American Scientist

When I began my academic career, I didn’t seek out the whale shark. Indeed, for a long time I didn’t expect I would ever see one, let alone study them. Then in the spring of 2008 I took a job as a resident marine biologist at the Four Seasons Resorts in the Maldives. This job opened up a world of marine delights to me, and a few weeks later, I swam with my first whale shark.

It was a strange encounter, and not how I imagined it would be. Squinting through my scuba mask into the inky depths, it took me several long seconds to recognize what I was seeing. Far below me, a carpet of white spots was moving slowly forward; no giant fins, no colossal tail, just the hint of a great shape fading into the blue.

This arcane first meeting, which in my mind epitomizes the mystery of the whale shark, immediately engaged my curiosity. Fortunately, I was in the right place at the right time to ask—and answer—questions about whale shark biology and conservation, and for the next two years I studied the species, initially as a pet project but later as part of my master’s degree.

Biology of an Ocean Giant

I am one of many to be intrigued by the enigmatic whale shark (Rhincodon typus). This ocean giant was first described by Scottish surgeon and naturalist Andrew Smith from a specimen caught off the South African coast in 1828. For much of the next century the whale shark seemed more legend than living, the subject of occasional mariners’ tales but little scientific scrutiny. Only since the 1990s, when a handful of scientists began to piece together the existing knowledge and set out to collect more, has research slowly begun to unravel the ecological story of the whale shark. But even today, much of the creature’s biology and ecology remains a mystery.

Whale sharks—the largest fish, thought to grow more than 15 meters long and possibly live up to 100 years—are ocean wayfarers. The whale shark’s enormous body, wing-like pectoral fins, and large, powerful tail are tailored to a life of almost perpetual movement. Yet this wandering lifestyle is in sharp contrast to its closest relatives, the carpet sharks (Orectolobiformes), which live a comparatively sedentary life on the sea floor. The most obvious clue to the whale shark’s heritage is its elaborate pattern of spots, which if viewed from above bears semblance to a luxurious and ornate carpet.

Although the extent of the whale shark’s wanderings is not fully understood, they are capable of epic journeys. Several tagged individuals have been tracked on journeys greater than 7,000 kilometers in a matter of months, moving through the waters of multiple countries along their route. Although the purpose of these journeys is not always clear, explanations may include movement between seasonal feeding grounds (the timing of the whale sharks’ annual arrival at some sites is uncanny) or migration to and from breeding sites.

Dissimilarities between the whale shark and its bottom-dwelling cousins do not end with its cosmopolitan lifestyle. Whale sharks are filter feeders, a feeding mechanism the species shares only with the unrelated basking and megamouth sharks. Its diet is made up of a variety of tiny organisms, filtered by driving water across its massive gills. Unlike other filter feeding sharks, which must move forward through clouds of prey to feed, whale sharks are able to hang motionless in the water column and suck water across their gills. This enables them to catch larger, more mobile prey, including small fish and squid.

Despite the whale sharks’ wandering nature, most of what we know about them has been learned from research at a dozen major coastal feeding sites around the world, where the sharks gather, often in immense numbers, for a few short weeks to feed. These aggregations coincide with periods of great microscopic bounty, although this menu can be diverse. In Gladden Split, Belize, whale sharks feed on the eggs of spawning snappers. In the Gulf of Aden between Yemen and Somalia, whale sharks vacuum up small schooling fish. In the Maldives, where I began researching the creatures, whale sharks visit the northern atolls during the southwest monsoon to feed on a super-abundance of tiny krill.

Curiously, the vast majority of the sharks that attend these feeding frenzies are relatively small, immature individuals. Large adults are occasionally encountered, but only rarely, begging the question, “Where, then, do they feed, and why?” We don’t yet know the answer, but with increasingly sophisticated and affordable research technology, it is only a matter of time until we do.

Monitoring Whale Shark Populations

In the Maldives, as in many other parts of the world, the status of the local whale shark population is an important unknown. We don’t know how big the population is, or whether it’s increasing or decreasing, but we do know that despite their size, whale sharks are not immune to human threats. The hunting of whale sharks for their meat and oil-rich livers was once globally widespread, although this practice has now largely stopped. But in a handful of places, including in some parts of the Indian Ocean, whale sharks continue to be exploited illegally for their immense and valuable fins. These are dried out and used to advertise shark fin soup (though not used for the soup itself).

Like most other elasmobranchs—a subclass of animals that includes sharks, rays, and skates—whale sharks have slow growth rates and reach sexual maturity late, making them particularly vulnerable to overfishing. Evidence for declining catches over the past 15 to 20 years has placed the species on the Red List of Threatened Species, generated by the International Union for Conservation of Nature, and on the appendixes of several international conservation conventions. In addition, the effects of climate change on the distribution and migration patterns of whale sharks are a major uncertainty, and prediction of how populations might respond in the future is hampered by a lack of information on whereabouts, numbers, and biology of the species.

