A conservation priority index to rank fish species within IUCN Red List categories: A case study of marine fishes from Réunion Island

Although the IUCN Red List is a crucial tool for assessing the extinction risk of species, further ranking species within each IUCN category is needed to refine conservation priorities. In practice, managers and policy makers often require to prioritize species according to IUCN categories, and subsequently rank them within these categories. The current IUCN classification system relies on quantitative criteria related to population dynamics such as population size, rate of decline, and geographic range, without considering the functional or evolutionary distinctiveness of species, nor directly accounting for their vulnerability to individual anthropogenic pressures like climate change or fishing. These pressures may instead be considered indirectly through the criteria used, which often reflect the combined effects of multiple, interrelated human-induced threats. Here, we developed an index to quantify the conservation priority of fish species with the same IUCN status by combining five criteria: functional distinctiveness, evolutionary distinctiveness, abundance scarcity, vulnerability to climate change, and vulnerability to fishing. This framework enables managers to customize priorities by assigning varying levels of importance to each criteria, offering a flexible tool for refining conservation plans. To demonstrate its applicability, we tested this framework on the marine fish community of Réunion Island, an area with a significant number of species classified as ‘vulnerable’ on the IUCN Red List. Finally, we discuss how this framework, formalized into an index, could substantially enhance fish species conservation and contribute to the long-term sustainability of ecosystems.