One of the last refuges for angel sharks revealed 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 refuges in Corsica, providing a better understanding of this very elusive shark and ultimately helping to protect it.
Eric Durieux, University of Corsica Pascal-Paoli; Caroline Bousquet and Johann Mourier, University of Montpellier
The angel shark is a flat shark that lives on sandy coastal bottoms. Once common throughout the Mediterranean and the Northeast Atlantic, it is now one of the most endangered species in the world (critically endangered according to the IUCN) and as such is a symbol of marine biodiversity in peril.
The angel shark, which even gave its name to the Baie des Anges in Nice, had been missing from the French coast for decades, but that was without counting on the Isle of Beauty, where it found a real refuge. If you strike up a conversation with the fishermen in the ports from Bastia to Solenzara, they are unanimous: the sea angel has always been and still is present on the coasts of Corsica. And that's good news.
This shark has large lateral fins, reminiscent of angel wings, and can spend several hours buried in the sand waiting for its next meal. Like many shark species, it has a slow reproduction rate (8 to 10 months of gestation), a low number of young per litter (7 to 25 pups), and late sexual maturity, making it particularly vulnerable to anthropogenic pressures.
Corsica, the angel's last refuge
The angel shark thus disappeared from more than 90% of its territory during the20thcentury, victim of intensive fishing, particularly bottom trawling, and the destruction of its habitat. Although it was once commercially fished, the species has been protected since 2010 in all European Union waters. It was long thought that the Canary Islands were the last refuge in the world for this shark, but the knowledge of local Mediterranean fishermen—including those in Corsica—tells a very different story.
In Corsica, this discreet yet fascinating species seems to prefer the vast sandy expanses of the eastern plain. And with good reason: low fishing pressure—mainly artisanal—coupled with an ideal habitat consisting of hundreds of square kilometers of sand, several estuaries, and one of the largest Posidonia meadows in the Mediterranean, classified as a Natura 2000 site and coordinated by the Corsican Environment Office. Artisanal fishermen, who are at sea as soon as the weather permits, are true sentinels 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 artisanal fishing
The aim was therefore to establish a partnership between scientists and small-scale fishermen to improve knowledge of this iconic species in Corsica. This project, entitled Corsic'Ange (launched in 2022), aimed in particular to better understand the distribution and spatio-temporal dynamics of the species in Corsica, on the one hand thanks to the empirical knowledge of fishermen and on the other hand by setting up specific monitoring at the population level through conventional tagging and at the individual level through electronic tagging to understand its movements and behavior.
The first step was to cross-reference fishermen's memories with scientific data to establish an inventory of the presence of angel fish in Corsica.
Imagine a coastline stretching 150 kilometers—the distance between Paris and the Normandy coast—and an area extending 5 kilometers offshore. This is where we are trying to study the angel shark. Needless to say, searching for a species on the brink of extinction in such a large 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 precious time: they know exactly when and where to find them. It is from some of them that we learn that the species is present all year round on the coast at depths of up to 100 meters, but that groups of several dozen individuals gather during the winter and spring.
How many are there and how is the population structured?
Identifying the functional areas of the species (breeding, nursery, resting, and migration corridors) is fundamental to improving conservation measures. The breeding habits of the angel shark, in particular, are very poorly documented, and only one nursery area has been located so far, in the Canary Islands.
The partnership with fishermen continued with monitoring through Capture/Marking/Recapture (CMR) on the east coast of Corsica, ultimately enabling the size of the population to be estimated. Sea angels caught accidentally were fitted with individual "spaghetti" tags and then released. During 18 months of monitoring, 214 angelfish were tagged and 19 were recaptured, representing a recapture rate of 9.7%, which already seems to indicate a significant population size.
This monitoring has also provided a better understanding of the structure of this population in terms of the distribution of young of the year, juveniles, subadults, and adults. Observations vary seasonally, with adults and subadults being more frequently observed in spring and summer, while juveniles are more often encountered in winter. Newborns have also been observed, as some females give birth on board fishing boats. These observations are the first evidence that could confirm the presence of a nursery area on the eastern coast of Corsica.
This data also made it possible to identify a new potential area of frequentation north of Cap Corse in the summer of 2023, with two individuals captured, one of which had already been tagged in the waters north of the eastern plain. This recapture revealed that angel sharks can travel up to 70 km in a year. The movements of tagged sharks vary, with some remaining loyal to one area from year to year, while others travel greater distances.
How can we study the behavior of this species?
We know that it likes sandy and muddy areas up to a depth of around 150 m, although it usually stays between 10 and 50 m, and that juveniles and pregnant females frequent estuary mouths and brackish waters. However, very little is known about the behavior of this species in the Mediterranean in particular.
To understand their lifestyle, acoustic telemetry tracking was carried out between July 2022 and November 2023. A transmitter (tag) was placed on 28 individuals, measuring between 63 cm and 1.20 m. Here again, the capture and electronic tagging were based on close partnership with fishermen.
The areas frequented by sharks, 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 been observed on the eastern plain to date, there is evidence of migration from deeper waters to the coast. The angel fish is thought to frequent Posidonia seagrass beds and deeper waters in summer, then move to the coast and river mouths in fall/winter.
The sea angel's home range is divided into two areas: the home range and the core activity area. The home range is the area where the bird spends most of its time, 95% of which is spent searching for food and reproducing. Its core activity area is the area where it spends 50% of its time and is its preferred area.
The analyses carried out show that the size of the home range is affected by the size of the shark. The larger the individuals, the larger their home ranges will be.
The sex of the animal is also a determining factor. Males tend to have a larger home range: on average 60km²compared to 35km²for females. The central areas of activity are smaller, with approximately 13km²for males and 8km²for females. These differences can be explained by the tendency of males to mate with several females and thus be more active in seeking partners, increasing their movements.
Throughout the study period, angel sharks were most often detected at rest, i.e., buried in the sand, both during the day and at night. This indicates 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... Some sharks, a minority, are most often detected when they are moving. For other rare individuals, such as a female tagged in the summer of 2022, activity rates are reversed, with significant time spent resting during the day and very high swimming activity at night.
These preliminary observations have not yet revealed all of the species' behaviors, but since then, additional data has been collected, which will allow us to improve our estimates of the species' vital territories and activity patterns.
The results of this study are promising! Working with professional fishermen is essential to better understand one of the last populations of this species on the brink of extinction on a global, Mediterranean, and national scale. Continuing the monitoring initiated in Corsic'Ange is essential and will enable us to estimate the size of the population, its genetic connectivity with Greek, Libyan, and Atlantic populations, as well as differences in behavior at the individual level and between related individuals. The partnerships established within theAngel Shark Project bring new perspectives for study and new methodologies. The improved knowledge provided by this partnership between scientists and fishermen helps promote the conservation of angel sharks in Corsica by directly involving local stakeholders, as well as more broadly in the Mediterranean and throughout its range.
This article is published as part of the Fête de la science (Science Festival), which takes place from October 4 to 14, 2024, and of which The Conversation France is a partner. This year's theme is "ocean of knowledge." Find all the events in your region on the Fetedelascience.fr website.
Eric Durieux, Associate Professor in Marine Ecology, University of Corsica Pascal-Paoli; Caroline Bousquet, Scientific Assistant, Corsican Environment Office; and Johann Mourier, Junior Professor in Fish Behavioral Ecology, University of Montpellier
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