Stanford (California): Scientists have compiled a sizable biologging dataset using sophisticated electronic tags to gain comparative insights into how "elasmobranchs," such as sharks, rays, and skates, use the ocean's depths.
Others dive hundreds of metres or more of the slope waters into the twilight zone, beyond where sunlight can penetrate, while some species spend their entire lives in shallow waters close to our shores on the continental shelf. The ability to examine the threats that these animals face and direct future management and conservation plans will be provided by this new understanding of how elasmobranchs use the ocean.
The largest global investigation of where and when a diverse group of elasmobranchs move vertically was conducted in a study led by Stanford University and ZSL (Zoological Society of London) researchers and published on August 19 in Science Advances. Two decades of data from satellite and archival tags that remotely tracked the movements and behaviours of 38 species in oceans around the world were compiled by a team of 171 researchers from 135 institutions in 25 countries.
Samantha Andrzejaczek, the co-lead author of the study and postdoctoral research fellow at Stanford University's Hopkins Marine Station, said, "For the first time, we have a standardised, global database that we used to fill important knowledge gaps about the diving behaviours of sharks and rays. This will make it possible to better understand how fisheries interact with elasmobranchs and how to manage many of these incredibly long-lived creatures.
Movement in three dimensions
Numerous marine species that live in the coastal ocean's near-surface regions have already been extensively studied by scientists for their movement patterns. To survey marine communities and populations, for instance, drones, scuba divers, and other techniques are used to a depth of about 50 metres. However, little is known about animal movement in three dimensions, particularly in the deeper, vertical spaces of the ocean.
Rays and sharks are well-known but endangered ocean species. Understanding their fundamental ecology is essential to their efficient management, according to David Curnick, co-author of the study and director of ZSL's Ocean Predator Lab. But for many species, our understanding of their basic behaviour is limited, and what we do know frequently only encompasses what can be seen in surface waters.
A variety of electronic tags that can be used to tag various elasmobranch species have developed over the past 20 years. Stanford researchers have been at the forefront of developing biologging tags and using these techniques on sharks and rays.
Elasmobranchs frequently move vertically, and this movement seems to correspond to the ocean's diel (twice daily) vertical migration. Small fish and invertebrates migrate from the light, topmost ocean layer to the relatively safe depths of deeper, darker water at dawn, followed by the animals that feed on them. They come back to the surface at night to eat.
According to Andrzejaczek, "we believe that during their diurnal migrations, sharks and rays follow food resources up and down the water column."
According to the study, about one-third of species frequently descend to cold, oxygen-poor depths that have little visibility because of biological activity related to productivity. White sharks (Carcharodon carcharias) have been observed to dive to depths of more than 1200 metres, while whale sharks (Rhincodon typus) have been observed to dive to depths of up to 1896 metres, which is close to the pressure limit of 2000 metres for current sensors.
Deep divers may be avoiding hunters as potential prey or searching for food in deeper water, according to Andrzejaczek. "Some of the biggest sharks and rays will feed on smaller sharks and rays. We discovered that 13 species have members who can dive more than 1000 metres, which is an incredibly deep depth. Some people might need to cool off while diving. "Large sharks may need to dive to cool off when they spend too much time in the warm surface waters, a form of behavioural thermoregulation," she continued.
The researchers also discovered species overlap in similar vertical spaces. Despite having very different evolutionary histories, the vertical distributions of whale sharks, tiger sharks, and oceanic manta rays were remarkably similar. Relationships between predators and prey probably cause this closeness. According to Andrzejaczek, plankton is a food source for both whale sharks and oceanic manta rays, and the tiger shark has been known to prey on both of those species.
A foundation for future management
The area of the ocean that receives sunlight, known as the photic zone or epipelagic, extends from the surface to a depth of about 200 metres and may be hazardous for elasmobranchs. They are most likely to come into contact with fishing gear there, either as bycatch or as a target species. Researchers discovered that 26 of the 38 species they examined spent more than 95% of their time in the top 250 metres of the water column.
According to the IUCN Red List of Threatened Species, more than one-third of all sharks and rays face extinction.
According to Barbara Block, the Prothro Professor of Marine Sciences at Stanford, whose tagging programmes like TOPP contributed 25% of the data set, "These data provide the foundation for future management of global elasmobranch resources. 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."
Elasmobranchs play important ecological roles in the ocean today and in the future, and their vulnerability to various threats depends on our understanding of how they use vertical habitats. The distribution of species may be affected by changes in ocean temperature and oxygen levels, which may also result in the creation of new conditions and threats, according to science.
Andrzejaczek stated that "human beings are unaccustomed to thinking of habitat in the vertical dimension. "We hope that this study will help people realise the importance of management plans that take into account this underappreciated aspect of elasmobranch behaviour. We could use these data, for instance, to learn more about the interactions between sharks and human fisheries.
Data from increasingly sophisticated and precise tags with sensitive sensors that can function in deep water and withstand the rigours of the environment were combined in this three-year study, along with data from improved analytical tools to incorporate various types of movement data. The tags were also mounted on a shark or ray. The international collaboration between biologging researchers has been a key component. Large-scale scientific investigations like this one require enormous levels of cooperation, according to Curnick. "We combine the expertise and knowledge of researchers from all over the world. The outcome is much greater than any one researcher or organisation could accomplish independently. —ANI