Monitoring the abundance of a species is a cornerstone of conservation science and has become a priority in whale shark research. To this end, my colleagues and I have been investigating the size of the population and the ways it is changing. To do this we used a technique known as capture-mark-recapture, which typically involves capturing an animal, marking it in some way, releasing it back into the population, and subsequently making a record each time it is seen again. This technique is frequently used in ecology to estimate survival rates, population decline or increase, risk of extinction, and other important life-history characteristics.

The capture-mark-recapture approach is not without its challenges, the greatest being that it requires the capture of an individual in some way for marking. Fitting a whale shark with a physical mark, such as a tag, is a difficult and potentially dangerous job. Although this has been done for whale sharks in many parts of the world, the process is slow and cumbersome. Moreover, unless large numbers of sharks are tagged, at considerable cost, capture marking is not particularly effective.

Fortunately, whale sharks are ideal candidates for another, far easier, method of tagging: photography. This noninvasive technique requires only two things: photographs of sharks and the ability to identify individuals from these images. On the first point, whale sharks are large and swim relatively slowly at the surface, so they can be easily photographed. On the second, an individual’s pattern of spots and stripes is unique, meaning that individual sharks can then be easily recognized in images.

There is good evidence that these markings remain unchanged over time, meaning that a photograph creates a visual record—in effect a virtual tag—that can be used to match individuals again and again within a reference library of photographs.

Possibilities through Social Media

Photographic mark-recapture comes with its own drawbacks. Most crucially, taking photographs of whale sharks, enjoyable as it may be, can be time consuming and expensive. This can at best slow down research and at worst result in insufficient data for mark-recapture analysis. We faced this stumbling block in the Maldives, but one morning, when posting some of my recent photos on Facebook, I realized that the Internet—and particularly the rapid growth of social media—could offer a potential solution.

Thousands of vacationers swim with, and photograph, whale sharks every year. These images are increasingly finding their way online, and websites such as Facebook, Flickr, and YouTube have consequently become vast, unappreciated databanks for monitoring the largest fish in the sea. In most cases, the social media sites offer open access for researchers to download these images. When subsequently contacted and asked for further details of their encounter, people are often delighted that their holiday snapshots might be useful for scientific research.

One crucial issue I worried about was the quality of these amateur images and the manner in which they are collected. Mark-recapture models make numerous assumptions regarding both the population under study and the process of marking and recapturing, which can usually be met where data are collected methodically. Publicly sourced data, which are opportunistic by nature, can easily violate these assumptions, and until recently the usefulness of this source of data was unknown.

In 2012 my colleagues and I published a study in Wildlife Research addressing these concerns by demonstrating the value of travelers’ data in whale shark research. In this study we compared estimates of whale shark abundance from two data sets, the first using photos collected methodically by researchers and the second using images that were sourced online from sightseers. We found no evidence that the data collected from travelers violated the mark-recapture model assumptions, and crucially the population estimates based on the two alternative data sets were almost identical.

These encouraging results were largely thanks to the sheer number of images we had access to, enabling us to capture a shark using the best quality image available, then go back through poorer quality images to “recapture” that same shark. That approach meant that we were not constrained by a linear time line of data collection, and could sift through images to build up extensive capture histories for many different sharks.

Our findings demonstrate that photographs sourced online can be used with confidence to both measure and monitor whale shark populations in the Maldives and potentially further afield, with minimal research costs. These results have paved the way for long-term monitoring of whale sharks in the region harnessing the power of citizen science, which is being taken forward by the Maldives Whale Shark Research Programme (MWSRP).

In 2013 the MWSRP designed a platform to increase the role of citizen science by working with tour guides from resorts, dive centers, and safari boats to photograph and collect useful information on whale sharks in the Maldives. The mobilization of this diverse citizen scientist network effectively allows more time on the water across a far larger area than researchers alone could cover.

In addition to increasing the scope of data collection, the MWSRP’s citizen science initiative aims to increase local guides’ knowledge of whale sharks and furnish them with the ability to identify the whale shark they encounter using the same open-source identification software used by many whale shark researchers worldwide. This further increases the public’s involvement in research and increases the pace at which data are collected and analyzed.

The Future of Citizen Science

More and more tech-savvy researchers are becoming aware of the potential wealth of data at their fingertips, as demonstrated by burgeoning citizen science research and crowdsourcing across many fields of science. And the culture of sharing information online is expanding and unlikely to go away. In the case of whale shark research, digital underwater cameras are getting cheaper and producing better quality images, and sharing images online is becoming ever easier.

For the whale shark—and for other species, such as manta rays, that can be monitored in a similar way using photographs—we may be entering an exciting new era of conservation biology. Before long, I believe citizen science will transform what we know about the populations of some of the most mysterious creatures in the ocean.