At Ningaloo Reef in Western Australia, whale sharks are frequently seen swimming amidst swarms of small carangid baitfish, roughly 10cm in size. These tiny fish were previously believed to accompany the massive sharks for protective reasons.
However, research conducted by Murdoch University’s Harry Butler Institute has unveiled startling revelations. Large schools of trevally, exceeding 30cm in length, have been observed engaging in rapid feeding frenzies, consuming entire schools of baitfish within intervals lasting from two to 45 seconds.
The study, published in Marine Biology, challenges the notion that baitfish cluster around whale sharks, which can reach lengths of up to 18 meters, solely for safety purposes. Christine Barry, the lead author and a Ph.D. candidate at HBI’s Center for Sustainable Aquatic Ecosystems and the Australian Institute of Marine Science, analyzed footage from cameras affixed to juvenile whale sharks, measuring up to 7 meters in length. An opportune video recorded by a Ningaloo tourism photographer also contributed to the findings.
According to Barry, rather than seeking refuge from predation, the association between baitfish and whale sharks may be driven by potential energy or food acquisition benefits.
“Whale sharks at Ningaloo Reef move throughout the region to locate dense patches of food,” Barry said. “By riding the bow wave, the accompanying baitfish save valuable energy by moving through the water with their whale shark taxis.”
“Furthermore, despite their massive size difference, baitfish and whale sharks eat the same food—such as plankton—but the amount eaten by the baitfishes barely impacts the resources available to the sharks.”
Barry said the advantages of traveling with whale sharks—movement and increased food opportunities—likely led to a commensal relationship that was energetically beneficial to the baitfishes.
“But they are still very vulnerable to predatory fishes when accompanying their giant friends, as the dramatic trevally feeding frenzy videos showed,” she said.
As fly fisherman and blue water fisherman in Australia, and specifically in the Ningaloo Reef, be sure to watch for whale sharks as you might get into some trevally!