Jennifer McKinney, from the Louisiana Department of Wildlife and Fisheries, led an international collaborative team of whale shark researchers working in the USA, Belize, Mexico, and Honduras, to come together and analyse the Caribbean and Gulf of Mexico database on Wildbook for Whale Sharks, the global whale shark photo-identification library.
Our previous genetic studies have shown that Atlantic whale sharks rarely, or never, move into the Indian or Pacific oceans, but we’re still trying to work out how much they swim around within each ocean. This new paper is a major step towards working that out, at least for the western Atlantic.
Whale sharks can be predictably seen – on a seasonal basis – at several sites along the Meso-American Reef system in the Caribbean, including Isla Mujeres off Mexico, Gladden Spit in Belize, and Utila in Honduras. Using the Wildbook library, we collated photographic identifications of 1, 361 sharks (submitted by over 300 people, thanks!) from these and adjacent countries.
Sightings and movements of identified whale sharks in the western Atlantic region
Half of the identified sharks were sighted only once over the course of the study (1999 to 2015). Ninety percent of the other 678 sharks were only re-sighted in a single country. That left 70 individuals that moved “internationally”, the majority between Honduras, Belize and Mexico. Sixteen sharks were sighted over more than a decade, with one being seen over almost the full duration of the study (16.5 years), and re-sightings were up to approximately 1, 600 km apart.
Population structure of identified whale sharks in the western Atlantic
It looks most of the sharks using this area have now been identified, with re-sighting rates of over 70% across the 2010–2015 period, although sharks probably enter and leave the region routinely. As is common in whale shark feeding areas – though still unexplained – there were 2.6 times more male than female sharks identified, and 89% of the sharks were estimated to be smaller than 8 m in length (and therefore likely immature).
While whale sharks swim all around the Caribbean and Gulf of Mexico, this bias towards juveniles, and males means that we are clearly still missing a lot of baby sharks, females, and adult sharks of both sexes. Our best guess, based on satellite tracking and other data, is that the western Atlantic whale sharks move off into the broader Atlantic Ocean when they’ve got breeding on the mind. Bow chicka wow wow…
If that’s piqued your interest, you might also like this post: The Science Behind The World’s Largest Whale Shark Aggregation: Isla Mujeres, Mexico.
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