Whale shark

The whale shark is the largest fish in the ocean, and a contender for the largest fish that has ever lived — recent size estimates for the extinct megalodon overlap with, and may exceed, the biggest whale sharks. Andrew Smith described the species in 1828 from a single animal harpooned in Table Bay, South Africa, and for the next century and a half it stayed close to a rumor: by the early 1980s, science had logged only a few hundred confirmed sightings. The largest widely accepted length estimate is 18.8 meters (62 feet), extrapolated from a shark landed in India. Read more in our guide to whale shark size.

Whale sharks are specialized filter feeders. They strain prey a few millimeters across from the water through 20 mesh-like filter pads in the gill chamber, and the throat behind those pads is only about the width of a baseball. Their teeth are tiny and play no part in feeding. Their skin, up to 15 centimeters (6 inches) thick and the thickest in the animal kingdom, gives the body structural support and insulation, and few predators can bite through it.

Measuring one is harder than it sounds, and it matters, because size is how we judge whether a population holds adults. We introduced parallel-laser photogrammetry to the species in 2011 and later used it to measure 122 sharks in Mozambique and 46 in Tanzania. Stereo-video work at Ningaloo Reef showed why it was needed: visual estimates get worse as sharks get bigger, systematically hiding the largest animals.

Two other things set whale sharks apart. Their spot patterns are unique to the individual and stable over decades — sharks at Ningaloo Reef have been re-identified more than 20 years apart — so photo-identification, matching individuals by their spots, underpins most of the population research on this page. And their bulk keeps them warm: muscle temperature changes far more slowly than the water around them, which may be what allows a tropical fish to make excursions below 1,000 meters (3,300 feet). More whale shark facts.

Whale sharks live in all tropical and warm temperate oceans, mostly between about 30°N and 30°S. Almost everything known about them, though, comes from a short list of coastal places where they gather predictably to feed. The global photo-identification database, which we co-authored the analysis of, pulled together nearly 30,000 encounters from 54 countries between 1992 and 2014, identified more than 6,000 individual sharks, and saw the number of known gathering sites rise from 13 to 20. That study also showed the pattern that shapes the whole field: more than two-thirds of identified sharks are male, most are juveniles, and individuals show strong fidelity to their site with little movement between neighboring countries. That database, now hosted on Sharkbook.ai, holds more than 17,000 identified whale sharks today.

Our own program works at several of those sites. Southern Mozambique, where our research began in 2005, is a year-round feeding area on the Inhambane coast where we have identified more than 700 sharks. At Mafia Island in Tanzania we have identified 268 over 12 survey seasons — a small catalog for a long-running site, because its sharks are highly resident within and across years. Off Nosy Be in Madagascar the Madagascar Whale Shark Project has identified over 540, arriving from September to December. In Oman, fieldwork from Muscat sits alongside a citizen-science catalog of more than 300 sharks. On the far northern Great Barrier Reef, a 2025 study led by the Biopixel Oceans Foundation and James Cook University, which we co-authored, documented Wreck Bay as the first known whale shark aggregation in the Coral Sea, the focus of the Coral Sea project we took part in. And Darwin Island in the Galápagos, where we work with the Galapagos Whale Shark Project, is the exception that proves the rule: of the 736 sharks identified there, over 95% are adult females of 10 to 14 meters (33 to 46 feet), a demographic rarely sighted elsewhere. The current Red List assessment also identifies St. Helena, in the central South Atlantic, as an important habitat for adult sharks, where reproductive behavior has been noted.

Elsewhere the picture fills in the same way, with local exceptions that matter. In the Arabian Gulf and the northern Gulf of Oman, 422 sharks were identified between 2011 and 2014, and the Al Shaheen field off Qatar, where our Qatar project has worked with Qatari partners since 2011, became the first site described as dominated by mature males. In the Red Sea, an aggregation off Saudi Arabia is split evenly between the sexes. The largest gathering ever recorded was off Mexico’s Yucatán Peninsula in 2009: up to 420 sharks in a patch of ocean about 18 square kilometers. The Philippines holds the second-largest known population, with one site in Palawan alone yielding 117 individuals in a single season. In the Caribbean, Utila in Honduras is one of the few places where whale sharks are seen in every month of the year: 95 individuals were identified there between 1999 and 2011, usually seen feeding at surface boils dominated by blackfin tuna.

