Using sophisticated electronic tags, a team of researchers led by Stanford University has collected a wealth of biological data to better understand how elasmobranchs – a group of marine animals that includes sharks, rays and skates – use the ocean’s depths.
The analysis showed that while some species spend their entire lives in shallow water near the coast, others dive hundreds of meters underwater. Finding out how Elasmobranchs use the ocean will allow resource managers and policy makers to study the threats these animals face and develop better management and conservation plans for the future.
The researchers collected data from two decades of satellite and archival tags to track the movements of 38 Elasmobranch species in oceans around the world.
“For the first time, we have a standardized global database that we’ve used to fill important knowledge gaps about the diving behavior of sharks and rays,” said study co-author Samantha Andrzejaczek, a postdoctoral fellow in the Department of Marine Biology at Stanford. “This will provide a better understanding of how fisheries interact with elasmobranchs and how to improve the management of many of these long-lived animals.”
According to scientists, one of the most common movements among elasmobranchs coincides with long (twice a day) vertical migration in the ocean. At dawn, small fish and invertebrates begin to migrate from the surface of the ocean into the safety of dark, deeper waters, where they are pursued by predators. At night, they come up to the surface again to feed.
“We think sharks and rays follow food resources up and down the water column in their diurnal migrations,” Dr. Andrzejaczek said.
About a third of elasmobranch species often dive deep. For example, white sharks (Carcharodon carcharias) dive to depths of over 1,200 meters, and whale sharks (Rhincodon typus) have reached a staggering depth of 1,896 meters.
“Deep-sea divers may seek food at greater depths or avoid hunters as potential prey,” says Dr. Andrzejaczek. “Some sharks and rays are small, and some of the largest sharks and rays will feed on them. We found that 13 species have individuals that dive over 1,000 meters, which is very deep.” Some of these animals can dive to the depths, not only in search of prey, but also to cool off after being in warm surface waters for too long.
The scientists also identified intersections between species in the same vertical spaces, largely driven by predator-prey dynamics. For example, the oceanic manta ray and the whale shark feed on plankton, while the tiger shark is known to prey on both of these species. These findings may help conservation efforts for elasmobranchs by identifying areas where they may be at risk of bycatch or other hazards.
“People are not accustomed to thinking about habitat in a vertical dimension. We hope this study will help people understand that we need management strategies that take into account this overlooked dimension of elasmobranch behavior. For example, we could use this data to better understand how sharks and human fisheries interact,” concluded Dr. Andrzejaczek.
The study is published in the journal Science Advances.