Rendezvous at Isla Mujeres

LET’S GET THIS OUT OF THE WAY EARLY. Mexico, or more specifically the north-eastern tip of the Yucatan Peninsula, offers the world’s most consistent sightings of large numbers of whale sharks over the summer months.
Little surprise then that the area is of interest to the whale-shark research team from Marine Megafauna Foundation (MMF) and diving and eco-travel company Aqua-Firma.
But why are there so many whale sharks there? And what do we know about them?
Whale sharks are a popular species with divers, but it’s easy to forget that, as recently as the 1980s, seeing a whale shark was a once-in-a-lifetime event for most people. Only 320 sightings had ever been documented, even though the sharks are distributed from New Zealand to New York. It turns out that we just didn’t know where to look.
Tropical surface waters are a biological desert. Sure, coral reefs are incredibly biodiverse, but they’re isolated oases in a literal sea of nothing. Whale sharks eat mostly plankton and, as the world’s largest fish, they eat a lot of plankton.
Most of the areas where seasonal whale-shark tourism has developed, such as Ningaloo Reef in Australia or Mafia Island in Tanzania, host some major biological event that rings the dinner gong for whale sharks.
Off Mexico, the attraction is fish eggs. Little tunny, a small tuna species that can produce up to 1.75 million eggs each breeding season, spawn in large numbers in the offshore waters north of Isla Mujeres.
Although local fishers knew about this annual phenomenon since at least the early 1990s, scientists and tourist operators caught on much more recently.

RAFAEL DE LA PARRA, a Mexican whale-shark scientist, first laid eyes on this offshore aggregation in 2006. Whale-shark tourism was already burgeoning off Isla Holbox, an island off the north coast of the peninsula, where whale sharks and manta rays were feeding in shallow, green, plankton-rich waters close to shore.
Rafael and his local collaborators organised five flights further out to sea that year, during which 480 whale sharks were recorded.
That changed everything. Repeated flights over this area – known as the Afuera, which means “outside” in Spanish – have now documented up to 420 sharks in a single survey. It is, by far, the largest documented whale-shark aggregation in the world.
Among the things that changed were the management requirements. Whale sharks are a protected species in Mexico, and the government created a special Whale Shark Biosphere Reserve in 2009.
Unfortunately, legislation could not keep up with the scientific results, and the Afuera zone was not included in the reserve. Back to that later.
I [Simon] have been studying whale sharks since 2005, initially in Mozambique and now around the world. Rafael, his wife Beatriz and I were all invited to participate in a research project off Utila, Honduras.
Learning more about their work in Mexico, I was determined to check out this amazing natural event for myself.
Aqua-Firma set up a trip to help fund me to do this and, since 2013, the company has been running trips that pay for MMF and Rafael’s team to conduct research off the Yucatan Peninsula during the peak whale shark season (July/August). Paying guests can join the team to take samples, lots of photos and generally revel in the presence of the hundreds of sharks that use this area as their seasonal home.
Every whale shark has a unique pattern of spots. It makes each individual identifiable, in much the same way as a human fingerprint. A photograph of the flank can be used to identify any whale shark, anywhere in the world.
However, that matching effort is a massive job. To speed the process, automation is required.
A serendipitous friendship between a software developer and astrophysicist, both of whom were interested in marine conservation, led to a solution.
An algorithm used in the processing of Hubble Space Telescope images was adapted, and whale-shark spots were used in place of stars. The Wildbook for Whale Sharks online database (www.whaleshark.org) was born.

