Habitat and Ecology
Tucuxis inhabit all three types of water of the Amazon basin: white water, clear water, and black water. Therefore, physical factors such as visibility and acidity appear not to affect their distribution directly. They seem to prefer the main channels of rivers and larger lakes where access is not limited by a narrow or shallow channel, while rapids and fast-moving turbulent water are avoided. Tucuxis also generally do not enter the flooded forest. They are mostly found within 50 m of the edges of rivers and channels (Martin et al. 2004). Similarly to the sympatric boto, the tucuxi shows a distinct preference for junctions of rivers and channels (da Silva and Best 1996, Leatherwood et al. 2000, Martin et al. 2004). The most preferred habitat is where a sediment-rich whitewater channel meets the low pH- carrying black water. The resultant mixing produces productive and obviously attractive conditions for dolphins (Martin et al. 2004). The large seasonal fluctuation in river levels (10m) influences the distribution of tucuxis. They enter lake systems during periods of high water but leave these environments as the waters recede, thus avoiding entrapment. In the Peruvian Amazon, tucuxis were not found in waters <3m depth in rivers or <1.8 m depth in lakes.
Individuals may occur in the same area year-round. Two tagged individuals in the Amazon were found within 5 km of the tagging site up to 1 year later (da Silva and Best 1994). A long-term photo-identification study revealed a maximum range for individuals of 130 km in Peru's Pacaya-Samiria Reserve (McGuire and Henningsen 2007). This relatively small range is probably due to limiting features such as small channels and seasonally shallow waters. According to McGuire (2002), encounter rates were highest in confluences, intermediate in lakes, and lowest in rivers and did not differ among seasons in the latter two. During the dry season, tucuxis persisted longer in the confluences and occurred in higher densities than in any rainy or intermediate season; the reverse pattern was observed during high water.
S. fluviatilis occurs most often in groups of one to six individuals. Groups of more than nine animals are rarely observed (da Silva and Best 1994, Faustino and da Silva 2006). The composition of groups is unknown. Vidal et al. (1997) reported overall mean group size of 3.9 in the upper Amazon. Tucuxis were most frequently seen as singles or pairs in rivers and lakes of Peru's Pacaya-Samiria Reserve; seasonal differences in group size were non-significant (McGuire 2002).
Information on reproduction is sparse. Males attain sexual maturity at approximately 140cm and females at between 132 and 137cm (da Silva and Best 1996). In Brazil, gestation lasts about 11 months and calves are about 80 cm long at birth, which occurs primarily from September to November during the low-water period (da Silva and Best 1996, Flores and da Silva 2009). Neonate tucuxi were observed in all seasons in the Peruvian Amazon, with a slight peak in encounter rates during high water (McGuire and Aliaga 2007).
At least 28 species of mostly small schooling fish belonging to 11 families are preyed upon by tucuxis in the Amazon region. The characoid family Curimatidae was represented in 52%, Sciaenidae in 39% and siluriforms in 54% of stomachs analysed (n = 29) (da Silva and Best 1994). During the dry season, fish concentrate in the main water bodies and thus are more vulnerable to predation. During the flood season, many species enter the floodplain and are largely out of reach of tucuxis.