In recent years, an unprecedented phenomenon has been unfolding in the Gulf of Trieste, a case unique in the entire Mediterranean. Large groups of bull rays, among the largest eagle rays in the Mediterranean, are being sighted with increasing frequency in the coastal waters off Trieste.
Over the past three years the phenomenon has grown in scale and become more frequent, even though this is a species already present in the Mediterranean Sea and recorded for years.
“This is the first year in which we are seeing a high number of individuals aggregated in groups that have reached as many as 50 animals,” says Saul Ciriaco, head of monitoring activities for WWF Miramare Marine Protected Area (MPA) and vice-president of the Shoreline cooperative.
The phenomenon has undoubtedly sparked the interest of researchers and enthusiasts, as this is a rare and protected species, but at the same time it is becoming a problem for local mussel farms, which have reported damage to their lines of mussels, as these have become an easy food source that attracts the rays.
“We had never seen bull rays interacting with human activities in this way,” explains Simona Clò, marine biologist and scientific director of MedSharks, an association dedicated to conserving marine species in the Mediterranean, stressing the need to find non-invasive technologies and solutions to protect mussel farmers while safeguarding bull rays.
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.
Changes in fisheries policy and warming seas among the causes
Changes in European fisheries policy have reduced the number of individuals caught, as they used to be taken as bycatch, or incidental catch, when another species was being targeted.
“Stricter, more selective fishing rules have also improved their reproductive performance,” Ciriaco notes. This combination is thought to be one of the reasons behind the increase in the number of animals sighted off the Adriatic coast.
“On top of that, winter water temperatures in the Mediterranean have reached a level that bull rays consider tolerable, as they prefer waters ranging from 11 to 28 degrees Celsius. In winter the temperature no longer drops below 10 degrees. So whereas the Gulf of Trieste used to be just a stopover, it has now become suitable for them to stay,” he adds.
Similar cases have been recorded in Greece and Turkey in recent years. Bull rays have also been reported around mussel farms off Olbia, but Trieste is the first location to have documented the phenomenon on a significant scale.
RelatedA species protected by the United Nations
Bull rays are cartilaginous fish, like other rays, belonging to the Myliobatidae family. They are native to tropical and temperate waters and grow to impressive sizes, reaching up to two metres across.
The species is considered to be at high risk across the Mediterranean and is protected both by the Barcelona Convention – which aims to protect the Mediterranean Sea from pollution and imposes measures to safeguard 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.
Monitoring and identification: the Life Eu Sharks project
“These sizeable groups of bull rays do not visit just one mussel farm; they move within a fairly wide area from Grignano almost to Monfalcone. We are talking about some twenty kilometres of coastline to monitor,” says Ciriaco, explaining that in this case the monitoring work carried out by Shoreline focuses mainly on counting individuals and applying a protocol to identify them.
These activities form part of the broader Life Eu Sharks programme, the European project aimed at promoting and conserving protected shark and ray species.
Last year, the Ministry of the Environment supplied several drones for tracking and photo-identifying individuals, since the only way to quantify accurately the presence of these animals in the waters of the Gulf of Trieste depends on recognising them.
“The largest group observed so far contained around 50 individuals, but the true numbers could be much higher,” says Clò, who works on the Life programme.
Thanks to the project “we are able to pinpoint individual animals using the distinctive pattern formed by the stripes on their back,” the biologist notes.
They may not be the same individuals every time, and their presence may also vary depending on the season, although the combination of factors considered suggests that bull rays may have found in the Gulf of Trieste a place suitable for settling.
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.
Threat to mussel farming: the Life Prometheus project
Bull rays are “durophagous” animals, feeding mainly on crustaceans and hard-shelled molluscs, although they usually forage along the seabed. They are also so-called “opportunistic” animals, meaning they “make maximum use of the resources available, and the presence of convenient farms is an ideal situation for them,” Clò explains.
However, this is a problem for mussel farms, which suffer significant economic losses, although it is difficult to quantify them, as other factors also threaten mussel farming, such as rising water temperatures and other predators like turtles.
To find a solution that protects both sides, Shoreline in the Miramare Marine Protected Area, in partnership with Life Eu Sharks, has launched the European project Life Prometheus.
The trial, conducted together with mussel farmers and researchers from the Universities of Padua and the Marche Polytechnic University, involves installing electromagnetic deterrents on the lines to drive bull rays away.
“These small magnets have a disturbing effect on the animals but are not harmful. We still need to see whether the system will actually work,” says Ciriaco.
In any case, Ciriaco believes it is “necessary to assess the effectiveness and applicability of the tool, as the cost and manpower required to install so many magnets would represent a major and expensive effort.”
RelatedA model for coexistence
One of the next steps is to set up a round table with the Friuli Venezia Giulia regional authorities and the harbour master’s office. “Bull rays are important animals for the environment. They also swim close to the shore and this is something that should be valued,” says Clò.
Designating specific areas of the Gulf of Trieste to promote snorkelling and underwater photography with these rare animals could provide a compensatory solution that would allow interaction between the species.
This text was translated with the help of artificial intelligence. Report a problem : [feedback-articles-en@euronews.com].
