Conservation scientists have unlocked the mysteries of how shark and ray species move up and down the ocean water column, in an effort to better understand them and secure their future.
A new global study published in Science Advances – led by the Zoological Society of London (ZSL) Institute of Zoology and Hopkins Marine Station at Stanford University – is the first ever global analysis of how the elasmobranch community (sharks, skates and rays) use the vertical dimension of the ocean. It covers some of the most mysterious deep-diving species, through to those spending more time in shallower water.
Using data from 989 biotelemetry 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 this new information will help improve conservation management plans previously hindered by a lack of data for certain species.
Thirteen species dive to depths greater than 1km beneath the surface. Whale sharks (Rhincodon typus) were found to dive to a staggering 1896m, while white sharks (Carcharodon carcharias) were recorded diving deeper than 1200m, providing new and important insights into the behaviour of these ocean giants.
The data also revealed that some species vary their depth in different parts of the world. It showed how this changes between night and daytime periods, with the predators moving up and down in the water to hunt their prey and, in some cases, avoid being hunted themselves.
Although reasons why species usually known to frequent shallower waters have been recorded diving into deep, dark waters is not confirmed, the study suggests a likely 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 250m of the water column, which is where they are most likely to interact with fishing gear.
Dr David Curnick, Research Fellow at the ZSL Institute of Zoology and co-lead author of the paper, says: “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. 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’.”
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 to understand 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.
Australian co-author, Flinders University shark and ray researcher Professor Charlie Huveneers, from the Southern Shark Ecology Group and Marine and Coastal Research Consortium at the College of Science and Engineering, says South Australian researchers collaborated on collecting some of the data.
“Flinders University, the University of Adelaide and SARDI collaboratively contributed data from bronze whalers and school sharks using pop-up satellite tags as part of PhD projects, which reached maximum depths of 129 m and 696 m, respectively,” he says.
“This study, combining depth data from 38 shark and ray species, showcases that the broad diversity in movement patterns is not limited to horizontal movements, but also extends to vertical movement.
“These datasets highlight the variability of depth use, with some species spending most of its time within 5m of the surface while others spent most of its time deeper than 200m.
“We also observed diurnal migrations in both directions, either shallow at night or shallow during the day, showing that one size does not fit all, and it illustrates how species can adapt to their environment to be as successful as possible.”
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 instance, a 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.”
The upcoming UN Climate Change Conference UNFCC COP27 and Convention on Biological Diversity COP15 will place a much-needed spotlight on big issues impacting biodiversity, and ZSL is calling for support from world leaders to put nature at the heart of all global decision making.