Group size and composition of Tursiops truncatus (Cetacea: Delphinidae), in a coastal insular habitat off southeastern Brazil

Abstract

The aim of the present study was to describe the characteristics of common bottlenose dolphins (Tursiops truncatus) group size and composition in the Cagarras Archipelago (23°01'S, 43°12'W), Rio de Janeiro, Brazil during a six year period study (2004 and 2006 to 2010). Group size (n = 51) ranged between three and 30 individuals (mean = 13.7 ± 7.1) and their frequency distribution showed two modes: one at the seven individuals group and the other at the 11 individuals group. The largest average group size occurred in 2004 (mean 21.4 ± 3.3) and the smallest in 2008 (mean 4.4 ± 0.8). The number of individuals/group decreased over the years which could be correlated with habitat quality indicating that common bottlenose dolphins could play a valuable role as sentry species. The average number of immature individuals (neonates, calves and juveniles) in relation to the total group size varied between 0.31 (2006) and 0.4 (2010) and differences between years were non significant. These findings suggest that Cagarras Archipelago should be considered as critical habitat for the survival and /or population growth of T. truncatus in southeastern Brazil.