Sink or swim? White sharks do both

Efficient hunting techniques give white sharks the edge in the wild. New research has surprisingly found that this top marine predator actually swims at low speeds while foraging.

While sluggishly (0.8 to 1.35 m s−1) swimming near seal colonies, the study suggests that white shark may benefit from moseying slowly along by increasing their chance of encountering a prey.

This strategy is as close to a ‘sit-and-wait’ strategy as is possible for perpetual swimmers, such as white sharks, says Flinders University Associate Professor Charlie Huveneers, from the College of Science and Engineering.

Researchers from Japan, Ireland, University of Tasmania, and Flinders University worked in collaboration with the cage-diving industry to study the swimming behaviour of white sharks (Carcharadon carcharias).

While cold-blooded animals such as fish and reptiles generally ‘live life in the slow lane’ to conserve energy, more active sea creatures such as sharks and sea turtles break the rule, travelling longer distances and swimming faster.

To maintain this relatively warm-bodied lifestyle, sharks are described as somewhere between true warm-blooded (‘endothermic’) animals such as birds and mammals and genuinely cold-blooded (‘ectothermic’) fish – therefore allowing then to swim at much higher speeds than other cold-blooded creatures of the oceans.

Quantifying the energetics and activity of the ‘regionally endothermic’ species will help us understand how a fundamental biophysical process (i.e. temperature-dependent metabolism) shapes animal ecology, the paper says.  

Returning to white shark territory near colonies of long-nosed fur seals (Arctocephalus forsteri) at the Neptune Islands Group in South Australia, Flinders Associate Professor Huveneers, who leads the Southern Shark Ecology Group research lab, joined Yuuki Watanabe from Tokyo’s National Institute of Polar Research, Nicholas Payne (Trinity College Dublin), Jayson Semmens (UTAS Institute for Marine and Antarctic Studies) and Andrew Fox (Fox Shark Research Foundation, Adelaide) to assess the swimming performance of white sharks.

Tags fitted with sensors were carefully attached to the dorsal fins of sharks as they swam next to the boat to detect and measure their movements, swim speeds and depth. After the tags detached automatically a few days later, the team successfully retrieved nine from the water, with the tenth was found washed ashore several weeks later.

Simultaneous measurements of depth, swim speed (a proxy for swimming metabolic rate) and body acceleration (indicating when sharks exhibited energy-efficient gliding behaviour) revealed their fine-scale swimming behaviour and allowed us to estimate their energy expenditure, the researchers says.

“White sharks often undertook repeated deep dives, but acceleration data showed that sharks did not swim down, but instead sank when diving,” Associate Professor Huveneers says.

“Although it leads to an energetic costs to go back up, this sink and swim yo-yo diving was found to be more energetically efficient than swimming horizontally.”

The project was funded by the Japan Society for the Promotion of Science, the Winifred Violet Scott Foundation, the Neiser Foundation, Nature Films Production, and a crowdfunding campaign on Pozible.

The paper, ‘Swimming strategies and energetics of endothermic white sharks during foraging’ (February 2019) by YY Watanabe, NL Payne, JM Semmens, A Fox and C Huveneers, has been published in the Journal of Experimental Biology (doi:10.1242/jeb.185603).

Professor Huveneers and other dedicated shark researchers were also acknowledged for their contribution to a WA and South African study just published in PLOS One – ‘Not all electric shark deterrents are made equal: Effects of a commercial electric anklet deterrent on white shark behaviour’ 

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