean
The term “shark-infested watersĒ makes me cringe. It’s ridiculous. Throughout most of the worldís oceans, sharks have been overfished to the point where even seeing a shark is rare to the point of statistical improbability.
The waters surrounding Darwinís Arch are a spectacular exception to that sad norm. The ocean here isÖ shark rich.
Scalloped hammerhead shark school
Darwin, and adjacent Wolf Island, have an average of 17.5 tonnes of fish per hectare. It’s the highest biomass of reef fish recorded anywhere, and most of these fish are sharks.
In any given direction youíre likely to see either a scalloped hammerhead, silky or Galapagos shark. Whale sharks cruise through sporadically to lord it over these pipsqueaks.
A (small) whale shark with hammerhead sharks at Darwin Arch
Itís a genuinely awesome place. To give you an idea of what I mean, I started leaving a remote camera down to capture the action:
Thatís a lot of hammerheads
What makes this place so sharktastic?
Iím glad you asked. I suspect it’s because of
- Magnetism, and
Whale sharks havenít been seen feeding around Darwin as yet, but the area is rich in food for other sharks. When deepwater currents hit the slope of this underwater volcano, they are forced upwards, with the water speeding up rapidly as it flows through a decreasing area.
This fast upwelling water transports deepwater plankton up to the surface, laying on a buffet for the fish that feed on the plankton, and the sharks that feed on them in turn. The hammerheads also enjoy a calamari meal, and there are plenty of Humboldt squid in the area taking advantage of the abundant prey.
Darwin Island from the air
Darwin Island, like the rest of the Galapagos archipelago, is volcanic in origin. The island is the eroded tip of an extinct volcano, rising from the seafloor over 2 km below. During eruptions, molten basalt flowed out of the cone and away from the volcano as lava. Basalt contains ferromagnesium minerals, such as magnetite, that align with the contemporary polarity of the planetís magnetic field.
The Earthís polarity has reversed itself repeatedly over geological time, on average every 500, 000 years. If the next eruption took place after a reversal, the magnetic particles in this more recent basalt would align with the opposite polarity.
The present seafloor around Darwin consists of a series of concentric rings in which the magnetic particles either align with the Earthís current polarity, and add to the areaís magnetic intensity, or else have the opposite polarity, thus subtracting from the areaís magnetic intensity.
The planet also wobbles irregularly along its north-south magnetic pole. This magnetic deviation is also recorded by the particles to give each ring its own unique magnetic signature.
The end result is a series of alternating stripes of strong and weak magnetic fields, each of which with their own distinct magnetic fingerprint, radiating outwards from the island.
These provide a detailed relief map that can be read by animals – if they have the right equipment. Itís like a street map for sharks, and it allows them to easily navigate from the Arch to offshore feeding areas at night, returning in the early morning.
For whale sharks, which likely do have a magnetic sense, Darwinís position close to the rift zone between the Nazca and Cocos plates makes the island a dirty big roundabout on the Eastern Pacific magnetic superhighway.
Thereís also the simple fact that people arenít killing sharks at Darwin. The Galapagos Marine National Park has been in place since 1998, and the waters around Darwin and Wolf islands have been fully protected from all fishing since March 2016.
Many shark species have been severely depleted by fisheries; whale sharks and scalloped hammerheads are both globally endangered species. Targeting them in areas where they are densely aggregated, like Darwin, would quickly wipe them out from a large area.
Some illegal fishing still occurs in the area, as evidenced by the number of sharks we saw that had fishing injuries or embedded hooks. Protecting this area will take constant vigilance, but it has the potential to rebuild populations at what is already one of the planetís most amazing spectacles.
Our Mission: Tag Pregnant Female Whale Sharks
Itís hard to miss a shark the size of a bus. Surprisingly, then, Darwin Arch is the only place where really large whale sharks – Iím talking over ten metres – are regularly seen.
The sharks at Darwin arenít just big, though. Theyíre almost universally pregnant females.
Why is this worth mentioning? Well, almost nothing is known about whale shark reproduction. Only a single pregnant female has ever been examined by scientists, and this ďmegamammaĒ shark was documented from Taiwan way back in 1995.
Thereís been little new information for over 20 years now.
Pregnant whale shark from the Galapagos
In September 2016, Chris Rohner and I from MMF were lucky enough to be able to join a research expedition to Darwin Island. The trip was organised by Jonathan Green from the Galapagos Whale Shark Project, and funded primarily by the Galapagos Conservation Trust and the BBC (shooting footage for the upcoming ďOceansĒ series). We were part of a team that also included the Galapagos National Park, Galapagos Science Centre, and Planeterra Foundation.
