The Atlantic spotted dolphin, Stenella frontalis is a tropical and warm temperature small cetacean endemic to the Atlantic Ocean. Here we report the first sighting of this species from the waters of Uruguay, further expanding the distribution of this species to 35°S.

The Atlantic spotted dolphin, Stenella frontalis, presents two geographically isolated populations in the western South Atlantic. This note reports on a 350 km northward extension range of the southern population. The sighted group of 80 animals was in waters 66 m deep and 75 nautical miles distant from the coast. The record was observed during a dedicated cetacean survey in Brazilian waters in the spring of 2008 and supports a discontinuous distribution along the coast of Brazil.

The taxonomy and distribution patterns of the common dolphins (genus Delphinus) are problematic among cetaceans and particularly complicated in the south-western Atlantic Ocean. On the basis of national collections, we found ten adult skulls of the genus Delphinus of Uruguayan origin. The two currently recognized species for the genus were identified based on craniometric measurements: the pelagic short beaked form (Delphinus delphis, N = 3) and the coastal long beaked form (Delphinus capensis, N = 4), although previous studies only recognized D. capensis for Uruguay. Three specimens could not be identified (Delphinus sp.) because of broken skull (2) and intermediate measures (1). This result is consistent with a recent review on the biogeography of the genus in the south-western Atlantic Ocean.

The present study demonstrates the influence of the time of the day and tide on the frequency of individuals and on the social and foraging behaviour of an equatorial population of Sotalia guianensis. A population of S. guianensis from enseada do Curral (6°13′00″S 35°3′36″W), north-eastern Brazil, was investigated from October 1999 to September 2003. Social and foraging activities oscillated significantly, being less intense at high tide in comparison to other analysed tides. Socialization was more intense between 1000 and 1300 hours and foraging was more intense in between tides, than at high tide. No diurnal or tidal fluctuation was found on the frequency of individuals, which remained constant throughout the day. These results demonstrate the influence of circadian factors on an equatorial population which is highly impacted by human activities and highlights a need for protective measures.