Whale sharks in Cendrawasih Bay, Indonesia feeding on baitfish beneath a bagan (lift net) fishing platform. (© Shawn Heinrichs)
Editor’s note: To kick off Shark Week 2017, we’re bringing you the latest science news about the world’s largest fish. This blog was co-written by shark researchers Mark Erdmann, Abraham Sianipar and Megan Meyers.
Last year about this time, we wrote on Human Nature about a ground-breaking new whale shark satellite tagging program we had just launched in Cendrawasih Bay, in the Bird’s Head region of eastern Indonesia.
Whale sharks in Cendrawasih are frequently captured inadvertently in fishing nets which target the silverside baitfish that the sharks normally feed upon. Taking advantage of this, we were able to get close to these gentle giants and give them some high tech “bling” — fin-mount satellite tags which transmit their position (as well as depth and temperature data from their recent dives) every time the sharks come to the surface. Fin-mount satellite tags have long been used on other shark species, including tiger sharks and great whites, but were not in common use on whale sharks because the animals are simply too big to easily catch and pull alongside a research ship while attaching the tags. (Check out the video below — shot by Megan Meyers — to see one of these sharks up close.)
We’ve now had tags deployed on five whale sharks for over a year, and have deployed an additional 11 tags over the past eight months. (Watch the sharks’ movements in real-time on our “whale shark tracker” page.)
So far the tags have functioned beyond our highest expectations, providing a wealth of new insights into the behavior of the world’s largest fish. Here are four things we’ve learned:
Sometimes whale sharks are inadvertently captured in bagan baitfish nets, allowing researchers a unique opportunity to affix and deploy fin-mounted satellite tags before releasing them. (© Shawn Heinrichs)
Researchers attach a Wildlife Computers finmount SPLASH10-346A satellite tag, custom-designed for our study with approximately two-year battery life. (© Shawn Heinrichs)
1. The whale sharks of Cendrawasih Bay are largely homebodies.
Reaching more than 15 meters (50 feet) in length, whale sharks are capable of long-distance migrations of potentially thousands of kilometers. The whale sharks in Cendrawasih Bay, however, seem largely content to stay put and feast upon the abundant silverside baitfish which seem to be their preferred food in the Bird’s Head Seascape.
This is of course great for tourism development, as this is one of the few places in the world with guaranteed whale shark sightings year-round. However, it goes against the common wisdom that whale sharks are highly migratory animals that move to target seasonally plentiful food sources like synchronous coral or tuna spawning. While we’ll never know for sure, we think there is a very good reason for this: The baitfish which the whale sharks feed on in Cendrawasih are abundant year-round. Why would you migrate away from an all-you-can-eat buffet?
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2. Sometimes they take lengthy, mysterious “road trips.”
Since late December 2015, we’ve witnessed some remarkable movements. Two of our sharks left Cendrawasih Bay and headed southeast along the coast of New Guinea, moving into Papua New Guinea’s waters. Others left the bay and went out past the island of Biak into the western Pacific, with one continuing up past the Micronesian island of Yap and into the southern Mariana Trench, and another past Palau and over to the east coast of the Philippine island of Mindanao. Two others traversed the top of the Bird’s Head and visited Raja Ampat.
Click on this map to watch whale sharks swim in real-time. While most of our tagged whale sharks have spent the majority of their time in Cendrawasih Bay, some have made some long-distance journeys out of the bay — though they always come back!
The map shows the track of “Moby,” a 5-meter (16-foot) male that swam north to Yap and the southern Mariana Trench before returning to the top of the Bird’s Head via Palau — a route of over 4,000 kilometers (2,485 miles). The green circles indicate the first location data, with the red circles indicating the most recent data.
Perhaps even more surprising than these long journeys is the fact that most of these animals have since returned to Cendrawasih Bay; it seems these travels were little more than quick “road trips.” We have no idea why they went so far — one shark covered 4,000 kilometers (2,485 miles) before returning to the bay) — for such short periods of time. Although we suspect that the journeys of some of the larger males might be related to mating, several of these traveling sharks have actually been juvenile males under 4.5 meters (15 feet), leaving us scratching our heads!
3. Whale sharks can dive really deep.
While at “home” in southern Cendrawasih Bay, the whale sharks are feeding in a relatively shallow area of the bay (never more than 100 meters, or 330 feet, deep), as the baitfish seem most abundant in nearshore areas. However, when the sharks move further afield, they are undertaking some truly amazing dives: 10 of the 16 have now dived below 625 meters (more than 2,000 feet), while two of the smaller sharks tagged have now dived below 1,800 meters (nearly 6,000 feet!). Are they diving to find food, or for other reasons? It’s still unclear.
Fortunately, whale sharks aren’t always in deep water, allowing researchers to free dive with them. (© Shawn Heinrichs)
4. We still don’t know a lot more than we do know about whale sharks.
Despite the fact that it’s hard to hide the world’s largest fish, many mysteries about whale shark ecology and life history remain. For instance, all 16 of the whale sharks we’ve tagged (and indeed, over 97% of the 100+ whale sharks that we’ve identified from the Bird’s Head) are adolescent males between 3 and 9 meters (10 and 30 feet). Where are all the females? Where are the large adults? And where are the babies? Where do they mate, and where do they give birth? It’s amazing that these very basic questions are still largely unknown for whale sharks not only in the Bird’s Head, but worldwide. It’s a reminder of just how little we know, even today, about life in the sea.
Three young lads looking for a meal. Where are all the girls? (© Shawn Heinrichs)
Since our tags have a battery life of two years, we’re looking forward to another year of data from these whale sharks. Who knows what exciting behaviors they might show? Indeed, one of the most interesting findings from our study was only possible thanks to this extended battery life. Had we been using standard “pop-up” satellite tags with a maximum deployment of only about six months, we would have never seen any of the sharks’ larger movements, which took place only after the tags had already been affixed for six months. Fortunately, these fin-mount tags are giving us the long-term data we need to begin to get a complete picture of these amazing sharks.
And now you can see for yourself: We’re delighted to announce that we’ve created a new feature on Conservation International’s website: the “whale shark tracker.” This tool will allow you to log in regularly and observe the movements of 10 of the tagged whale sharks of Cendrawasih Bay! In so doing, we hope to further promote understanding and appreciation of sharks as some of the most misunderstood but truly incredible creatures on our blue planet. Enjoy!
Mark Erdmann is Conservation international’s (CI) vice president of Asia-Pacific marine programs, Abraham Sianipar is CI-Indonesia’s elasmobranch conservation specialist and Megan Meyers is a master’s student at the University of Auckland focusing her thesis work on analyzing these sharks’ movements.
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