Sotalia guianensis (Van Bénéden 1864)

English: Guiana dolphin; Costero
German: Küsten-Sotalia
Spanish: Costero
French: Sotalia côtier

Family Delphinidae

Sotalia guianensis © Würtz-Artescienza (see "links")

1. Description

The appearance of the Guiana dolphin resembles that of a smaller bottlenose dolphin. It is light grey to bluish-grey on the back and pinkish to light grey on the belly, with a distinct boundary between the mouth gape and the flipper's leading edge. On the sides, there is a lighter area between the flippers and the dorsal fin. The dorsal fin is triangular and may be slightly hooked at the tip. The beak is moderately slender and long. The Guiana dolphin is significantly larger than the tucuxi. Body size reaches 210-220 cm and body mass 80 kg (Flores and da Silva, 2009).

2. Distribution

Dolphins of the genus Sotalia are found along the Caribbean and Atlantic coasts of Central and South America and in the Amazon River and most of its tributaries. Until recently, the taxonomy of these dolphins was unresolved. Although five species were described in the late 1800s, only one species was recognized prior to 2007 (Sotalia fluviatilis) with two ecotypes or subspecies, the coastal subspecies (Sotalia fluviatilis guianensis) and the riverine subspecies (Sotalia fluviatilis fluviatilis) (Rice, 1998 and refs. therein; Culik, 2004).

Distribution of Sotalia guianensis: coasts of north-eastern South America and eastern
Central America (Reeves et al. 2008; © IUCN; enlarge map).

Recent morphometric analyses, as well as mitochondrial DNA analysis, suggested recognition of each subspecies as separate species (e.g. Furtado, 1999). Caballero et al. (2007) and Cunha et al (2005) reviewed the history of the classification of this genus. Caballero et al. (2007) presented new genetic evidence from ten nuclear and three mitochondrial genes as well as evidence from previous studies supporting the elevation of each subspecies to the species level under the Genealogical Lineage Concordance Species Concept and the criterion of irreversible divergence. The authors proposed the common name 'costero' for the coastal species, Sotalia guianensis (Van Beneden 1864), and accept the previously proposed 'tucuxi' dolphin, Sotalia fluviatilis (Gervais, 1853), for the riverine species. More recently, "Guiana dolphin" has been used for Sotalia guianensis (Flores and da Silva, 2009).

S. guianensis (P.-J. van Bénéden, 1864) is found in inshore coastal waters, estuaries, and the lower reaches of rivers along the western Atlantic from Nicaragua (14°35'S) south to Floreanópolis, Santa Catarina, Brazil (27°35'S) (Flores and daSilva, 2009). This subspecies includes Sotalia brasiliensis E. Van Bénéden, 1875 (Rice, 1998 and references therein). In a recent paper Carr and Bonde (2000) extended the known range to the northwest in northeastern Nicaragua, north of the mouth of the Layasiksa River, west side of Waunta Lagoon (13°40'N) where one individual was positively identified.

3. Population size

The species appears to be relatively abundant throughout its range. Numerous estimates exist of relative abundance in small areas, such as minimum number sighted, encounter rate, and estimates of minimum density (IWC, 2000). The following summary moves from north to south. Edwards and Schnell (2001) found that in the Cayos Miskito Reserve, Nicaragua, 49 Sotalia inhabited the portions of the Reserve studied. Mean overall density was 0.6 individuals/km², with slightly lower densities in lagoons (0.5 ind/km²).Bossenecker (1978, in da Silva and Best, 1994) estimated 100-400 dolphins near the mouth of the Magdalena River in Colombia, and noted that they were abundant in the Gulf of Cispata, near San Antero (Colombia). In Suriname, they were described as "rather common" in the mouths of the larger rivers, and in Guyana they were reported as "frequent" in the lower reaches and mouth of the Essequibo river.

Sotalia were reported to be common in the Baia de Guanabara (Rio de Janeiro), by Geise (1991, in da Silva and Best, 1994) who estimated the population at 418 individuals in about 109 groups. Geise (1989, in da Silva and Best, 1994) estimated the total number of individuals for the area around Cananéia Island to be 2,829. In Sepetiba Bay, Southeast Brazil, a population density of 2.79 dolphins/km² and a population of 1,269 individuals (CI=739-2,196) were estimated. Density and abundance were similar for the entrance and interior of the Bay (Flach et al. 2008).

