Gray Dolphin - Sotalia fluviatilis

This small dolphin resembles the bottlenose dolphin in body shape; it is rather chunky. The snout is longer and narrower, the flippers are broader, and the dorsal fin is shorter and more triangular than in the bottlenose dolphin. Dorsally, dolphins of the genus Sotalia are dark bluish or brownish gray, fading to light gray or white on the belly. Much of the light ventral area may be pinkish. There is a broad, somewhat indistinct stripe from the eye to the flipper and often light zones on the sides above the flippers. The mouth contains 26-35 teeth in each row. Adult dolphin of the genus Sotalia are up to 2.1 m (coastal) and 1.6 m (riverine) in length. They reach weights of up to at least 40 kg.

In the Amazon and Orinoco river systems, it is often difficult to distinguish tucuxi from Amazon River dolphins at a distance. Up close, however, differences in dorsal fin shape, head shape, and behavior are the best clues to distinguishing them. Bottlenose dolphins could be mistaken for Sotalia along the coast, but they are much larger, with taller dorsal fins. Franciscana might also be difficult to distinguish from Sotalia in coastal waters. The franciscana often has a larger body, much longer snout, and squarish (rather than pointed) flippers.

This dolphin’s range is in the Atlantic Ocean and is found mostly nearshore and in estuaries along the Atlantic coast, from Panama (perhaps Honduras) to southern Brazil. There are separate marine and freshwater populations. The latter are found in the Amazon and Orinoco drainage basins, as far inland as southern Peru, eastern Ecuador, and southeastern Colombia.

Dolphins of the genus Sotalia live in nearshore and estuarine waters, mostly in groups of four or fewer, although they are found in groups of up to 20 (in freshwater) or 50 (in marine waters). They are generally shy and difficult to approach. During the flood season, riverine animals may move into smaller tributaries, but apparently do not move into the inundated forest to feed (as Amazon River dolphins do), staying mainly in the main river channels.

At least in Brazil, calving in the riverine form apparently occurs primarily during the low water period, October to November. There is little or no evidence of calving seaonality in the marine form. Sexual maturity is reached at about 1.3-1.4 m in the riverine form, and 1.6.1.8 m in the coastal form. Size at birth is between 0.7 and 0.9 m.

A wide variety of fish, mostly small schooling species, are eaten by riverine tucuxi. Those along the coast consume pelagic and demersal fish and cephalopods.

Main threats include:

• Entanglement in gillnets and seines through much of its range

• Direct killing for human consumption and shark bait

• Damming of rivers

• Damaging effects of gold mining with mercury

• Habitat loss/destruction

• Vessel collisions

• Environmental contaminants

• Behavioral disturbance

• Hand-feeding by tourist boat

The species is not uncommon, and in many parts of the Amazon River system is actually quite abundant. The marine form is also relatively common throughout much of its range along the coast. Currently, it is listed as “Data Deficient” (IUCN) and “Not Listed” (ESA).

Borobia, M., S. Siciliano, L. Lodi and W. Hoek. 1991. Distribution of the South American dolphin Sotalia fluviatilis. Canadian Journal of Zoology 69:1025-1039.

Da Silva, V.M.F. and R.C. Best. 1996. Sotalia fluviatilis. Mammalian Species 527:1-7.

Flores, P.A.C. 2002. Tucuxi Sotalia fluviatilis. pp. 1267-1269 in W.F. Perrin, B. Würsig and J.G.M. Thewissen, eds. Encyclopedia of marine mammals. Academic Press.

Reeves, R.R., T.L. McGuire and E.L. Zuniga. 1999. Ecology and conservation of river dolphins in the Peruvian Amazon. IBI Reports 9:21-32.

Silva, V.M.F.D. and R.C. Best. 1994. Tucuxi Sotalia fluviatilis (Gervais, 1853). pp. 43-69 in S.H. Ridgway and R. Harrison, eds. Handbook of marine mammals, Volume 5: The first book of dolphins. Academic Press.