Scientists have assembled a large biologging dataset to gather comparative insights on how sharks, rays, and skates use the ocean depths. Some species spend their entire lives in shallow waters close to our shores on the continental shelf, while others jump hundreds of meters off the slope into the twilight zone. This new understanding of how elasmobranchs use the ocean will allow policymakers and resource managers the opportunity to examine the threats these animals face.
The largest investigation of where and when a diverse group of elasmobranchs move vertically was published in August. A group of researchers from 135 institutions across 25 countries brought together two decades of data from satellite and archival tags that tracked the movements and behaviors of 38 species in oceans across the globe.
For the first time, we have a standardized, global database that we used to fill important knowledge gaps about the diving behaviors of sharks and rays. Improved management of long-lived animals and better understanding of what fisheries interact with elasmobranchs will be enabled by this.
There is movement in 3 dimensions.
Scientists have a lot of movement data about many marine species that live near the ocean's surface. Animal movement in three dimensions, especially in the deeper, vertical spaces of the ocean, is less understood than surveys of marine communities and populations using drones, scuba divers, and other methods.
The ocean's sharks and rays are in danger. David Curnick is the head of the Ocean Predator Lab at ZSL and co-lead author of the paper. For many species, we don't know much about their behavior, and what we do know is limited to what we can see in the water.
Many elasmobranch species have been tagged with electronic tags over the past two decades. biologging tags have been developed and applied to sharks and rays.
One of the common vertical movements among elasmobranchs is similar to the ocean's diel. The animals that prey on small fish and invertebrates begin to migrate from the bright, uppermost ocean layer to the darker, deeper water at daybreak. They come back to the surface to eat.
sharks and rays are following food resources up and down the water column
The study found that a third of the species dive to depths where the water is cold and low in oxygen due to biological activity. White sharks can dive deeper than 1,200 meters, while whale sharks can reach 1896 meters, which is close to the pressure limit.
Divers might be looking for food in deeper water or avoiding hunters themselves as potential prey. Some sharks and rays are small and some of the biggest are feeding on them. 13 species had individuals that were able to dive to more than 1000 meters. cooling off periods may be required. She said that large sharks may have to dive to cool down if they spend too long in the warm surface waters.
There are overlaps among species in the same space. There are very different evolutionary histories of whale sharks, tiger sharks, and oceanic manta rays. This proximity is likely driven by predatory and prey relationships. There are two shark species that feed on plankton, the whale shark and the oceanic manta Ray.
Future management is a foundation.
The ocean region receiving sunlight stretches from the surface to about 200 meters and can be a dangerous place for elasmobranchs. They are most likely to be exposed to fishing gear at that location. The top 250 meters of the water column was where 26 of the 38 species spent most of their time.
More than one-third of sharks and rays are at risk of extinction.
"These data provide the foundation for future management of global elasmobranch resources, and it has taken a team of scientists thousands of hours to tag and track the sharks with global satellite and biologging systems to make this possible," said Barbara Block, the Prothro Professor of Marine
Understanding how elasmobranchs use vertical habitats is important to understand their current and future ecological roles in the ocean. Scientists can use this and future databases to investigate how changing ocean temperature and oxygen levels can affect species distributions.
Humans are not used to thinking of habitat in the vertical dimensions. This study will hopefully make people realize that we need management strategies that incorporate elasmobranch behavior. We can use the data to understand how sharks and humans interact.
This three-year study brought together data from increasingly sophisticated and more accurate tags with sensitive sensors that can endure the rigors of the environment while riding along on a shark or ray, as well as improved analytical tools to incorporate different types of movement data. The key ingredient is the cooperation of scientists from all over the world.
Large-scale scientific studies are not possible without a monumental collaborative effort. Researchers from around the world come together to reconcile their knowledge. The result is much better than any single researcher or institution could achieve on their own.
More information: Samantha Andrzejaczek et al, Diving into the vertical dimension of elasmobranch movement ecology, Science Advances (2022). DOI: 10.1126/sciadv.abo1754. www.science.org/doi/10.1126/sciadv.abo1754 Journal information: Science Advances
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