Trevally rally around whale sharks to travel and chow down

It’s bad luck being a baitfish.

There’s been a long-standing belief these small fishes associate with large whale sharks (Rhincodon typus) for protection, but researchers observing the relationship have found it’s not true.

Instead, they’re literally bait for bigger fish.

A group of researchers led by ecologists from Murdoch University made that discovery when studying whale sharks and the swarms of comparatively tiny Carangidae fishes swimming around them.

Trevally gorging on baitfishes swimming with juvenile whale shark.
Trevally gorging on baitfishes swimming with juvenile whale shark. Credit: Ollie Clarke Photography

When reviewing the footage from cameras tagged to juvenile whale sharks in Ningaloo Reef off the northwest coast of Western Australia,  and a tourism photographer based in the region, researchers found baitfish provided an easy target for bigger predators.

Schools of trevally – which grow to more than a metre in length – were found to opportunistically attack and devour these tiny fish in flash-feeding frenzies sometimes lasting 45 seconds.

It suggests baitfish flock to whale sharks for reasons other than safety. After all, there’s nothing safe about being dive-bombed by a predator.  More likely, says marine ecologist Christine Barry, baitfish are merely using whale sharks as a rideshare.

“By riding the bow wave, the accompanying baitfish save valuable energy by moving through the water with their whale shark taxis,” says Barry.

“But they are still very vulnerable to predatory fishes when accompanying their giant friends, as the dramatic trevally feeding frenzy videos showed.”

It’s also likely whale sharks act as a food delivery service for baitfish, given their appetite for plankton found in the region.

Probing the mystery spread

Other ecologists studying relationships between whale sharks and smaller fish have found this oceanic taxi service extends to larger predators.

The golden trevally (Gnathanodon speciosus) is prevalent across the Pacific and Indian Oceans, including across varied biological and geographic boundaries.

It turns out, according to Dr Nathan Waltham, an associate professor at James Cook University’s Centre for Tropical Water and Aquatic Ecosystem Research, their widespread is probably thanks to a long history of hitchhiking with their larger brethren.

“Golden trevally appear not to be affected by biogeographic boundaries that constrain the distributions of most other species with reef or coastal associated adults,” Waltham says.

In separate studies to the Murdoch research, scientists from JCU studied online images to ascertain which species golden trevally commonly associate with. Whale sharks appeared in 60% of the records.

“All of the trevally close to whale sharks were juveniles or sub-adults, and in many cases, they were seen close to the sharks’ mouths,” says Distinguished Professor Marcus Sheaves.

“Whale sharks are known to undertake extremely long-distance migrations over short periods of time, so it could be possible for juvenile golden trevally to make those same migrations by essentially hitchhiking with whale sharks.”

Sign up to our weekly newsletter

Please login to favourite this article.