Whale Shark Fingerprints
Updated: Apr 27, 2021
The whale shark (Rhinocodon typus) is the largest shark species alive today, reaching lengths of 18m. Yet, it is not just their size which makes whale sharks so recognisable... they have remarkable patterns of spots and stripes, which are as unique to each shark as a fingerprint is to a human. Not only does this make these animals startlingly beautiful, but can also aid scientists in identifying and tracking individual whale sharks, through a method akin to fingerprint analysis, called photographic identification.
Photographic identification (photo-ID) has been used to identify specific individuals in several other species of marine animals, including turtles, whales and dolphins. This technique allows animals to be tracked through time and space, similarly to tagging studies, but is advantageous as it is not invasive. Whatsmore, as whale sharks are sedate, not a threat to humans and the target species for many ecotourism companies, photo-ID data can be collected through citizen scientist initiatives; generating large, powerful datasets, for minimal cost. An online database, called Wildbook for Whale Sharks, allows the general public to become involved in photo-ID data collection and the data can be used by scientists all over the world!
A recent study used photo-ID in conjunction with Wildbook for Whale Sharks to study the demographics of the whale shark aggregation at Donsol, in the Philippines. Images of whale sharks were taken during shark watching ecotourism excursions and individual sharks were identified by researchers, using software called I3S. This program highlights the unique pigmentation patterns on the flank of the shark, near the gills, known as a "dorsal fingerprint", which could be used to accurately identify an individual if it was photographed again. It also meant that the individual could be added to the Wildbook for Whale Sharks database to determine where else in the world the shark had been seen (McCoy et al, 2018).
They found that, in the Philippines, males were sighted more commonly that females, and 53% of these were mature males (meaning they were of an age/size that they were able to reproduce). The sharks were considerably larger than found at other aggregations in the Philippines, suggesting that this area is may be used for pupping or mating. This is very exciting, because scientists have yet to find the elusive whale shark mating grounds!
The researchers also used these data to study the residency periods of whale sharks in the area. They found that whale sharks stayed at Donsol for around 50 days and had high "site fidelity" (aka "philopatry"), meaning they returned to very specific locations in the area over multiple years. Some identified sharks were even found to return to the site consistently for 10 years! This highlights that the area is very important for whale sharks, supporting a population of 1,767 individuals. The region should therefore be a priority for marine conservation (McCoy et al, 2018).
This study highlights how citizen scientist initiatives and collaboration with ecotourism operators can have incredible valuable for science! They managed to perform 1,985 surveys of whale sharks over a 9 year period, during which they had 6,786 encounters with whale sharks! Initiatives of this kind create robust datasets, which can be used to answer scientific questions about sharks, but can also be critical for engaging the general public and local community to take interest in sharks. During this work, members of the public's submissions to Wildbook for Whale Sharks were responsible for the identification of 12 new whale shark individuals! It's pretty cool that members of the public can get involved in science in this way!
If you are ever lucky enough to see a whale shark in the wild, you can become a part of the data collection team by submitting your photographs at to Wildbook for Whale Sharks.
Take note that whale shark sighting rates in Donsol vary considerably in different seasons because the sharks move into the area to target diatom blooms which peak between February through May. So if you hope to see whale sharks there, that is the time to go!
References
McCoy E, Burce R, David D, Aca EQ, Hardy J, Labaja J, Snow SJ, A Ponzo & Araujo G (2018). Long-Term Photo-Identification Reveals the Population Dynamics and Strong Site Fidelity of Adult Whale Sharks to the Coastal Waters of Donsol, Philippines. Frontiers in Marine Science, 5, doi: 10.3389/fmars.2018.00271. Access online.
By Sophie A. Maycock for SharkSpeak.