4. Biology and Behaviour

Habitat: Marine Sotalia show a preference for shallow protected estuarine waters or bays. In the Baia de Guanabara (Rio de Janeiro), they prefer the deeper channels (25 m depth) and avoid areas with less than about 6m of water. Where the rivers that feed such areas are large enough, dolphins may penetrate up to 130 km or more upriver; however the species identity of these dolphins is still uncertain. The major restriction to the south seems to be low sea-surface temperature (Reyes, 1991 and refs. therein) of the Malvinas current. They may range as far as 70 km offshore, e.g. in the Abrolhos Archipelago (Flores and daSilva, 2009).

Sightings of Sotalia groups in the Cayos Miskito Reserve, Nicaragua indicates that some areas were preferred. In coastal areas Sotalia were sighted most often within 100m of shore and the animals were seldom observed in more than 5m of water (Edwards and Schnell, 2001).

Schooling: According to da Silva and Best (1994) Guiana dolphins occur in groups of as many as 30 individuals, with a mode of 2 per group in the Baia de Guanabara and Cananéia. Group size varies in these two areas according to the time of day and type of activity. Borobia (1984) and Geise (1984, 1989, both in da Silva and Best, 1994) reported that in the marine form, calves are usually observed in small groups of three (one calf and two adults) or four (two calves and two adults). Azevedo et al. (2005) investigated group characteristics in Guanabara Bay, south-eastern Brazil. Group size ranged between 1 - 40 individuals and groups of 2 -10 were most common. There was no variation in group size between seasons, but nursery groups were twice as large than non-calf groups. Flores and daSilva (2009) reported that Guiana dolphins do not associate with bottlenose dolphins in Brazilian waters.
Edwards and Schnell (2001) found a mean group size of 3 in the Cayos Miskito Reserve, Nicaragua.

Reproduction: Males reach sexual maturity at 7 years and at body lengths estimated at 170-175 cm. Females mature at 5-8 years of age and at body lengths of 164-169 cm. The reproductive cycle is estimated at 2 years, with no marked seasonality in ovulation or timing of birth. Gestation is about 12 months, fetal growth rate was 9 cm/month, and length at birth is estimated at 92.2 cm. Females older than 25 years have senescent ovaries (Rosas and Monteiro-Filho, 2002).

Food: Marine Sotalia from southeast Brazil feed on a diet of pelagic clupeids (Trichurus lepturus and Pellona barroweri), demersal sciaenids (Cynoscio spp., Porichthys porosissimus, Micropogonias furnieri) and neritic cephalopods (Loligo spp. and Lolliguncula brevis). In Santa Catarina these dolphins are known to feed on the anchovies which are abundant in this area (da Silva and Best, 1994). Back-calculation of prey lengths indicated that fish ranged from 1.2 to106.9 cm and cephalopods from 3.4 to 22.2 cm in mantle length (Di Beneditto and Arruda Remos, 2004). Demersal fishes usually associated with estuarine sandy bottoms were the main prey items. Sciaenid fishes that produce relatively loud sounds by swimbladder muscular contraction were observed as common prey items. The dolphins also prey on shrimps (Penaeus schmitii and P. paulensis; Penaeidae) (De oliveira Santos et al. 2002).

5. Migration

General patterns: Marine Sotalia may penetrate up to 130 km or more upriver. The marine form probably also has a defined home range, although the area covered may be large because of the distances between one estuary or protected bay and another (Reyes, 1991). Geise (1989) and Andrade et al. (1987, both in da Silva and Best, 1994) observed individuals identified by natural marks in the same area for over 1 year. Home ranges are probably among the smallest for small cetaceans, with 15 km² in southern Brazil and up to 265 km² in another location.

Movement patterns vary among cold and warm seasons in the temperate regions, whereas in warmer waters no seasonal movements are known (Flores and daSilva 2009). Furthermore, Guiana dolphins show strong site fidelity. In Guanabara Bay, southeastern Brazil, which is surrounded by a metropolitan complex and is the most degraded area of the species' distribution, individuals were seen for 4.5 consecutive years, with a range of 1 to 8 years, and calves remained in the area beyond sexual maturity (Azevedo et al. 2004).

