Scientists take a deep dive into how sharks use the ocean
IMAGE: SAMANTHA ANDRZEJACZEK, A POSTDOCTORAL RESEARCH FELLOW AT STANFORD’S HOPKINS MARINE STATION, (SECOND FROM RIGHT) TAGGING A TIGER SHARK. view more
CREDIT: ALEX KYDD
Using sophisticated electronic tags, scientists have assembled a large biologging dataset to garner comparative insights on how sharks, rays, and skates – also known as “elasmobranchs” – use the ocean depths. While some species spend their entire lives in shallow waters close to our shores on the continental shelf, others plunge hundreds of meters or more off the slope waters into the twilight zone, beyond where sunlight penetrates. This new understanding of how elasmobranchs use the ocean will enable policymakers and resource managers the opportunity to examine the threats these animals face, and guide future management and conservation plans.
A study published Aug. 19 in Science Advances, led by Stanford University and ZSL (Zoological Society of London) researchers, is the largest global investigation of where and when a diverse group of elasmobranchs move vertically. A team of 171 researchers from 135 institutions across 25 countries brought together two decades of data from satellite and archival tags that remotely tracked the movements and behaviors of 38 species in oceans across the globe.
“For the first time, we have a standardized, global database that we used to fill important knowledge gaps about the diving behaviors of sharks and rays,” said Samantha Andrzejaczek, co-lead author of the study and a postdoctoral research fellow at the Hopkins Marine Station of Stanford University. “This will enable better understanding of what fisheries interact with elasmobranchs and how to improve management of many of these long-lived animals.”
Movement in three dimensions
Scientists already have a wealth of movement data about many marine species that inhabit the near-surface spaces of the coastal ocean. Drones, scuba divers, and other methods, for example, survey marine communities and populations to a depth of about 50 meters, but animal movement in three dimensions, especially in the deeper, vertical spaces of the ocean, is far less understood.
“Sharks and rays are iconic but threatened ocean species. Key to their effective management is an understanding of their basic ecology,” said David Curnick, head of the Ocean Predator Lab at ZSL and co-lead author of the paper. “Yet, for many species, we know relatively little about their fundamental behavior, and what we do know is often restricted to what we can observe in surface waters.”
Over the past 20 years, a variety of electronic tags have come of age that provide the opportunity to tag numerous elasmobranch species. Scientists at Stanford have been at the forefront of biologging tag development and applying these technologies on sharks and rays.
One of the common vertical movements among elasmobranchs appears to match up with the ocean’s diel (twice a day) vertical migration. At daybreak, tiny fish and invertebrates – followed by the animals that prey on them – begin migrating from the bright, uppermost ocean layer to the relative safety of darker, deeper water. At night, they return to the surface to feed.
“We think that sharks and rays in their diel migrations are following food resources up and down the water column,” said Andrzejaczek.
The study found that about one-third of species frequently dive to depths where the water is cold, often low on oxygen due to biological activity related to productivity, and visibility is limited. Sensor records show that white sharks (Carcharodon carcharias) dive deeper than 1200 meters while whale sharks (Rhincodon typus) have reached 1896 meters, which is near the pressure limit of 2000 meters for today’s sensors.
“Deep divers might be looking for food in deeper water or avoiding hunters themselves as potential prey,” said Andrzejaczek. “Some sharks and rays are small, and some of the biggest sharks and rays will feed on them. We found that 13 species had individuals that dive to more than 1000 meters, which is extremely deep.” Some may require cooling-off periods while at depth. “When large sharks spend too long in the warm surface waters, they may have to dive to cool down, a form of behavioral thermoregulation,” she added.
The researchers also identified overlaps among species in the same vertical spaces. Whale sharks, tiger sharks, and oceanic manta rays showed strikingly similar vertical distributions, although they have very different evolutionary histories. Predator-and-prey relationships likely drive this proximity. “The oceanic manta ray and whale shark both feed on plankton, and the tiger shark has been known to predate on both those species,” said Andrzejaczek.
A foundation for future management
The photic zone or epipelagic – the ocean region receiving sunlight – stretches from the surface to about 200 meters and can potentially be a dangerous area for elasmobranchs. That is where they are most likely to be exposed to fishing gear as either target species or as bycatch. Of the 38 species in the study, researchers found that 26 spent more than 95% of their time in the top 250 meters of the water column.
More than one-third of all sharks and rays are threatened with extinction, according to the IUCN Red List of Threatened Species.
“These data provide the foundation for future management of global elasmobranch resources, and it has taken a team of scientists thousands of hours tagging and tracking the sharks with global satellite and biologging systems to make this possibility happen,” said Barbara Block, the Prothro Professor of Marine Sciences at Stanford, whose tagging programs such as TOPP, contributed 25% of the data set.
