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

Abstract

Although bottlenose dolphins Tursiops truncatus are among the most common delphinid species, global population trends remain poorly understood. To improve the knowledge of the species in the Southwest Atlantic Ocean, thispaper reviews all available data related to the distribution and occurrence, abundance, residency and ranging patterns, group size and composition, survival and reproduction and population structure of the coastal bottlenose dolphin in Argentina. Most information proved to be scattered in time and space. Based on the available data, total abundance of coastal bottlenose dolphins in Argentina appears to be low. Data show sightings decreased notably since the 1990s in the northern province of Buenos Aires and the province of Chubut, with the species having disappeared completely from the former region. Data also indicated that two genetically and morphologically distinct coastal populations occur in Argentinean coastal waters, with a sympatric distribution in the provinces of Río Negro and Chubut. One is an isolated ‘Evolutionary Signifcant Unit’ within the larger Southwest Atlantic, whereas the other population appears to be genetically related to the Southwest Atlantic offshore ecotype. In the absence of more substantial data, the present information is pertinent to our scientifc knowledge of the species in the country, collating all published information as well as information from grey literature and previously unpublished data. However, the available information appears to remain insufcient to explain the apparent decline in sightings and to assess the remaining abundance nationwide accurately. Therefore, we strongly recommend increased research efforts for an in-depth assessment of the species’ population status in Argentina.

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

“…Presently, Lahille's Bottlenose Dolphins only occur in the southern part of the province of Buenos Aires, in Bahía Blanca and Bahía San Blas. 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.…”

“…Surveys conducted during the 1990s, failed to record any individuals in the same study area and it was concluded that these dolphins had completely disappeared in the past decade Rodriguez 2003, Vermeulen et al2017). No other coastal areas were identified where the abundance has increased substantially over time, dismissing any hypothesis of a distribution shift (Vermeulen et al 2017). Possible explanations for this disappearance relate to overfishing and other drastically increased anthropogenic pressures on the marine environment in that time period (e.g., pollution, habitat degradation, etc; Bastida and Rodriguez 2003), but no empirical data are available to sustain this hypothesis.…”

“…Simões-Lopes and Fábian 1999, Bastida and Rodriguez 2003, Goodall 1989, Goodall et al 2011. The Argentina subpopulation (for justification of the subpopulations, see section "Population") ranges within Argentinean waters from the south of the province of Buenos Aires to the province of Chubut (Vermeulen et al 2017). The southern Brazil-Uruguay subpopulation ranges between the state of Parana until the coast of Uruguay.…”

“…The southern Brazil-Uruguay subpopulation ranges between the state of Parana until the coast of Uruguay. The geographical gap between the two subpopulations relates to the Río de La Plata Estuary and the northern coast of the province of Buenos Aires (Vermeulen et al 2017).…”

“…Lahille's Bottlenose Dolphins occur in two genetically and geographically isolated subpopulations: one located in the north of the range of the subspecies in southern Brazil and Uruguay, and a second in the south of the subspecies' range in Argentina (genetically sampled in Bahía San Antonio; Fruet et al 2014). The combination of both genetic differences (Fruet et al 2014) and a 700 km gap between the groups (Bastida and Rodriguez 2003;Vermeulen et al 2017) makes them meet the IUCN definition of "subpopulation": geographically or otherwise distinct groups in the population between which there is little demographic or genetic exchange (typically one successful migrant individual gamete per year or less) (IUCN 2001(IUCN , 2012. So far, it remains unknown if the observed genetic separation is caused by the adaptation to different biogeographic regions (see Fruet et al 2014), or due to the relatively recent disappearance of the subspecies along the northern coast of the Buenos Aires province south to Río de La Plata estuary (area in between Uruguay and Patagonia), or their product.…”