The Atlantic spotted dolphin, Stenella frontalis (G. Cuvier, 1829), occurs in coastal waters over the continental shelf of the Atlantic Ocean, usually in depths up to 200 m (Perrin, Reference Perrin2001; Moreno et al., Reference Moreno, Zerbini, Danilewicz, Santos, Simões-Lopes, Brito and Azevedo2005). On the east coast of South America, the species shows a possibly unique distribution pattern, presenting two geographically isolated populations (Moreno et al., Reference Moreno, Zerbini, Danilewicz, Santos, Simões-Lopes, Brito and Azevedo2005). The ‘southern' Atlantic spotted dolphin population is distributed on the coast of southern and south-eastern Brazil (from 33^oS to 21^oS), while a ‘northern' population ranges from northern Brazil 06^oS northwards. It is not yet known whether the distribution of spotted dolphins in northern South America, the Caribbean, the Gulf of Mexico and the central North American coast, where the species is known to occur regularly (e.g. Perrin et al., Reference Perrin, Mitchell, Mead, Caldwell, Caldwell, Van Bree and Dawbin1987; Jefferson et al., Reference Jefferson, Leatherwood and Webber1993), is continuous. The allopatric distribution of these two populations in Brazilian waters was well documented after an extensive review of strandings, sightings and incidental catches (Moreno et al., Reference Moreno, Zerbini, Danilewicz, Santos, Simões-Lopes, Brito and Azevedo2005), and is very likely not the result of reduced research effort in that region.

This note reports on a 350 km northward extension range of the southern population of S. frontalis. A school of spotted dolphins was sighted on 17 September 2008 (18^o52.06′S 38^o24.73′W) during a research cruise in the coastal waters of Brazil designed to assess the distribution and abundance of humpback whales, Megaptera novaeangliae, in their wintering ground. The survey was carried out in August–September 2008 and was designed to collect sighting data using line transect methods. The survey area (123,000 km²) comprised the continental shelf and shelf break up to the 500 m isobaths between Cabo de São Roque (~05°S) and Cabo Frio (~23°S). Survey effort consisted of 2346 km of trackline. The ‘flying bridge’ was used as the observation platform. Cetaceans were continuously searched from 05:30 hours to 17:00 hours. Seven or eight scientists rotated through three observation positions.

Despite extensive observation effort throughout the surveys area, only one sighting of Atlantic spotted dolphins was recorded. The group was composed of approximately 80 individuals, including calves, juveniles and adult animals (Figure 1). The sighting was located in the southern portion of the Abrolhos Bank, in a location where depth was 66 m and distance from the coast was 75 nautical miles (Figure 2). This record extends the northern range of the southern Atlantic spotted dolphin population to ~350 km.

Fig. 1.

Part of the school of Atlantic spotted dolphins, Stenella frontalis, sighted in the southern Abrolhos Bank, Brazil.

Fig. 2.

Distribution of Atlantic spotted dolphins, Stenella frontalis, in Brazil, evidencing the sighting in Abrolhos Bank. Shaded areas correspond to the previously known distribution of the species off eastern South America.

The Atlantic spotted dolphin has not been previously reported for the Abrolhos Bank, despite the relatively high survey effort in the area since 1988 (Siciliano, Reference Siciliano1995; Martins et al., Reference Martins, Morete, Engel, Freitas, Secchi and Kinas2001; Andriolo et al., Reference Andriolo, Kinas, Engel, Martins and Ruffino2010). This region represents the main breeding ground for humpback whales in the western South Atlantic. In addition to humpback and southern right whales, Eubalaena australis, only three small cetaceans had been previously observed offshore in this area: the Guiana dolphin, Sotalia guianensis; the rough-toothed dolphin, Steno bredanensis; and the bottlenose dolphin, Tursiops truncatus (Rossi-Santos et al., Reference Rossi-Santos, Wedekin and Sousa-Lima2006). The franciscana, Pontoporia blainvillei, also inhabits the coastal waters in the southern limit of Abrolhos Bank (Moreno et al., Reference Moreno, Martins, Andriolo and Engel2003). Therefore, this record adds a new cetacean species for this important marine ecosystem in Brazilian waters.

The results of this dedicated scientific cruise also support the conclusions of Moreno et al. (Reference Moreno, Zerbini, Danilewicz, Santos, Simões-Lopes, Brito and Azevedo2005) of a substantial latitudinal gap (~6–18^oS) in the distribution of the species off Brazil and of the existence of an isolated population further to the south. Studies on abundance, population structure, ecology and conservation problems faced by this southern population are strongly recommended given the various anthropogenic activities taking place in this region (e.g. gillnet fisheries, oil and gas development and habitat degradation).