Lucileine de Assumpção

Empirical evidence integrating multiple biological levels on freshwater biodiversity persistence in regulated river basins remains scarce for Neotropical systems. We evaluated fish assemblage structure, reproductive strategies, and life-cycle support for migratory species across lotic and lentic habitats in three tributaries of the Porto Primavera Reservoir, Upper Paraná basin, Brazil, over a 12-year monitoring period (2007–2018), encompassing 104 species and a Life-cycle Support Index (LSI) based on occurrence of eggs, larvae, juveniles, and adults. Marginal lagoons functioned as biodiversity hotspots, with higher richness and diversity than river channels; reproductive guilds were habitat-partitioned, with opportunistic and parental-care strategists in lentic environments and periodic/migratory strategists in lotic habitats. Tributary identity was the strongest predictor of assemblage composition: the Aguapeí River, partially protected, showed the highest LSI values, whereas the Verde River provided the strongest evidence of reproduction for conservation-priority taxa, including the critically endangered Brycon orbignyanus, whose adults occurred exclusively there, and the vulnerable Pseudoplatystoma corruscans, for which adults, eggs, and larvae co-occurred. Three proposed hydropower developments on the Verde River would fragment a 200-km free-flowing stretch, threatening these conditions. Undammed tributaries are irreplaceable for freshwater biodiversity and should be recognized as conservation priorities in hydropower licensing and basin governance.

Empirical evidence integrating multiple biological levels on freshwater biodiversity persistence in regulated river basins remains scarce for Neotropical systems. We evaluated fish assemblage structure, reproductive strategies, and life-cycle support for migratory species across lotic and lentic habitats in three tributaries of the Porto Primavera Reservoir, Upper Paraná basin, Brazil, over an 11-year monitoring period (2007–2018), encompassing 104 species and a Life-cycle Support Index (LSI) based on occurrence of eggs, larvae, juveniles, and adults. Marginal lagoons functioned as biodiversity hotspots, with higher richness and diversity than river channels; reproductive guilds were habitat-partitioned, with opportunistic and parental-care strategists in lentic environments and periodic/migratory strategists in lotic habitats. Tributary identity was the strongest predictor of assemblage composition: the Aguapeí River, partially protected, showed the highest LSI values, whereas the Verde River provided the strongest evidence of reproduction for conservation-priority taxa, including the critically endangered Brycon orbignyanus, whose adults occurred exclusively there, and the vulnerable Pseudoplatystoma corruscans, for which adults, eggs, and larvae co-occurred. Three proposed hydropower developments on the Verde River would fragment a 200-km free-flowing stretch, threatening these conditions. Undammed tributaries are irreplaceable for freshwater biodiversity and should be recognized as conservation priorities in hydro-power licensing and basin governance.

Dams severely affect aquatic environments and block the longitudinal migration of fish. In order to mitigate the negative effects generated by these developments, fish passes, or fishways, are implemented in dams with the purpose of restoring river connectivity and allowing the movement of migrants. Nevertheless, fishways in neotropical areas often face design and construction issues that can reduce their efficiency and selectively disadvantage species with limited swimming capabilities. This study analyzes how a fish ladder on the Paraná River influences the black armored catfish (Rhinelepis aspera), a benthic, long-distance migratory species important to commercial fisheries. A total of 200 individuals were PIT-tagged and monitored for four months. The results showed that although many fish successfully located the fishway, only a small portion (3.5%) managed to complete the ascent. The interaction between the hydraulic characteristics of the fishway and the fish condition factor played a significant role in ascent performance. Our findings underscore the importance of assessing fishway suitability for benthic neotropical species to support conservation efforts in the Upper Paraná River Basin. To improve passage rates for R. aspera, we recommend optimizing flow conditions by adjusting orifice and notch configurations, incorporating roughness elements, and modifying resting pool designs. These adaptations would reduce energy expenditure for ascending fish, enhancing fishway performance and contributing to the sustainability of migratory species in this region.

Steindachneridion melanodermatum is the largest catfish of the Lower Iguaçu River and is endangered due to the habitat fragmentation caused by dams. Currently, the wild population’s last refuge is restricted to an area of 190 km. This study presents the first analysis of its genetic diversity and population structure, using microsatellite loci and mtDNA. The population has an adequate level of genetic diversity, but signs of a recent bottleneck were observed. The Baixo Iguaçu Hydroelectric Power Plant has recently fragmented the population and threatened it with extinction in a reduced area of nearly 30 km. Based on our results, we strongly advise against the stocking of breeding specimens below the Salto Caxias HPP to not compromise the integrity of the native gene pools at the receptor sites. In addition, we recommend manual fish transposition, trap-and-haul, to maintain the genetic connectivity of individuals upstream and downstream of the dam as a conservation strategy. Furthermore, studies on behavior and swimming capacities, and suitable fishways for this species must be developed. We strongly recommend that the Lower Iguaçu River and its tributaries be protected and preserved as free from additional barriers to prevent future habitat disruption for the benefit of S. melanodermatum and several other endemic and endangered species.

