A Whale (Shark) of a Mystery

If you have ever sat through Jaws or watched Discovery Channel’s annual lovefest, perhaps you have imagined your reaction at the sight of a massive shark heading straight for you.  Worse still, perhaps you have imagined yourself among a swirling frenzy of sharks, each awaiting its shot.  I have imagined this.  Between the dolphin-pitch squeaks of terror and a soiled wetsuit, it’s not a pretty picture.

As one approaches, I remember the advice I heard somewhere…go for the eyes and snout.  Stick and move!

Before I can lift a finger to fend it off, though, the moment passes by with little more than a glancing nudge from an animal whose front-end looks something like a 1990s Honda Civic.

Whale shark, Rhincodon typus (image from Wikimedia Commons).

Huh?

Oh, PHEW!  It’s a whale shark frenzy I’m imagining, not one of their toothier cousins.

But if it’s not me they want, then what is this collection of Rhincodon typus after?

This is the question for scientists studying such gatherings, termed “afueras,” off the coast of the Yucatan peninsula near the Mexican island of Isla Mujeres.  The simple answer is food, just not of the large-game (or human) variety. 

Whale sharks, one of only a few species of filter-feeding sharks (along with the Basking and the rare Megamouth), are known to consume relatively tiny prey including plankton, krill, cephalopods (octopus, squid, etc.), small fish, and fish spawn by filtering large quantities of water through sieve-like apparatuses in their mouths.  While they spend most of their time roaming the open ocean in solitude, they occasionally aggregate in coastal areas when there is a feast to be had.

Researchers from Mexico’s Proyecto Domino and the Mote Marine Lab in Sarasota, FL, among others, have documented annual swarms of whale sharks off another Mexican barrier island, Holbox, where they gather to feed each summer on enormous plankton blooms that fog the ocean.

But, as Jim Tharpe recently reported in the Washington Post, afueras are something “extraordinarily different.” 

Tharpe, who accompanied scientists in search of the afuera last summer, wrote that whereas “the Holbox sharks are spread over dozens and even hundreds of square miles in often murky waters,” the sharks in an afuera “are concentrated in clear water in an area that can be as large as a few square miles or as small as a football field.”

The particular explanation for the afuera (Spanish for “outside”), so named because they occur outside the typical geographic extent of the annual Holbox gatherings, has remained a mystery to researchers.  That the answer has been so elusive seems remarkable considering the afueras host the highest concentrations of the world’s largest fish.  Hundreds of whale sharks, typically in the vicinity of 20 feet but which can grow to 40 feet or more, have been observed at a time.

The dearth of knowledge about these events exemplifies how much is yet to be learned about whale sharks, and, indeed, about sharks in general.

"Amazingly, the largest fish in the world, which is the whale shark, is one of the least known," Rafael de la Parra of Proyecto Domino told LiveScience.

Relatively little is understood, for example, with respect to whale sharks’ behavior including their mating and reproductive habits, their migration and aggregation patterns, why they make prolonged dives hundreds of meters away from their food sources on the surface.  Also not well known is the vulnerability of their population to humans from fishing and even (ironically) ecotourism boats, which occasionally collide with the sharks.

The lack of knowledge is explained, in part, because of the challenge of studying sharks in their natural habitats.  Whereas land animals and birds may be observed in person and tracked over time in the relative comfort of the above-ground world, similar observation of ocean-going sharks is substantially more hindered.

(image from Wikimedia Commons)

Data from tagged sharks indicates travel patterns, but the motivation for these patterns may not be readily apparent or easily decipherable.  Some species may be baited and viewed up-close, but the range of activities that can be studied in this manner is limited.

Studying the afueras presents its own challenges for researchers. 

Although they broadly occur within certain geographic boundaries, locating the particular site on the open ocean “can be a daunting challenge,” as Tharpe wrote.  They may also occur only a few days out of each summer or not at all in a given year.

To overcome these challenges, de la Parra, Robert Hueter of Mote Marine, and others have used a variety of techniques.  By diving with the sharks and observing them from boats, they can witness the sharks’ behavior up close and in detail, they can document the lengths and genders of the aggregating sharks, photograph individual animals, and even take genetic samples.  Water samples are collected to test for temperature and salinity, among other characteristics, and to determine what other sea life is present.

Aerial surveys complement the surface studies by providing population estimates and physical dimensions of the afuera, as well as larger-scale behavioral patterns.  Anecdotal observations from locals and fishermen also provide the researchers with insights about the sharks’ historical occurrence in the afuera zone and the presence of other fish in the area.

Their efforts to study the mystery of the afueras appear to be paying off.

According to Tharpe, “Some scientists believe the afuera is directly tied to another spectacular event, one that goes unseen by humans.  Every year, huge schools of little tunny, an abundant member of the tuna family, arrive to spawn in the warm Yucatan waters.”

It’s thought that the scores of whale sharks cruising at the surface with mouths agape off Isla Mujeres may be dining on the tunny spawn that is concentrated in the area.  Similar feeding on fish spawn near Australia and Belize has been documented before, so it seems a reasonable hypothesis.

While a piece has been added to the whale shark puzzle, research at the afueras will likely continue in order to further understand their feeding habits and to uncover new information about this giant (though gentle) predator.

Now, the next time I imagine myself amid a whale shark frenzy, at least I’ll know they’re probably more interested in gobbling up the tunny than the Timmy.

For more information and some great photos, check out the links above.

Additional Reference:

• Stevens, J.D. (2007) Whale shark (Rhincodon typus) biology and ecology: A review of the primary literature. Fisheries Research, 84, 4-9.

And last but not least, an incredible video from BBC's Planet Earth series (via YouTube):