Diurnal rhythms: An apparent diurnal behaviour rhythm has been observed in whereby Guiana dolphins entered the Bahía de Guanabara between 06:00 and 08:00h and left between 13:00h and 18:00h but were rarely seen entering and leaving the bay on the same day (12% of the observations). A similar behaviour was reported for S. guianensis in the Cananéia region (da Silva and Best, 1994 and references therein; Geise et al. 1999).

At Enseada do Mucuripe in Fortaleza, Brazil, the distribution of sightings and displacement routes of Sotalia suggested preferential uses of the sites Praia Mansa and Praia de Iracema at different times, suggesting movement patterns between resting and feeding areas. Highest and lowest frequencies of sightings at Praia de Iracema occurred respectively in the first and fourth quarters of the day. The higest frequencies happened at low tide (Oliveira et al. 1995).

6. Threats

Direct catches: There are no records of past or recent commercial fisheries for Sotalia (IWC, 2000). On the coast of Brazil they may occasionally be killed for use as bait for sharks or shrimp traps or for human consumption, although the extent of this practice is unknown.

Incidental catches: Modern fishing practices and the greatly increased intensity of fishing in both the marine and freshwater habitats are the greatest direct threats to the species. Guiana dolphins are easily captured in monofilament gill nets as well as in shrimp and fish traps and seine nets. Beltran (1998, in IWC, 2000) recorded 938 animals taken in drift nets from the port of Arapiranga during the summer of 1996 and a further 125 taken during the winter. These data were collected by interviewing fishermen in the port after trips and collecting carcasses. The IWC sub-committee on small cetaceans expressed its concern about the magnitude of these catches. More recently, Monteiro-Neto et al. (2004) estimated that approximately 90 Guiana dolphins are killed every year in the passive gill net fisheries along the Brazilian coast. In the metropolitan area of Fortaleza, the capital of Ceara State, 32 bycaught animals were recorded. The use of pingers in fishing nets may assist in the mitigation of entanglements.

Habitat degradation: Another potential threat to Sotalia spp., in both riverine and coastal environments, is the damming of rivers for hydroelectric projects, with future plans for up to 200 such dams in series along many of the main Amazon tributaries. Where such dams are built on rivers that empty directly into the sea, the altered flux of freshwater may affect both the primary and secondary productivity in the estuaries and reduce the feeding potential of these areas for Guiana dolphins (da Silva and Best, 1994, Jefferson et al. 1993).

Pollution: Pollution from industrial and agricultural activities may be considered a threat both directly, through the destruction of habitat, or indirectly, through contamination of the food chain. Large harbours like the Baia de Guanabara (Rio de Janiero) and Santos (São Paulo) are extremely polluted with effluent, including heavy metals, posing a serious potential threat. The continued use of insecticides containing substances banned elsewhere is common in South America (da Silva and Best, 1994 and ref. therein). However, Guiana dolphins from the Cananeia Estuary, an important biological area on the southeast coast of Brazil, had lower organochlorine concentrations in their blubber than small cetaceans from developed areas elsewhere in the world, although the estuary is known to have been impacted by both chlorinated pesticides and polychlorinated biphenyls (PCBs)(Yogui et al. 2003).

Mercury is used in the refining of fluvial gold and then, like the pesticides, probably enters the aquatic food chain of the rivers and coasts. Mercury and selenium were found in the livers of two Sotalia from Suriname (da Silva and Best, 1994 and ref. therein). The detection of Cd, Hg and Pb in tissue samples of S. guianensis off the coast of Ceara, Brazil, indicated that heavy metals are locally available in the waterand bioaccumulation may be occurring through the food web. Contamination levels were not considered critical, but they could be related to Ceara's growing industrial development. The associated risks of pollution outfalls may pose a threat to marine organisms in a near future, especially for top predators such as the Guiana dolphin (Monteiro-Neto et al. 2003).

