The La Plata basin is the second largest basin of South America and has supported important river fisheries for more than a century. In this paper, we evaluate for the first time the historical trends of landings of 21 fish taxa and the recent population trends of 27 species of commercial fishes in the lower La Plata basin (Argentina). We compiled three kinds of data sets: Total fishery landings (between 1934 and 1986) and exports (1994‒2019), fisheries monitoring programs of Chaco and Santa Fe provinces in the Paraná River (2009‒2019), and surveys of fish populations in the Upper (Corrientes, 1993‒2020) and Middle (EBIPES, 2005‒2020) Paraná River. The analysis of the historical landings showed more species declining in the lower portion of the basin than in the upper basin. Regarding recent population trends, Pimelodus spp., Hoplias spp., Salminus brasiliensis, Luciopimelodus pati, and Ageneiosus spp. declined in more than one region, while Megaleporinus spp., Pterodoras granulosus, and Oxydoras kneri showed stable to positive trends, with the other species varying in their trends between regions. These tendencies could be associated to a combination of factors such as overfishing and environmental changes that would require an ecosystem approach for their adequate management.
En este trabajo se genera un modelo arqueológico de base biogeográfica para el suroeste rionegrino y neuquino, focalizando en el área del lago Nahuel Huapi. Se establece un ranking de ambientes considerando topografía, productividad y estacionalidad. Se propone una primera contrastación del modelo en relación con la evidencia rupestre publicada y se analizan la frecuencia de sitios, motivos, técnicas, colores y diseño considerando las grandes unidades ambientales disponibles en el área: bosque, ecotono y estepa. Los resultados indican un paisaje regionalmente heterogéneo y fragmentado con condiciones de tránsito y habitación humana muy disimiles. La contrastación inicial de estos datos con el registro rupestre indica cierta correspondencia, observándose indicadores distribucionales de técnicas, colores y diseños diferenciables en cada una de las unidades ambientales disponibles en el área del lago Nahuel Huapi y los sectores ecotonales y esteparios hacia el este.
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.
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.
The age and growth of Pseudoplatystoma corruscans, is analyzed on specimens landed in Puerto Antequera, Province of Chaco, Argentina. The study is based on length frequency distribution of 1192 individuals and growth marks of 293 pectoral spines. Previously to age assignation based on spines readings, we performed analyses that ruled out age-associated resorption of rings and corroborated the annual periodicity of mark formation. The average sizes of the radius of each ring were obtained, and the total length of fish were back-calculated to the time of the ring formation, by the regression model fitted between the total radius of the spines to the respective sizes of each fish. Such data showed a good fitting to growth models of von Bertalanffy, Gompertz and logistic for both sexes separately. Results indicate that the study of the species growth must be carried out for each sex separately and that the fishing regulations must consider this characteristic of the species since the current management guidelines could be promoting differential capture by sexes.
The Manguruyú Zungaro jahu, endemic to the La Plata Basin, is one of the largest neotropical fish species whose life history is virtually unknown. The objectives of this study were to determine if Z. jahu is a large-scale migratory species and to describe habitat use throughout ontogeny using the otolith Sr isotope ( 87 Sr/ 86 Sr) ratio. Surface water samples were collected in 2020 at four sites representing the main water courses of the Great Pantanal in Brazil (upper Paraguay, Cuiabá, São Lourenço and Vermelho rivers). Eighteen fish individuals were collected from the Paraná River (Argentina). New water 87 Sr/ 86 Sr data were incorporated into previously established statistical models to infer the fish habitat use through the otolith 87 Sr/ 86 Sr ratios. The water 87 Sr/ 86 Sr ratio of the upper Paraguay tributaries varied (0.7209 and 0.7229) and were the highest in the La Plata Basin. The dispersal area of Z. jahu was restricted to the surroundings of the confluence of the Paraná and Paraguay rivers. Fish showed chemical signatures compatible with those of the middle Paraná (88%), Paraguay-Pilcomayo-Bermejo (50%) and the lower/upper Paraná (66%). The discriminant analysis based on otolith core indicated natal origins from the middle Paraná (37%), upper Paraná (32%), and Paraguay sub-basin (32%). The dispersal area of Z. jahu migrations could be restricted to the surroundings of the confluence of Paraná and Paraguay, which highlights the need to review regional fishing regulations, especially between latitudes 30 -34 S.
Surubim (Pseudoplatystoma corruscans, Pimelodidae) are migratory catfish native to the rivers in the La Plata and São Francisco basins. They are piscivores that attain considerable body sizes and are a valuable economic resource. Surubim exhibits extensive migrations during its life cycle that may affect the population structure at vast geographic scales. The authors examined the genetic diversity and population genetic structure of P. corruscans using microsatellite markers from a comprehensive sampling of 260 individuals from the Upper and Lower Paraná River. They identified two well-differentiated genetic clusters corresponding to a natural geographic barrier historically separating Upper and Lower Paraná regions. They also demonstrated temporal variation in population genetic structure at a site in Lower Paraná close to the confluence with the Paraguay River, most likely explained by the influx of migrant fishes at certain times of the year.
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