Our objective was to find these huge sharks, film them, and satellite-tag them to study their movements and diving behaviour.
Jonathan and whale shark
We came equipped with eight Wildlife Computers MiniPat satellite-linked tags, set for six-month deployments. These tags contain light sensors, to estimate the time of sunrise, sunset and peak light, so that latitude and longitude can be roughly determined, as well as measuring water temperature, and ambient pressure to calculate depth.
The satellite connection only works in air, so after the 180 days is up an electronic release detaches the tag from the shark. The tag then floats to the surface and drifts for a few days, while transmitting data to the ARGOS satellite network, before the battery runs out.
Thatís the plan, anyway.
Where do huge whale sharks actually live?
Adult female whale sharks are almost never seen in coastal feeding areas. These areas, where most whale shark tourism tends to occur, are normally dominated by juvenile male whale sharks for some reason.
Iím working on the assumption that whale sharks move out into the open ocean once theyíre close to hitting adulthood (at about nine metres length). I donít actually know that though. Iím pretty much making an inference based on the fact that we donít see them. But absence of evidence is not evidence of absence, if youíll excuse the cliche.
This trip was my chance to find out what is going on.
Spoiler Alert: We Did Good
The average school bus is 13.7 m in length, and 8-12 tons in weight. Whale sharks are actually bigger, and heavier, than a bus. Iím just pointing this out so you donít go thinking Iím prone to fits of hyperbole.
You can trust me. Iím a Doctor.
Pregnant whale shark and silky shark
Anyway, I bring this up because, well, itís an awesome fact, but also because the main ledge at Darwin Arch, in about 16 m of water, is commonly referred to as ďThe Bus StopĒ.
We often spent the first portion of the dive either hanging off this area, or being beaten against it (currents were variable). Here, you can watch what seems like every shark in the ocean slowly swimming by.
When a whale shark does materialise, a frantic pursuit begins. The cruising speed for a large whale shark is amazingly similar to slightly faster than my top speed, if I kick so hard that my lungs might explode.
We photo-identify each shark, based on its unique individual spot pattern, and sex it (easy with the pregnant sharks!).
Chris photo-identifying a large shark
My main job was to deploy our satellite tags. Tagging whale sharks is simple, but not easy. We typically get around 30 seconds with each shark, in total. The sharks are really big, and whale sharks have the thickest skin of any animal in the world.
To plant a dart tag head in the connective tissue, just under the outer layer of the skin, we use a pneumatic speargun. The sharks donít even react. The shark then swims off with the small tag attached near the base of the dorsal fin, and I’m†left behind with a depleted tank and the hope that the tag will report back in a few months time.
We did 35 dives over 12 days from the M/Y Queen Mabel. We identified 14 whale sharks, and tagged eight. Jonathan and I teamed up to double-tag three of these sharks with tethered SPOT satellite tags, which give us good positional data when the shark is at the surface, along with the pop-offs.
Tagged whale shark
What happens next?
If the SPOTs stay on (no guarantees, as silky sharks like to eat them), weíll be able to combine data to look at their diving behaviour in specific areas. Hopefully, by the end of March 2017 weíll have received all the data back from our pop-up tags.
Jonathan and I did a liveaboard dive trip immediately after our research expedition. We saw one of the sharks, tagged at Darwin on September 26th, 38 km away at Wolf Island on the 4th October. She looked fine, and said hi. We havenít heard anything from the other tagged sharks, but no news is good news until March…
Sometime in 2017 the Galapagos whale sharks will also be featured in the new BBC Blue Planet 2 series. The original was filmed way back when I wasÖ finishing school. Sigh. Iím so old now. Should be an awesome documentary series though!
Iíll update when we know more ūüôā
Want more shark geekery? Check out the†worldís largest whale shark aggregation off Mexico.
If youíd like to read new stories as theyíre published, get exclusive updates on new expeditions, and that sort of thing, subscribe to my newsletter:
Short video from the trip by Chris Rohner
My work on this project is supported by two private trusts, the Shark Foundation, Aqua-Firma and Waterlust. Thanks a million to the awesome crew of the M/Y Queen Mabel, and my fantastic science buddies on the trip!
Like geeky animal facts and dad jokes?
I write a few articles just for my mailing list. They normally focus on something interesting, and possibly hilarious, that I've learnt about sharks (or other random animals) that week. There may also be groan-inducing jokes.
Real talk: there will be groan-inducing jokes.