A new global study published by the Institute of Zoology and Zoology of the Zoological Society of London (ZSL) Hopkins Marine Station at Stanford University is the world’s first-ever analysis of how sharks and their relatives (rays and stingrays) use the vertical dimension of the ocean.
Found worldwide in our oceans and some freshwater bodies, the elasmobranch (sharks, rays, and rays) community is most diverse and abundant in the shallow tropics, although you can find them in the coldest and deepest waters. With around 500 species of sharks and 600 species of rays in the world – and new species continuing to be discovered – scientists know that no two shark species are the same. Just how you and I are both Homo sapiens and don’t live identical lives, the elasmobranch community (sharks, rays, and rays) live their days in different ways.
But how do they differ? And do they differ as they move through the water column of our oceans?
The water column is a concept used in oceanography to describe the physical (temperature, salinity, light penetration) and chemical (pH, dissolved oxygen, nutrient salts) characteristics of seawater at different depths. There are five vertical zones of the ocean’s water column that you’ve probably heard of – the epipelagic, mesopelagic, bathypelagic, abyssopelagic, and hadalpelagic zones. This water column is home to the largest migration on the planet: the daily vertical migration (DVM). Daily and synchronized movement of marine animals between the surface and the deep layers of the ocean, some sharks participate in this migration (such as bigeye thresher sharks, Alopias superciliosus).
“How large marine animals use the horizontal space of our ocean has been well studied. However, so far comparative studies in vertical planes have been limited, despite the ocean having an average depth of 3 .5 kilometers (2.17 miles) and that elasmobranchs occupy all levels of this dynamic environment,” said Dr. David Curnick, researcher at the ZSL Institute of Zoology and co-lead author of the paper. elasmobranchs use the vertical dimensions of their habitat is key to understanding their way of life, but also how anthropogenic stressors affect them.This helps us find ways to better protect them through more informed monitoring strategies, for example. example.
Using data from 989 biotelemetry tags – electronic tags that allow an animal’s behavioral activity to be measured remotely – the team analyzed 38 species of elasmobranchs from the North Pacific to the Indian Ocean, and from the Arctic to the Caribbean. It was a monumental effort: 171 researchers from 135 institutions were involved in this collaboration, many hoped that this study would help improve conservation management plans for all species (but especially those that lacked data). Stanford postdoctoral researcher and co-lead author of the paper, Dr Samantha Andrzejaczek, said: “This massive dataset provides for the first time new insights into the vertical movement patterns of sharks and rays at sea. global scale. This is an important step both for understanding which sharks and rays are most likely to be at risk, but also for considering how changes in temperature and oxygen levels may influence their vertical distributions.
Scientists have found that 13 species have dived to depths greater than one kilometer (0.62 miles) below the surface. For example, whale sharks (type of rhinoceros) were found to dive to 1,896 meters (1.17 miles) while white sharks (Carcharodon carcharias) have been recorded diving over 1,200 meters (0.75 miles)! Meanwhile, 26 of 38 species – including the oceanic whitetip shark (Carcharhinus longimanus), tiger shark (Galeocerdo cuvier), scalloped hammer (Sphyrna lewini) and the silky shark (Carcharhinus falciformis) – spent more than 95% of their time in the top 250 meters (0.15 miles) of the water column, the depths where they are most likely to interact with fishing gear.
And that’s the problem. “By examining a wide range of elasmobranch species in this study, we demonstrate how they face overlapping risks, such as targeted fisheries and capture in nets, also known as ‘bycatch,'” explained Curnick.
According to the International Union for Conservation of Nature (IUCN) Red List of Threatened Species, more than a third of sharks and rays are now estimated to be at risk of extinction. This makes it one of the most threatened taxa in the marine environment. But what is causing this dramatic global decline? A recent study found that global populations of deep-sea sharks and rays have declined by 71% since the 1970s due to an 18-fold increase in relative fishing pressure. Largely due to overexploitation, half of oceanic shark and ray species are now either “critically endangered” or “endangered”, according to the IUCN Red List. Many have called for immediate action to reduce the risk of overfishing or the extinction of oceanic sharks and rays.
The data also revealed some interesting patterns, such as how certain species vary their diving depth in different parts of the world. The researchers were amazed that they were able to spot some changes between night and day periods as predators moved up and down the water to chase their prey and, in some cases, avoid being chased. Although the reasons why some species of sharks and rays that were generally known to frequent shallow waters have been recorded diving deep are unconfirmed, scientists believe it is likely a combination search for food sources, body temperature regulation, reproduction and predator avoidance.
As our planet continues to warm due to climate change, the forecast for our oceans is grim. Anthropogenic climate change is projected to lead to warming, acidification, ocean deoxygenation, near-surface nutrient reductions, and changes in primary production, all of which are expected to affect marine ecosystems. This would eventually change the structure of the ocean, and the scientists believe this work will help inform predictions of how reduced oxygen availability at certain depths could limit the vertical movements of sharks, rays and rays and help predict the wider implications of climate change.
Ernesto Bertarelli from Bertarelli Foundation told a ZSL press release, “I have seen firsthand the terrible threats that shark populations face around the world and how they have been decimated over the past few decades. I hope this incredible research will help scientists, conservationists and fisheries managers better protect these amazing – and hugely important – species in the future so they can keep their rightful place in the world. ‘ocean.