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Abstract Conservation of fish assemblages in severely impounded rivers has often focused on providing longitudinal and lateral connectivity along the main stem. Less attention has been given to tributaries, some of which remain unimpounded. This study shows that the biodiversity of ichthyoplankton in tributaries to the largest reservoir in the Paraná River, Brazil, is similar to that of tributaries of the few remaining unimpounded stretches of the Paraná River. Annual and seasonal variability in discharge within and among tributaries attracts and supports a diverse mix of species in each spawning season. Tributaries enrich the fish biodiversity of the entire system by providing a variety of environmental conditions, access to spawning habitat, refugia for early life stages, and by promoting ecological heterogeneity. Thus, tributaries can mitigate the adverse influence of an impounded main stem. Tributaries also support populations of endangered and threatened species. A focus on tributaries offers conservation alternatives that are more feasible given the scale and large number of impoundments constructed on the main stem. Conservation in the main stem may focus on maintaining passage through dams and access to large floodplains. Conversely, in tributaries conservation may focus on managing the quality of inflows from catchments, preserving suitable instream habitats and flows, and preserving access to smaller but more numerous floodplains.

Brown trout uses river flow and thermal regimens as main stimuli for initiating and maintaining behavioral reactions such as migration and spawning. Therefore, anthropogenic alterations on these factors may have strong impacts on its populations. The aim of this work is to understand these consequences by assessing potamodromous brown trout movements in the past and present, and to model future responses. For this, brown trout movements in a fishway in the Marin River (Bidasoa basin, Northern Iberian Peninsula) have been monitored from 2008 to 2017. Random forest regression has been used to assess the influence of environmental variables on brown trout movements and to model the response under hypothetical climatic and hydrological scenarios. Results show that brown trout uses the fishway during the whole year, with more upstream movements during the spawning season. The model is able to predict accurately the timing and number of migrants. Its use under hypothetical climate change and flow regulation scenarios shows a delay in the migration time. Therefore, modelling using large time series can be a powerful tool to define management and conservation strategies and prepare compensation measures for future scenarios.

Abstract When engineers and ecologists face a fishway design, many issues need to be considered, the type of fishway being the first and foremost. It is an especially complex issue in areas with species whose migratory and swimming behavior are as yet poorly known as Mediterranean barbels. The present study focuses on the fish passage of two of the most common types of technical fishways: Vertical Slot (VS), and Submerged Notch with Bottom Orifice (SNBO). Both types were studied and compared in terms of ascent ability (as the success rate and transit time) and motivation (as the proportion of attempts and attempt rate), with Iberian barbel (Luciobarbus bocagei) as the target species. Ascent ability in VS and SNBO were similar. More than 90% of fish ascended the fishways successfully and the median transit time to ascend a total water height of 2.25 m was less than 23 min. Fish length had an effect on ascent time, with the biggest ones being faster. Motivation was greater for VS, although not seeming to have a relevant influence in the passage. These results support the use of VS and SNBO in areas with Iberian barbel and provide new data of fishways performance helping ecologists and engineers with their decision making, mainly in Mediterranean areas with similar habitats and species.