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Understanding the Ecology and Life History of Whale Sharks
By Don Silcock, May 19, 2023 @ 06:00 AM (EST)

Large, seemingly pregnant whale sharks come to the remote Galápagos Islands from May to December every year

I believe our technologies and techniques can contribute to understanding the timing of reproduction and sexual maturity
—Rui Matsumoto

The whale shark (Rhincodon typus) is both the world’s largest fish and one of the ocean’s greatest puzzles. A true enigma, these pelagic gentle giants have no real predators and seem to wander the world for no apparent reason. Where they go and why they go there is simply not known.

What we do know is that while they are predominantly solitary creatures, they will gather in key locations at certain times of the year. They seem to do this primarily to feed on locally occurring phenomena, such as the “Afuera” spawning of little tunny at Mexico’s Yucatán, or the coral spawning at Ningaloo Reef in the northwest of Australia.

But of the known aggregations, probably the most intriguing is the one that occurs in the remote Galápagos Islands from May to December each year, when large, seemingly pregnant, females arrive in quite significant numbers.

We know surprisingly little about the life history of the ocean’s biggest fish


Nature’s Creche

That these huge animals have traveled incredible distances while pregnant to gather at such an iconic location seems like a perfect script for a TV documentary! It certainly sends a strong emotional appeal to those of us who love the ocean: a special place, far from land, where female whale sharks can give birth and nurture their young safely—nature’s creche for the ocean’s behemoths.

Scientists don’t work on emotions though; they need facts. But how do you gather facts on large wild animals that are underwater in a remote location swept by strong currents and where contact is fleeting at best? The short answer: with great difficulty. But a team of marine scientists led by Rui Matsumoto set out to do exactly that.

Okinawa Churaumi Aquarium

Churaumi (meaning “beautiful ocean”) in Okinawa, Japan is one of the largest aquariums in the world. It is renowned for its almost two-million-gallon main tank called “The Kuroshio Sea,” with its captive whale sharks, manta rays and several species of large sharks, together with its successful in-house breeding programs for mantas and bottlenose dolphins.

Despite the huge size of the facility, hosting large pelagic creatures in a confined environment remains a controversial topic, but peel that onion a bit and it becomes clear that a great deal of work goes into monitoring the health of those animals. Interestingly, the techniques developed to do that monitoring are playing a major role in understanding the health and condition of these animals in the wild.

Divers with a tagged whale shark


Whale Sharks—“We know so little about their ecology”

To better appreciate what scientists are doing to understand the life history of whale sharks, I spoke to Rui Matsumoto about his work at the Okinawa Churaumi Aquarium and how it prepared his team for collecting blood samples and taking ultrasound readings from whale sharks in the wild.

Rui, could you briefly explain a little about yourself?

I am from Kobe in Japan and studied the phylogeny (evolutionary history) of hammerhead sharks at Hokkaido University. Over time, I became increasingly interested in whale sharks and was fascinated that despite being such popular creatures, we know so little about their ecology.

As I understand it, the techniques of in-water blood sampling and ultrasonography were developed at the Churaumi Aquarium in Okinawa to better monitor the health of the animals under care. Basically, in much the same way as people are checked by their doctor. It sounds like something that is much easier said than done… What were the practical difficulties you had to overcome to make those procedures successful?

To be successful with these in-water techniques, we need a combination of good diving skills and an understanding of shark structure. I studied shark anatomy in my PhD program, so I have a good understanding of the structure, and Kiyomi Murakumo, who is working with me, is a licensed nurse and very capable with the medical equipment we use.

Elasmobranch expert Rui Matsumoto


What role does blood sampling and ultrasonography have in Churaumi’s program to breed whale sharks in captivity for the first time?

To encourage breeding, animals must first be healthy, and so the main purpose of these techniques is to monitor and manage their health. Over time, the vast amounts of data we have collected enabled us to understand the physiological state of the animals in our care. In addition, because we are monitoring their hormonal levels, blood chemistry and the development of internal reproductive organs we can understand their sexual maturity and reproductive cycles.

Were the same procedures used with Churaumi’s breeding programs for manta rays and bottlenose dolphins?

Yes, we have successfully bred manta rays and bottlenose dolphins in captivity. However, in the case of dolphins, there is no need to take blood samples or perform ultrasound underwater because they breathe through their lungs—so the procedure can be performed at the surface. On the other hand, manta rays are fish, and so we developed underwater ultrasound to detect pregnancy at an early stage and to safely monitor the condition of the fetus without inducing stress on the animals.

