Guiana Dolphin (Sotalia guianensis) in the southern Gulf of Venezuela: Seasonal distribution, group size, and habitat use

Abstract

Small cetaceans, such as the Guiana dolphin (Sotalia guianensis), do not migrate long distances to forage but use rather small areas subject to food source variations. Information on their distribution, behaviour, habitat use, and estimated population density play an important role in the identification of critical areas and regulating human activities within. Here we examine the seasonal variations, group size, and distribution of Guiana dolphin in the southern Gulf of Venezuela, to identify their critical habitats and thus aid mitigation planning. We used boat-based surveys with a follow-group protocol where observers monitor groups of animals for 30 min, from a distance of 20 to 30 m, gathering the following data: date, time observed, GPS position, estimated number of individuals, presence of calves, behavioural observations (when possible); and environmental parameters (sea-surface temperature, water transparency, salinity, sea state [Beaufort], and cloud cover). We found that Guiana dolphins did not use the studied area (6.33 km2) homogeneously but rather aggregated in north of Zapara Island’s retaining wall. The highest density of dolphins was between 301 and 573 individuals within 4.22 km2 approximately off north of Zapara Island, and the lowest (1 - 150 individuals) at the entrance of the main navigation channel. The group size was considerably larger in the wet season (mean ± SD = 6.4 ± 1.9 individuals, range = 1 to 80) than the dry season (2.2 ± 2.7 individuals, range = 1 to 27). The number of sightings differed among seasons (dry/wet), there were more sightings in the wet season (1.3414 ± 0.4389 groups, range = 1 to 5) than dry season (1.1233 ± 0.7418 groups, range = 1 to 5). Our data suggest this population might be highly adapted to local conditions and prey movements, and use the areas intensely year around, particularly during the wet season.