Hydropower Development and Fishways: A Need for Connectivity in Rivers of the Upper Paraná Basin

Endoparasitic helminth fauna and diet of Geophagus sveni (Pisces) in Upper Paraná River basin

In freshwater ecosystems, parasite infection patterns are influenced by factors including spatial-temporal variations, host diet, and habitat. Fish often change diets, affecting their parasite communities. This study focused on non-native host fish Geophagus sveni, aiming to characterize diet and endoparasitic helminth fauna patterns in the invaded area, investigating spatial and seasonal possible differences of endoparasite infections and correlating with host diet, in São José dos Dourados River and Tietê River areas. The host fish were collected in these areas during the dry and rainy season using gillnets. The endoparasites were collected and preserved in alcohol and identified using taxonomic methods, and stomach contents were examined for diet analysis. Parasitism descriptors were calculated and evaluated spatially and seasonally by ANOVA and the Kruskal-Wallis tests. PERMANOVA assessed G. sveni diet differences, and RDA correlated the endohelminth abundance with the host diet. Two endoparasites were recorded: metacercariae of Austrodiplostomum compactum (Trematoda) and larvae and adults of Raphidascaris (Sprentascaris) lanfrediae (Nematoda). Spatial differences were observed for the mean abundance and prevalence of R. (S.) lanfrediae and A. compactum prevalence. Seasonal variations of parasitic descriptors occurred for the nematode in the Tietê River area. The detritus and aquatic insects were the most consumed items by G. sveni. Detritus consumption positively correlates with nematode abundance. The findings indicate that factors such as artificial channels and rainfall, which can influence resource availability, may affect the fish's diet and potentially influence the structure of its endoparasite community. The study emphasizes the importance of understanding trophic chain-transmitted parasites and calls for further research in Neotropical environments.

Effectiveness of New Rock-Ramp Fishway at Miyanaka Intake Dam Compared with Existing Large and Small Stair-Type Fishways

The migration of fish is influenced by the unique environmental characteristics of the destination and migratory habitat preferences. There are three fishways in Miyanaka Intake Dam. The rock-ramp fishway was newly established in 2012, creating an environment with different flow velocities and water depths. The purpose of this study was to investigate the effectiveness of the new rock-ramp fishway for native fish through two surveys. In the first survey, traps were installed during the survey period in all three fishways and all fish were caught. The run-up environment was quantified by measuring the flow velocity. In the second survey, fish were caught by spectrum methods upstream and downstream from the dam. It was found that bottom-dwelling fish and swimming fish not bound to the bottom with low migration abilities used the rock-ramp fishway for migration and as a habitat. After the new rock-ramp fishway was built, catches increased upstream from the dam. Further, the rock-ramp fishway is a potential habitat for certain species, such as Cottus pollux. As this is the first study to demonstrate the effectiveness of rock-ramp fishways, the research results are expected to be valuable to fishery managers and those planning river engineering projects.

Ecohydraulic Characteristics of a Differential Weir-Orifice Structure and Its Application to the Transition Reach of a Fishway

A transition reach is usually necessary to connect two channels with different cross-sections to facilitate a gradual variation of the water depth and a mean flow velocity profile. A modified weir–orifice structure named differential weir-orifice (abbreviated DWO) is proposed here and applied to the transition reach of a fishway. Considering the preferred flow velocity and body shape of specific migrating fish, the design guidelines of a DWO plate are firstly discussed in terms of basic hydraulics. Then, by means of hydrodynamic numerical simulation and scale model tests, the design of a non-prismatic transition reach in a proposed large fishway is studied and optimized with a DWO. The simulation results indicate that the velocity profile in conventional weirs varies in the range of 2.5 to 3.2 m/s, and the head drop of each stage is up to 0.4 m, which is not suitable for fish migration. However, the flow could be improved by adopting the DWO: the split ratio of each weir gradually increases from 13% to 40%, with weir height decreasing along the reach, which allows the water drop of each stage to decrease by 35% to 50%. This causes the velocity over the weir to reduce in the range of 1.2 to 2.1 m/s, while the velocity in the orifice flow remains limited to 0.8 to 1.3 m/s. The flow velocity profile in the DWO also presents a bimodal distribution, with different flow layers increasing the suitability for fish migration. Moreover, the DWO also performs well in decreasing the vertical turbulent intensity together with the turbulent kinetic energy, reduced by about 33% at the weir crest. The energy at the bottom orifice is only about 28% of that at the weir crest. This indicates that the turbulent kinetic energy profile could be adjusted by the DWO, part of which is transferred from the weir crest to the bottom orifice. The model test results also show that the water surface experienced only gradual variation along the DWO. The adjusting functionality of the DWO can enable the fishway to be more effective, promoting an ecohydraulic design solution for fishway structures.

Ichthyofauna in the last free-flowing river of the Lower Iguaçu basin: the importance of tributaries for conservation of endemic species

The fish fauna from the Lower Iguaçu River and tributaries upstream of the Iguaçu Falls, the last free-flowing river stretch, were investigated. Twenty five sites in tributaries and the main channel were sampled between 2010 and 2016 using several kinds of fishing gear. The species were categorized according to their size, origin, and conservation status. Species richness and abundance in the main channel and tributaries were compared. In total, 87,702 specimens were recorded, comprising 76 species, 25 families, 53 genera, and eight orders. Characiformes and Siluriformes were the richest orders, representing 92% of the total specimens; Characidae, Cichlidae, Pimelodidae, and Loricariidae were the richest families. The fish fauna was composed of small and medium-sized species and included endemic (42%), autochthonous (24%), allochthonous (21%), and exotic (9%) species, as well as hybrids (4%). Significant differences in the relative numerical abundance of species were found among sites. Ancistrusmullerae and Rhamdiabranneri (endemic) were indicator species for tributaries inside of Iguaçu National Park (INP), while Phallocerosharpagos (autochthonous) and Ictaluruspunctatus (exotic) for tributaries outside of INP and Odontesthesbonariensis (allochthonous) for the main channel. The last dam-free stretch of the Lower Iguaçu River and tributaries upstream the Iguaçu Falls exhibits a rich endemic fish fauna, including some rare, endangered species (Steindachneridionmelanodermatum, Gymnogeophagustaroba, and Psalidodongymnogenys). These findings are essential to predict and understand the effects caused by the new Baixo Iguaçu Hydroelectric Power Plant and highlight the importance of tributaries and Iguaçu National Park for conservation of endemic species.