Residency Patterns, Abundance and Social Composition of Bottlenose Dolphins (Tursiops truncatus) in Bahía San Antonio, Patagonia, Argentina

Residency patterns, abundance and social composition of bottlenose dolphins (Tursiops
truncatus) were assessed during 2008-2010 in Bahía San Antonio (BSA), Patagonia,
Argentina. A total of 462.3 survey hours resulted in 80.7 contact hours with in total 107
dolphin groups. Data indicated that dolphins spend most of their time resting, feeding and
travelling in BSA, although their time-budget changed over the different seasons. As such,
dolphins increased their time feeding and socializing during winter and spring, whereas
during summer they spent up to 46% of their time diving, a behaviour presumably
associated with a tail out/peduncle-dive foraging strategy. Dolphin groups had a median size
of 4 individuals ranging between 1 and 40. Group size seemed to vary significantly over the
different seasons and behaviours, with groups being larger during winter and during
socializing and feeding. Group size was further positively correlated with the presence of
calves. A total of 63 dolphins were individually identified in the bay and re-identified up to
35 days. Of these, 57% could be regarded as resident in the area (year-long or seasonally),
including all mother and calf pairs. Using the closed time heterogeneity model (Mth), and
accounting for the proportion of unidentifiable individuals, calculations resulted in a
maximum corrected abundance estimate of 97 and 83 individuals during winter, and a
minimum abundance of 34 and 38 individuals during autumn of 2009 and 2010 respectively.
At all times, between 25% and 68% of the population consisted out of unidentifiable
individuals (juveniles and calves) depending on the season. Results further revealed that
bottlenose dolphins in BSA associated at random and that the entire community existed out
of two levels of casual acquaintances.
Data presented herein suggest that the shallow and protected waters of BSA support a
resident community of bottlenose dolphins, between 35 to 97 individuals depending on the
season, living in a fission-fusion society in which companionships frequently change. It was
further suggested that dolphins specifically use BSA to rest and feed, and that the region is
preferred by females to give birth and raise their young, with a possible increased calving
period during winter and spring based on an increasing prey availability.