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"Afuera" The aggregation of whale sharks off Isla Mujeres

Updated: Aug 12, 2023

These majestic giants can grow to up to 12 meters. Their concentration on feeding allows snorkelers to have respectful close encounters with these behemoths of the ocean
Swimming with the whale sharks of the Afuera

Every year, from May until September, you can view something truly magical happening off the coast of Isla Mujeres. Literally, hundreds of Whale sharks (Rhincodon typus) gather to feed in what local fishermen call the "Afuera" (Outside, or in this case, offshore), so named for the deeper waters off the tip of the Yucatan.

A dozen locations around the world attract whale sharks in moderate numbers, such as Western Australia, Indonesia, and Belize but off the coast of Quintana Roo, more numbers are attracted than anywhere else in the world, with an estimate of around 800 in a season.

Afuera attracts mainly male whale sharks to the area to gorge themselves on an unlimited supply of nourishment. Generally, these gentle giants are quite solitary, and aggregations of 15 to 20 sharks are considered a large gathering. However, the largest aggregation ever recorded happened here, off the Yucatan Peninsula 2009, with at least 420 spotted in a single survey. With the size of a whale shark compared to a bus, imagine what it would be like to be surrounded by this many buses with their hoods open.


Although this massive gathering might have been happening for many years, known only to local fishermen, it was only as recently as 2002 that the first rumours emerged to the scientific community. Generations of fishermen knew about the whale sharks and followed them to good tuna or cobia fishing. Still, the scientific community was blissfully unaware that something extraordinary was taking place.

Shark Biologist Roberto Hueter, director of the Center for Shark Research at the Mote Marine Laboratory, had been travelling to Quintana Roo nearly every year since 1994 to study, almost exclusively, the blacktip shark until a fateful conversation with a local Mexican fisherman in 2002. The fisherman casually mentioned that hundreds of whale sharks gathered nearby each year, to which Hueter famously replied, "Come again?"

Scientists began to survey from the sea and sky, electronically tagging sharks and pinning ID tags on them, documenting the size of the local whale shark population, the sex ratio, and the individual shark sizes. What followed was the most extensive surveying of sharks ever, spanning the next nine years. The sheer amount of data compiled was incredible. Surveying had also commenced in Australia, and while there is still so much to learn, because of this information, many of the whale sharks' secrets have been divulged, and conservation efforts have been significantly aided.

In 2003, due to this sudden increase in interest in the whale shark, Willy Sabatina and other local fishermen began experimenting with organized whale shark tours that had already proven popular in Australia. Holbox transformed quickly from a fishing economy to an ecotourism destination to view these whale sharks, which soon extended to Isla Mujeres and Cancun. The large number of whale sharks in this area prompted CONANP to establish the Domino Project, a project-based and sponsored by CONANP / SEMARNAT from 2003 to 2009. To help improve conservation and management as a sustainable resource, a biosphere was established in 2009 called Reserva de la Biosfera Tiburòn Ballena

The triangular leg to the east of 86°45′W was added to the original survey design to incorporate the newly-discovered Afuera whale shark aggregagtion. Waypoints were marked on GPS instrumentation to ensure accurate repeatability of the same path.
Flight path followed on each aerial survey for whale sharks off the coast of Quintana Roo in September 2006

It was in September 2006 that a larger, separate aggregation, the Afuera, was sighted in the deeper offshore waters between Isla Mujeres and Isla Contoy. These sightings confirmed information from local fishermen from as early as 1991. Led by Rafael de la Parra Venegas, 5 ariel surveys were made; what the surveys revealed astonished everyone involved, and during which over 480 sightings were confirmed.

The Afuera is not the gathering of whale sharks near Holbox but something altogether different and unique. This unparalleled biological congress occurs in the deeper, crystal-clear turquoise waters of Isla Mujeres and Isla Contoy.

In 2009 the largest aggregation of whale sharks ever recorded was spotted in a single survey flight, with over 421 whale sharks recorded.

Why are they there?

