One of the last remaining habitats of the angel shark, discovered thanks to Corsican fishermen
Once common in the Mediterranean, the angel shark is now one of the most endangered species in the world. A partnership between scientists and fishermen has led to the discovery of one of its last remaining habitats in Corsica, with the aim of learning more about this very elusive shark and , ultimately, better protecting it.
Eric Durieux, University of Corsica Pascal-Paoli; Caroline Bousquet and Johann Mourier, University of Montpellier
The angel shark is a flat shark that inhabits sandy coastal seabeds. Once common throughout the Mediterranean and the northeastern Atlantic, it is now one of the world’s most endangered species (listed as Critically Endangered by the IUCN) and, as such, serves as a symbol of marine biodiversity under threat.
The angel shark, which even gave its name to the Baie des Anges in Nice, had been missing from French coasts for decades, but that was without counting on the “Isle of Beauty,” where it has found a true refuge. If you strike up a conversation with the fishermen in the ports from Bastia to Solenzara, they are unanimous: the angel shark has always been and still is present along the coasts of Corsica. And that’s good news.
This shark has large pectoral fins, reminiscent of an angel’s wings, and can spend several hours buried in the sand waiting for its next meal. Like many shark species, it has a slow reproductive rate (8 to 10 months of gestation), a small litter size (7 to 25 pups), and reaches sexual maturity late, making it particularly vulnerable to human-induced pressures.
Corsica, the angel's last refuge
The angel shark has thus disappeared from more than 90% of its range during the20thcentury, a victim of intensive fishing—particularly bottom trawling—and the destruction of its habitat. Although it was once sold commercially, the species has been protected since 2010 in all waters of the European Union. It was long believed that the Canary Islands were the last refuge in the world for this shark, but the knowledge of local Mediterranean fishermen—and thus Corsican fishermen—tells a very different story.
In Corsica, this species—as elusive as it is fascinating—appears to have a preference for the vast sandy expanses of the eastern plain. And for good reason: low fishing pressure—primarily from small-scale fishing—combined with an ideal habitat consisting of hundreds of square kilometers of sand, several estuaries, and one of the largest Posidonia seagrass beds in the Mediterranean, classified as a Natura 2000 site and managed by the Corsican Environmental Agency. Small-scale fishermen, who head out to sea as soon as the weather permits, serve as true guardians of the sea and, as such, are regular observers of this species. https://www.facebook.com/plugins/video.php?href=https%3A%2F%2Fwww.facebook.com%2FTF1Info%2Fvideos%2Fcorse-le-dernier-refuge-du-requin-ange%2F1916073955444067%2F&width=500&show_text=false&height=281&appId
The valuable expertise of small-scale fishing
The goal was therefore to establish a partnership between scientists and small-scale fishermen to improve our understanding of this iconic species in Corsica. This project, titled Corsic’Ange (launched in 2022), aimed specifically to better understand the distribution and spatio-temporal dynamics of the species in Corsica, on the one hand through the empirical knowledge of fishermen and, on the other hand, by implementing specific monitoring at the population level using conventional tagging and at the individual level using electronic tagging to track its movements and behavior.
The first step was to compare the fishermen’s recollections with scientific data in order to assess the current status of the sea angel population in Corsica.
Imagine a coastline stretching 150 kilometers—the distance from Paris to the Normandy coast—and a 5-kilometer-wide stretch of ocean off that coast. This is where we’re trying to study the sea angel. Needless to say, searching for a species on the brink of extinction across such a vast area is like looking for a needle in a haystack.
We therefore turn to small-scale coastal fishermen to study this species. Their years of experience at sea save us valuable time: they know exactly when and where to find them. It is from some of them that we learn that the species is present year-round along the coast at depths of up to 100 meters, but that gatherings of several dozen individuals occur during the winter and spring.
How many are there, and what is the population structure?
Identifying the species’ functional areas (breeding, nursery, resting, and migration corridors) is essential for improving conservation measures. The breeding habits of the angel shark, in particular, are poorly documented, and only one nursery area has been identified so far in the Canary Islands.
The partnership with fishermen continued with a Capture-Mark-Recapture (CMR) study on the eastern coast of Corsica, ultimately enabling an estimate of the population size. Accidentally caught angel sharks were fitted with individual “spaghetti”-type tags and then released. Over the 18-month monitoring period, 214 angel sharks were tagged and 19 were recaptured, representing a recapture rate of 9.7%, which already suggests a population of significant size.