Jonathan Green, Founder and Director of Galápagos Whale Shark Project, swims with a tagged whale shark in the Galápagos

Jonathan with a freshly collected blood sample


Did you and your team conduct those procedures on free-swimming whale sharks anywhere else before using them in the Galápagos in 2017 and 2018?

No, the Galápagos was the first time the program was conducted in the wild, and it was quite a challenge, as the whale sharks were so big compared to the ones at Churaumi. They were a bit like towering walls in front of you, plus their skin was so thick it made collecting blood samples and taking ultrasound scans quite difficult.

How did you deal with the strong currents in the Galápagos?

Because it is in a robust waterproof housing the size of a large briefcase, the ultrasound system is very heavy and difficult to maneuver underwater, so I don’t use any diving weights and use a thruster (underwater propeller) attached to my tank. It took some getting used to, but it allows me to keep up with the whale sharks and stay in position while we perform our procedures.

Did the whale sharks “cooperate” in any way or were they unaware of what was happening?

Our impression was that the whale sharks had no reaction at all, and they probably felt almost nothing.

Whale sharks are big! Were you, or any of the team, ever in any kind of danger?

Once Kiyomi was struck by a caudal fin when she was next to a whale shark as it moved away. The right half of her body was covered with blue bruises—large whale sharks appear to move slowly, but they are very strong and powerful animals!

Rui takes an ultrasound reading

Rui keeps up with the whale shark using a thruster attached to his tank


The Galápagos and the annual appearance of the large, seemingly very pregnant, females appeared to indicate the area was a key piece in the incredible puzzle of where whale sharks go to and why. And if the Galápagos really was a key birthing area for the species, then it would provide a profoundly important foundation upon which to eventually solve that puzzle. What were your thoughts about all that as you prepared for the expeditions to the Galápagos?

A primary aim of our Galápagos Whale Shark Project (GWSP) research team is to better understand the migratory routes of the giant female whale sharks. We have been able to attach satellite tracking devices to many of the sharks and by integrating that migration data with all the physiological information we also collect, we have been able to get a much better understanding of female whale shark reproductive biology.

What that all means is that we can determine the reproductive status (breeding season, rest breeding season, pregnancy status) of the giant female whale sharks.

How surprised were you with the results of the blood sampling and ultrasonography?

I was surprised that the hormone levels were lower or equal to those of the young female whale sharks (8 meters) we had tested at Churaumi. We had expected that mature, if not pregnant, whale sharks would have higher hormone levels than those of immature individuals. This result defied our expectations, but also, the skin, muscle tissue and liver were significantly thicker than we had expected, and we struggled to reflect the ultrasound.

It must have been a big call to disclose and then publish the results of the in-water procedures in the Galápagos. How important was the data and experience gained with those procedures on the captive whale sharks at Churaumi in making that call?

We have been able to publish the results of our work in the Galápagos (see PDF), but to be scientifically correct, we need to compare our data with the results of similar testing at other locations. But we are the only institute that has conducted such testing, and so we had to compare the results from the Galápagos with the data we have from our testing of immature female whale sharks at Churaumi.

Jenny Waack taking a blood sample


How would you summarize the overall experience and learning gained from what you and your team did in the Galápagos?

The results to date have demonstrated that these techniques can be used to noninvasively acquire anatomical and physiological data from living organisms. But we need to collect a lot more data with these techniques going forward.

From a real-world perspective, what does all this mean for our overall understanding of the oceans and the role that whale sharks play?

Unfortunately, and contrary to their growing popularity, these sea giants are believed to be declining in population. To stem this concern, we need to conserve them, but we do not yet have a good understanding of their life history.

I believe our technologies and techniques can contribute to understanding the timing of reproduction and sexual maturity. In the future, we want to try and match the physiological data with the migration data and better understand their life history in the ocean.

Now that the global pandemic seems to be behind us, what are you planning for the next steps in solving the whale shark enigma?

The first step is to collect more data, and in the Galápagos, the whale shark season is from May to December, but so far, we have only sampled in July and September. It is important to sample in all the months the animals are present so we can understand if there are any seasonal influences.

With whale shark populations on the decline, more data need to be collected to help create effective conservation programs

Don Silcock is based in Bali, and his website www.indopacificimages.com has extensive location guides, articles and images on some of the world’s best diving locations and underwater experiences. This interview with Rui Matsumoto was conducted with the help of Dr Simon Pierce, co-founder of the Marine Megafauna Foundation (MMF) and the world’s top whale shark conservation biologist. MMF’s mission is to save threatened marine life using pioneering research, education, and sustainable conservation solutions.

Simon Pierce documents his travel and underwater adventures, plus his tips on underwater photography and dive gear on his site Nature Tripper.


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