The whale sharks near Cabo Catoche (Holbox aggregation) had been known to be feeding in dense patches of seasonal spawning crustacean zooplankton (copepods and Sergestiod shrimps) in waters that were closer to shore, more green, turbid, and distinctly shallower; being 6-20 meters deep. So what was attracting the larger aggregation to the deeper blue waters of the Afuera?

Investigations by Rafael de la Parra Venegas revealed vast numbers of fish eggs, namely Little Tunny, which was identified by DNA testing at the Smithsonian using a technique called barcoding. The density of eggs had to be large, considering the incredible number of whale sharks attracted to the area in preference to the abundance of zooplankton at the Cabo Catoche feeding site. The density was so large upon collection that it clogged the plankton net in as little as 30 seconds.

The whale shark is gorging itself on the abundant Little Tunny eggs.
Whale shark ram feeding on Little Tunny eggs at the Afuera

Little Tunny

The Afuera begins not with the whale sharks but with tuna. The Little Tunny mentioned above, to be precise. The Little Tunny (Euthynnus alletteratus) are prolific breeders. They are the most common tuna in the Atlantic Ocean, also called Bonita, False albacore, and Little tuna.

Spawning generally occurs in the Atlantic between April and November when water reaches 25ºC (77ºF) in waters 30-40 mt deep, with the most intense spawning occurring between July and August; this perfectly describes the conditions and timing of the Afuera.

Little Tunny - Euthynnus alletteratus

The females are prolific fish and can release 1.75 million eggs in multiple clutches over a mating season. The females release their eggs to fertilize in the water column after the males release their sperm.

The eggs are round, transparent, and buoyant because they contain a droplet of oil that allows them to be pelagic and float in the ocean current.

The spawning happens at night in an as-yet-unseen shoal, and it must be quite a substantial group because it supports hundreds of whale sharks for several months of uninterrupted gorging. Thus it can be concluded that the whale shark gathering is not only an incredible occurrence, but an important scombrid spawning event of goliath proportions, previously unknown to science, is also taking place.


So just how much DOES a whale shark eat? According to one survey by Motta et al. in the journal Zoology, a huge amount!!

The authors studied whale sharks from the Yucatan aggregation and careful measurements taken into consideration the amount of time feeding (approx 7.5 hours a day), the amount of water they took in (600 cubic mt per 6 mt sharks), and the amount of food filtered (2.8 kg for the above-mentioned shark). With all these variables, it was calculated that a 6mt shark would eat around 21 kilograms (46lb) of plankton a day. Imagine!! This is just a juvenile.

How do they eat?

The whale shark is one of only three living species of filter feeders, along with the basking shark and megamouth shark; despite their apparent similarities, the other sharks are only distant relatives.

The whale shark has many rows of teeth called vestigial teeth (300-350 rows of them) that play no role in feeding.

Feeding occurs through two methods.

Ram Filtration - the whale shark opens its mouth and, by swimming forward, pushes all the food and water into the mouth.

Active suction - the whale shark opens and closes its mouth and sucks in vast volumes of water, which it then expels through the gills.

Gill rakers

In both cases, the food is separated from the water by filter pads that are black sieve-like structures, presumed to be modified gill rakers. The whale shark has 20 unique filter pads quite different from the other filter-feeding sharks.

Cross-flow filtration or tangential flow filtration

Food separation is caused by cross-flow filtration (or tangential flow filtration). This extremely efficient, anti-fouling filter works by passing water parallel across the filter pads rather than perpendicular. Water passes to the outside, and the food particles continue to the back of the throat. Whale sharks have been observed to be "coughing", which is assumed to be clearing a build-up of particles on the filter pads.


The whale shake is an active feeder. It can either ram the food (Ram filtration) as discussed above or gulp (Active suction) in a stationary position called "Botello" (or bottling). The whale shark assumes a vertical position and vacuums up every last morsel of food, be it zooplankton, crustaceans, or tiny tunny eggs. They are so fixated on this task that they can appear entirely oblivious to anything else, allowing for a close encounter with these gentle giants. However, specific rules must be respected to enjoy this privilege; we will discuss this later.

