Mysterious lives of sharks and rays revealed in new study

A Galapagos Shark and a Devil Ray © Mark Royer
A Galapagos Shark and a Devil Ray

Conservation scientists have unlocked the mysteries of how shark and ray species move up and down the ocean water column, in efforts to better understand them and secure their future.

From some of the most mysterious deep-diving species, to those that spend more time in shallower water, a new global study published today (Friday 19th August) involving Lancaster University and led by ZSL’s Institute of Zoology and Hopkins Marine Station at Stanford University details the first ever global analysis of how the elasmobranch community (sharks, skates and rays) use the vertical dimension of the ocean.

Using data from 989 biotelemetry tags – tags which allow remote measurements of behavioural activity - the global team of 171 researchers from 135 institutions analysed 38 species of elasmobranchs from the North Pacific to the Indian Ocean, and the Arctic to the Caribbean.

ZSL’s researchers hope that this new information will help improve conservation management plans that were previously hindered by lack of data for certain species.

Thirteen species were found to dive to depths greater than one kilometre beneath the surface. Whale sharks (Rhincodon typus) were found to dive to a staggering 1896 metres while white sharks (Carcharodon carcharias) were recorded diving deeper than 1200 metres, providing new and important insights into the behaviour of these ocean giants.

The data also revealed how some species vary their depth in different parts of the world. It showed how this 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 hunted themselves.

Although the reasons why species usually known to frequent shallower waters were recorded diving into deep, dark waters is not confirmed, the study suggests it is likely a combination of seeking food sources, body temperature regulation, reproduction, and predator avoidance.

The team found that although many species can and will undertake deep dives, 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 metres of the water column, depths where they are most likely to interact with fishing gears.

Dr David Curnick, Research Fellow at the ZSL Institute of Zoology and co-lead author of the paper, said: “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 kilometres deep and elasmobranchs occupying all levels within this dynamic environment.

“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.”

Dr David Jacoby, Lecturer in Zoology at Lancaster University contributed data to the study from a number of shark species tagged with satellite transmitters.

He said: “This paper breaks new ground in our understanding of how different species of elasmobranchs - many of which are heavily threatened - occupy and move between different depths within our oceans. Importantly, this global analysis allows us to begin to understand how sharks might overlap with fishing gears that target different depths, complementing existing studies that consider horizontal rather than vertical overlap.”

More than one third of all sharks and rays are threatened with extinction, according to the IUCN Red List of Threatened Species. Having a three-dimensional map of how elasmobranchs use the ocean is vital in understanding the roles they play in wider ecosystems and to determine their individual exposure to threats.

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.”

As the world warms due to climate change, it is predicted that the structure of the ocean is also going to change. With many areas suffering oxygen depletion and shifts in ocean chemistry, many species have already been driven into unfamiliar territory and habitats. However, a better understanding of their fundamental ecology can 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 said: “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.”

Dr Curnick concluded: “Knowing just how deep some species dive (or don’t dive), will help us to inform much needed conservation plans for these species and their relatives – for example, more widespread use of bycatch avoidance strategies. It will also help us understand how these animals are likely to respond to the predicted climate induced changes to our oceans.”

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