Accessibility and appeal jointly bias the inventory of Neotropical freshwater fish fauna.

Inventorying biodiversity is fundamental to overcoming knowledge shortfalls in species discovery and description (Linnean shortfall), their distributions (Wallacean shortfall) and abiotic niches (Hutchinsonian shortfall). However, inventorying efforts often show spatial biases, and the underlying causes have only been explored at large spatial extents for the most well-known terrestrial taxa. Improving our understanding of these biases and their drivers is particularly crucial in the speciose tropics, where biodiversity knowledge suffers the most from knowledge shortfalls. Using the most complete information available on freshwater fish distributions in three regions in the Neotropics (Orinoco and Trans-Andeans, the Amazon and the La Plata basins), first, we evaluated environmental biases in inventory effort and completeness across sub-basins. Further, we examined three hypotheses (Accessibility , Appeal , and Safety) to explain the differences in contemporary (2000–2018) inventorying efforts. The Accessibility and Appeal hypotheses were well-supported and conjointly explained a moderate proportion of the spatial variation in the inventorying effort. By contrast, the Safety hypothesis predictors showed limited support. The most accessible sub-basins (i.e., lower travel time from cities or closer to fluvial ports) experienced higher inventorying effort regarding the number of unique inventorying locations and years inventoried. Our results suggest that differences in Accessibility alone may be insufficient to explain inventorying biases. Inventorying efforts were also biased against sub-basins with lower biodiversity and habitat diversity, showing preferential sampling driven by sub-basins' Appeal. The imprint driven by Appeal elucidates opportunities to coordinate future efforts to fill the shortfalls of biodiversity knowledge more efficiently for the Neotropical fish fauna. • Freshwater fish inventory effort and completeness is environmentally biased in the Neotropics. • A small percentage of sub-basins have near-complete fish inventories. • The accessibility and appeal of sub-basins drive contemporary inventory efforts. • Sub-basins with lower biodiversity and habitat diversity have received less inventorying effort. • Opportunities exist to coordinate efforts to fill shortfalls in Neotropical fish fauna knowledge. [ABSTRACT FROM AUTHOR]