Exploration for oil in the offshore regions of Brazil, Venezuela and Colombia may not pose a direct threat to S. guianensis. Nevertheless, the apparent dependence of this dolphin on estuaries means that an oil spill near such an area could contaminate the food chain and affect local populations (da Silva and Best, 1994 and ref. therein).

7. Remarks

Range states (Reeves et al. 2008):
Brazil, Colombia, Ecuador, French Guiana, Guyana, Nicaragua, Panama, Peru, Suriname and Venezuela.

Sotalia sp. is listed in Appendix II of CITES and S. guinanensis is listed in Appendix II of CMS. The species is listed as "Data Deficient" by the IUCN.

On the Atlantic coast of South America, large rivers are geographical limits for countries along the coast. Because of the estuarine preference of tucuxis in the area, it is likely that the dolphins move between some of these countries (Reyes, 1991, and refs. therein). According to Monteiro et al. (2000) the small number of individuals in conjunction with long gestation and nursing periods, suggest that an increased mortality due to dolphin-fisheries interactions could severely impact local populations. The IWC sub-committee (IWC, 2000) recognised that incidental catches of S. guianensis are widespread. Little information exists regarding marine Sotalia populations, and in many areas, such as the lower Orinoco, it is not clear which species is present (IWC, 2000, and refs. therein).

The IWC sub-committee on small cetaceans (2000) recommended:
- that research should be directed towards detecting trends in abundance by making repeatable and statistically rigorous estimates of density in a range of regions and habitats,
- that information be collected to allow evaluation of the relative levels of incidental mortality associated with different fishing methods,
- that research be directed to determine which species of Sotalia occurs in areas such as the Orinoco and Amazon estuaries.

