Until now, more horizontal movement patterns were known – now an international consortium of researchers is presenting for the first time an overview study on the up and down movements of sharks and rays in the sea. It is based on data collected over the past 20 years by animals equipped with biologging systems. There are discernible differences between species in the use of sea depths, but also some overarching patterns. The scientists say that the information can now be used for further research and, above all, for the protection of these particularly endangered marine animals.
From the great white shark to the manta ray – the numerous shark and ray species are assigned to a common group of cartilaginous fish: the elasmobranchs, also known as squamous gills, play important roles in the ecosystems of the world’s oceans as predators and plankton eaters. “These are particularly impressive but also critically endangered marine animals,” says co-author David Curnick of the Zoological Society of London. According to the Red List, more than a third of all shark and ray species are threatened with extinction. “If we are to protect them, the key is understanding their basic ecology. However, we know relatively little about the behavior of many species, and what we know is often limited to observations in surface waters,” says the marine biologist. In comparison, the movements in the vertical dimension, especially in the deeper regions of the oceans, have hardly been systematically documented.
In the last 20 years, however, data has been generated on this: Various research groups have equipped specimens of numerous shark and ray species with biotelemetry systems, which have also provided information about the vertical movements of the animals. For the current overview study, 171 researchers from 25 countries have now compiled this data. They come from 38 species of sharks and rays from the oceans around the world. “For the first time, we now have a standardized, global database that has enabled us to fill important knowledge gaps about the diving behavior of sharks and rays,” says co-author Samantha Andrzejaczek of Stanford University.
Diving behavior at a glance
As the researchers report, certain patterns are emerging in addition to the species-specific features. One of the most common vertical dynamics seems to be associated with a typical diurnal pattern of movement in the oceans: at daybreak, tiny fish and invertebrates—followed by the animals they prey on—begin their migration from the upper ocean layers to the relative safety of the darker, deeper ones water At night they return to the surface to feed. “There seems to be evidence that many sharks and rays follow the food resources up and down the water column on their daily migrations,” says Andrzejaczek.
The study also shows that many species show special behaviors and sometimes dive very deep: For example, recordings from sensors show that great white sharks (Carcharodon carcharias) dive deeper than 1200 meters, while whale sharks (Rhincodon typus) have even reached 1896 meters, which is close the pressure limit of modern sensors. “We found that 13 species have individuals that dive to more than 1,000 meters, which is extremely deep,” says Andrzejaczek. Some of them may use the depth to cool down: “If large sharks spend too long in warm surface water, they may need to dive to cool down – a form of behavioral thermoregulation,” says Andrzejaczek.
The researchers also found overlap between species in the same vertical spaces. Whale sharks, tiger sharks and manta rays therefore showed strikingly similar vertical distributions. This closeness is likely due to the relationship between predator and prey. “The manta ray and whale shark both feed on plankton, and the tiger shark is known to prey on both of these species,” says Andrzejaczek.
Important basis for research and protection
According to the team, the study now forms an important knowledge base. Information about vertical habitat use is crucial to assessing the current and future ecological role of elasmobranchs in the ocean. Scientists could use the databases to study how changes in ocean temperature and oxygen levels might affect species distribution.
In addition, there is information about the background to their endangerment: Of the 38 species examined, 26 spent more than 95 percent of their time in the upper 250 meters of the water column, the researchers report. In this area there is a high probability that they will either be a target species or accidentally come into contact with fishing gear. “We can use the new data to better understand how sharks and fisheries interact,” says Andrzejaczek. The researchers hope that the information could thus help to better identify threats and create future management and protection plans.
Source: Stanford University, professional article: Science Advances, 10.1126/sciadv.abo1754