Diversity patterns and their causes remain important questions, especially for ecosystems that preserve natural conditions. This is the case of fish diversity in large Neotropical rivers. In this context, we investigated fish diversity patterns along an extensive unregulated river section (ca. 450 km) in the Middle Uruguay River Basin. Sampling was conducted seasonally between May 2017 and March 2018, at six sites (patches) with contrasting environmental conditions. We collected 3,008 individuals belonging to 90 species. Nine were migratory, which summed relevant biomass in local assemblages (20 to 60%). We observed spatial variation in assemblage structure, but weak seasonal effects. Overall, biomass was similar among sites, but richness tended to increase downstream. Migratory fishes showed opposed trends, with higher richness and biomass upstream, particularly catfishes (Siluriformes). Ordination analyses separated sites in three groups based on variations in composition and abundance, and revealed associations between these groups and specific hydro-geomorphic conditions (i.e., flow, depth, channel width and the presence of riparian vegetation). Our study revealed that fish diversity distributes heterogeneously along the main channel, where hydro-geomorphic patches select for distinct assemblages along riverine gradients.

The spotted surubim Pseudoplatystoma corruscans is a large migratory catfish native to the La Plata and San Francisco basins. Nonetheless, its reproductive dynamics and behaviour remain poorly understood. This brief note provides the first record of the mating behaviour of P. corruscans in natural conditions. This behaviour was observed in the main channel of the Middle Uruguay River (Brazil). A school of approximately 25 fish, 5 females (with protruding abdomen and larger body size) and 20 males, were followed and videotaped. The largest female led the school, followed by other fish. Some males disputed repeatedly the posterior region of the females, whereas other males continuously repelled them. Eventually, some males, apparently exhausted, floated belly up. During this event, males were in a state of pronounced agitation, and some had abrasions in the head. Polygamy characterized the mating behaviour of P. corruscans.

1. Hydrological connectivity is a basic feature of river ecosystems, responsible for important ecological processes. Human activities, however, have progressively modified natural connectivity, threatening the maintenance of aquatic biodiversity.2. Neotropical potamodromous fish depend heavily on fluvial connectivity to complete their life cycles. However, important ecological aspects remain poorly understood, especially in river systems with limited floodplain development (i.e. channelized rivers and drylands), where the environment poses constraints to fish recruitment. The Uruguay River Basin is such a system, with several migratory fish, small floodplain areas, and limited information about recruitment dynamics and nursery grounds. 3. The lack of information on critical habitats limits conservation and management actions, especially in scenarios of increasing environmental degradation and river regulation. This study investigated recruitment dynamics of migratory fishes in the Middle Uruguay River, a long fluvial segment targeted for hydropower development. The study investigated spatial variations in the abundance, composition and size structure of juvenile fish among different sites along an extensive fluvial gradient (ca. 650 km), comparing the main channel with tributaries.4. Juveniles of migratory fish were widely distributed in the region, totalling nine species, with a predominance of Characiformes. Spatial distribution was variable among species and size classes, with a differential use of tributaries, wetlands and the main channel. Juvenile composition differed between sampling sites and environments, and was correlated with different limnological and spatial variables.Greater abundances occurred in the mouth of a tributary where a small floodplain develops during the wet season.5. The results emphasize the importance of fluvial connectivity for fish migration, dispersal and recruitment, providing valuable information about migratory fishes in river systems with limited wetland development. These findings have significance for the management and conservation of migratory fish under scenarios of hydropower expansion, as comprehensive studies on recruitment are rare in free-flowing rivers.

Length‐Weight relationship (LWRs) were estimated for four fish species occurring in the Ibicuí River drainage basin, southern Brazil. Samples were captured monthly during the spring and summer seasons (October 2018 to March 2019). The fish were caught with gillnets of different meshes (15, 20, 25, 30, 40 mm between adjacent nodes) and a trawl net (5 mm mesh between adjacent nodes) Eighteen excursions were realized in three sites, with a length of approximately 100 km, in third‐order flux, all distributed in sub‐basin Ibicuí River. The captured tools were installed at the end of the day (6 p.m.) and removed around 6 a.m. the next day. The present study provides the length‐weight relationship for four species (Astyanax dissensos, Galeocharax humeralis, Hypostomus roseopunctatus e Hypostomus uruguayensis).

No abstract

Currently, there are several types and models of plankton nets available on the market. However, it is not known for sure how the differences in size and shape of these nets influence the capture of eggs and larvae. Therefore, the objective of this study was to investigate the influence of mouth size and collection cod-end type on the capture of ichthyoplankton organisms. Collections were carried out in October and November 2019 on the Ibicuí River (Rio Grande do Sul, Brazil) using four different plankton trawl nets: T1: Wide mouth pelagic net with closed collection cod-end; T2: Wide mouth pelagic net with opened collection cod-end; T3: Narrow mouth pelagic net with closed collection cod-end and T4: Narrow mouth pelagic net with opened collection cod-end. At the laboratory, the samples were screened and the larvae were classified in the Yolk-Sac, Pre-Flexion, Flexion and Post-Flexion stages. To evaluate the variation of eggs, total larvae and developmental stages in each of the trawl nets, a two-way Analysis of Variance (ANOVA) was used. When the ANOVA results were significant, Tukey's a posteriori test was performed to detect differences. There was no statistically significant difference (P>0.05) in the capture of eggs and total larvae in the different nets in October. However, November has registered the highest number of total larvae in the widest mouth nets. In both months of sampling, there was greater capture of the Flexion and Post-Flexion stages on the wide mouth and opened collection cod-ends nets. The efficiency of ichthyoplankton capture seems to be related to larval developmental stages, which demonstrates the importance of choosing a net that adapts to the objectives of each study.

Length‐weight relationship (LWRs) were determined for 13 fish species from the Uruguay River. The fish were caught in six locations and sampled at each location during each season, totaling 72 samplings, starting in May 2016 and ending in February 2018. Various materials were used such as gillnets (15; 20; 25 30; 40 and 80 mm, triple mesh 150.30.150 150.40.150 and 150.50.150 mm, 8 mm trawl). We also used two longlines, 100 m long, 30 hooks each (hooks 5/0 and 12/0), baited with small fish, corn and snail. Length‐weight relationship were adjusted appropriately for all species (r2 > .95). The b values of the LRW equations ranged from 2.38 in Steindachnerina brevipinna to 3.62 in Pimelodus atrobrunneus.

Resumo O tamanduá-bandeira Myrmecophaga tridactyla ocorre amplamente ao longo da região Neotropical. No entanto, é provável que esteja extinto em parte de sua distribuição original, levando esta espécie a ser classificada como Vulnerável à extinção (VU) pela IUCN. Um novo registro de ocorrência da espécie foi obtido no município de Santo Antônio das Missões, localizado na região de transição entre o bioma Mata Atlântica e Pampa, no estado do Rio Grande do Sul, Brasil. O registro de um indivíduo atropelado foi obtido durante o monitoramento de fauna realizado na rodovia BR-285 em novembro de 2019.