Riverine habitat preferences of botos (Inia geoffrensis) and tucuxis (Sotalia fluviatilis) in the central amazon

The distribution and density of the Amazon's two contrasting endemic dolphins - boto, or Amazon river dolphin, Inia geoffrensis, and tucuxi, Sotalia fluviatilis - were examined on two adjoining large rivers in western Brazil. Using a 17-m river boat as a sightings platform, strip transects were used to cover areas within 150 m of the river margin and line transects were used in all other areas. Highest densities of both dolphins occurred near the margin, and lowest in the center of rivers. Seven different habitats were identified along river margins. The boto and the tucuxi differed in some elements of habitat choice, but they shared a preference for areas with diminished current and where two channels joined. Neither species favored the two most common edge types in this region of the Amazon - mud banks and flooded forest margins. Overall, the most preferred habitat type was the least common, and known as "meeting of the waters." In these areas a channel of sediment-rich white water meets one carrying acidic black water; the resultant mixing produces particularly productive, and obviously attractive, conditions for dolphins. These results demonstrate that Amazonian dolphins selectively occur in areas known to be favored for gill net deployment by local fishermen, and may explain why entanglement is apparently a common cause of mortality