Sotalia fluviatilis

The riverine and marine forms of Sotalia were recently split into two species, S. fluviatilis in the Amazon and S. guianensis in marine and estuarine waters of eastern South and Central America and the Caribbean.  Both genetic (Cunha et al. 2005, Caballero et al. 2007) and morphological (Monteiro-Filho et al. 2002, Fettuccia et al. 2009) evidence support recognition of the two species.

Two issues remain unclear for Sotalia. The first is the taxonomic status and range of Sotalia in the Orinoco River. There are records of Sotalia dolphins at Ciudad Bolívar, some 300 km upstream of the Orinoco mouth. These animals are suspected be S. guianensis (da Silva and Best 1996, Meade and Koehnken 1991, Flores and da Silva 2009). It is believed that Sotalia dolphins cannot traverse the rapids at the Casiquiare channel, which connects the Orinoco and Amazon River basins (da Silva and Best 1996). This barrier has existed since the uplift of the Mérida Cordillera (10 million years ago - Mya; Lundberg et al. 1998), which predates the divergence of the two Sotalia species (Cunha et al. 2005, Caballero et al. 2007). Thus, Sotalia dolphins in the Middle Orinoco are likely to be an isolated population of S. guianensis (Cunha et al. 2010).

The second issue is the taxonomic status of Sotalia dolphins in the southern freshwater portion of Maracaibo Lake. Those individuals differ morphologically from the marine Sotalia that inhabit the northern portion of the lake, adjacent to the Gulf of Venezuela. Dolphins from southern Maracaibo Lake are smaller than marine Sotalia and about the same size as S. fluviatilis (Casinos et al. 1981, da Silva and Best 1996, León 2005). However, there is no connection between Maracaibo Lake and the present-day known range of riverine Sotalia, and Maracaibo Lake has been isolated from the Amazon basin for the last 8-10 Mya (Hoorn et al. 1995, Días de Gamero 1996, quoted in Cunha et al. 2010). The morphological distinctiveness of the southern Maracaibo Lake population could reflect phenotypic plasticity, unlike the difference between S. guianensis and S. fluviatilis. It may also indicate a lack of gene flow with the marine Sotalia around the mouth of the lake and in the Gulf of Venezuela. Indeed, Caballero et al. (2006) observed some exclusive haplotypes in samples from the lake, but they did not attribute the variation to species differentiation.

The split of Sotalia into two species has conservation relevance. The Tucuxi can be considered the world's only exclusively freshwater delphinid. This endemism jeopardizes its persistence because its restricted habitat is shared with increasing human populations. Tucuxis are vulnerable to threats such as incidental mortality in fisheries, habitat deterioration and fragmentation of populations by dam construction (Reeves at al. 2003). No formal assessment to evaluate the risks of population decline has been performed. Such assessment will require a better understanding of population structure so that population-specific abundance, non-natural mortality rates and other relevant parameters can be estimated and considered. Due to a lack of good data on these parameters, Sotalia fluviatilis is listed as Data Deficient.

Tucuxis are found in the Amazon drainage as far inland as southern Peru, eastern Ecuador, and southeastern Colombia. They occur in the main tributaries of the Amazon/Solimões River basin and they cross international boundaries in areas such as Leticia, between Brazil and Colombia. The species does not occur in the Beni/Mamoré river basin in Bolivia nor in the upper Rio Negro. Its putative presence in the Orinoco is controversial because of a stretch of rapids and waterfalls that are suspected to block the species movements to this area (Flores and da Silva 2009). During the flood season, Tucuxis may move into smaller tributaries, but apparently they do not move into the inundated forest to feed (as botos, Inia geoffrensis, often do), staying mainly in the main river channels, tributaries and lakes (da Silva and Best 1996). Tucuxis are largely sympatric with the Boto in the Amazon and Orinoco systems but generally do not interact with that species.

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.

Sotalia fluviatilis is listed in Appendix I of CITES and in Appendix II of CMS. The species is legally protected in most of the range countries.