Hammerhead sharks swim ‘side-stroke’ to save energy

By Jared Richards 3 August 2016
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Previously considered an anomaly or sign of sickness, a new study has found hammerheads swim on their side due to an evolutionary trait.

GREAT HAMMERHEAD SHARKS spend up to 90% of their time swimming rolled on their side as a way to conserve their energy, according to a collaborative study between marine biologists and scientists from Australia, Japan, Belize, Israel and the USA.

Deploying video cameras and acceleormetres on five hammerhead sharks (Syhyrna mokarran) living in waters off Australia, Belize and the Bahamas, the study found each shark regularly swam at rolled angles, using their large dorsal fin as a lift. All swimming between 50 to 70 degrees, the sharks ‘side-stroke’ reduced drag to actively avoid exerting the energy upright swimming requires.

Dr Adam Barnett, a co-author of the study and researcher at James Cook University, noted that the surprising discovery is a reminder of how so many of the details of ocean life remain unknown.

New Australian dinosaur

Image: Adam Barnett

“At first we thought the shark must be sick, but it would switch from one side to the other and once we began looking, we found other examples of this behaviour,” he said.

The scientific community has responded enthusiastically to the paper published in Nature Communications.

“The side-swimming is something people have observed in aquariums or in the wild that they assumed was a one-off thing, but evidently it’s not,” said Colin Simpfendorfer, Director at the Centre for Sustainable Tropical Fisheries and Aquaculture at James Cook University.

The question now, Colin added, is whether the hammerhead’s dorsal fin’s large size is an evolutionary trait to enable them to swim on their sides, or whether it is for some yet-unknown reason.

Considered a globally endangered species on the IUCN Red List, the larger question for the hammerhead shark is its future. Regularly fished and captured for its fin, population numbers have declined from 50-90 per cent in South Africa, as well as in the northwest and western central Atlantic. Regulations regarding hammerhead fishing fluctuate across the globe.

“[The study] gives us a new understanding but [when it comes to] the reality of being threatened, whether hammerhead sharks swim on their sides or the right way up won’t change that a great deal,” said Colin.