Several whale sharks are swimming close to the surface in the Gulf of Mexico off Florida’s west coast. With their mouths wide open, they filter food from the water. New footage from the Florida Fish and Wildlife Conservation Commission shows the animals feeding and forms part of a research project addressing a surprisingly difficult question: What exactly do the world’s largest fish eat there?
According to information published by Reuters Connect, researchers collected samples from such feeding groups this summer. In the laboratory, genetic sequencing should reveal which organisms the material comes from. The Fish and Wildlife Research Institute, a laboratory at the University of Florida, NOAA, the Blue World Research Institute and the universities of Southern Mississippi and South Florida are involved.
DNA instead of identification by sight
Whale sharks are filter feeders. Water passes through the mouth and over specialised filtering structures on the gills, where small prey is retained. Yet video often reveals only that an animal is feeding. Tiny fish, invertebrate larvae, eggs and plankton are difficult to distinguish reliably in moving seawater.
Genetic traces can narrow this gap. Instead of identifying each prey animal with the naked eye, DNA from a sample can be compared with known sequences. The method can also detect organisms that are damaged, digested or microscopic. However, it does not automatically provide a complete menu: sampling, DNA degradation, reference databases and species with varying levels of detectability influence the result.
Previous sample revealed new food source
An earlier find from the same research area shows why the work may prove worthwhile. NOAA Fisheries describes eggs collected from a feeding aggregation and analysed genetically. The analysis assigned them to a species not previously documented as whale-shark prey in the Gulf, although NOAA does not name the species in its public account.
Findings like these may explain why whale sharks gather temporarily at particular sites. The important factor is not only where they swim, but also when concentrated food becomes available. Summer aggregations have long been known from the northern Gulf of Mexico. The current sampling effort focuses on prey off Florida’s west coast.
Sighting reports lead researchers to the animals
Whale sharks range across vast areas of ocean and cannot easily be found on demand. The project therefore relies on reports from boaters and other people at sea. According to NOAA, local sightings have already helped researchers locate a feeding aggregation near Tampa Bay and fit one whale shark with a satellite tag.
The whale shark, later named “Larry”, travelled more than 4,800 kilometres in one year. Satellite-tag data reveal movement routes and the marine areas used by an animal, while water and prey samples address a different question: which resources it uses at a particular location. Combining sightings, telemetry and laboratory analyses provides a clearer picture of the species’ ecology.
Study results are not yet available
The current footage is impressive, but it is not a report of results. So far, public information confirms only that samples are being collected and analysed genetically. It remains unclear which prey the whale sharks consumed in this year’s feeding aggregations, how often individual organisms occurred and whether diet varied by place or time.
The number of animals in the footage also cannot be used to estimate the population in the Gulf of Mexico. Whale sharks may gather at food-rich sites and later disperse over much larger areas. The footage therefore documents a local feeding event, not population growth. Future laboratory results may reveal which food makes such temporary hotspots attractive to this endangered species.
