Abstract Off Patagonian coasts, Argentina, the dusky dolphin is one of the most common small cetaceans. This species is the aim of newly developed watching activities during summer in Golfo Nuevo. However, the real occurrence and movement pattern are unknown. The objectives of this work were to investigate the predictability of dusky dolphin distribution, group structure and behaviour in the western portion of Golfo Nuevo, with respect to environmental features (bottom depth, bottom slope, distance from shore and substrate). Random transects in the bay were searched by small boat during the summer and autumn of 2001–2004. 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 and milling) were recorded and thereafter at 2 min intervals. A grid of 1.5 1.5 km was constructed and each cell was characterised by environmental features, Area Use Index (percent of total annual search effort) and Activity Index (predominant behaviour of groups observed in that cell). Mothers with calves and smaller groups and resting behaviour occurred in shallowest waters supporting the idea/hypothesis that movement to shallower water is related to increased safety for individuals. Travelling occurred in the deepest areas with other behaviours observed in intermediate depths. Dolphin distribution within the bay differed significantly between years, but this was not related to any of the factors analysed in this study. Although there was considerable variation between years, in general, dolphins were found in deeper waters further from shore (except for mother–calf groups) and over areas with steeper seafloor gradient. The high variability in the distribution of the animals does not allow for the generation of a simple, area-specific management plan.
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