Whale sharks gather where prey is dense enough to make filtering worthwhile, which in warm, nutrient-poor water is rare. At Mafia Island we sampled the water beside feeding sharks and found plankton biomass roughly ten times higher than background — 25 against 2.6 milligrams per cubic meter — and dominated by a single 10-millimeter sergestid shrimp, Belzebub hanseni. Nearly three-quarters of the sharks we watched there were feeding, most of them actively at the surface. The same inshore waters support a local ring-net fishery in which whale sharks have been entangled, so the sharks and the boats share the same patch of water.

What happens at the surface is only part of the diet. Combining stomach contents from stranded sharks with fatty-acid profiles from living ones, we found evidence that Mozambican whale sharks also feed at night, and may feed at depth: the dominant prey were mysids (small, shrimp-like crustaceans) that rise into the water column after dark, and the fatty-acid signatures, chemical traces of diet stored in the sharks’ tissue, were consistent with deep-water shrimp and lanternfish. The chemical evidence suggests deep feeding rather than demonstrating it — our feeding guide explains why those signals stay open to interpretation. Stable isotope work at Mafia later found sharks feeding largely within the local food web, with individual differences in strategy inside one small aggregation.

Feeding style varies with the food. In Qatar, sharks gather in an offshore oil field to feed on mackerel tuna eggs, and off Mexico on the eggs of little tunny. At Ningaloo Reef they alternate between passive ram filtering, swimming forward with the mouth open, and active surface feeding, including at night.

Mafia is unusual in another way. Acoustic tags on 51 sharks showed that when they stop appearing at the surface between March and September, they do not actually leave. They move deeper and slightly offshore, concentrating their activity in a core area of about 13 square kilometers (5 square miles). A core habitat that small and that predictable is good news for management: boat speed limits and net restrictions in a well-defined area could deliver real protection.

Whale sharks are capable of ocean-basin crossings, but most of the time they don’t. Our satellite tags on 15 juveniles off Praia do Tofo tracked movements of 10 to 2,737 kilometers, with habitat modeling showing the sharks choosing cooler, shallower, more productive water than chance would predict — most stayed in Mozambican coastal waters. Photo-identification and stable isotopes together showed negligible connectivity between Mozambique, Tanzania, and Qatar and site fidelity within a few hundred kilometers, which is why we argue for regional management units rather than treating the Indian Ocean as one stock.

Some individuals go much further. Tags fitted to large adult females at Darwin Island by the Galapagos Whale Shark Project recorded movements of up to 2,747 kilometers (1,707 miles), west into the open Pacific or east toward South America, several ending off northern Peru. Juveniles can range widely too: off Madagascar, two sharks tagged by the Madagascar Whale Shark Project covered at least 3,414 and 4,275 kilometers (2,121 and 2,656 miles). In the Philippines, 17 juveniles tagged with our partners at LAMAVE all stayed in Philippine waters, yet spent nearly three-quarters of their time offshore, over water deeper than 200 meters (660 feet). Globally, genetics separates Atlantic from Indo-Pacific whale sharks clearly enough to show the two rarely mix, while sightings and tracks together support basin-scale connectivity over years rather than seasons.

Their vertical range is even more striking. Pop-up tags in the Gulf of Mexico recorded sharks spending most oceanic time in the upper 500 meters (1,640 feet), with a maximum dive of 1,928 meters (6,325 feet), pausing on the way down at around 475 meters (1,560 feet) in what looks like foraging in the deep scattering layer, a dense band of small midwater animals that shows up on sonar. A shark tagged off Mozambique dived to 1,286 meters (4,219 feet) and tolerated water from 3.4 to 29.9 °C (38 to 86 °F) while crossing the Mozambique Channel. A 2026 study led by Dr. Ingo Miller of our Coral Sea partners, and co-authored by MMF, used a deep-rated tag on a 7-meter juvenile male to record a dive of at least 1,978 meters (6,490 feet), possibly as deep as 2,527 meters (8,291 feet) — the deepest yet recorded for a whale shark. Diving is rhythmic too, tracking day, night, and the lunar cycle of the reef spawning events the sharks turn up for.

