Sightings of this protected species, unprecedented in the Mediterranean, are threatening local mussel farms, prompting trials in the Gulf of Trieste with magnetic deterrents.
In recent years, an unprecedented phenomenon has been observed in the Gulf of Trieste, a case that is unique across the Mediterranean. Large groups of bull rays, known locally as vaccarelle and among the biggest eagle rays in the Mediterranean, are being sighted with increasing frequency in the coastal waters off Trieste.
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Over the past three years the phenomenon has grown in scale and frequency, even though this is a species already present in the Mediterranean and recorded for many years.
“This is the first year in which we have recorded large numbers of individuals grouped together, with aggregations reaching as many as 50 animals,” says Saul Ciriaco, head of monitoring activities for WWF at the Miramare Marine Protected Area (Amp) and vice-president of the Shoreline cooperative society.
The phenomenon has naturally attracted the interest of researchers and enthusiasts, as it involves a rare protected species, but at the same time it is creating problems for local mussel farms, which have reported damage to their mussel lines, as these have become an easy source of food that attracts the animals.
“This kind of interaction between bull rays and human activities had never been seen before,” explains Simona Clò, marine biologist and scientific coordinator of MedSharks, an association for the conservation of marine species in the Mediterranean, highlighting the need to develop non-invasive technologies and solutions to protect mussel farmers and safeguard the bull rays.
The Gulf of Trieste has thus become a testing ground for coexistence between these species, managed jointly by Amp Miramare and, at European level, by Life Eu Sharks and Life Prometheus, two projects co-financed under the European Union’s Life programme.
Changes in fishing policy and warming seas among the causes
Changes to European fisheries policy have reduced the number of animals being caught, as they used to be taken as bycatch, or incidental catch, when other species were being targeted.
“More selective tightening of fishing rules has also improved their reproductive success,” Ciriaco explained. This combination is one of the reasons that could explain the increase in animals spotted off the Adriatic coast.
“On top of this, winter sea temperatures in the Mediterranean have reached a level that bull rays find tolerable, as they prefer water between 11 and 28 degrees Celsius. In winter the temperature does not drop below 10 degrees. So whereas the Gulf of Trieste used to be a transit area, it has now become suitable for them to stay,” he added.
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 location to have documented the phenomenon on a significant scale.
A 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 are of impressive size, reaching up to two metres across.
The species is considered highly 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 conservation measures for threatened species – and by the United Nations’ Bonn Convention on migratory species.
It is also listed on the Red List of the International Union for Conservation of Nature (IUCN), which records the conservation status of animal and plant species worldwide.
Monitoring and identification: the Life Eu Sharks project
“These large groups of bull rays do not visit just one mussel farm, but move around a fairly wide area from Grignano almost as far as Monfalcone. We are talking about some twenty kilometres of coastline to be monitored,” says Ciriaco, explaining that in this case the monitoring activity 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 species of sharks and rays.
Last year the Ministry of the Environment supplied several drones for tracking and photo-identification of the animals, since the only reliable way of quantifying the presence of these animals in the waters of the Gulf of Trieste depends on recognising them individually.
“The largest group sighted so far numbered around 50 individuals, but the actual figures could be much higher,” says Clò, who collaborates on the Life programme.
Thanks to the project, “we can identify individual animals by the distinctive ‘fingerprint’ formed by the stripes on their backs,” the biologist pointed out.
They may not be the same animals all the time, and their presence can vary with the seasons, although the range of factors considered suggests that bull rays may have found the Gulf of Trieste to be a suitable place to settle.
This means “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 farms: 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. They are also so-called opportunistic animals, meaning “they make full use of available resources, and the presence of convenient farms is an ideal situation for them,” explains Clò.
However, this poses a problem for mussel farms, which suffer significant economic damage, even though it is difficult to measure, since other factors also threaten mussel cultivation, such as rising water temperatures and other predators, for example turtles.
To find a solution that protects both sides, Shoreline in the Miramare Marine Protected Area, in collaboration with Life Eu Sharks, has launched the European project Life Prometheus.
The trial, carried out together with mussel farmers and researchers from the universities of Padua and Marche Polytechnic, involved installing electromagnetic deterrents on the lines to keep bull rays away.
“These small magnets have a disturbing effect on the animals but are not harmful. We now need to see whether the system will actually work,” says Ciriaco.
For Ciriaco it is nevertheless “necessary to assess how effective and applicable the device really is, because the cost and labour needed to deploy so many magnets would demand a major and expensive effort.”
A model for coexistence
One of the next steps is to set up a round table with the Friuli Venezia Giulia region and the harbour master’s office. “Bull rays are important animals for the environment. Moreover, they swim close to the shore and this is something that should be valued,” says Clò.
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 interaction between species.
