Obligate freshwater elasmobranchs in decline due to overfishing
Many elasmobranchs – sharks, skates and rays – are declining mainly due to overfishing. To date, all the evidence showing that populations of elasmobranchs are shrinking came from marine species. This is not surprising, as most elasmobranch species (about 95% of them) live in the sea. Few of them, like bull and river sharks and sawfishes, are famous for their ability to enter rivers. However, the 32 species of river stingrays of the family Potamotrygonidae take that ability to the extreme. They are obligate freshwater species, meaning that they can only survive in fresh water. Until now, scientists thought that obligate freshwater elasmobranchs, whose habitat is more affected by human activities than the oceans, might be threatened. However, the scientific evidence needed to assess the status of river stingrays was inexistent. Now, a new paper featured in the journal Biological Conservation has shown that river stingray populations in Argentina's Paraná River are being affected by fishing.
The study revealed a steep decline in the abundance of river stingrays between 2005 and 2016 in the middle and lower Paraná River and its associated floodplain, an extensive wetland of approximately 35,000 km2. Six species of river stingrays inhabit the Paraná River, but their taxonomy was not resolved until 2012, which made species-specific estimations of abundance trends impossible for five of the six species. The researchers were able to isolate the analysis for the ocellated river stingray (Potamotrygon motoro). The population of this species was stable, the study concluded, but taking it out from the pool of remaining species increased the declining rate of this pool from 15 to 25% annually.
The analysis also concluded that an excessive fishing pressure is related to the decline. It is common practice among South American fishermen to cut the tail of the stingrays they catch. Most of the times, stingrays are returned to the river with a mutilated tail and survive. In fact, a healed mutilated tail is an indicator that a stingray has been caught before; therefore the incidence of tail mutilation can be used as a proxy for fishing pressure. The study showed that, along the Paraná River, stingray abundance decreased with increasing incidence of tail mutilation.
Biologists from the National Scientific and Technical Research Council (CONICET), the Ministry of Agro-industry, Chaco's Department of Fish and Wildlife, and three universities, all from Argentina, conducted the study. Dr. Luis Lucifora, lead researcher of the study at the Institute of Subtropical Biology of CONICET and National University of Misiones, says "most river stingrays are listed as Data Deficient, but the observed trends in the Paraná River confirms that many obligate freshwater elasmobranchs are threatened". Very large species, like the giant river stingray (Potamotrygon brachyura) that may weigh more than 200 kg, or very rare species, like Potamotrygon schuhmacheri, are of high concern; on the other hand, the ocellated river stingray is able to withstand fishing levels that are too high for other species. Dr. Lucifora adds "the difference in response to the same fishing level among species leaves a possibility for a sustainable use of some river stingrays".
More information:
Lucifora, L. O., L. Balboni, P. A. Scarabotti, F. A. Alonso, D. E. Sabadin, A. Solari, F. Vargas, S. A. Barbini, E. Mabragaña & J. M. Díaz de Astarloa. 2017. Decline or stability of obligate freshwater elasmobranchs following high fishing pressure. Biological Conservation 210: 293-298. DOI: 10.1016/j.biocon.2017.04.028
Provided by National Scientific and Technical Research Council