Much of what we know about whale shark pregnancy comes from a single animal. A 10.6-meter (35-foot) female caught off Taiwan in 1995, nicknamed “megamamma,” carried about 300 embryos at every stage from egg case to near-term pup — the largest litter recorded in any shark, and the finding that settled a long argument over whether whale sharks lay eggs. They don’t: the eggs hatch inside the mother and the pups are born free-swimming. Genetic work on 29 of those embryos showed they were all full siblings sired by one male, suggesting females can store sperm and fertilize a litter over an extended period. Nothing comparable has been examined since. See our guide to whale shark reproduction.

Maturity comes late and differs by sex. From our laser measurements in Mozambique, half of males are mature by about 9.2 meters (30 feet); at Ningaloo the figure is closer to 8 meters (26 feet). Maturity size may be larger in cooler water, though the cause of that variation is unclear. Repeat measurements of 54 sharks over up to a decade at Ningaloo produced growth curves in which males level off at 8 to 9 meters (26 to 30 feet) while females are predicted to keep growing toward 14.5 meters (48 feet) — a divergence that may help explain why coastal feeding sites fill up with young males while the largest animals are almost never seen. Ten years of measurements in the Maldives put male age at maturity at roughly 25 years and longevity at about 130; vertebral band counts from Taiwanese fisheries gave a lower longevity estimate of 80 years. A male at the Okinawa Churaumi Aquarium matured at 8.5 meters (28 feet) and over 25 years old.

Where females give birth is still unknown. Females suspected to be pregnant have been reported in the Gulf of California, where 15 were identified at Gorda Banks alongside very small juveniles nearby. At Darwin Island, with the Galapagos Whale Shark Project and Okinawa Churaumi Aquarium, we used a purpose-built underwater ultrasound and drew blood from free-swimming sharks — both firsts for the species — and found developing follicles (structures containing immature eggs) in two females of 11 to 12 meters (36 to 39 feet), but no embryos or egg cases. That cuts against the assumption that the swollen abdomens of large Galápagos females mean pregnancy, so the reproductive state of these sharks, and where they give birth, remain unresolved.

The whale shark is Endangered on the IUCN Red List. Dr. Simon Pierce led the current global assessment and, with Brad Norman, the 2016 assessment that first moved the species from Vulnerable to Endangered. The assessment infers that numbers have fallen by 50 to 79% over the past three generations, about 120 years, driven mainly by fisheries. Roughly three-quarters of the remaining population lives in the Indo-Pacific and a quarter in the Atlantic.

Mozambique is the clearest case. Our logbook analysis at Praia do Tofo recorded a 79% decline in standardized whale shark sightings between 2005 and 2011. Extending that to a 15-year series of 706 identified sharks, we documented an 87% decline in sightings and an 89% fall in abundance. Females declined faster than males, 92% against 81%, and sharks of 7 meters or more declined by 99%. It is a pronounced decline at one of the world’s largest whale shark aggregations.

Mafia Island is the counterexample. Capture-mark-recapture models, which estimate population size and survival from repeat identifications of the same individuals, fitted to eight years of photo-identification there show a stable population with high survival — with a warning attached: only a change of about 30% would have been detectable over that period, and more transient aggregations may need 15 to 20 years of data before a trend can be called at all. That is the argument for long-term monitoring, and why we keep going back.

Elsewhere, mark-recapture models estimated that 681 sharks used the waters off Nosy Be between 2015 and 2019, from 408 identified individuals, and 2,837 across the Arabian Gulf and Gulf of Oman, from 422. Those are modeled estimates of the sharks using a region, not headcounts. A global study drawing on more than 50 aggregation sites and over 13,000 identified sharks gives the best sense of scale. Its figures are identification catalogs built over many years, not counts of animals present at once: at Ningaloo Reef, the largest known aggregation site, about 2,500 individuals have been identified since the 1980s from more than 30,000 encounters. Genetics adds an independent signal: genetic diversity at Ningaloo fell across six consecutive years, consistent with historic harvesting elsewhere in the Indo-Pacific.

