Red List Index of Western Palearctic freshwater fishes

The Red List Index (RLI) is a key indicator for global biodiversity targets, but its application to freshwater fishes has been limited by infrequent reassessment and sparse monitoring data. To address this gap, we developed the first comprehensive RLI for freshwater fishes of the Western Palearctic (Europe, West Asia, and North Africa), based on two comparable global assessment rounds. We reconciled taxonomy, classified category changes as nongenuine (reflecting taxonomic revision or improved knowledge) or genuine, and calculated RLIs with bootstrapped 95% confidence intervals. Of 1,157 native, described species, 727 (63%) had two comparable assessments and formed the index. The RLI declined across the region (-2.4%), with subregional declines ranging from -0.8% in West Asia to -7.1% in North Africa; genuine declines outnumbered improvements by approximately 6:1 and included losses to extinct in the wild or possibly extinct. Most species showed no change or a nongenuine change. The main constraint on the trend was not data deficiency (eight species) but the absence of a comparable earlier assessment for 337 species recently assessed. Of these species, the assessments showed that 163 were threatened and concentrated in West Asia, and coverage was lowest in West Asia (51%) and for small-bodied, range-restricted families. The index should therefore be read as a regional benchmark and not as a test of the RLI's sensitivity because the true number of genuine changes that should have occurred is unknown. Building on this analysis, we developed a prioritized, data-informed agenda for strengthening future iterations: completing a second assessment of the already-threatened species currently outside the trend, focusing the resurvey on under-covered subregions and families, and matching monitoring technique to taxon. Documented recoveries show that targeted conservation can reverse declines, often in data-poor areas, but without such effort, both decline and recovery of these faunas will be detected slowly.