This monitoring also provided a better understanding of the population structure, in terms of the distribution of this year’s calves, juveniles, subadults, and adults. Sightings vary seasonally, being more frequent in spring and summer—primarily involving adults and subadults—while juveniles are more commonly encountered in winter. Newborns have also been observed, as some females occasionally give birth aboard fishing boats. These observations provide the first evidence that could confirm the presence of a nursery area off the eastern coast of Corsica.
This data also helped identify a new potential habitat north of Cap Corse in the summer of 2023, with two individuals captured, one of which had previously been tagged in the waters north of the Eastern Plain. This recapture revealed that angel sharks can travel up to 70 km over the course of a year. The movements of tagged sharks vary; some remain in the same area from year to year, while others travel much greater distances.
How can we study the behavior of this species?
It is known that this species prefers sandy and muddy areas at depths of up to approximately 150 meters, although it most often remains between 10 and 50 meters, and that juveniles and pregnant females tend to frequent estuary mouths and brackish waters. However, very little is known about the behavior of this species, particularly in the Mediterranean.
To understand their lifestyle, acoustic telemetry monitoring was conducted between July 2022 and November 2023. A transmitter (tag) was attached to 28 individuals, ranging in size from 63 cm to 1.20 m. Once again, the capture and electronic tagging were made possible through close collaboration with local fishermen.
The areas frequented by sharks, as identified by fishermen, were covered by a grid of 58 acoustic receivers (fixed hydrophones) between Bastia and Solenzara. Of the 28 tagged sharks, 24 were detected by the hydrophone network: 7 males and 17 females.
Although no north-south migration has yet been observed in the eastern plain, a migration from deeper waters toward the coast does occur. The sea angel is believed to inhabit Posidonia seagrass beds and deeper waters in the summer, and then move to coastal areas and river mouths in the fall and winter.
The home range of the angel shark is divided into two zones: the home range and the core activity zone. The home range is the area where it spends most of its time—95% of the time—to meet its needs for foraging and reproduction. Its core activity zone, on the other hand, is the area where it spends 50% of its time and represents its preferred habitat.
The analyses conducted show that the size of the home range is influenced by the shark's size. The larger the individual, the larger its home range will be.
The animal’s sex is also a key factor. Males tend to have larger home ranges: an average of 60km²compared to 35km²for females. Core activity areas, however, are smaller, at about 13km²for males and 8km²for females. These differences can be explained by the tendency of males to mate with multiple females and thus be more active in seeking out mates, leading to increased movement.
Over the entire study period, oceanic whitetips were most often detected while stationary—that is, buried in the sand—both during the day and at night. This suggests that their swimming activity is not greater at night, as one might assume, and that these sharks spend nearly 75% of their time buried in the sand.
There are, however, a few exceptions… A small number of sharks are most often detected when they are moving. For other rare individuals, such as a female tagged in the summer of 2022, activity patterns are reversed, with significant periods of inactivity during the day and very high swimming activity at night.
These preliminary observations have not yet revealed all aspects of the species’ behavior, as additional data has since been collected, which will enable us to refine our estimates of the species’ home ranges and activity patterns.
The results of this study are encouraging! Collaboration with professional fishermen is essential to better understand one of the last remaining populations of this species, which is on the brink of extinction at the global, Mediterranean, and national levels. Continuing the monitoring initiated in Corsic’Ange is essential and will allow us to estimate the population size, its genetic connectivity with Greek, Libyan, and Atlantic populations, as well as behavioral differences at the individual level and among related individuals. The partnerships established within theAngel Shark Project bring new research perspectives and methodologies. The improved knowledge gained through this partnership between scientists and fishermen helps promote the conservation of the angel shark in Corsica by directly involving local stakeholders, as well as more broadly in the Mediterranean and throughout its entire range.
This article is published as part of the Fête de la Science (taking place from October 4 to 14, 2024), of which The Conversation France is a partner. This year’s edition focuses on the theme “Ocean of Knowledge.” Find all the events in your region on the Fetedelascience.fr website.
Eric Durieux, Associate Professor (HDR) of Marine Ecology, University of Corsica Pascal-Paoli; Caroline Bousquet, Research Assistant, Corsican Environmental Agency; and Johann Mourier, Assistant Professor of Fish Behavioral Ecology, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Readthe original article.