An Insight Into How Sharks Use Our Oceans

A new global study published by the Zoological Society of London (ZSL)’s Institute of Zoology and Hopkins Marine Station at Stanford University is the first ever global analysis of how sharks and their relatives (skates and stingrays) use the vertical dimension of the ocean.

Found worldwide in our oceans and some freshwater bodies, the elasmobranch community (sharks, rays, and skates) are most diverse and abundant in shallow tropical regions even though you can find them in the coldest of waters and deepest of locales. With about 500 species of sharks and 600 species of rays globally – and new species continuing to be discovered – scientists know no two shark species are exactly alike. Just how you and I are both Homo sapiens and don’t live identical lives, the elasmobranch community (sharks, skates, and rays) carry on about their days in different ways.

But how do they differ? And do they differ while moving about the water column in 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 have probably heard of — the epipelagic, mesopelagic, bathypelagic, abyssopelagic, and hadalpelagic zones. This water column is home to the largest migration on the planet: the diel vertical migration (DVM). A daily, synchronized movement of marine animals between the surface and deep layers of the ocean, some sharks participate in this migration (such as bigeye thresher sharks, Alopias superciliosus).

“The way that large marine animals use the horizontal space in our ocean has been well studied. However, until now, comparative studies in the vertical planes have been limited, despite the ocean being an average 3.5 kilometers (2.17 miles) deep and elasmobranchs occupying all levels within this dynamic environment,” said Dr. David Curnick, Research Fellow at the ZSL Institute of Zoology and co-lead author of the paper. “Investigating how elasmobranchs use the vertical dimensions of their habitat is key in understanding the way they live, but also how anthropogenic stressors are impacting them. This helps us to find ways to better protect them through more informed monitoring strategies for example.”

Using data from 989 biotelemetry tags – electronic tags which allow remote measurements of an animal’s behavioral activity – the team analyzed 38 species of elasmobranchs from the North Pacific to the Indian Ocean, and the Arctic to the Caribbean. It was a monumental effort: 171 researchers from 135 institutions were involved in this collaboration, many hoping this study will help improve conservation management plans for all species (but especially those were data deficient). Stanford Postdoctoral Research Fellow and co-lead author of the paper, Dr Samantha Andrzejaczek said: “This massive dataset provides new insights into the vertical movement patterns of sharks and rays on a global scale for the first time. This is an important step for both understanding which sharks and rays are most likely to face threats, but also to consider how changing temperature and oxygen levels may influence their vertical distributions.”

The scientists found 13 species dove to depths greater than one kilometer (0.62 miles) beneath the surface. For example, whale sharks (Rhincodon typus) were found to dive to a staggering 1,896 meters (1.17 miles) while white sharks (Carcharodon carcharias) were recorded diving deeper than 1,200 meters (0.75 miles)! Meanwhile, 26 of 38 species – including the oceanic whitetip shark (Carcharhinus longimanus), tiger shark (Galeocerdo cuvier), scalloped hammerhead (Sphyrna lewini), and silky shark (Carcharhinus falciformis) – spent more than 95% of their time in the top 250 meters (0.15 miles) of the water column, depths where they are most likely to interact with fishing gears.

And that’s the problem. “By looking at a wide range of elasmobranch species in this study, we demonstrate how they face overlapping risks, such as targeted fisheries and getting caught in nets, also known as ‘bycatch,’” explained Curnick.

According to the International Union for Conservation of Nature (IUCN) Red List of Threatened Species, it is now estimated that over one-third of sharks and rays are threatened with extinction. This makes them one of the most threatened taxon in the marine environment. But what is causing this dramatic global decline? A recent study revealed that global populations of open-ocean sharks and rays have declined by 71%since the 1970s due to the 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 urged for immediate action to reduce the risk of further overfishing or extinction to oceanic sharks and rays.

The data also revealed some interesting patterns, such as how some species vary their diving depth in different parts of the world. It astounded the researchers that they were able to spot some changes between night and daytime periods as the predators move up and down in the water to hunt their prey and, in some cases, avoid being the hunted. Although the reasons why some sharks and ray species who were usually known to frequent shallower waters were being recorded taking deep dives is not confirmed, the scientists believe it is likely a combination of seeking food sources, body temperature regulation, reproduction, and predator avoidance.

As our planet continues to warm due to climate change, the predictions for our oceans are grim. Anthropogenic climate change is projected to lead to ocean warming, acidification, deoxygenation, reductions in near-surface nutrients, and changes to primary production, all of which are expected to affect marine ecosystems. This would ultimately change the structure of the ocean, and the scientists believe this work will help inform predictions on how reduced oxygen availability at certain depths could limit shark, ray and skate vertical movements and help to predict the wider implications of climate change.

Ernesto Bertarelli of the Bertarelli Foundation told a ZSL press release: “I’ve seen for myself the terrible threats that shark populations face around the world and how they have been decimated in recent decades. I hope that this incredible research will help scientists, conservationists and fisheries managers better protect these astonishing – and hugely important – species in the future so that they can retain their rightful place in the ocean.”