Seasonal drift patterns of fish eggs and larvae were analysed along the river section downstream the Iguaçu River Falls to the confluence with Paraná River, over three spawning seasons. This river section is situated in a narrow rocky canyon characterized by a steep slope, which leads to highly turbulent and fast flowing conditions in a series of rapids. Water transit time in this river stretch of 20 km was short (5–11 h), the flow‐velocity in rapids can reach values of more than 6 m s⁻¹. Densities of drifting eggs and larvae varied between three spawning seasons, and differences of eggs were more pronounced compared to larvae. Eggs and larvae were observed all along the river section. A spatial, longitudinal distribution pattern of larvae was found, with higher numbers in the upper section. A distinct spatial sequence of different embryonic stages was observed, with the highest abundance of early stages. Larval densities were low in comparison to egg densities, and preflexion and flexion stages predominated. High abundance of buoyant eggs in drift indicates fish spawning in rapid and turbulent waters of the Iguaçu River, particularly downstream from the Iguaçu Falls. Rainfall and turbidity may be the main trigger for spawning. Spawned eggs drifted quickly towards the confluence, most are likely to hatch in the Paraná River, while larvae disperse to nursery habitats even in the river section below the falls. Thus, inshore zones along the whole stretch provide nursery habitats for larval development of fish species exhibiting different reproductive strategies.

Steindachneridion melanodermatum, an endemic and endangered species, is the largest catfish in the Lower Iguaçu River basin. Currently, the wild population of this species no longer exists in most of this basin, probably due to the loss of habitat by successive hydroelectric dams. This study evaluated the spatial distribution and abundance of S. melanodermatum in the last free-flowing river stretch of approximately 190 km downstream from the Salto Caxias Dam, upstream of the Iguaçu Falls, as well as some tributaries. Hydroacoustic and fishing campaigns were performed between September 2010 to December 2016 to characterize the preferred habitat. A total of 180 specimens was sampled with higher abundances in a protected area near Iguaçu Falls in the Iguaçu National Park. Twenty-four deep pools were identified with maximum depths ranging from 5 to 25 m: 21 pools were along the main channel of the Iguaçu River, and three pools were in tributaries. Deep pools are preferential habitats for this species, especially the deep pool called Poço Preto (P23) and its surroundings. Conservation strategies are required to protect these habitats and prevent S. melanodermatum from becoming extinct, such as establishing deep pools as ecological sanctuaries, intensifying the inspection of illegal fishing, and maintaining dam-free tributaries.

The population structure and reproductive biology of the surubim of Iguaçu, Steindachneridion melanodermatum, a large endemic and endangered catfish from Iguaçu River basin, were evaluated. In total, 182 specimens of this species were sampled from September 2010 to December 2016 in the Lower Iguaçu River, including some tributaries. The determination of the cycle and reproductive period was undertaken by assessing the mean variation in the values of the gonadosomatic index (GSI) of females and males, as well as by the frequency distribution of the stages of gonadal development on the basis of macroscopic and microscopic analyses. The total length and bodyweight ranged from 21 to 102 cm and from 82.8 to 15 670 g respectively. The reproductive period of this species was from June to September (winter and early spring), and may extend until November, with GSI peaking in August for males and September for females. The size at first maturation (L50) corresponded to 39.5 cm for females and 43.9 cm for males. Spawning in this species was associated with lower water temperatures, and high discharge and dissolved oxygen concentration. Our findings are crucial in assisting the development of effective measures and specific criteria for the conservation of this endangered species.

The seasonal pattern and the composition of downstream drift of fish eggs and larvae through the fish ladder of the Porto Primavera dam, located in the Upper Paraná River, Brazil, were studied. Additionally, the condition of individual larvae with regard to injuries was examined. Samples were collected monthly over three reproductive seasons at upper and lower pools at the twilight (nightfall and dawn). Ichthyoplankton densities were low, especially for eggs, and the downstream passage of fish eggs and larvae through the fish ladder was negatively correlated to water residence time of the reservoir. Highest larvae densities occurred between December and March, with highest densities in February. The ichthyoplankton abundance was similar between sites and times of day, with the predominance of partially damaged larvae (60%). Seventeen taxa were identified, especially non-migratory species. Long-distance migratory species represented less than 1% of the total. Since many migratory species have found favorable habitats for spawning and development in the tributaries of Porto Primavera Reservoir, the dispersion of larvae from these free-flowing tributaries located far from dam through reservoir may not be beneficial for their survival. At the same time, despite the low density and diversity of migratory species larvae drifting downstream from the dam, this fish ladder may have dispersive potential as long as the reservoir displays a shorter water residence time.

This study characterized the morphological differences among migratory fish species from the Paraná River Basin. A total of 177 adult fish of Leporinus elongatus, Leporinus macrocephalus, Prochilodus lineatus, Salminus brasiliensis, Pimelodus maculatus, Pinirampus pirinampu, Pseudoplatystoma corruscans, Pterodoras granulosus, and Rhaphiodon vulpinus were sampled in the Canal da Piracema, a fish passage system at Itaipu Dam, Paraná River. The migratory species were analyzed through 22 morphometric measurements and fineness ratio to identify morphological patterns related to swimming performance. Four species groups were indicated by the Cluster analysis: I) R. vulpinus; II) P. corruscans; III) L. macrocephalus, L. elongatus, P. lineatus, P. maculatus, and S. brasiliensis - measurements of head height and anal fin length formed this group; and IV) P. granulosus and P. pirinampu - measurements of maximum body width and caudal peduncle width contributed to form this group. The morphometric variables that most contributed to the distinction were head length and mouth width for groups I and II. The species R. vulpinus and P. granulosus differed from other species, showing fineness ratios of 9.4 and 3.7, respectively. Leporinus elongatus, L. macrocephalus, P. lineatus, P. maculatus, and S. brasiliensis showed significant differences from other species, with ratios ranging from 4.57 to 5.19, indicating that these species may be better swimmers. Long-distance migratory species using the Piracema Canal to ascend upstream areas differed morphologically, morphological characteristics such as a narrow caudal peduncle and maximum body length, besides to the values of the fineness ratio, were essential to differentiate the migratory species.