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Tagging Giants: Studying Whale Sharks in Cendrawasih Bay

After a successful whale shark  satellite tagging expedition last November, I’ve returned to breathtaking Cendrawasih Bay in the far eastern reaches of Indonesia’s Bird’s Head Seascape to assist the Cendrawasih Bay National Park Authority in further investigations of their recently discovered whale shark (Rhincodon typus) population.

Along with colleagues from WWF-Indonesia, Hubbs Sea World Research Institute and the State University of Papua, we are aiming to do something which has never been tried before: insert radio-frequency identification (RFID) tags into every shark we encounter, in order to rapidly determine the size of the bay’s population while also allowing us to monitor individuals’ movements in the bay over the coming years.

These tags are now widely used in the U.S. for keeping track of pets. The tiny pill-sized transmitter is injected beneath the skin and serves as a unique, permanent “ID card” which can be scanned with a receiver wand — allowing us to quickly determine a shark’s history whenever (and wherever) it is next encountered.

This technology has never been tried before with whale sharks, in large part because it‘s fairly impractical to swim after the giants (they can reach over 15 meters, which is almost 50 feet in length!) with a receiver wand underwater. However, the whale sharks of Cendrawasih Bay have a unique habit that makes this technique possible: they aggregate at bagan (lift net) fishing platforms to feast upon the small silverside baitfish that the fishers are targeting. The sharks frequently stay for hours at a time, gorging themselves on baitfish(see video above). During this time, they are incredibly easy to approach.

Our team is led by Dr. Brent Stewart, a global authority on tagging whale sharks and other large sea creatures who had this crazy idea during our satellite tagging mission last year. Brent is hopeful we’ll be able to tag 10-15 sharks, and has come with various equipment, from syringes to spears, we hope to use to insert the RFID tags, as well as a receiver wand in a waterproof casing.

We arrive at midday after a long 27-hour steam from the town of Manokwari. The park rangers have already checked with all the bagan fishers and found one platform that has three sharks lazily feeding beneath it.

Within minutes, we’ve successfully tagged all three sharks and taken a full set of identification photos of each animal. Whale sharks have a variable pattern of white spots on their side, which is believed to be unique to each individual — much like human fingerprints. One goal of this research is to use the RFID tags to see if an individual’s spot pattern remains the same (and uniquely identifiable) over a period of years.

Over the next four days, we manage to tag a total of 30 individuals, far exceeding Brent’s expectations. Interestingly, 29 of the 30 sharks are adolescent males between 3-8 meters (10-26 feet) in length. It seems that Cendrawasih Bay is a bachelor pad!

Actually, Brent explains, this dramatic skew towards young males is common at most known whale shark aggregations around the world. Amazingly, we learn that despite all of our technology and science, humans have very little understanding of the world’s largest fish. Where are the females, adult males and babies? Where do they mate and give birth?

Based on data he’s recovered from depth-recording satellite tags on whale sharks around the world, Brent believes that the answer to all of these questions is simple: deep! Even the young males that gather at certain spots on a seasonal basis and can be viewed by snorkeling tourists spend most of their time below 60-100 meters (196-328 feet) depth — and frequently below 1,000 meters (3,280 feet)! He reckons that the females and adult males probably also spend most of their time at these depths as well, likely feeding on plankton or small fish.

During the five-day trip, we are fortunate to spend many hours with these graceful giants. We also learn that the sharks’ interactions with the bagan fishing platforms are not completely positive. On the third day of tagging, the fishers from one bagan call us over to help: two overly excited whale sharks have swum into the fishers’ nets just as they were lifting them, and are now trapped within.

It takes us about 10 minutes to coax both sharks out of the large lift net, but we learn from the fishers that this situation is now increasingly common. It seems that the sharks are no longer content to wait outside the nets for a free meal, but increasingly swim right into them! They have also learned how to “suck” the fish out of holes in the nets!

We have a long discussion with the fishers and the park authority about this, both of whom are very interested in finding a solution. My colleagues at WWF-Indonesia agree to work with the fishers to consider modifying the lift net design in a way that prevents the sharks from entering. The fishers are amenable to this solution, as they also like the sharks (considering them good luck) and prefer to avoid the damage to both nets and sharks that can result from entanglement.

We’re also quite surprised to find that even after tagging 30 sharks, over half of the individuals we see on our last dive are still untagged! How many sharks are there in Cendrawasih? My colleagues on the expedition are already planning the next tagging trip later this year, and hope that we’ll soon be able to answer this and other questions about these gentle giants. Stay tuned!

Dr. Mark Erdmann is the senior advisor to CI-Indonesia’s Marine Program.