"Afuera" The aggregation of whale sharks off Isla Mujeres
Updated: Aug 30, 2022
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.
There are a dozen locations around the world that 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 what is an unlimited supply of nourishment. Generally, these gentle giants are quite solitary and aggregations of 15 to 20 sharks are thought to be a large gathering. However, the largest aggregation ever recorded happened here, off the Yucatan Peninsula, in 2009 with at least 420 spotted in a single survey. With the size of a whale shark being 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 is only as recently as 2002 that the first rumors started to emerge to the scientific community. Generations of fishermen knew about the whale sharks and followed them to good tuna or cobia fishing but the scientific community was blissfully unaware that something incredibly special was taking place.
Shark Biologist Roberto Hueter, director of the Center for Shark Research at the Mote Marine Laboratory, had been traveling 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 happened to casually mention that each year hundreds of whale sharks gathered nearby, 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 largest 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 aided greatly
In 2003, due to this sudden increase of interest in the whale shark, Willy Sabatina and other local fishermen began to experiment with organized whale shark tours that had already proven to be popular in Australia. Holbox transformed quickly from a fishing economy to an eco-tourism destination to view these whale sharks and this 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
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 by 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 special, an unparalleled biological congress that takes place 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 huge 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 in fact, that it clogged the plankton net in as little as 30 seconds.
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 and are 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 that are 30-40 mt deep, with the most intense spawning occurring between July and August, This perfectly describes the conditions and timing of the Afuera.
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 and transparent and they are buoyant because they contain a droplet of oil that also 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 not only is the whale shark gathering 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 shark), 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 will 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 obvious 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 huge volumes of water which it then expels through the gills.
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 that are quite different from the other filter-feeding sharks
Food separation is caused by cross-flow filtration (or tangential flow filtration). This is an extremely efficient, anti-fouling filter that 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 the clearing of 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 at hand that they can appear quite oblivious to anything else, which can allow for a close encounter with these gentle giants. However, certain rules must be respected to be able to enjoy this privilege and we will talk about 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
GROUP NAME: School
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 favorite food is plankton. Not only is the whale shark the largest fish, but it is also the heaviest, rivaling 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.
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)
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 close by, 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 powers of healing, healing from deep gashes with no sign of scarring whatsoever.
The eyes are small for a creature that spends much of its life in darkness, suggesting eyesight is not an important 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 the ability to detect chemicals in the water is of paramount importance 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 that 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 reasons unknown, most of the whale sharks in these aggregations around the world 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 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 own 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.
The females give birth to live young, rather than laying eggs in leathery cases like many shark species. By birthing at different times they may avoid giving birth to lots of 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 every year and unite at meeting points around the world 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 centered 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. You cannot have a stable population of sharks with this many males 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 and 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. The tracking only ended when her tag fell off, after five months of observation.
She was retagged in August 2018 by Rafael de la Parra Venegas, who originally tagged her in 2007, and traveled 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 ghritracking.org
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.