Researchers refute hypothesis that schools of baitfish travel alongside whale sharks for protection from predators

Barry, C., Gleiss, A.C., Ferreira, L.C., Thums, M., Davis, R.W., Fuiman, L.A., Roche, N.R., Meekan, M.G. (2023). Predation of baitfishes associated with whale sharks at Ningaloo Reef. Marine Biology, 170(140).

You’ll often see large animals such as whale sharks (Rhincodon typus) surrounded by crowded schools of small fish, called ‘baitfishes’ (Carangidae spp.).

School of baitfish (Carangidae spp.). Photo by William Warby, CC BY 2.0.

While it’s clear that baitfishes don’t understand the concept of personal space, we don’t actually know why they like to hitch a ride with these gentle giants. There are three leading hypotheses for this phenomenon: 1) The sharks provide shelter from predators; 2) the sharks’ massive bodies push a bunch of water out of the way (called a ‘pressure wave’), making travel much easier and less tiring for the baitfishes; and/or 3) the baitfishes take advantage of feeding opportunities from the sharks.

To gain more insight into the association between whale sharks and baitfishes, researchers analyzed videos captured by a tourism photographer and camera tags attached to whale sharks at Ningaloo Reef, Western Australia from 2009, 2017, and 2018.

Attack of the trevally

The videos showed schools of trevally (Carangidae spp.), a common predatory fish, feeding on the swarming baitfishes around the whale sharks head. One of the videos, recorded in 2009, captured a predation event of over 20 trevally consuming more than 200 baitfishes in a span of only 19 seconds. This surely puts hot dog eating contest records to shame! In another video, there was a total of 15 separate predation events. Using statistical analyses, the researchers found that the average time between attacks was 17.4 minutes and most raids lasted from 2 to 45 seconds long. The sharks, meanwhile, seem unbothered, usually slowing down and assuming a tail-down, vertical position until the feast ended.

Pair of bluefin trevally (Caranx melampygus). Photo by U.S. Fish and Wildlife Service, CC BY-NC 2.0.

Poor bodyguards (but helpful in other ways?)

Since the videos revealed that baitfishes are regularly ambushed by trevally despite aggregating around a whale shark, this suggests they don’t gain protection from the huge sharks after all. Ultimately, this footage is evidence that predation shelter is not the driving force behind the baitfish-whale shark association, refuting one of the three hypotheses presented above. Because whale sharks search for areas with lots of prey, it could be that baitfishes like to tag along to benefit from these feeding sites. Perhaps they also enjoy the less resistant swim as they ride the pressure wave generated by the whale sharks, similar to drafting in car races.

Whale shark (Rhincodon typus) surrounded by remoras (echeneids) and baitfishes (Carangidae spp.). Photo by Olivier Roux, CC BY-NC 2.0.

It’s exciting to be one step closer towards solving the mystery behind whale sharks’ clingy travel companions. While we still have a ways to go, narrowing the explanation from three to two hypotheses gives us a deeper understanding of the interaction between whale sharks and baitfishes. Sadly, the global whale shark population has declined by over 50% in the last 75 years and they are considered an endangered species. Learning more about their interactions with other species such as baitfishes can aid in their conservation efforts. Additionally, baitfishes are critical to commercial fisheries by serving as bait for target species and in some instances, by being commercially valuable themselves. Knowing why they like to travel alongside large animals can help inform fisheries management and conservation strategies for both parties.

Cover photo by Mike Johnston, CC BY 2.0.

Leave a Reply

Your email address will not be published.