Whale Shark Spot Patterns | Ocean Calling Retreats

Huge spotted whale shark gliding through open blue water. Photo: NOAA, Unsplash

What a whale shark's spot pattern reveals

Look closely at a whale shark's flank and the pale spots on its grey skin look almost decorative, like something designed rather than grown. A diver drifts beside it for a moment, takes one photo of the area behind the gills, and swims away. Months later, somewhere else in the world, a computer reads that photo and says: I know this shark.

That is the short answer to how whale shark spot pattern identification works. Each shark's spots are unique and stable enough to work like a fingerprint. And the method used to read them did not come from marine biology at all. It came from astronomy. Thanks to it, ordinary holiday photos have become one of the most useful tools researchers have for studying the world's largest fish.

A pattern that works like a fingerprint

Whale sharks are hard to study. They travel long distances, spend much of their time deep or far offshore, and you cannot easily put a tag on every one. Telling individuals apart is the first step to some simple but important questions. How many sharks use a site? Do the same ones come back? How long do they stay? Is a population growing or shrinking?

Researchers realised that the spots and stripes on the skin, especially just behind the gills, could tell individuals apart in photographs. A photo is cheap, harmless when taken properly, and anyone in the water can take one, not only scientists.

How does an astronomy algorithm identify a whale shark?

In 2005, Zaven Arzoumanian, Jason Holmberg and Brad Norman published a paper in the Journal of Applied Ecology describing a way to identify individual whale sharks by matching their spot patterns with a computer. They borrowed the idea from astronomy. They treated the spots like bright stars in the night sky and adapted a star pattern-matching method, often described as the Groth algorithm, that astronomers had used to match star fields in telescope images, including from the Hubble Space Telescope.

The logic transfers surprisingly well. A star chart is a set of points at fixed positions relative to each other. It can be matched to another image of the same sky even if the angle or exposure is different. A patch of whale shark skin is also a set of points at fixed positions. Another diver, on another day, at another angle, can photograph the same shark, and the pattern still matches. The software only needed to look for spots instead of stars.

The method became the engine behind a global photo-identification library, first built by the conservation group ECOCEAN and now part of the Wildbook platform. Anyone can upload a clear photo of a whale shark's flank, and the software checks it against the database. One later account reported that the library held more than 26,000 photos of over 2,100 sharks identified this way. It is a lovely example of citizen science working as it should: a diver shares a photo, and that photo becomes a real data point, with no biologist needed in the water.

What this data has told researchers

Years of matched sightings really pay off. A 2025 study in Frontiers in Marine Science, led by Edy Setyawan, used photo-identification data collected between 2010 and 2023 in Indonesia's Bird's Head Seascape, across Cenderawasih Bay, Kaimana, Raja Ampat and Fakfak. From 1,118 sightings, the team identified 268 individual whale sharks by their spot and stripe patterns, using photos from researchers and citizen scientists. About 98% of the sightings were in Cenderawasih Bay and Kaimana.

Sharks in Cenderawasih Bay stayed for about 77 days on average, much longer than in Kaimana, and one shark was seen again across a span of more than ten years. The population is mostly young males, around 4 to 5 metres long, which suggests the region may be an important nursery area.

The researchers also recorded injuries. About 77% of the sharks carried some scar, mostly minor abrasions and nicks in the fins. Science news coverage summed it up with a sobering headline: most of the whale sharks in the area carry scars caused by humans, while serious injuries from natural causes were much less common. The authors linked many injuries to contact with fisheries and tourism. The lead author recommended seasonal tourism closures in Cenderawasih Bay and better protection of migration routes and feeding areas. None of this would be knowable without a reliable way to tell one shark from another, across years of encounters by different people.

Why it matters: conservation status and ethics

All of this feeds into conservation. In July 2016 the IUCN Red List moved the whale shark from Vulnerable to Endangered, in an assessment led by Simon Pierce with Brad Norman. As reported at the time, the assessment estimated a global decline of around 50% over three generations, about 75 years, with a steeper decline in the Indo-Pacific, where most whale sharks live. The main threats named were fishing, both targeted and accidental, and collisions with boats. Photo-identification is one of the main tools for estimating how big regional populations are and whether they are recovering.

There is an ethical point here too. A photo is only a gift to science if taking it did not harm the animal. Crowding a shark, touching it, blocking its path or chasing it for the perfect angle turns a research contribution into a disturbance. Sites where whale sharks are fed to keep them in one place raise their own questions, which we look at in how to tell a wild whale shark tour from a fed one. Ocean Calling never feeds, chases or touches wildlife, and we never promise a sighting.

What this means if you ever swim near one

If you are ever in the water with a whale shark during a properly regulated encounter, a clear photo of the area behind the gills, taken calmly and without crowding the animal, is a small, real way to help. Keep your distance, stay to the side and never in front, and follow the local code and your guide's briefing. Wildlife encounters are unpredictable. Swimming with whale sharks should always follow local regulations and experienced professional guidance. Our overview of ethical whale shark encounters worldwide explains how rules differ between places.

A compact underwater or action camera is enough for an ID photo. You do not need to get close. You need a steady, side-on shot. A rash guard protects your skin from the sun during long surface time, so you can wait calmly instead of reaching for sunscreen between passes. If you are curious about other animals researchers study with photo-ID, our guide to where to see manta rays responsibly is a good next read.

A whale shark's spots were never meant to be read. But a method built to chart the night sky turned out to be the right tool anyway, and two decades of ordinary divers' photos have quietly become one of the better datasets science has for this giant fish. If this made you curious about meeting the ocean this way, you can explore our retreats.

Questions people ask

Is every whale shark's spot pattern unique?

Researchers use the spots and stripes behind the gills to tell individuals apart in photos, the same way a fingerprint is used. The method was published in the Journal of Applied Ecology in 2005.

How does the computer match the spots?

It uses a pattern-matching method adapted from astronomy, which treats the spots like stars and compares their relative positions between photos.

Can I submit my own whale shark photo?

Yes. Global photo-ID libraries built on this method accept photos from the public. A clear, side-on shot of the area behind the gills is the most useful.

Is the whale shark endangered?

Yes. The IUCN Red List moved it from Vulnerable to Endangered in 2016, with fishing and boat strikes named among the main threats.

What did photo-ID reveal in Indonesia?

A 2025 study identified 268 individual whale sharks from 1,118 sightings in the Bird's Head Seascape between 2010 and 2023. Most were young males, and about 77% carried scars, many linked to contact with people.

Wildlife encounters are never guaranteed and never forced. Ocean Calling works only with vetted, ethical local operators, and every animal is free to approach, or not, entirely on its own terms.

Sources & further reading

Further reading on Ocean Calling: More from the journal · Wildlife & the law · See the retreats · Ocean Atlas

Whale Shark Spot Patterns: How Scientists Identify Sharks