The gilthead seabream, a species of commercial interest that migrates seasonally, does not form independent local populations in the north-western Mediterranean but instead constitutes a single, functionally connected population on a large scale. Individuals spend the summer feeding in the coastal lagoons of the Gulf of Lion and, every autumn, undertake reproductive migrations of hundreds of kilometres to shared spawning grounds in the open sea, mainly in the Marseille region, but also in areas of the Catalan coast. They repeat this pattern year after year.

This is one of the most notable findings to emerge from the RESMED project, led by Professor Bernat Hereu from the Faculty of Biology and the Biodiversity Research Institute (IRBio) at the University of Barcelona, and CONNECT-MED, led by expert Jérôme Bourjea from the French Research Institute for Exploitation of the Sea (IFREMER). The conclusions are now published in a study in the journal Movement Ecology.
The gilthead seabream (Sparus aurata) is an emblematic Mediterranean species and plays a prominent role in the fishing industry and in the structure of marine ecosystems. Despite its ecological and economic importance, its reproductive migrations — a key phase in its life cycle — remain poorly understood by the scientific community.
The unknown migration of sea bream to the Medes Islands to reproduce
Year after year, the gilthead seabream follows a marked seasonal rhythm: from April to September, the species remains in a lagoon in the Gulf of Lion to feed, and in autumn, it leaves the lagoons and moves to the marine breeding grounds, where it gathers from October to December before returning to its lagoon of origin.
The main breeding area is the Marseille region, which includes the Calanques National Park and the Côte Bleue Marine Park. “Everything indicates that the vast majority of gilthead seabream in the Gulf of Lion gather in the Marseille region from October to early December to reproduce, a vital phase for the populations, but also a phase of great vulnerability to fisheries if there is no effective and sustainable management,” emphasizes Jérôme Bourjea.
The study now reveals for the first time — using direct tracking data using acoustic telemetry — a surprising fact that changes the migratory map of gilthead sea bream during the breeding season: two-thirds migrate to the waters off Marseille to reproduce, “but one-third of the population travels in the opposite direction to the Catalan coast, to a location that has yet to be precisely identified,” says Professor Bernat Hereu, from the UB’s Department of Evolutionary Biology, Ecology and Environmental Sciences.
Since the gilthead seabream is a commercially exploited species on both sides of the Pyrenean coastline, experts note that it would be important to agree on fish stock management processes shared between countries, as has been done with pelagic species such as bluefin tuna and swordfish.
The study is based on an exceptional acoustic telemetry system: 222 gilthead seabream have been tracked using small acoustic transmitters for three years (2019-2022), and more than 180 underwater sensors or listening stations were deployed, from Spain to the Calanques of Marseille, from lagoons to the open sea. In total, more than 700,000 detections were recorded to reconstruct the movements of individuals along some 400 kilometres of coastline on both sides of the Pyrenean marine region.
The study confirms the potential of regional acoustic telemetry networks to study the connectivity of coastal species of interest — that is, the links between marine animal populations in different marine regions — such as gilthead sea bream or the European seabass (Dicentrarchus labrax), and to understand the impact of offshore wind farms on fish communities.

Further information:
https://www.ifremer.fr/en/press/migration-gilthead-seabream-mediterranean-study-provides-new-insights-and-raises-questions

Video:
https://www.youtube.com/watch?v=y1F071Jq_nI