Occurrence of bottlenose dolphins (Tursiops truncatus) in the Río Negro Estuary, Argentina, and their mid-distance movements along the northeastern Patagonian coast

Abstract

A systematic study was carried out on bottlenose dolphins (Tursiops truncatus) in the Río Negro Estuary (RNE), Patagonia, Argentina, to analyze their occurrence and activity patterns in this region. The photo-identification data of this study was further compared to data from an adjacent region to gain information on the animals' movements along the northeastern Patagonian coast. Information was gathered through land-based observations between the months of March and July of 2008 up to 2011. Data on dolphin activity patterns were collected via an ad libitum focal-group sampling mode. At the same time, dorsal fin images were obtained from as many dolphins as possible for identification and subsequent re-identification of individuals. Total effort equaled 188h, resulting in 58h of observation of 124 dolphin groups [sightings per unit effort (SPUE) = 0.66 group/h]. Most of the groups observed contained between one and five individuals, and two main activity states could be determined, namely traveling (65%) and foraging (26%). The photo-identification effort, which started opportunistically in 2006, resulted in a catalogue of 17 individual dolphins, with a total mean re-identification rate of nine days (max. = 24 days). When comparing these pictures to the existing catalogue of Bahía San Antonio (BSA; approximately 200km west from the study area) dorsal fins of 15 individuals could be matched and most (n = 12) could be subsequently re-identified in both areas, indicating their long distance movements along the northeastern Patagonian coast during the austral autumn months. This season coincides with the lowest dolphin abundance and feeding activity in BSA. This study indicates that bottlenose dolphins enter the RNE to forage at least during autumn. It further suggests that the search for food resources is the main trigger for their movement patterns along the northeastern Patagonian coast during this season, at least for certain individuals. More research is needed to accurately confirm these hypotheses.

A review on the distribution, abundance, residency, survival and population structure of coastal bottlenose dolphins in Argentina

“…Group size and composition (see also Table 2) In El Cóndor, Failla et al (2016) found that most groups observed contained 1-5 individuals (37%), although occasional aggregations of up to 20 dolphins (2% of groups) were recorded. The authors further reported that 31% of the observed groups contained calves, but never more than one calf per group.…”

“…1990-2016 (Fig. 2C): Despite the general presence of coastal bottlenose dolphins in the area for the past decades, dedicated studies only commenced in 2006, documenting the presence of a resident community ranging from Bahía San Antonio to El Cóndor (Vermeulen and Cammareri, 2009b;Vermeulen et al, 2016;Failla et al, 2016). In this area, 13 strandings were recorded between 2001 and 2015 (E. Vermeulen and M. Failla, pers.…”

Tursiops truncatus ssp. gephyreus: Vermeulen, E., Fruet, P., Costa, A., Coscarella, M. & Laporta, P.

“…Although no precise abundance estimates are available in this region specifically, the numbers are believed to be low based on photo-identification data for the time period 2010-2016, not exceeding 50 individuals (Vermeulen et al 2017). In the province of Río Negro, abundance was estimated at 83 (95% CI = 73 to 112) Lahille's Bottlenose Dolphins in Bahía San Antonio between 2009 and 2011 (using mark-recapture data and Pollock's Robust Design; Vermeulen and Bräger 2015), ranging at least along the entire coast of the San Matias Gulf (Failla et al 2016, Coscarella et al 2016. Based on a viability analysis, numbers were estimated to be declining at a rate of 1.1% per year during the study period 2009 to 2011 due to a low recruitment rate related to a low number of reproducing females (Vermeulen and Bräger 2015).…”