Trieste bull ray sightings: unique in Mediterranean but threat to mussel farms

Sightings of this protected species are unprecedented in the Mediterranean in scale and frequency, but they threaten local mussel farms. In the Gulf of Trieste tests are under way to identify and drive the animals away using magnetic deterrents. In recent years, a peculiar phenomenon has been observed in the Gulf of Trieste, a case unique in the entire Mediterranean. Large groups of bull rays (known locally as vaccarelle), among the biggest eagle rays in the Mediterranean, are being sighted with increasing frequency in the coastal waters off Trieste. It is a new development that over the past three years has reached a significant scale, even though this was a species already present in the Mediterranean Sea and recorded for years. “In recent years sightings have become steady, around one a week in the summer season,” says Saul Ciriaco, head of monitoring activities for WWF Miramare Marine Protected Area (MPA) and vice‑president of the Shoreline cooperative. “This is the first year in which we have recorded a high number of animals grouped together, with aggregations reaching as many as 50 individuals. That, as well as the frequency, is what makes it distinctive,” Ciriaco adds, noting that these aggregations are thought to be linked to feeding, but also to reproduction, as males and females come into contact. The phenomenon has undoubtedly aroused the interest of researchers and marine enthusiasts, as this is a rare protected species, but at the same time it is becoming a problem for local mussel farms, since mussel lines have become an easy source of food that attracts these animals. Evidence of this comes from numerous reports by mussel farmers, who in recent years have recorded damage to their mussel farms coinciding with the rising frequency of sightings. “Such interaction between bull rays and human activities had never been seen before,” explains Simona Clò, marine biologist and scientific director of MedSharks, an association devoted to conserving marine species in the Mediterranean. Clò stressed the need to find non‑invasive technologies and solutions to protect mussel farmers and safeguard their livelihood, without putting at risk the survival of bull rays, a protected species that must be preserved. The Gulf of Trieste has thus become a testing ground for coexistence between these species, a process led jointly by the Miramare MPA and, at European level, by Life Eu Sharks and Life Prometheus: two projects co‑funded by the European Union’s Life programme. Similar cases have been recorded in Greece and Turkey in recent years. Bull rays have also been reported near mussel farms off Olbia, but Trieste is the first place to have documented the phenomenon on a significant scale, with substantial aggregations. A protected species Bull rays are cartilaginous fish belonging to the Myliobatidae family. They are a species typical of tropical and temperate waters, which is why their presence is well documented in the eastern Atlantic Ocean, the south‑western Indian Ocean, the Mediterranean Sea and the Black Sea. These animals reach impressive sizes and can grow to up to two metres across. The species is considered seriously at risk on a Mediterranean scale and is protected both by the Barcelona Convention, which aims to protect the Mediterranean Sea from pollution and imposes protection measures for threatened species, and by the UN Bonn Convention on migratory species. It is also included on the Red List of the International Union for Conservation of Nature (IUCN), which sets out the conservation status of animal and plant species worldwide. Changes in European fisheries policy have led to fewer animals being caught, as they used to be taken as bycatch, or incidental catch, when another species was being targeted. “Stricter and more selective fishing rules have also improved their reproductive performance,” Ciriaco explained. This is thought to be one of the reasons for the increase in animals sighted off the Adriatic coast. “On top of this, winter water temperatures in the Mediterranean have reached a level that bull rays find tolerable, as they prefer waters ranging from 11 to 28 degrees Celsius. In winter, temperatures no longer drop below 10 degrees. So whereas the Gulf of Trieste used to be a place they passed through on migration, it has now become suitable for them to stay,” he added. Monitoring and identification: the Life Eu Sharks project “These sizeable groups of bull rays do not visit just a single mussel farm; they move across a fairly wide area from Grignano almost as far as Monfalcone. We are talking about some 20 kilometres of coastline to monitor,” says Ciriaco, explaining that in this case Shoreline’s monitoring work focuses mainly on counting individuals and applying a protocol to identify them. These activities are part of the wider Life Eu Sharks programme, the European project for promoting and conserving protected species of sharks and rays. Last year, the Environment Ministry supplied several drones for tracking and photo‑identifying animals, because the only way to quantify precisely how many of these animals are present in the waters of the Gulf of Trieste depends on recognising them. "The largest group sighted so far numbered about 50 animals, all adults and sub‑adults, but the real figures could be much higher,” says Clò, who works with the European Life programme. All the reports collected and the images obtained from the drones are uploaded to a database, where software then identifies the animals. Thanks to the project, “we are able to identify individual animals through the distinctive pattern of stripes on their back. This will give us an estimate of the actual population in the Gulf of Trieste,” the biologist pointed out. They may not always be the same animals, and their presence may change with the seasons. “They could start arriving in late spring and then leave in autumn, or not,” she specified. The particular shape of the Gulf of Trieste and the frequency of sightings, combined with rising sea temperatures, suggest that bull rays may have found a place suited to settlement. This means that “they have more time to realise that it is simpler and more efficient to eat mussels directly from the ropes,” Ciriaco suggests. Threats to mussel farming: the Life Prometheus project Bull rays are durophagous animals, meaning they feed mainly on crustaceans and molluscs with hard shells, although they usually forage on the seabed. However, the ready availability of mussel farms off Trieste has encouraged bull rays to come closer in order to obtain food more easily, while “not losing the habit of rooting around on the seabed,” Ciriaco notes. “Bull rays are so‑called ‘opportunistic’ animals, like sharks, meaning they make maximum use of the resources available; if they find lines covered in mussels, they feed directly from them,” Clò explains. “For them it is an ideal situation.” However, this is a problem for mussel farms, which suffer significant economic damage, even though it is difficult to quantify, because other factors also contribute to threatening mussel farms, such as rising water temperatures and other predators, for example turtles. To find a solution that protects Trieste’s mussel farms without harming these protected species, Shoreline in the Miramare Marine Protected Area, in partnership with Life Eu Sharks, has launched the European project Life Prometheus. The trial, carried out together with mussel farmers, involves installing electromagnetic deterrents on the lines to keep bull rays away. “These small magnets have a disturbing effect on the animals, but they are not aggressive or harmful,” says Ciriaco. The first magnets were installed at the beginning of July, so at least a month will be needed to check whether the system actually works with bull rays, given that these deterrents are normally used to keep sharks away from fishing nets. If the system works, Ciriaco believes it will be “necessary to assess the cost‑benefit ratio of its effectiveness and practical applicability compared with other tools, such as fitting sleeves or sticks on the lines, since the cost and labour required to deploy large numbers of magnets would demand substantial, costly effort.” A model of coexistence One of the next steps is to set up a round‑table with the Friuli Venezia Giulia region and the coastguard to discuss the issue and possible solutions. “Bull rays are important animals for the environment. They also swim close to the shore and this is something that should be valued and managed together,” says Clò. “I hope we can intervene without causing damage,” avoiding a situation in which individual farmers are forced to cope alone with strategies that jeopardise the survival of this protected species. Identifying dedicated areas of the Gulf of Trieste where snorkelling and underwater photography with these rare animals can be promoted could be a compensatory solution that would allow coexistence and interaction between species.