Geographic distribution of the short-tailed river stingray (Potamotrygon brachyura): assessing habitat loss and fishing as threats to the world’s largest obligate freshwater elasmobranch

Abstract

Freshwater elasmobranchs are threatened but little is known about them. Potamotrygon brachyura is the world's largest obligate freshwater elasmobranch. This makes it popular among fishermen and suggests a susceptibility to anthropogenic threats. We collected records of P. brachyura (n ¼ 70) from media, fishermen and scientific sampling, to estimate its global geographic distribution (using both generalised additive and MaxLike models). Then, we estimated the species' exposure to habitat modification and fishing pressure, by applying multinomial ordinal models with threat levels as response and the presence or absence of P. brachyura as an independent variable. Distance to coast, depth, water temperature range, salinity range and mean water temperature were the main determinants of the distribution in the Río de la Plata. This resulted in a narrow coastal distribution. In the Uruguay and Paraná River basins, the probability of occurrence was positively associated with flow accumulation, percentage of open water and submerged vegetation, and lowland ecoregions, and negatively to wetness index and altitude. This limited the distribution to large lowland rivers. Approximately 41% of the range of P. brachyura in the Río de la Plata was subject to high habitat modification. In the Uruguay and Paraná River basins, this percentage reached 7%; however, the overlap with high fishing pressure was 59%. For conservation purposes, P. brachyura may function as an umbrella species.

Decline or stability of obligate freshwater elasmobranchs following high fishing pressure

“…The relative importance of habitat degradation and fishing as threats may differ regionally. A recent assessment of potential threats to P. brachyura shows that the main threat to this species in the Paraná-Paraguay riverine axis would be fishing rather than habitat modification, since its geographic distribution significantly overlaps with areas of high fishing pressure (Lucifora et al, 2016). This is explained by the relatively good conservation status of the aquatic habitat of the middle Paraná-Paraguay riverine axis, which is free from major infrastructure developments (e.g.…”

“…For example, P. brachyura achieves a very large size, a trait that is usually correlated with low biological productivity (Dulvy et al, 2014). In recent years, this species became actively hunted by both recreational and commercial fishermen (Lucifora et al, 2016), which suggests that it might be an important contributor to the genus-level observed decline. The very rare P. schuhmacheri, which is endemic to the middle Paraná and Paraguay rivers (Rosa et al, 2010) should also be considered a species of global conservation concern.…”

“…Six species of river stingrays of the genus Potamotrygon occur in the middle and lower Paraná River: P. amandae, P. brachyura, P. falkneri, P. histrix, P. motoro, and P. schuhmacheri (Rosa et al, 2010;da Silva and de Carvalho, 2011;Loboda and de Carvalho, 2013;Lucifora et al, 2016). Five of these species are categorized as Data Deficient by the IUCN (Charvet-Almeida and Drioli and Chiaramonte, 2005;de Araújo, 2009;Charvet-Almeida et al, 2009;Soto et al, 2009), and P. amandae has not been assessed.…”

“…Some of these species are of particular conservation concern because they have traits that correlate positively with high extinction risk, such as endemicity, large body size and rarity (Pimm and Jenkins, 2010;Dulvy et al, 2014). Four species (P. amandae, P. brachyura, P. histrix and P. schuhmacheri) are endemic to the Río de la Plata basin (Rosa et al, 2010;Loboda and de Carvalho, 2013;Lucifora et al, 2016). P. brachyura attains a very large body size, reportedly exceeding 200 kg (Lucifora et al, 2016), and P. schuhmacheri is one of the rarest elasmobranch species (either freshwater or marine), since it is known from less than 5 specimens (Rosa et al, 2010).…”

“…Four species (P. amandae, P. brachyura, P. histrix and P. schuhmacheri) are endemic to the Río de la Plata basin (Rosa et al, 2010;Loboda and de Carvalho, 2013;Lucifora et al, 2016). P. brachyura attains a very large body size, reportedly exceeding 200 kg (Lucifora et al, 2016), and P. schuhmacheri is one of the rarest elasmobranch species (either freshwater or marine), since it is known from less than 5 specimens (Rosa et al, 2010).…”