About the whale sharks

COMMON NAME: Whale Shark

SCIENTIFIC NAME: Rhincodon typus

TYPE: Fish; Actually, the world's largest fish and largest shark

ARE NOT: Whales

DIET: Carnivore, mostly plankton and small fishes


AVERAGE LIFE SPAN IN THE WILD: 70 years, possibly up to 100 years

SEXUAL MATURITY: Reached between 18-30 years

REPRODUCTION: Possibly giving birth and/or incubating up to 300 pups at a time

HABITAT: Open ocean in tropical seas around the world

SIZE: 18 to 32.8 feet (5.4 - 11.5 mt)

WEIGHT: 20.6 tons

SIZE: Relative to a bus

The whale shark belongs to the subclass Elasmobranchii which includes sharks, skates, and rays, and there are three main features of this subclass.

1 Their skeleton is made of cartilage

2 Their skin has denticles instead of scales that make the skin extremely tough

3 They have five to seven gill slits on each side of their head

Also, internally they do not have a swim bladder for buoyancy, but a large oily liver is thought to compensate.

For more information about shark anatomy, click here.

This largest fish of the sea can reach up to 12 mt (40 feet) or more, with a mouth span of up to 1.5 mt (5 ft). Their favourite food is plankton. Not only is the whale shark the largest fish, but it is also the heaviest, rivalling Tyrannosaurus rex and other giant dinosaurs in size and weight. They are also the largest animal that can be approached without real danger, according to Hueter and Moet's colleague John P Tyminski. The largest whale shark ever recorded was accidentally caught in Bombay in 1983 and was 12.18 mt long. A larger specimen was caught in 1925 in the Gulf of Siam, but the 18 mt lengths may have been overestimated.

Anatomy of a whale shark

The whale shark has a flattened head with a blunt snout above the mouth and short barbels that protrude from its nostrils. It is grey to brown on its back and sides with white spots along pale vertical and horizontal stripes. The belly of the whale shark is white. It has two dorsal fins rearward of its body and ends in a large dual-lobbed caudal fin (or tail)

Vestigial teeth
Vestigial teeth

Of the 6000 vestigial teeth (although some say 27.000 while others state 3000, so who really knows?) that the whale shark has, they are rarely used. Their appearance is like a rasp on each jaw - hence the name Rhineodon (the original spelling), which means "rasp-tooth". A harsh noise often heard by pearl fishermen when the whale shark is nearby is thought to be produced by grinding these together.

The skin on the back of a whale shark is the thickest and toughest of any species in the world. It is also said that whale sharks have extraordinary healing powers, healing from deep gashes without scarring.

The eyes are small for a creature that spends much of its life in darkness, suggesting eyesight is not an essential sense to the whale shark. They have no eyelids but can withdraw the eye into the head, rotating it as it does, as a protective measure.

The nasal grooves above the mouth of the whale shark allow for a continuous stream of water to flow past the olfactory apparatus that attribute to an extremely sensitive sense of smell. Foraging in the night and dark when vision is of little use means detecting chemicals in the water is paramount in locating prey.

The most important sense is the whale shark's sixth sense. They have nerve endings that can detect electromagnetic fields in the water. On both sides of the shark exist small pores which communicate with the lateral nervous system. These openings are known as the Ampullae of Lorenzini.

Although whale sharks are massive, they are docile fish, and sometimes people take advantage of this to ride them. DO NOT DO THIS! Just because a creature is docile does not mean we have the right to harass, injure or aggravate them.

Unfortunately, they are currently listed as a vulnerable species and continue to be hunted in parts of Asia, such as the Philippines.


For unknown reasons, most of the whale sharks in these global aggregations are male. It is hypothesized that the females complete long journeys to give birth, but where the fish mate and where they give birth remains a mystery.

On the 15th of July 1995. an 11 mt whale shark was harpooned off the East coast of Taiwan while swimming Southward in the Kuroshio current. 304 embryos were taken from the twin uteri, all in different stages of development.

29 of the embryos, representing 10% of the original litter and spanning all stages of development and size, were studied, and it was concluded that they were all fertilized by the same father suggesting that this same male had sired the entire litter. This led to cautious conclusions by Jennifer Schmidt, a biologist at the University of Illinois, that female whale sharks mate once, store the sperm, and fertilize their eggs.