Our Global Whale Shark Program runs long-term monitoring in Mozambique, Tanzania, the Galápagos, and Oman, contributes to partner-led projects in Madagascar and Qatar and off Mexico’s Yucatan Peninsula, and has completed studies in the Coral Sea in Australia, the Philippines, at Utila in Honduras, and on Mexico’s Pacific coast. The results feed into global assessments. Photo-identification is the backbone: we contribute images from our surveys, dive operators, and visiting divers to Sharkbook.ai, Wild Me’s shark photo-identification platform (see our whale shark publications).

Around that catalog we run laser photogrammetry and stereo-video for size and demography, satellite tags for large-scale movement and diving, acoustic arrays for fine-scale residency, tissue sampling for genetics and stable isotopes, and underwater ultrasound and blood hormones for reproduction. Dr. Simon Pierce leads the program, the Oman project, and our contributions to the partner-led Galápagos project, and took part in the completed Coral Sea project; Dr. Chris Rohner leads the Tanzania project and the population modeling and tracking; Dr. Clare Prebble leads the Mozambique project and the biochemical ecology; Dr. David Robinson leads the Qatar project; and Stella Diamant leads the Madagascar Whale Shark Project.

The questions now are specific: whether the Tofo population can recover under national protection, whether Mafia Island’s 12-season time series is long enough to detect a change in that population, what a 20-station acoustic array says about year-round use of Madagascar’s proposed Tandavandriva corridor, where the Galápagos females go to give birth, and why half of the 30 sharks we identified in Oman in 2025 bore vessel-strike injuries.

Fisheries did the historic damage. Targeted fisheries operated off Taiwan, India, and elsewhere through the 1980s and 1990s. India granted whale sharks its highest legal protection in 2001, yet 79 were still landed between 2001 and 2011 as bycatch, and a survey in China documented 186 sharks caught and traded. They are also landed irregularly in Indonesian artisanal fisheries and encircled in tuna purse-seine nets, where tagging shows they can survive release if handled well. In 2025 the Indian Ocean Tuna Commission adopted a conditional ban on retaining whale sharks caught in its fisheries, which takes effect only once its Scientific Committee recommends it; in 2026 we co-authored a paper led by James Bell of Cefas setting out the evidence for that recommendation.

Ships are the threat that took longest to see. Combining satellite tracks with vessel traffic, an international study we contributed to found that 92% of whale sharks’ horizontal space use overlaps with persistent large-vessel traffic, that collision risk correlated with reported ship-strike deaths, and that depth records from several tags showed sharks sinking — probably dead — after passing through busy routes. That “cryptic” mortality may explain why populations keep falling despite international protection and low fishing pressure. A follow-up ranked priority sites for collision management and found most core habitats small enough that, in the study’s simulations, rerouting or slowing ships would have potentially minimal impact on industry. Wounds do heal — major injuries close quickly, at rates that vary with the type of wound — but the scarred sharks we can photograph are the ones that survived, so visible injuries cannot measure the real death toll: whale sharks are unlikely to survive propeller or impact wounds from large vessels. In the Maldives, injured sharks stayed longer in the tourism area, which is not a good sign for their condition.

Tourism is both a threat and the reason many countries protect the species. Our work at Tofo Beach showed that boat proximity predicts avoidance and shortens encounters, and produced Mozambique’s first code of conduct; work in the Philippines found that motorized vessels close by shortened encounters, and touching changed the sharks’ behavior. Managed well, the value is real: US$1.5 million from one three-month season at Nosy Be, and an estimated US$2.4 to 3.6 million in visitor spending at Mafia Island in the 2025 season, where 46% of visitors surveyed said they would not have come without the sharks.

Legal protection is now strong on paper. Whale sharks have been on Appendix II of the Convention on Migratory Species since 1999 and were uplisted to Appendix I in 2017, obliging the range states that are parties to the convention to protect the species and its habitat; MMF led the technical proposal behind that uplisting.Trade was regulated under CITES Appendix II from 2003, and at the 20th Conference of the Parties in November 2025 whale sharks were uplisted to Appendix I — a full ban on commercial international trade — in force from 5 March 2026. Mozambique banned their capture under its 2020 marine fisheries regulations, in force from January 2021, after years of our research and advocacy, and Madagascar wrote a tourism code of conduct into law in December 2023. Protection on paper is a starting point, and the scale of the Mozambique decline, recorded before that country’s protection took effect, shows why it has to be enforced.

You can support this work by adopting a whale shark, or see the sharks yourself on one of our expeditions.