Decline or stability of obligate freshwater elasmobranchs following high fishing pressure

Abstract

Despite elasmobranchs are a predominantly marine taxon, several species of sharks and rays are regularly found in fresh water. Although there is ample evidence of declining elasmobranch populations around the world, this evidence comes exclusively from marine and euryhaline species; the ecology and conservation status of obligate freshwater elasmobranchs is far from being understood. River stingrays (Potamotrygoninae, 32 species) live exclusively in South American rivers and represent the overwhelming majority of freshwater elasmobranch diversity. Here, we present evidence of a decline in the abundance of river stingrays in the middle and lower Paraná River, an extensive wetland mosaic of approximately 35,000 km 2. By taking advantage of a stingraymanipulation procedure widespread among South American fishermen, we were able to estimate spatial differences in relative fishing pressure and found that the observed decline is related to fishing pressure. The highest fishing effort and lowest relative abundance occurred in areas where fisheries operate on the river floodplain. The lowest fishing effort and highest relative abundances occurred in areas where fisheries operate in the main channel. The only species with a stable trend was Potamotrygon motoro. This evidence confirms the long-presumed vulnerability of obligate freshwater elasmobranchs and suggests that some species, e.g. P. motoro, can be exploited sustainably. Our results also indicate that negative effects on freshwater elasmobranchs can be minimized by adjusting fishing grounds.

Estimating intrinsic susceptibility to extinction when little ecological information is available: The case of Neotropical freshwater stingrays (Chondrichthyes: Potamotrygoninae)

“…In addition, their relatively high r max enables these species to respond better to exploitation, provided it is regulated or not too intense. For example, in the middle Paraná River, a group of potamotrygonins composed mainly of P. motoro and subjected to bycatch has a stable trend, whereas the less productive P. brachyura declines (Lucifora et al, 2017). However, molecular evidence indicates that both P. motoro and P. orbignyi may be species complexes (Fontenelle, Lovejoy, et al, 2021; Toffoli et al, 2008).…”

“…Neotropical freshwater stingrays (Chondrichthyes: Potamotrygoninae; hereafter potamotrygonins) exhibit the general life history strategy of chondrichthyans, yet they live in a habitat more prone to human alterations than the marine one (Compagno & Cook, 1995; Kyne & Lucifora, 2022). As a result, the susceptibility of potamotrygonins to overfishing does not differ from the rest of chondrichthyans (Lucifora et al, 2022), and evidence of population declines due to overfishing is starting to emerge (e.g., Lucifora et al, 2017; Ortega et al, 2016; Scarabotti et al, 2021). It is known that threats acting to degrade habitat quality and extent are greater in freshwater compared with marine environments (Su et al, 2021; Tickner et al, 2020), and there are concerns that habitat degradation and its effects on potamotrygonins is an understudied, but possibly considerable threat.…”

Murky waters: Assessing the vulnerabilities of Indo‐West Pacific non‐marine elasmobranchs to inform future conservation planning priorities

“…In support of this assumption, Lucifora, Scarabotti & Barbini (2022) found that the maximum body size appears to be a good general indicator of productivity for potamotrygonid rays in South America. However, it should be noted that smaller bodied potamotrygonid species have still been subject to fisheries-induced population declines (Lucifora et al, 2017). This highlights the need for greater research into the life history and demography of freshwater rays to estimate productivity accurately and to provide informed fisheries management advice.…”

Emergent research and priorities for shark and ray conservation

“…Far less common are assessments of deep-water (e.g. Graham et al 2001, Devine et al 2006, Barnett et al 2012) and euryhaline or freshwater populations (O'Connell et al 2007, Lucifora et al 2017.…”