"If mating occurs randomly when a male and female happen to meet, this would provide the female with a sort of reproductive' insurance policy,'" Schmidt told LiveScience. "She has sperm available to continue to fertilize her own eggs even if she doesn't encounter another adult male."

No one has ever seen whale sharks mate, and pregnant females are not a common sight, with only three ever reported. It is, therefore, difficult to draw concrete conclusions, but it is unheard of to have monogamy amongst sharks as most sharks mate with multiple males. The study above, however, seems to indicate otherwise in whale sharks.

Like many shark species, the females give birth to live young rather than laying eggs in leathery cases. By birthing at different times, they may avoid giving birth to many babies all at once, becoming a buffet for predators. Spreading them out over different locations spreads out that risk.


Researchers have learned that whale sharks migrate thousands of miles yearly and unite at global meeting points to gorge on plankton, krill, and fish eggs.

The largest-ever study of whale sharks, previously mentioned, spanned a nine-year time frame. The pure amount of data received is mind-blowing. Hueter's studies were centred around the aggregations off the Yucatan coast. "From this one feeding area, these animals spread out over vast parts of the region—throughout the Gulf of Mexico, down into the Caribbean Sea, through the Straits of Florida, up into the open Atlantic Ocean," said study co-author Robert Hueter, director of the Center for Shark Research at Mote Marine Laboratory. "We found animals coming back for as many as six years at a time. Clearly, they are returning to this site to fuel up on the rich food that's there to carry them through much of the rest of the year."

As we know, most whale sharks in the aggregations are males. With this many males, you cannot have a stable population of sharks, and it is hypothesized that mature females go on long migrations to the middle of the ocean to give birth. Giving birth in the open ocean, away from high-traffic coastal areas, would ensure the young sharks, which are up to two feet long, would be less vulnerable to predators.

Over 800 animals were studied during this period; one particular one stood out. A presumably pregnant female named Rio Lady was tagged and tracked for a journey of around 7200km (4800 miles) to a region mid-Atlantic just below the equator, where it is proposed that she gave birth. After five months of observation, the tracking only ended when her tag fell off.

She was retagged in August 2018 by Rafael de la Parra Venegas, who originally tagged her in 2007, and travelled over 15,483.5km (9621 miles) in 620 days. She is the only marine animal to have been tagged twice. Her contribution to understanding whale sharks is invaluable. She and other satellite-tagged sharks can be followed at

A whale shark was also recorded diving 1928 mt, and it is hypothesized that by gliding to the ocean's bottom while moving forward, they manage to continue their journey, cool off, and conserve energy.

Snorkelling with the whale sharks

It is an incredible and humbling privilege to get close to these docile ocean behemoths and one that should not be taken lightly or abused. There are rules and regulations intended to limit the impact of ecotourism on whale sharks. It is worth noting that your chances of seeing Manta rays are very high on these tours. These rays are on the endangered list, and there is a species of Manta ray that can only be found in this region.

  • Each season has an opening and closing date; tours outside these dates are illegal.

  • Visitors are not allowed to touch the whale sharks

  • A distance of 2 mt (6.5ft) from the whale sharks is required at all times, although sometimes this can be difficult to maintain as there are so many, and they swim toward you, making you move to get out of their way.

  • There is a set amount of snorkelers in the water at a time (generally 2), and a guide must lead them.

  • You must wear a life vest or wetsuit, although you must be skin-diver trained to wear a wetsuit.

  • No sunscreen

  • No Scuba diving

  • No flash photography

If you would like some hints on taking photos of the whale sharks, check out this link here.

This pattern behind the gill is like the whale sharks fingerprints. Each one is unique and can be used to identify individual whale sharks.
Identifying skin patterns behind the gill

Registering a sighting and aiding in research

To help research and catalogue individual whale sharks, you can register your photos and sightings on and Project AWARE. Each whale shark can be identified by photographing the skin patterns behind the gills (like the sharks` fingerprints) and any scars. Individual whale sharks can be identified using pattern recognition software and photo management tools.