8. Sources

· Azevedo AF, Lailson-Brito JJr, Cunha HA, van Sluys M A (2004) A note on site fidelity of marine tucuxis (Sotalia fluviatilis) in Guanabara Bay, southeastern Brazil
· Azevedo AF, Viana SC, Oliveira AM, Van Sluys M (2005) Group characteristics of marine tucuxis (Sotalia fluviatilis) (Cetacea: Delphinidae) in Guanbara Bay, south-eastern Brazil. J Mar Biol Assoc UK 85: 209-212
· Caballero S, Trujillo F, Vianna JA, Barrios-Garrido H, Montiel MG, Beltran-Pedreros S, Marmontel M, Santos MC, Rossi-Santos M, Santos FR, Baker CS (2007) Taxonomic status of the genus Sotalia: species level ranking for 'tucuxi' (Sotalia fluviatilis) and 'costero' (Sotalia guianensis) dolphins. Mar Mamm Sci 23: 358-386.
· Carr T, Bonde RK (2000) Tucuxi (Sotalia fluviatilis) occurs in Nicaragua, 800km north of its previously known range. Mar Mamm Sci 16: 447-452.
· Culik BM (2004) Review of small cetaceans: distribution, behaviour, migration and threats. UNEP/CMS Secretariat. Marine Mammal Action plan/ regional Seas Reports and Studies no. 177. 343 pp.
· Cunha HA, Silva VMF, Lailson-Brito J, Santos MCO, Flores PAC, Martin AR, Azevedo AF, Fragoso ABL, Zanelatto RC, Sole-Cava AM (2005) Riverine and marine ecotypes of Sotalia dolphins are different species. Mar Biol 148: 449-457.
· Da Silva VMF, Best RC (1994) Tucuxi - Sotalia fluviatilis (Gervais 1853) In: Handbook of marine mammals (Ridgway SH, Harrison SR, eds.) Vol. 5: The first book of dolphins. Academic Press, London, pp. 43-70.
· De Oliveira Santos MC, Rosso S, Dos Santos RA, Lucato SHB, Bassoi M (2002) Insights on small cetacean feeding habits in southeastern Brazil. Aquat Mamm 28: 38-45
· Di Beneditto APM, Arruda Remos RM (2004) Biology of the marine tucuxi dolphin (Sotalia fluviatilis) in south-eastern Brazil. J Mar Biol Assoc UK 84:1245-1250
· Edwards HH, Schnell GD (2001). Status and ecology of Sotalia fluviatilis in the Cayos Miskito Reserve, Nicaragua. Mar Mamm Sci 17: 445-472.
· Flach L, Flach PA, Chiarello AG (2008) Density, abundance and distribution of the Guiana dolphin (Sotalia guianensis van Benéden, 1864) in Sepitiba Bay, southeast Brazil. J Cetacean Res Manage 10:31-36.
· Flores PAC, Da Silva VMF (2009) Tucuxi and guiana Dolphin - Sotalia fluviatilis and S. guianensis. In: Encyclopedia of marine mammals, 2nd Ed. (Perrin WF, Würsig B, Thewissen JGM, eds.) Academic Press, Amsterdam, pp. 1188-1192.
· Furtado NM (1999) Molecular systematics and population genetics of marine vertebrates from Brazil. Dissertation Abstracts International Part B: Science and Engineering 60: 463:463.
· Geise L , Gomes N, Cerqueira R (1999) Behavior, habitat use and population size of Sotalia fluviatilis (Gervais, 1853) (Cetacea, Delphinidae) in the Cananeia estuary region, Sao Paulo, Brazil. Rev Brasil Biol 59: 183-194.
· IWC (2000) Annex K: Report of the sub-committee on small cetaceans. IWC, Cambridge, 2000. J Cetacean Res Manage 6: 265-268
· Jefferson TA, Leatherwood S, Webber MA (1993) FAO Species identification guide. Marine mammals of the world. UNEP/FAO, Rome, 320 pp.
· Monteiro NC, Alves JTT, Avila FJC, Campos AA, Costa AF, Silva CPN, Furtado NMAA (2000) Impact of fisheries on the tucuxi (Sotalia fluviatilis) and rough-toothed dolphin (Steno bredanensis) populations off Ceara state, northeastern Brazil. Aquat Mamm 26: 49-56.
· Monteiro-Neto C, Avila FJC, Alves TT Jr, Araujo DS, Campos AA, Martins AMA, Parente CL, Furtado-Neto MAA, Lien J (2004) Behavioral responses of Sotalia fluviatilis (Cetacea, Delphinidae) to acoustic pingers, Fortaleza, Brazil. Mar Mamm Sci 20: 145-151
· Monteiro-Neto C, Itavo RV, Moraes LEU (2003) Concentrations of heavy metals in Sotalia fluviatilis (Cetacea:Delphinidae) off the coast of Ceara, northeast Brazil. Environ Pollut 123: 319-324.
· Oliveira JA, Avila FJC, Alves Junior TT, Furtado Neto MAA, Monteiro Neto C (1995) Monitoring of the gray dolphin, Sotalia fluviatilis (Cetacea: Delphinidae), off Fortaleza, Ceara State, Brazil. Arq Cienc Mar 29: 28-35.
· Reeves RR, Crespo EA, Dans, Jefferson TA, Karczmarski L, Laidre K, O'Corry-Crowe G, Pedraza S, Rojas-Bracho L, Secchi,ER, Slooten E, Smith BD, Wang JY, Zhou K (2008). Sotalia fluviatilis. In: IUCN 2009. IUCN Red List of Threatened Species. Version 2009.2. <www.iucnredlist.org>.
· Reyes JC (1991) The conservation of small cetaceans: a review. Report prepared for the Secretariat of the Convention on the Conservation of Migratory Species of Wild Animals. UNEP/CMS Secretariat, Bonn.
· Rice DW (1998) Marine mammals of the world: systematics and distribution. Society for Marine Mammalogy, Spec Publ 4, Lawrence, KS. USA.
· Rosas FCW, Monteiro-Filho ELA (2003) Reproduction of the estuarine dolphin (Sotalia guianensis) on the coast of Parana, Southern Brazil. J Mammal 83: 507-515
· Yogui GT, Santos MCU, Montone RC (2003) Chlorinated pesticides and polychlorinated biphenyls in marine tucuxi dolphins (Sotalia fluviatilis) from the Cananeia Estuary, southeastern Brazil. Sci Total Environ 312: 67-78.

© Boris Culik (2010) Odontocetes. The toothed whales: "Sotalia guianensis". UNEP/CMS Secretariat, Bonn, Germany. http://www.cms.int/reports/small_cetaceans/index.htm
© Illustrations by Maurizio Würtz, Artescienza.
© Maps by IUCN.