About eighty kilometres off the north-east coast of Qatar, above the Al Shaheen oil field, something remarkable happens between May and September. Whale sharks arrive — sometimes more than a hundred at once, occasionally many hundreds. It is one of the largest known gatherings of the species anywhere in the world.
The largest fish, and the gentlest
A whale shark, Rhincodon typus, can grow beyond 12 metres. Despite the size and the mouth more than a metre wide, it is a filter feeder. It swims forward with its mouth open, passing water across spongy filtering pads that trap plankton and small prey, then pushes the water out through its gills. Thousands of tiny teeth play no role in feeding at all.
Following the eggs
Marine biologists studying the Qatari aggregation collected samples from the water where the sharks were feeding and found it dense with the eggs of mackerel tuna, which spawn in the area in the same months. Analysis of the sharks' stomach contents and the isotopes in their tissue supported the same conclusion: the gathering is a feeding event that tracks a seasonal food pulse.
The sharks are also mostly juveniles, and males outnumber females — a pattern found at several other aggregation sites and still not fully explained.
Why counting them is difficult
Whale sharks carry a unique pattern of pale spots behind the gills, as individual as a fingerprint. Researchers photograph the left flank of each animal and match it using software originally developed by astronomers to compare star fields. Photographs from tourists and boat crews feed into the same shared database, making this an unusually successful example of citizen science.
Satellite tags add movement data, showing that individuals seen in Qatari waters also travel to the Red Sea, the Gulf of Oman and beyond.
Protecting a visitor
Whale sharks are listed as endangered globally. Their threats are collisions with ships, entanglement in fishing gear, and disturbance from boats crowding feeding animals. Qatar has protected the species in its waters and set rules for how closely boats and swimmers may approach.
Conservation of a migratory animal has an awkward feature: protection in one country is undone by harm in another. That is why regional agreements, not just national ones, decide whether a summer gathering keeps happening.