AS OF THIS WRITING, there have been more than 33,000 encounters with more than 7000 individual whale sharks on the database. Photographic submissions from both researchers and the public allow the movements of individual sharks to be tracked around the world, population sizes to be calculated, and increases or declines in sightings to be identified and investigated.
The trillions of tuna eggs on the menu here may draw in whale sharks from all over the Atlantic. The Yucatan coast, including both the inshore and Afuera sharks, was the first region to reach 1000 identified whale sharks.
Fully 75% of identified whale sharks from the Atlantic Ocean have been sighted in this area. It has to be one of the highest densities of sharks occurring anywhere in the world. The little tunny spawn overnight, and their eggs float gently upwards to carpet the surface. The sharks swim around vacuuming the eggs up for hours at a time.
Once the day’s spawn has dissipated, the sharks switch their behaviour and swim deeper overnight. It may well be that they are dissipating heat following hours of swimming and exposure to the sun in the hot surface water.
Back-of-an-envelope calculations reveal that an average-sized whale shark, surface feeding for 11 hours, would ingest 142.5kg of tuna eggs. That’s around 43,000Kcal, equivalent to more than 8kg of Dairy Milk chocolate.
Shifting to cooler water overnight might also slow their metabolism, helping to maximise the absorption of this massive meal.
With that much food on offer, it’s no wonder that the sharks stick around. Local tagging work has found that some individual sharks stay in the area for up to six months each year, with most having finally left by late August to mid-October. Research from 2003-2012 found that many sharks visited the Afuera repeatedly, with some returning for six consecutive years.
Where do they go in between? Well, it seems to vary between individuals. Rafael and co-authors recently published a study on 31 satellite-tagged whale sharks from Mexico, which dispersed into the Gulf of Mexico or the Caribbean Sea. When they moved away from land, and their reliable supply of tuna eggs, the sharks’ behaviours changed as well.
Because whale sharks are fish, they don’t have to come to the surface to breathe. Although most of their time was spent near the surface, from zero to 200m depth, one of the tagged sharks remained at more than 50m for three days straight.
Occasionally they dived much, much deeper, and the maximum dive by one of these sharks, 1928m, was the deepest recorded by a whale shark to date.
It’s not easy to establish why the sharks are swimming so deep. There are potential reasons, or it could be a combination of several.
A few clues were apparent. Rather than occurring randomly, the deepest dives often occurred around sunrise and sunset. Increasingly, we suspect that whale sharks forage on deepwater zooplankton, which typically migrate between the surface at night and a few hundred metres deep during the day.

FOR THE WHALE SHARKS, diving around these times may allow them to prey on the zooplankton during this migration, when some light is still available to make their hunt easier.
Deep dives could also have a navigational function. Dawn and dusk are when the Earth’s magnetic field intensity reaches its peak, and – because the geomagnetic intensity gradient also increases with depth – these dives could help to improve their ability to determine their location.
Whale sharks are born at around 50-60cm, and may grow to 20m. The Afuera aggregation is composed of mostly (72%) male whale sharks, ranging in length from 2.5 to 10m. The sharks present are predominantly juveniles: not babies, but few are reproductively active.
Where is the rest of the population? Well, somewhere else. Genetics work has shown that Atlantic whale sharks are a separate sub-population to those found in the Indian and Pacific oceans, so we assume that the adults – and the majority of females – may live in the open ocean. There isn’t a great deal of evidence to support this; it’s more that they are rarely seen along the coast.
One tagged female, thought to be a young adult, made a huge migration from the Afuera zone, across the Equator into the mid-Atlantic.
This 7000km swim, at an average speed of around 50km per day, is one of the largest ever recorded for a whale shark.
This celebrity shark, now called “Rio Lady”, has been seen back at the Afuera since. In fact I’ve seen her every year since I first went there in 2013, so this was a truly huge loop.
Rafael is fairly confident that she was pregnant when she was first tagged, though it is difficult to tell, so this single track is tantalising in that it could suggest that whale sharks give birth in the mid-Atlantic. We hope that further work will provide more evidence.

IT IS A HUGE PRIVILEGE for us to be able to swim with so many of these threatened sharks, and we all need to respect that they use the Afuera for their own purposes. Their huge calorie intake of tuna spawn may help to fuel their movements for months afterwards.
It’s a shame that the Afuera site was properly delineated only after the Whale Shark Biosphere Reserve was created, as this means that the primary aggregation site is poorly protected.
Huge shipping vessels hug the tip of the Yucatan, coming dangerously close to the whale sharks.
Although it is difficult to quantify, many whale sharks are likely killed on impact. This shipping lane needs to be moved further offshore, and this is a key aim of this project.
After seeing up to 180 sharks in a day myself [says Simon], I can truly say that this is one of the world’s most amazing wildlife experiences. The Afuera may be the best site in the world for seeing and photographing whale sharks.