Key facts
- Large groups of bull rays are being sighted with increasing frequency in the Gulf of Trieste.
- This phenomenon is unique in the Mediterranean and has reached a significant scale in recent years.
- The bull rays are damaging mussel farms by feeding on them, creating economic losses.
- Warming sea temperatures and stricter fishing regulations are contributing to the increase in bull ray populations.
- Projects are underway to find non-invasive solutions for coexistence, including electromagnetic deterrents.
- The bull ray is a protected species under international conventions.
An unprecedented increase in bull ray sightings has been observed in the Gulf of Trieste, a phenomenon unique to the Mediterranean. These large, protected eagle rays, locally known as 'vaccarelle', are gathering in groups of up to 50 individuals, posing a significant threat to local mussel farms. Researchers attribute the surge in bull ray populations to a combination of factors, including stricter European fisheries policies that reduce bycatch and warming sea temperatures that make the Gulf of Trieste a more suitable habitat year-round.
The interaction between the bull rays and human activities, particularly mussel farming, is a novel challenge. The rays are attracted to the mussel lines as an easily accessible food source, leading to economic losses for farmers. Simona Clò, scientific director of MedSharks, emphasizes the need for non-invasive technologies to protect both the species and the livelihoods of mussel farmers.
Two European Union co-financed projects, Life Eu Sharks and Life Prometheus, are actively involved in addressing this situation. Life Eu Sharks focuses on monitoring and identification, using drones to photo-identify individual rays based on unique stripe patterns on their backs. This monitoring covers a 20-kilometer stretch of coastline.
Life Prometheus, in cooperation with Shoreline and researchers from the Universities of Padua and Marche Polytechnic University, is testing electromagnetic deterrents. These devices aim to disturb the rays without causing harm, though their cost-effectiveness and applicability are still under assessment.
Bull rays are cartilaginous fish native to tropical and temperate waters, reaching up to two meters in wingspan. They are classified as highly at risk in the Mediterranean and are protected by the Barcelona Convention and the UN Bonn Convention, as well as being listed on the IUCN Red List. They are durophagous, feeding on hard-shelled mollusks, and are opportunistic feeders, readily exploiting easily accessible resources like mussel farms.
Future steps include establishing a round table with regional authorities to discuss coexistence strategies, potentially including the designation of specific areas for responsible interaction with the bull rays through activities like snorkeling and underwater photography.