Guiana dolphins (Sotalia guianensis) displaying beach hunting behavior in the Cananéia Estuary, Brazil: social context and conservation issues

Abstract

To elaborate a fuller description of the beach hunting behavior displayed by Guiana dolphins (Sotalia guianensis) in the Cananéia Estuary (CE) (25ºS, 48ºW), a photo-identification study was conducted from May 2000 to July 2003. Three land platforms were chosen at the main entrance of the above-mentioned estuary, where scientists enjoy a unique opportunity to observe dolphins at greater proximity, undisturbed. Observations were opportunistic and unevenly distributed in time and space. To enhance the identification efforts, an 80-pound crossbow was used to collect skin samples in order to determine the sex of the eight individuals thus monitored. In 67 days of surveys, rendering approximately 80h of direct observations, 4,102 photographs were taken, of which 1,098 (26.8%) were considered useful for identification purposes. A total of 103 groups were reported (3.7 ± 2.6 individuals), ranging from solitary dolphins to 15 individuals. Female-calf pairs were observed in 92.4% of all the groups investigated. On the first occasion, a calving interval was observed for the species in the CE: approximately 3 years and 9 months. Of the 40 individuals identified at least once, eleven used the sloping beaches regularly: females KN #s 10, 30, 255, 268 and 279; males KN #s 86 and 257; and four individuals of unknown sex. The characteristics of the associations among beach hunters were evaluated by the analyses of association indices. A total of 83 possible associations between dyads were analyzed using the half-weight index and two selective criteria (5+ and 8+ sightings/individual). Mean association indices varied from 0.12 to 0.16, evidencing weak bonds among beach hunters. Most regular beach hunters were females, showing that the beach hunting behavior should probably be considered more typically female. Beach hunting in the CE might be considered another example of cultural transmission in cetacean societies.