Although Astrospartus mediterraneus (Risso, 1826) is an emblematic Mediterranean species, limited information is currently available, as it has not received much attention from the marine scientific community to date. In this context, an unusually high abundance of this basket star was observed in 2018 as part of the by-catch of local artisanal fishers in the Cap de Creus area (NW Mediterranean Sea). Indeed, fishers reported that this species had increased in abundance and expanded its distribution in recent years, ultimately interfering with their fishing activity. As such, this study aimed to elucidate the abundance, distribution, size, and structure of A. mediterraneus populations, as well as to evaluate the potential impact this species has on artisanal fisheries’ performance in the study area. To such aim, this work benefited from the Local Ecological Knowledge (LEK) of the fishers, by-catch analyses of regular fishing events and video transects recorded employing Remote Operated Vehicles (ROV).

Results show that A. mediterraneus has a specific distribution in the study area, mainly driven by habitat requirements. High abundances of the basket star were associated with rocky substrates with the presence of gorgonians, located between 50 and 80 m depth and preferentially occurringon sloping areas. Despite the lack of long-time monitoring data, A. mediterraneus' aggregations recorded in the studied area are the densest ones so far reported for the whole Mediterranean Sea with peakvalues of 18 ind. m⁻² and a mean density of 0.45 ind. m ⁻² ± 0.71 ind. m⁻². Additionally, the average size of their central disks (2.67 ± 0.97 cm), suggests that this is rather young population, which could be possibly linked with the beginning of a massive outbreak, which would be in accordance with the fishers’ own experiences, as it seems apparent that the species has been on the rise in recent years. Furthermore, they unanimously consider this proliferation to be a considerable handicap in terms of time and monetary losses.

Overall, additional studies are needed to increase current knowledge on the ecology of this species. Understanding the species will be paramount to monitor and assess this phenomenon with stakeholders to avoid further ecological and economic consequences.