Where is the largest constellation of whale sharks in the world?
MMF Principal Scientist, Dr Chris Rohner, hosted two Aqua-Firma groups on a whale shark research and photography trip to Mexico in July/August 2017. Together with our local collaborator, Rafael de la Parra from Ch’ooj Ajauil, we conducted eight boat surveys to the world’s largest known aggregation of whale sharks east of Isla Contoy on the Yucatán Peninsula. There, whale sharks come together to feed on fish eggs, and in the past over 400 individual sharks were counted in one spot from the air.
A snorkeler swims alongside a whale shark. @Chris Rohner
Fish spawning seems to attract the largest concentrations of whale sharks. In Qatar, where the sharks also eat fish eggs, we regularly see more than 100 sharks too. Unlike in Qatar, Mexico has a huge whale shark tourism industry and in 2017 there were 240 licensed whale shark boats. Finding the aggregation was easy, as there were dozens of boats floating in one spot at any given time. Whale shark interactions are highly regulated in Mexico. For example, only one guide with two tourists per boat are allowed in the water at once. This means that on a good day with as many sharks as boats, tourists have a good encounter, with just three people per shark.
Whale sharks come together to feed on fish eggs. @Chris Rohner
Whale sharks come from around the Caribbean to take part in this feeding bonanza. As Simon and friends describe in their recent paper, there is higher connectivity among aggregations along the Meso-American Barrier Reef than at any other adjacent whale shark hotspots. We identified 61 individual whale sharks during our first week (week 2 is still being processed) and we saw whale sharks that were first identified in Honduras, Belize and in Mexico, underlining this high connectivity in the region.
One interesting aspect of their feeding ecology in Mexico is the high incidence of stationary feeding. Whale sharks sometimes stop moving and actively draw water into their mouths, presumably in areas with a high density of fish eggs. Because the sharks are negatively buoyant, they eventually sit vertically in the water column with their head at the surface and feed. Fish eggs cannot escape from a predator and hence the vortex created by actively drawing in water is enough to capture their prey. Elsewhere, where whale sharks feed on more active prey, such as sergestids or mysids, this technique would not work and instead the sharks actively swim and draw water into their mouths at the same time.
A vertical feeder. @Chris Rohner
In addition to the photo-ID collection, we also recorded scars of whale sharks to track potential threats and see how they heal over time. We collected several biopsies for Alexandra’s upcoming genomics PhD project and attempted to fin-mount a satellite tag. We also participated in the annual whale shark festival, where Chris gave a talk on the feeding ecology of whale sharks. It was a productive time in Mexico and we are looking forward to being back again in 2018.
Chris giving a presentation at the local Whale Shark Festival. @Ralph Pannell