Understanding how elasmobranchs use vertical habitats is crucial to understanding their current and future ecological roles in the ocean and their risks to various threats. Scientists could utilize this and future databases to investigate how changing ocean temperature and oxygen levels may influence species’ distributions and create new conditions and threats.
“Human beings are unaccustomed to thinking of habitat in the vertical dimension,” said Andrzejaczek. “We hope this study can make people realize that we need management strategies that incorporate this overlooked dimension of elasmobranch behavior. For example, we could use these data to better understand how sharks and human fisheries interact.”
This three-year study brought together data from increasingly sophisticated and more accurate tags with sensitive sensors that can withstand the rigors of the environment while riding along on a shark or ray and function in deep water, as well as improved analytical tools to incorporate different types of movement data. The key ingredient has been cooperation among biologging scientists from around the world.
“Large-scale scientific studies like this one are not possible without a monumental collaborative effort,” said Curnick. “We reconcile the collective knowledge and expertise of researchers from around the world. The result is far greater than any one researcher or institution could achieve on their own.”
This work was led by Stanford University and the Zoological Society of London. Additional Stanford co-authors include graduate student Maurice Goodman; research data analyst Mike Castleton; research scientist Jonathan Dale; and researcher Robert Schallert. Former Stanford graduate students and postdoctoral scholars George Shillinger, Andre Boustany, Kevin Weng, Taylor Chapple, and Sal Jorgensen are also co-authors. Block is also a professor of biology and a senior fellow at the Stanford Woods Institute for the Environment.
This research was funded by the Bertarelli Foundation, the Moore Foundation, the Sloan Foundation, Instituto Politecnico Nacional, the Darwin Initiative, the Georgia Aquarium, the Rolex Awards for Enterprise, and the Whitley Fund for Nature.
JOURNAL
Science Advances
ARTICLE TITLE
Diving into the vertical dimension of elasmobranch movement ecology
ARTICLE PUBLICATION DATE
19-Aug-2022
Shark depredation impacts support for shark conservation
Conflict between shark populations and fishing communities is reported to be rising within one of the world’s first shark sanctuaries.
Fishers say increasing shark populations have reduced catch and income, according to Newcastle University research.
Shark populations around the world are in rapid decline, with 24% of all shark species at risk of extinction. While efforts to protect sharks vary, 17 countries have declared their waters as ‘shark sanctuaries’, typically banning commercial shark fishing. In some regions this has led to increasing reports of negative fisher-shark interactions, particularly depredation – when a shark preys on a fishers catch.
New research led by Newcastle University presents a unique overview of fisher-shark interactions, focusing on fisher perspectives within the Maldives shark sanctuary. Their results show that most fishers reported an increase in shark depredation, with substantial catch and income losses (>21%) reported by reef fishers. They also found that those experiencing greater losses due to depredation showed reduced support for the Maldives shark sanctuary.
Mapping exercises with fishers and underwater camera stations (BRUVS) were used to identify areas of high shark abundance and ‘conflict hotspots’ where sharks and fishing activity overlapped. This provided valuable information as to why fishers believe depredation is increasing despite stable populations in the region.
Published in the journal Conservation Letters, the findings suggest that management of shark depredation needs to be addressed sensitively to avoid negative implications for fisher welfare and shark population recovery.
Study lead author, Dr Danielle Robinson, of Newcastle University’s School of Natural and Environmental Sciences, said: “The study highlights the importance of more inclusive approaches to help us understand and manage depredation conflicts. Research and conservation frameworks tend to focus on commercial fisheries due to their economic value, yet our findings highlight the urgent need to consider and manage negative perceptions of small-scale fisheries when developing policy solutions”.
“Fishers and sharks are competing for the same thing – fish – and I think a really exciting aspect of this study is that we were able to collect spatial data from fisher interviews and underwater cameras (BRUVS) at the same spatial scale to map areas with high conflict potential”.
The study is one of few in the field to shed light on the socio-economic costs of shark sanctuaries and the importance of accounting for diverging perceptions among fisher groups.
Findings raise important issues relating to trade-offs between policies to protect biodiversity and those related to human welfare.
Reference
Robinson, D., Newman, S. P., Whittingham, M. J., Francksen, R. M., Adam, M. S., & Stead, S. M. (2022). Fisher–shark interactions: A loss of support for the Maldives shark sanctuary from reef fishers whose livelihoods are affected by shark depredation. Conservation Letters, 00, e12912. https://doi.org/10.1111/conl.12912
JOURNAL
Conservation Letters
METHOD OF RESEARCH
Observational study
SUBJECT OF RESEARCH
Animals
ARTICLE TITLE
Fisher–shark interactions: A loss of support for the Maldives shark sanctuary from reef fishers whose livelihoods are affected by shark depredation
ARTICLE PUBLICATION DATE
19-Aug-2022
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