Whale sharks hide in plain sight

The whale shark is the largest known fish species, with the biggest specimens spanning more than 14 meters and weighing more than 20 metric tons. Despite their size, these majestic creatures are vulnerable to being caught in fishing nets, both incidentally and deliberately. A study by KAUST researchers has revealed migratory patterns that could assist in efforts to protect whale shark populations¹.

These fish can travel thousands of kilometers to satisfy the demands of their plankton diet, and some data indicate a semi-migratory lifestyle. “Whale sharks that we have studied in the Red Sea, for example, clearly leave the study area outside their ‘aggregation season’,” says Michael Berumen from KAUST’s Red Sea Research Center. 

A similar migration pattern has been noted by ‘citizen scientist’ observers in Tanzania, where the animals are a major draw for ecotourism and interested visitors report their observations. Although useful, this information from visitors and tour boat operators is not systematic enough to accurately map shark behavior.

Working in collaboration with the Marine Megafauna Association (MMA), KAUST researchers employed an acoustic telemetry strategy in Tanzania that had been used in Berumen’s Red Sea-based research. They tagged 30 whale sharks in Tanzania’s Kilindoni Bay with acoustic transmitters, each of which generated a distinct signaling pattern. These were in turn detected with a set of 19 receivers, placed at sites where the sharks had been previously known to congregate. “This technology provides a very high-resolution picture about animal movement within a study area,” says Berumen, “which we wanted to couple with MMA-led studies of plankton distribution.”

The data revealed that there is more to whale shark migration than meets the eye. Visual reports of sharks were infrequent between January and March, suggesting that these fish had departed the area. But acoustic telemetry showed that many sharks remained local during this time span, but had simply moved to slightly deeper water further from the coast, where they were less likely to be seen. The perceived ‘return’ migration of these fish later in the year was actually their movement back to shallow coastal waters. “We found very regular detection of these whale sharks throughout the year,” says Berumen. 

He notes that there are more than a dozen other sites worldwide where whale sharks are known to congregate. Demonstration that other populations also maintain quite a stable residency could inform targeted monitoring and protection strategies close to whale shark areas.

References

1. Cagua, E.F., Cochran, J.E., Rohner, C.A., Prebble, C.E., Sinclair-Taylor, T.H. Pierce, S.J., Berumen, M.L. Acoustic telemetry reveals cryptic residency of whale sharks. Biological Letters 11, 20150092 (2015).| article