Tips for photographing the whale sharks for identification.

  • Focus on the area around the gill and the primary dorsal fin. These are key areas for individual identification.

  • Remember that the patterns on each side of the whale shark are different, so take note of which side of the whale shark the photo was taken from

Note the following

  • Date of sighting

  • Time

  • Region/Sea area

  • Dive site name (if Scuba diving) or specific location (Isla Mujeres, Afuera location), for example. GPS coordinates are best

  • What you saw

  • Approx length of the whale shark

  • Sex of the whale shark *

  • How many you saw

  • Any visible scars and where

  • Visible tags and details

* Male whale sharks can be identified by the presence of pelvic fins, which form claspers underneath the animal. Female whale sharks do not have these extensions.

The information will be used to match with an existing whale shark in the database or create a new profile and will help in the global conservation of this vulnerable species. You can also adopt a whale shark and follow its progress and sightings worldwide while supporting individual research programs in different regions.

Impact of ecotourism on whale sharks

A clear photo that can be used to identify its unique gill pattern and any scars
Identifying whale sharks

Research in Australia shows that the average body size of sharks has shrunk by 2 mt since the mid-1990s, and the overall population has declined. Some say, including Hueter, that ecotourism, in part, is to blame and that boats and snorkelers inhibit feeding and returning to feeding sites.

Fish biologist Mark Meekham argues that the decline in Australia has nothing to do with ecotourism and is contradicted by direct evidence stating that whale sharks that have swum with tourists are no less likely to return than those that have not. That, in fact, ecotourism helps safeguard the animals, and fishing practices, especially near Australia, could be held to blame. While Mexico and its surroundings have never fished whale sharks because they dislike the meat, other countries such as Taiwan and Asia prize the flesh and fins of whale sharks. Many countries have closed whale shark fishing, such as Taiwan, India, and many others, but China's appetite is still growing.

The human impact on whale sharks can take many forms, from sound pollution in the ocean, actively feeding the whale sharks and disturbing their instincts, to whale shark trade and environmental damage.

In the aggregation near Isla Mujeres, the sharks do not seem to mind tourists, being so focused on their feeding that they ignore you. This allows for an extended close encounter with them. If they are startled, they are known to resort to "banking" - a defensive reflex where the shark dives down and turns its back on you.

The problems mainly come from a large number of boats, and because they filter feed close to the surface, accidental propeller accidents can occur.

Problems also arise from "pirate" operators that work to their own laws. If you find a tour with a price "too good to be true", it's because it generally is. The prices of the tours are pretty much set, with the money going to the conservation of whale sharks, training guides, whale shark operation licenses and permits, local economy, and covering costs. These "cheap alternatives" are more than likely a pirate company with little or no respect for the region's conservation, rules and regulations, or economy. Many of the problems and accidents that occur during whale shark season occur because of these illegal operators.

Holbox and Isla Mujeres are home to the fastest-growing whale shark ecotourism in the world, and systems are in place to ensure that the animals are protected, and the strict rules and regulations are abided by.

In conclusion

In Isla Mujeres, we are fortunate to be privy to not just one but possibly two phenomenal natural events, not just the Afuera, the gathering of whale sharks, but also the unseen scombrid spawning that initiates all of this. Therefore the Yucatan region is an important area of biological activity worthy of significant and sustained conservation efforts. How we respect these wonders of nature will determine whether an eco-touristic approach is successful and beneficial to these creatures.

It will take many more years of research and data to separate the issues of tourism and fishing. Still, protecting these magnificent creatures is in everybody's best interest. The question is, can we respect and protect them while entertaining ourselves at their expense? So if you decide to go on a whale shark tour, you must choose wisely the company you book with so that they will honour the sharks and abide by the rules and regulations to snorkel with them appropriately.

Use your opportunities to help research and identify these whale sharks by reporting your findings to

We know what we know about these graceful creatures because of the work of people such as Simon Pierce, Mark Meekham, Rafael de la Parra Venegas, and Robert Hueter.

Please click here if you want more information on whale shark tours and help book with a company that abides to correct eco-touristic methods.

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