In the Amazon basin, data are available on abundance and encounter rates for only a few river segments: 107 individuals counted (0.19 dolphins/km; ± 0.06) in a 490-km segment of the Solimões River, Brazil in August 1979 (Magnusson et al. 1980); 0.22 dolphins/km (± 0.04) observed in a 1525-km segment of the Solimões River, Brazil during four surveys in March 1983- February 1984 (Best and da Silva 1989b); 78 individuals counted and 346 individuals estimated (CV=0.12) using line and strip transect methods in a 120-km segment (ca. 250 km²) of the Amazon River bordering Peru and Colombia, including the Atacuari, Loretoyacu, and Amacayacu tributaries, and the Caballo Cocha and Tarapoto-El Correo lake systems in June 1993 (Vidal et al. 1997); 0.147 dolphins/km (± 0.121) observed in a 232-km segment of the Amazon-Marañon River, Peru during 11 surveys in March-August 1991-1993 (Leatherwood and Reeves 1995a); 0.41-0.50 dolphins/km observed in a 95-km segment of the Tigre River, Peru during 14 surveys in March-August 1991-1994 (Leatherwood and Reeves 1995b); and 260 ± 50 individuals in 225 km² of the Mamirauá Lake system of Brazil - extrapolated to about 13,000 individuals in the 11,240 km² Mamirauá Sustainable Development Reserve (Martin and da Silva 2004a).

Counts and encounter rates for segments of the Orinoco basin include: 122 individuals counted (1.16 dolphins/km) in a 105-km segment of the Apuré River, Venezuela in 1981 (Pilleri et al. 1982); 0.20 dolphins/km observed in a 201-km segment of the Apuré River, Venezuela in July-August 1989 (Schnapp and Howroyd 1992); 14-15 individuals counted (0.024 dolphins/km) in a 450-km segment of the Orinoco River, Venezuela in 1981 (Pilleri et al. 1982); 8 individuals counted (0.025 dolphins/km) in a 340-km segment of the Casiquiare River, Venezuela in 1981 (Pilleri et al. 1982).

Unknown