Building a baseline: a survey of the composition and distribution of the ichthyofauna of Guanabara Bay, a deeply impacted estuary

Abstract Biodiversity baselines are essential subsidies to evaluate how environmental changes and human impacts affect the special and temporal patterns of communities. This information is paramount to promote proper conservation and management for historically impacted environments such as Guanabara Bay, in southeastern Brazil. Here, we propose an ichthyofaunal baseline for this bay using gathered past data from 1889 to 2020, including literature records, scientific collections, biological sampling, and fisheries landing monitoring. A total of 220 species (203 teleosts and 17 elasmobranchs), distributed in 149 genera (136 teleosts and 13 elasmobranchs) and 72 families (61 teleosts and 11 elasmobranchs) were recorded, including the first record of a tiger-shark, Galeocerdo cuvier, in Guanabara Bay. Although the employed sampling effort was sufficient to represent the ichthyofauna in the middle and upper estuary, the Chao2 estimator indicates an even greater richness regarding the bay as a whole. Evidence of reduced abundance and probable local extinction over the decades was found, supporting the importance of implementing management and conservation strategies in the area. The ichthyofaunal distribution analyses revealed that areas close to conservation units are richer compared to their surroundings, indicating that this is an effective strategy to mitigate human impacts in the bay.