Density and Abundance Estimation of Amazonian River Dolphins: Understanding Population Size Variability

Renan Lopes Paitach,Mariana Escobar Wilson White,Paul André Van Damme,Miriam Marmontel,Heloise Pavanato Julião,Federico Mosquera-Guerra,Alexandre Novaes Zerbini,Fernando Trujillo,André Giovanni De Almeida Coelho,Gabriel Melo Alves Dos Santos,Mariana Paschoalini

doi:10.3390/jmse9111184

Abstract

The dolphins Inia geoffrensis—boto and Sotalia fluviatilis—tucuxi are threatened cetaceans inhabiting river ecosystems in South America; population numbers are still lacking for many areas. This paper provides density and abundance estimations of boto and tucuxi in 15 rivers sampled during the past nine years as part of a multinational research alliance. Visual boat-survey data collection protocols and analyses have been developed since 2012 (based on Distance Sampling methods) and recently reviewed (2019) to improve robustness and comparability. Differences across the sampled rivers and the analyzed river basins (Amazon and Orinoco) pointed to a density/population size gradient with lower densities and abundances observed in the Orinoco basin (0.9–1.5 ind./km²), passing through the eastern Amazon basin (2–5 ind./km²), and the largest numbers found at the central Brazilian Amazon (lower Purus River—2012 (14.5 boto/km², N = 7672; 17.1 tucuxi/km², N = 9238)). However, in other parts of the central Amazon, the density of dolphins was smaller than expected for high productive whitewater rivers (1–1.7 ind./km² in the Japurá and Solimões rivers). We attributed these differences to specific features of the basin (e.g., hydro-geomorphology) as well as to the cumulative effects of anthropogenic activities.

Highlights

The Inia geoffrensis, commonly known as boto, and the tucuxi Sotalia fluviatilis are freshwater cetaceans inhabiting complex ecosystems throughout their range: the major neotropical rivers of the Amazon, Orinoco, and Tocantins-Araguaia basins in six countries (Brazil, Bolivia, Colombia, Ecuador, Peru, and Venezuela) [1,2,3,4,5,6]
The highest densities and abundances for both species of river dolphins were found in the lower Purus River (Central Brazilian Amazon), 14.5 boto/km2 N = 7672 (CV = 0.37), and 17.14 tucuxi/km2 N = 9238 (CV = 0.49) (Table 2)
Similar densities were estimated for boto in the Iténez River (Brazilian/Bolivian border) with the lower abundances estimated, where no tucuxis are found

Summary

Introduction

The Inia geoffrensis, commonly known as boto, and the tucuxi Sotalia fluviatilis are freshwater cetaceans inhabiting complex ecosystems throughout their range: the major neotropical rivers of the Amazon, Orinoco, and Tocantins-Araguaia basins in six countries (Brazil, Bolivia, Colombia, Ecuador, Peru, and Venezuela) [1,2,3,4,5,6]. Variations in the water level influence the availability of aquatic habitats and the levels of dissolved oxygen, resulting in important seasonal changes in productivity and biodiversity [11]. This heterogeneity results in constantly changing distribution patterns of prey and, of river dolphins across this complex mosaic of aquatic habitats [12,13,14]

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