An Unprecedented Aggregation of Whale Sharks, Rhincodon typus, in Mexican Coastal Waters of the Caribbean Sea

Whale sharks, Rhincodon typus, are often perceived as solitary behemoths that live and feed in the open ocean. To the contrary, evidence is accumulating that they are gregarious and form seasonal aggregations in some coastal waters. One such aggregation occurs annually north of Cabo Catoche, off Isla Holbox on the Yucatán Peninsula of Mexico. Here we report a second, much denser aggregation of whale sharks (dubbed “the Afuera”) that occurs east of the tip of the Yucatán Peninsula in the Caribbean Sea. The 2009 Afuera event comprised the largest aggregation of whale sharks ever reported, with up to 420 whale sharks observed in a single aerial survey, all gathered in an elliptical patch of ocean approximately 18 km2. Plankton studies indicated that the sharks were feeding on dense homogenous patches of fish eggs, which DNA barcoding analysis identified as belonging to little tunny, Euthynnus alletteratus. This contrasts with the annual Cabo Catoche aggregation nearby, where prey consists mostly of copepods and sergestid shrimp. Increased sightings at the Afuera coincide with decreased sightings at Cabo Catoche, and both groups have the same sex ratio, implying that the same animals are likely involved in both aggregations; tagging data support this idea. With two whale shark aggregation areas, high coastal productivity and a previously-unknown scombrid spawning ground, the northeastern Yucatán marine region is a critical habitat that deserves more concerted conservation efforts

Aerial photographs of whale sharks feeding at the Afuera aggregation in August 2009.

<p><i>Figure 4A</i> was taken from approximately 600 m altitude and shows 220 whale sharks and 4 tourist boats. <i>Figure 4A</i> was taken from lower altitude and shows 68 whale sharks, 1 tourist boats and 2 pairs of tourists snorkeling.</p

The flight path followed on each aerial survey for whale sharks off the coast of Quintana Roo, México.

<p>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.</p

Total number or whale sharks and whale sharks per flight, compiled from aerial surveys of the Afuera whale shark aggregation off the coast of Quintana Roo, México between 2005 and 2009.

<p>Total number or whale sharks and whale sharks per flight, compiled from aerial surveys of the Afuera whale shark aggregation off the coast of Quintana Roo, México between 2005 and 2009.</p

Time series of whale shark observations during the 2009 Afuera whale shark aggregation in the coastal waters of Quintana Roo, México.

<p>Each column represents a single aerial survey.</p

Aerial survey effort and whale sharks observed per month between May and September, from 2006 to 2009, in the coastal waters of Quintana Roo, México.

<p>Aerial survey effort and whale sharks observed per month between May and September, from 2006 to 2009, in the coastal waters of Quintana Roo, México.</p