Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Chordata Mammalia Cetartiodactyla Delphinidae

Scientific Name: Sotalia fluviatilis
Species Authority: (Gervais & Deville, 1853)
Common Name(s):
English Tucuxi, Guianian River Dolphin, Estuarine Dolphin, Gray Dolphin
French Dauphin De L'Amazon , Sotalie
Spanish Bufeo Negro , Bufo Negro
Taxonomic Notes:

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.

Assessment Information [top]

Red List Category & Criteria: Data Deficient ver 3.1
Year Published: 2012
Date Assessed: 2010-04-09
Assessor(s): Secchi, E.
Reviewer(s): Reeves, R. & Perrin, W.F.

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.

Previously published Red List assessments:
2010 Data Deficient (DD)

Geographic Range [top]

Range Description:

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.

The map shows where the species may occur. The species has not been recorded for all the states within the hypothetical range as shown on the map. States for which confirmed records of the species exist are included in the list of native range states.

Countries occurrence:
Brazil; Colombia; Ecuador; Peru
Range Map: Click here to open the map viewer and explore range.

Population [top]

Population: To date, there is no information on the population structure of S. fluviatilis. The only data available suggest that the species has moderate to high genetic diversity, since 12 individuals from the same location in the central Brazilian Amazon had five different control region haplotypes (Cunha et al. 2005), and 21 dolphins from the Peruvian, Colombian and Brazilian Amazon had 13 haplotypes (combining the control region and ND2, Caballero et al. 2007). Microsatellite variation is also larger in S. fluviatilis (H = 0.531) than in S. guianensis (H = 0.364; Cunha and Watts 2007). The reason for a higher level of genetic variation in S. fluviatilis, in spite of its probably smaller population size, remains to be determined. There are no estimates of the total population size for S. fluviatilis, though it appears to be relatively abundant throughout most of its range (Flores and da Silva 2009). Local estimates of relative abundance exist for some areas. In the Amazon drainage, an average density (encounter rate) of approximately 1.1 dolphins/km of river was estimated between Manaus and Tefé in the Solimões River (Magnusson et al. 1980). Four boat surveys of about 1,525 km each, from Manaus to Letícia, resulted in a mean estimate of 768 (± 104.7 SD) dolphins per trip or 1.02 individuals/km² (da Silva and Best 1994). More recently, mean density along the margins of main rivers in the central Amazon, Brazil (1,320 km of strip survey) was estimated at 3.2 individuals/km². About 54% of the individuals were found within 50 m of the edge of rivers and channels (Martin et al. 2004). Faustino and da Silva (2006) recorded on average 24 Tucuxis/km in the Mamirauá System, Central Amazon. Higher densities were found in deeper channels with high turbulence and productivity.

About 350 Tucuxis were estimated to inhabit the Samiria River system in Peru (Leatherwood 1996). Recent studies indicate that encounter rates in this area were within the range for these dolphins elsewhere in South America and that population numbers were stable over 10 years between 1991 and 2000 (McGuire 2002). Mean encounter rates in the Peruvian Amazon were 0.01-080 individuals/km in rivers and 0.05-2.17 individuals/km² in lakes (28 surveys over a four-year time period; McGuire 2002). The species is reportedly common in Colombia in the Loretoyacu River, in the Tarapoto River and in the El Correo Lake system (da Silva and Best 1994). Vidal et al. (1997) estimated that in 1993 there were 409 Tucuxis (CV=13%) along 120 km of the Amazon River bordering Colombia, Peru, and Brazil. Density (dolphins/km²) was highest in lakes (8.6), followed by areas along main banks (2.8) and around islands (2.0). Those estimates of density or abundance in small portions of the range are of limited use for assessment of the conservation status of the species as a whole.
Current Population Trend: Unknown
Additional data:
Population severely fragmented: No

Habitat and Ecology [top]

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. 

Sotalia 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.

Systems: Freshwater

Use and Trade [top]

Use and Trade: This species is used at a low level as bait in fisheries, and its eyes and genital organs are used as aphrodisiacs.

Threats [top]

Major Threat(s): There are no records of past or recent commercial fisheries for Sotalia spp. (IWC 2001). River dolphins in the Amazon region are threatened primarily by incidental mortality in fishing gear (IWC 2001). Sotalia fluviatilis consumes 14 of the 30 species of commercially exploited fish in the Amazon, and thus incidental captures during fishing are frequent (da Silva and Best 1994, 1996; Martin et al. 2004). In one study in the central Amazon of Brazil, 74% of 34 Tucuxis examined had been killed in gill nets and 15% in seine nets (da Silva and Best 1985). By-catch in fishing gear, caused by the proliferation of gill nets, and some evidence of poisoning by commercial fishermen, seem to be the main conservation threats to Tucuxis in Peru (Reeves et al. 1999).

Other potentially important threats are oil spills, boat strikes, chemical and noise pollution, overfishing of prey, and damming of rivers for hydroelectric projects (da Silva and Best 1994, Denkinger 2001, McGuire 2002). Dams can interrupt gene flow and create isolated groups of dolphins with reduced genetic variability and lowered demographic resilience. The use of pesticides banned in many countries continues to be common in some parts of South America (PNUMA 2002). Mercury is used to refine fluvial gold and then, like the pesticides, enters aquatic food webs. Dams, in addition to fragmenting dolphin populations, can have serious effects on migratory fish populations on which Tucuxis feed (such as some siluriform fishes) (da Silva and Best 1994). The increased use of outboard engines and illegal fishing with explosives are also sources of concern in parts of these dolphins' range (e.g. Utreras et al. 2001, Zapata-Rios and Utreras 2004).

Although freshwater dolphins have been protected by superstitions in parts of Amazonia (Leatherwood and Reeves 1997), in Colombia and Brazil there was and may still be a small market for the eyes, teeth and reproductive organs of dolphins, used as love charms or aphrodisiacs when prepared in a special manner (da Silva and Best 1994, 1996; Trujillo and Diazgranados 2002; Alves and Rosa 2008). According to V. da Silva (pers. comm.), this market is no longer a significant threat in the Brazilian Amazon, although according to M. Borobia (in IWC 2007) the practice of selling dolphin parts continues in at least some local areas. However, according to V. Da Silva (pers. comm.), fishermen in the central Amazon do not kill Tucuxi deliberately or use parts of its body as love charm. Teeth, eyes and body parts are from S. guianensis, most captured accidentally in the Para coast. Eyes of S. guianensis, killed in the nets at the Amazon estuary are often sold in the markets as Boto eyes. Recent molecular analyses have shown that a high proportion of dolphin products sold as amulets and love charms are derived from Guiana dolphins (S. guianensis), even in the Amazon region (Cunha and Solé- Cava 2007, Gravena et al. 2008, Sholl et al. 2008).

Recently, Loch et al. (2009) found two Tucuxis and one Boto wounded by perforating and cutting objects, probably harpoons and machetes. Such kills indicate potential conflicts with locals using tucuxi and boto as bait in a catfish fishery (V. da Silva, pers. comm.). Trujillo and Diazgranados (2002) reported one death by harpooning in the Colombian Amazon, in a ten-year monitoring period.

Conservation Actions [top]

Conservation Actions:

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.

Citation: Secchi, E. 2012. Sotalia fluviatilis. The IUCN Red List of Threatened Species 2012: e.T190871A17583369. . Downloaded on 04 October 2015.
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