A decade of tracking whale sharks

Whale sharks are among the most fascinating creatures on the planet. The largest fish in the ocean, reaching sizes of up to 18 metres, the future of these sharks is now hanging in the balance. Sharks as a taxonomic group are incredibly vulnerable to human pressures – their slow growth, long lives and low reproductive output mean that their populations find it hard to rebound after a catastrophic event, and right now, that catastrophic event is us! It is thought that humans kill between 70-100 million sharks each year. Fortunately, whale shark fishing is banned in most countries, although some allow them to be landed if caught accidentally – a few years ago, a group of undercover journalists found that a single factory in China was processing around 600 whale sharks each year.

While this shark is mostly a solitary, open ocean animal, it does form predictable seasonal aggregations at around two dozen sites across the globe, timed to coincide with reef spawning events, which provide rich foraging for this plankton feeder. These aggregations are mostly made up of immature males. Galapagos is different. Here, scuba divers come from all over the world to dive with mostly large adult females at the remote island of Darwin between the months of July and October each year.

Together with Jonathan Green, we formed the Galapagos Whale Shark Project to try to understand what these sharks were doing there. We began fieldwork in 2011, and almost immediately we discovered that what we thought we knew was all wrong. We had been told that there were only a handful of sharks up there, hanging around Darwin’s Arch for the season, and that we would have difficulty getting a decent sample size. Over 150 sharks later, we now know that this was not the case!

We found that rather than an aggregation site, Darwin was more of a stopover point. While it was correct that there were only a handful of sharks there on any given day, each day there were different sharks. They were moving through. And they didn’t seem to be feeding. We placed satellite tags on them to track their movements and found that early on in the season, they would head out into the Pacific, along the Equatorial Front – an area where two surface currents slide past each other, creating vortices called “tropical instability waves” that propagate westwards along the Front. After 1000-1500 kilometres, they would turn back, and return near Darwin again. But again, they wouldn’t stay. This time however, they would continue east, all the way to the shelf-break of mainland Ecuador and northern Peru. But we struggled to get any tags to stay on the sharks later than January, so we don’t know what they do after that.

They were also much larger than at any other aggregation site, averaging 10-12 metres. And over all these years, we’ve only seen a handful of males. Their size and their distended abdomens led us to speculate that they might be pregnant. Almost nothing is known about whale shark reproduction, but a single female caught in Taiwan in the 1990s had over 300 embryos inside her. Some were in early stages of development, and others were so close to full development that scientists were able to keep them alive in tanks. Whale shark pups are around 60 centimetres long, and are almost never seen. The few that have been reported tend to be close to very deep water, or in the bellies of predators such as blue sharks or billfish. Might we have stumbled upon the holy grail of whale shark research? The almost mythical ‘pupping grounds’?

In 2017, we partnered with the Okinawa Churashima Foundation and were able to draw blood from free-swimming whale sharks for the first time ever, and take ultrasounds from nearly two dozen sharks. Imagine: ultrasound and blood from a 12-metre shark at 20 metres depth and in a 3-knot current! We were convinced that we would find evidence of pregnancy. But it was not to be. Neither blood hormonal analyses nor the ultrasounds showed any evidence that the sharks were pregnant. We did observe follicles developing in the ovaries, so you would think that we’d see the embryos and egg cases if they were there. We also found that the bulge behind the pelvic fins seems to be composed of a fairly compact, fatty deposit (and in any case is in the wrong place for bearing developing embryos).

So where do we go from here? Well, we are following several lines of investigation. Clearly, the whale sharks are not going to give up their secrets easily. All our ultrasounds were done at the back end of the season, so maybe they were pregnant earlier on, and have completed their pupping by the time they loop back to Darwin. So might we try getting ultrasounds in June or July?

Or maybe their migration has nothing to do with their reproduction and more to do with feeding – after all, those tropical instability waves are natural plankton traps. If females have different dietary requirements to males, they might migrate seasonally between feeding grounds. The shelf-break off northern Peru is quite productive… but where are they between January and June? If the tags don’t stay on that long, could we tag them elsewhere at other times?