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

Aquatic mammals from the Mexican Caribbean; a review

We present a review of the aquatic mammal species occurring in the Mexican Caribbean. Several published sources were reviewed to find information about aquatic mammals reported for the Mexican Caribbean. Additionally, we consulted 29 national and international collections and museums. Based on documents, collections, direct records and local news, we analyzed 18 confirmed species of aquatic mammals for the study area [Tursiops truncatus, Stenella clymene, S. frontalis, S. longirostris, Steno bredanensis, Grampus griseus, Globicephala macrorhynchus, Peponocephala electra; Pseudorca crassidens, Orcinus orca, Physeter macrocephalus, Kogia breviceps, K. sima, Ziphius cavirostris, Mesoplodon europaeus, Trichechus manatus manatus, Lontra longicaudis annectens and Monachus tropicalis (extinct)]. In order to gather solid baseline information that enhances efficient long-term management, regular and systematic population censuses of the aquatic mammal community are recommended. We recommend the use of the Mexican Caribbean area as a separate unit of management and conservation, differently as has been included in the macro region Gulf of Mexico/Mexican Caribbean

Vertical Movements and Patterns in Diving Behavior of Whale Sharks as Revealed by Pop-Up Satellite Tags in the Eastern Gulf of Mexico

<div><p>The whale shark (<i>Rhincodon typus</i>) is a wide-ranging, filter-feeding species typically observed at or near the surface. This shark’s sub-surface habits and behaviors have only begun to be revealed in recent years through the use of archival and satellite tagging technology. We attached pop-up satellite archival transmitting tags to 35 whale sharks in the southeastern Gulf of Mexico off the Yucatan Peninsula from 2003–2012 and three tags to whale sharks in the northeastern Gulf off Florida in 2010, to examine these sharks’ long-term movement patterns and gain insight into the underlying factors influencing their vertical habitat selection. Archived data were received from 31 tags deployed on sharks of both sexes with total lengths of 5.5–9 m. Nine of these tags were physically recovered facilitating a detailed long-term view into the sharks’ vertical movements. Whale sharks feeding inshore on fish eggs off the northeast Yucatan Peninsula demonstrated reverse diel vertical migration, with extended periods of surface swimming beginning at sunrise followed by an abrupt change in the mid-afternoon to regular vertical oscillations, a pattern that continued overnight. When in oceanic waters, sharks spent about 95% of their time within epipelagic depths (<200 m) but regularly undertook very deep (“extreme”) dives (>500 m) that largely occurred during daytime or twilight hours (max. depth recorded 1,928 m), had V-shaped depth-time profiles, and comprised more rapid descents (0.68 m sec^-1) than ascents (0.50 m sec^-1). Nearly half of these extreme dives had descent profiles with brief but conspicuous changes in vertical direction at a mean depth of 475 m. We hypothesize these stutter steps represent foraging events within the deep scattering layer, however, the extreme dives may have additional functions. Overall, our results demonstrate complex and dynamic patterns of habitat utilization for <i>R</i>. <i>typus</i> that appear to be in response to changing biotic and abiotic conditions influencing the distribution and abundance of their prey.</p></div

Crepuscular pattern of extreme dives observed in Shark 9 while in the central Gulf of Mexico.

<p>(A) Depth-temperature profiles during the period of October 29 through November 2. Tag measured light levels are shown as gray trace. (B-E) Close-up depth profiles of extreme dives coinciding with sunrise or sunset (marked with arrows in A). Note the stutter steps during descent at depths of 400–500 m.</p

Composite histograms from high resolution data of recovered tags comparing day vs night depth and temperature during inshore and offshore phases.

<p>(A) Time-at-depth inshore. (B) Time-at-depth offshore. (C) Time-at-temperature inshore. (D) Time-at-temperature offshore. Error bars represent SEM.</p

Statistical results of day vs night comparison of whale shark mean depths from recovered PSAT tags.

<p>The inshore and offshore phases of their tracks were analyzed separately.</p

Changes in the diel vertical migration (DVM) pattern for Shark 9.

<p>(A) Typical reverse DVM demonstrated inshore near the northeast Yucatan Peninsula tagging site. (B) Contrasting pattern of protracted surface time at night observed offshore in the central Gulf of Mexico, north of the Yucatan Peninsula.</p

Composite histograms for satellite-tagged whale sharks (<i>Rhincodon typus</i>) off the northeast Yucatan Peninsula and west Florida.

<p>(A) Time-at-depth. (B) Time-at-temperature. Values are mean ±SE. Broken lines in A delineate the epipelagic, mesopelagic, and bathypelagic zones.</p