University of Western Australia: Scientists investigate the ups and downs of the lives of sharks and rays

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An international team of scientists, including from The University of Western Australia and the Australian Institute of Marine Science (AIMS), have carried out research into how sharks, rays and skates – also known as elasmobranchs – use the ocean depths.

“The ocean varies more quickly in the vertical plane than it does in the horizontal and when these animals are moving vertically in their three-dimensional habitat, they’re also going through extreme environmental changes. “

Dr David Tickler, UWA
The team of 171 researchers from 135 institutions across 25 countries brought together two decades of data from 989 sophisticated electronic tags that remotely tracked the movements and behaviours of 38 species in oceans across the globe, with their findings published in Science Advances.

Scientists have a wealth of movement data about marine species that inhabit the near-surface spaces of the coastal ocean but animal movement in three dimensions, especially in the deeper, vertical spaces of the ocean, is less understood.

Dr David Tickler, from the Marine Futures Lab at UWA, said understanding how elasmobranchs use vertical spaces was crucial to understanding their current and future ecological roles in the ocean and risks from various threats. More than one-third of all sharks and rays were threatened with extinction according to the International Union for Conservation of Nature Red List of Threatened Species.

“Think of human interaction with the ocean such as shipping, fishing and exploration for oil and minerals and proposed activities such as deep sea mining – if we don’t understand how animals use vertical space, we don’t understand the threats to them,” Dr Tickler said.

“The ocean varies more quickly in the vertical plane than it does in the horizontal and when these animals are moving vertically in their three-dimensional habitat, they’re also going through extreme environmental changes.

“How they use that vertical habitat tells us a lot about what’s going on in the ocean, but also how the animals are adapting to different niches in that vertical space.”

The study found the ocean region receiving sunlight — which stretches from the surface to about 200 metres in depth – was potentially a dangerous area for elasmobranchs and was where they were most likely to be exposed to fishing gear as either target species or bycatch.

Of the 38 species in the study, researchers found that 26 spent more than 95 per cent of their time in the top 250m of the water column.

Dr Samantha Andrzejaczek, co-lead author of the study and a postdoctoral research fellow at the Hopkins Marine Station of Stanford University, said one of the common vertical movements among elasmobranchs appeared to match up with the ocean’s diel (twice a day) vertical migration.

At daybreak, tiny fish and invertebrates – followed by the animals that prey on them – begin migrating from the bright, uppermost ocean layer to the relative safety of darker, deeper water. At night, they return to the surface to feed.

“We think that sharks and rays in the diel migration are following food resources up and down the water column,” Dr Andrzejaczek said.

Researchers hope their findings will enable policy makers and resource managers the opportunity to examine the threats these animals face, and guide future management and conservation plans.

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