Searching for the ocean’s largest shark in Mexico

Whale sharks are the largest fish in the ocean, so it is somewhat surprising how little we actually know about them. One could be forgiven for thinking they would be easy to find, follow, and track, but this isn’t the case. Unlike whales (similar in size), whale sharks don’t need to continually surface to breath so they can vanish for long periods of time.

Spot patterns act like fingerprints making identification of individuals possible. Photo: Simon Hilbourne

La Paz, on the southern tip of the Baja peninsula in Mexico, is one of the best places in the world to see whale sharks. Close to the self-imposed marine sanctuary of Cabo Pulmo and within the impressively productive Sea of Cortez, La Paz is a mecca for pelagic species. During the winter months whale sharks, mobula rays, hammerheads, orcas and whales can be seen in the region. Juvenile whale sharks regularly aggregate in a shallow region of the Bay of La Paz to feed on plankton in the water column. Quite incredibly, this shallow region is within 5-10 km of the city of La Paz.

Juvenile whale sharks aggregate in the shallow waters of the Bay of La Paz

Whale Shark Mexico, an NGO run by Dr. Deni Ramirez, has been monitoring the whale shark population in the Bay of La Paz for over a decade.

Choosing when and where you search for whale sharks can impact your chances of finding them. Although there is no conclusive data that whale sharks are more common early in the day, this is normally when the winds are weakest. As soon as the wind picks up and there is any swell, it becomes incredibly hard to spot the sharks’ dorsal fin. Sighting trips are therefore often run in the mornings when the Bay of La Paz is still calm. Getting an aerial view of the bay can make spotting the whale sharks a whole lot easier. Whale Shark Mexico occasionally get the help of a local pilot and spotter plane to cover a much greater distance and direct the research team to the whale sharks.

Spotter plans are often used to help find the whale sharks. Photo: Scubography

Once a whale shark has been sighted it’s a race against time to get into the water and collect the data needed before the shark swims off. Photo identification images are taken of the whale shark’s left hand side, enclosing and area between the last gill slit and the start of the pectoral fin. Additional pictures of the right hand side, and dorsal side are taken as well and injuries are noted. Researchers then free dive under the sharks to try to determine the sex of the individual. Males can be identified by the presence of claspers. In sexually immature males, the claspers have not yet elongated and calcified and are harder to identify quickly.

Face to face with a whale shark. Photo: Scubography

Back on dry land the photo ID images are compared with a database of previously sighted individuals. Using software similar to that used by astrologists to study star constellations, the spot patterns are compared. The work that Whale Shark Mexico has conducted has helped unravel some of the mysteries about whale sharks. Many of the sharks sighted in the Bay of La Paz return year after year and some have been re-sighted in different locations along the length of the Sea of Cortez.

Surfacing whale shark. Photo: Scubography

This work gives a snapshot of where the sharks are on the handful of surveyed days. Photo identification does not explain where the sharks go when they leave the bay. For that satellite tagging is required, something that is all too often financially prohibitive for small NGOs.

 

Check out the work of Whale Shark Mexico

 

 

Guest article written by Simon Hilbourne

Simon Hilbourne’s love for the ocean stemmed from his childhood in Sri Lanka and Thailand. Upon becoming a scuba diving instructor he felt his passion was in line with marine science and research rather than tourism. He graduated from the University of Southampton with a masters in Marine Biology and has been involved in whale shark research in Mexico and the Philippines so far. Whenever he gets the chance, he continually pursues his love for underwater photography.

Photo Credits: Simon Hilbourne (Scubography)