Uncovering changes in the Ormer (Haliotis tuberculata) population in northeastern Brittany (France) using participatory science

Abstract Similarly to other abalone species, the ormer ( Haliotis tuberculata ), considered a delicacy, faced multiple anthropogenic pressures, including overfishing from legal and illegal harvesting. While the ormer is extensively studied in aquaculture, limited information has been available on the status of the wild population H. tuberculata in France since the Vibryo harveyi pandemic occurring in the late 1990s. Such a lack of data may contribute in the gradual shift in the perception of stock changes, known as the Shifting Baseling Syndrome (SBS). To address this gap, we combined historical monitoring data collected in the 1980s in the Emerald Coast (North Brittany; France) with recent participatory science diving surveys conducted by a local NGO in the same area. Using quantile regression and mixed models, we investigated changes in density and size structure over three decades, and further explored the ecology of H. tuberculata . Results showed a size-depth relationship, suggesting an age-related vertical distribution. Models indicated no change in density between the two periods, highlighting population recovery from the pandemic that cause high mortality in the region. However, mean individual size declined by approximately 2 cm compared to the 1980s, mainly related to the decline of individuals larger than the legal catch size (9 cm). Such changes could potentially result from anthropogenic pressures including overfishing, ocean warming or acidification. These preliminary findings highlight the interest of combining participatory science initiative along with historical records to tackle shift baseline syndrome, and to inform conservation and management strategies for overlooked and exploited marine species.