Ocurrence and habitat use of dusky dolphins, Lagenorhynchus obscurus, in summer and fall, in Golfo Nuevo, Argentina

GRISELDA V. GARAFFO; MARIANA DEGRATI; SILVANA L. DANS; SUSANA N. PEDRAZA; ENRIQUE A. CRESPO

Liege

Congreso; 16th EUROPEAN CETACEAN SOCIETY CONFERENCE; 2002

Off Patagonian coasts, in the Southwestern South Atlantic, dusky dolphins are common small cetaceans. This species is the aim of newly developed watching activities during summer in Golfo Nuevo, Argentina. However the real occurrence and movement pattern are unknown. The objectives were to determine the occurrence of dusky dolphins in the western portion of Golfo Nuevo, to determine the relationships between environmental features (bottom depth, bottom depth gradient, substrate) and sighting data and to determine whether group behaviour is related to environmental features. Shipboard surveys were conducted from January to May 2001, through random transects. When a group of dolphins was sighted, estimates of group size, composition (mothers with calves, adults and juveniles only, and mixed groups) and the predominant activity (feeding, travelling, socialising, resting, social-travelling and milling) were recorded and thereafter for each 2min intervals. The location of each group was tracked and recorded by a GPS. A grid of 1.5x1.5km squares was constructed and each square was characterised by depth, depth gradient and substrate. The tracks were overlapped to the study area. Two indices were used, Area Use Index and Activity Index. Eighty-eight trips were performed, watching 151 groups, from which 93 were tracked. Mothers with calves were more frequent in January while mixed groups were more frequent at the end of the study period (G=12.7, p=0.048). Areas with steepest gradient showed highest values of Area Use Index (G=14.36, p=0.006). Mothers with calves occurred in shallowest waters (KW=14.02, p=0.0009). Smallest groups occurred in shallowest waters (KW=23.77, p=0.0002). “Resting” occurred in shallowest areas and “travelling” occurred in deepest areas (KW=13, p=0.019). Bottom depth and bottom depth gradient were the more important environmental features determining dolphins distribution. These variables could be related to preys distribution, although “feeding” is not the only activity developed in the study area.