Honduras · Global Whale Shark Program

Utila Whale Shark Project

Twelve years of sightings identified 95 whale sharks at Utila, and later regional work showed that these sharks move between Honduras, Belize, Mexico, and the United States.

Led by Utila Whale Shark Research, with MMF

95whale sharks identified at Utila, 2001–2011

65%of the sharks of known sex were male

~12 daysestimated mean residency of a shark at Utila

1,600+ kmtraveled by two sharks between Honduras or Belize and the US Gulf coast (990+ miles)

Utila, at the southern end of the Mesoamerican Barrier Reef, is one of the few places in the Caribbean where whale sharks (Rhincodon typus) can be seen in any month of the year. Working with Utila Whale Shark Research and regional partners, we documented the identity and movements of these sharks.

Utila is a small island 30 km (19 miles) off the Honduran mainland, close to the edge of the continental shelf; the seabed drops to 1,000 m (3,300 ft) just 6 km (4 miles) off its north coast. Between 2011 and 2013, MMF’s Dr. Simon Pierce spent six weeks on the island running whale shark research weeks with Deep Blue Resort and helping Steve Fox of Utila Whale Shark Research analyze more than a decade of sightings. That work was published in 2013 and contributed to two larger studies of whale shark movements across the western Atlantic and around the world.

What the project has done

Analyzed 479 whale shark sightings at Utila between 1999 and 2011 and identified 95 individual sharks from their spot patterns, publishing the results in the Journal of Fish Biology in 2013.
Estimated the sharks’ residency at Utila and how often they return.
Contributed Utila’s sightings to a 2017 study of whale shark connectivity across the western Atlantic, and to a 2017 global study of whale shark biology built from citizen science photographs.

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What the study found

Size, sex, and seasonality

During boat surveys around the island, snorkelers photographed each shark’s left flank and recorded its sex, and each shark’s length was estimated. Of the 95 sharks identified, 65% of those whose sex could be determined were male. They ranged from 2 to 11 m (7 to 36 ft) long, and most were juveniles. Sharks were seen in every month, with a peak in March and April; however, there was no clearly defined season, and numbers varied considerably from year to year. Three sharks carried large propeller scars.

A whale shark swims in blue water off Utila, Honduras

Feeding at tuna boils

Most of Utila’s whale sharks were found feeding on small baitfish and larval fish at “boils” of blackfin tuna, where the tuna drive their prey to the surface. The whale sharks either hang vertically below the boil, suction-feeding on the fish, or swim through it with their mouths open. Up to three sharks fed at a single boil, alongside silky sharks, mobula rays, dolphins, and seabirds. Boat crews locate the boils by watching for jumping tuna and circling terns and boobies.

Residency and return

Most sharks were transient: 78% were seen in only one calendar year. The study’s model estimated that about four or five sharks were present around Utila at any one time, staying for an average of about 12 days before leaving for an average of about three months. Some returned in later years, a few in up to five different years, and one 7 m (23 ft) male was photographed again seven years after it was first seen.

Connections across the western Caribbean

The 2017 western Atlantic study, led by Jennifer McKinney of the Louisiana Department of Wildlife and Fisheries, combined more than 4,000 encounters from Mexico, Belize, Honduras, the United States, and elsewhere, identifying 1,361 individual sharks, 146 of them in Honduras. It estimated the regional population at about 2,170 sharks, with a plausible range of roughly 1,590 to 2,910. Movements were most frequent between Honduras and Belize and between Honduras and Mexico, and sharks seen in Honduras or Belize in spring were often seen in Mexico a few months later, at the aggregation off the Yucatan Peninsula. Two sharks traveled 1,600 km (990 miles) or more between Honduras or Belize and the US Gulf coast. One shark first cataloged in Honduras was recorded in four countries over 14 years. The 2013 Utila paper had already noted that the similar size and sex structure of the sharks in Honduras, Belize, and Mexico, together with the oceanographic links along the reef, raised the possibility of managing them as a single unit. It also suggested limiting the access or speed of large vessels in areas the sharks use heavily, and adherence to a code of practice for whale shark tourism.