Paulo Branco

Introduction: The Guadiana basin harbours one of the most threatened freshwater fish faunas in Europe, concentrating a high number of Iberian endemisms and species of high conservation interest. Among these, the Iberian minnowcarp (Anaecypris hispanica) and the Iberian Arched-Mouth Nase (Iberochondrostoma lemmingii) stand out as threatened species whose populations are in serious decline. Methodology: In March 2023, an obsolete river barrier, the Galaches weir, was removed from the Odeleite River (Guadiana basin), releasing an 8 km lotic stretch previously inaccessible to downstream fish fauna. This study aimed to evaluate the effects of weir removal on the composition and structure of the fish community, using a Before-After design with fish sampling conducted before (2022) and after (2024) weir removal. Fish sampling took place in early summer by means of electrofishing in two 150 m long river sites, one upstream and one downstream of the weir. Results: Before removal, both upstream and downstream sites were dominated by native leuciscids (A. hispanica, I. lemmingii, Squalius pyrenaicus, and Pseudochondrostoma willkommii) and cyprinids (Luciobarbus spp.), with non-natives Lepomis gibbosus and Australoheros facetus occurring in lower abundance. After removal, native species remained clearly dominant at both sites, while A. facetus was no longer detected and L. gibbosus did not increase in abundance, suggesting that barrier removal did not favour the spread of non-native fish. Species size–structure revealed the presence of multiple size classes, possibly supporting a regular recruitment of key native taxa: A. hispanica occurred almost exclusively with total lengths of 4 to 6 cm, whereas I. lemmingii and other native leuciscids displayed broader size ranges, spanning juveniles to adults, consistent with sustained reproduction and growth in the restored river stretch. Conclusions: These findings support the hypothesis that weir removal has improved the sustainability of native fish populations such as A. hispanica by improving habitat connectivity and increasing recruitment. The results highlight the importance of weir removal for the conservation of threatened species in Mediterranean river systems.

Introduction: Despite more than two decades of implementation, the European Water Framework Directive (WFD) still faces major challenges in achieving good ecological status across European water bodies. Key limitations persist in connectivity restoration, transboundary harmonization, monitoring network design, and biological assessment of complex systems such as large rivers, reducing the Directive’s capacity to provide consistent ecological diagnoses and support effective river basin management. Objective: This work had four objectives: (I) incorporate ecological status into connectivity assessments; (II) evaluate harmonization in Iberian transboundary basins; (III) optimize the national fish monitoring network through co-creation; (IV) develop a fish-based multimetric index for Portuguese large rivers. Methodology: The work combined four approaches: (1) graph-based connectivity analysis integrating the probability of achieving good ecological status to evaluate functional connectivity across European river networks; (2) cross-border comparison of ecological classifications between Portugal and Spain in shared Iberian basins; (3) optimization of the Portuguese fish monitoring network through a co-creation approach involving the national authority; (4) development of a fish-based multimetric index designed for Portuguese large rivers. Results: Integrating ecological status into connectivity analyses reduced estimated connectivity and highlighted the combined effects of fragmentation and degradation. Cross-border comparisons showed that formal harmonization does not ensure consistent ecological classification. The optimized monitoring networks improved ecological representativeness without increasing sampling effort, while co-creation ensured operational feasibility. The new fish index for large rivers captures spatial variation in ecological quality and responds to pressure gradients, addressing a recognized methodological gap. Conclusions: Improving WFD implementation requires progress across multiple complementary components rather than isolated advances. More effective river management depends on integrating ecological processes, comparable assessment outputs, representative monitoring networks, and system-specific tools. These approaches provide transferable pathways for strengthening freshwater assessment and supporting more coherent river restoration and management across Europe.

Introduction: Freshwater ecosystems are among the most threatened worldwide, with river fragmentation, primarily caused by dams and weirs, identified as a major driver of biodiversity loss. This issue is particularly acute in Europe, where more than one million instream barriers disrupt longitudinal connectivity and compromise the movement of migratory fish. Fishways are widely implemented to mitigate these impacts, yet attraction efficiency at fishway entrances remains poorly understood, especially for Iberian potamodromous cyprinids, a group facing severe conservation pressures. Objective: This study aims to investigate how hydraulic conditions and social context influence the attraction and passage behavior of Luciobarbus bocagei, a rheophilic potamodromous cyprinid endemic to the Iberian Peninsula, in an experimental Vertical slot fishway (VSF) entrance. Methodology: Experiments were conducted in a controlled flume equipped with a VSF entrance design. Two hydraulic scenarios were tested, a Low Head Drop (LD) and a High Head Drop (HD), under a constant discharge of 34 L/s. A computational fluid dynamics (CFD) model was used to characterize and compare the flow field hydrodynamics. Fish were tested individually and in groups of three to assess the role of social dynamics. The metrics collected included time to first approach, first attempt, time to first successful passage, attraction efficiency, and passage efficiency. Cox proportional hazards models were applied to evaluate treatment effects. Results: Preliminary results showed that social context influenced fish attraction behavior. In the two hydraulic scenarios, individuals tested alone tend to exhibit lower likelihoods of approaching, attempting, and successfully negotiating the fishway compared to fish in schools. Delays were also evident for attempts and successful passages, with LD_Ind performing the worst. Conclusions: These findings highlight the importance of hydraulic conditions and social behavior in shaping attraction efficiency. They underscore the need to integrate species-specific behavioral ecology into fishway design, operation, and attraction assessment, acknowledging that fish attractivity is influenced by environmental and ecological factors beyond fishway structure, particularly in Mediterranean river systems where fragmentation pressures are high and potamodromous cyprinids are at risk.

Introduction: Freshwater systems are increasingly being impacted by a plethora of pressures. Freshwater fish are thus periled, urging the need to investigate the drivers of population decrease to better counteract them, in order to provide some conservation relief to these pressured species. Methodology: To facilitate freshwater research, the Dammed Fish Project developed a series of free tools that simplify procedures and facilitate the access of correct data. Results: RivTool+ is a free software that evolved from RivTool (used in over 75 countries) and that integrates new functions and acts as a tool hub to host additional software apps. The computing engine of RivTool, that allows along the river network calculations and summarizations, is now able to be used by new tools. RivConnect—River network connectivity app that allows graph-based quantification of structural and functional connectivity, using several metrics and understanding network directionality. RivFish—App that contains the corrected, spatially and taxonomically, occurrence, at the basin and sub-basin level, of more than 600 native freshwater fish species of Europe. RivOpt—Optimization tool that allows for river network connectivity restoration optimization. RivOpt accounts for conflicting multiple objectives and is able to deal with different restoration actions for each barrier (removal, partial removal, fishway construction and retrofitting or no action). Conclusions: Dammed Fish tools facilitate research procedures and access to verified data, improving the information baseline, increasing the accuracy of results and accelerating research. Thus, it contributes to an improved understanding of the mechanisms controlling species vulnerability and contributes to their conservation.

Introduction: Fragmentation is potentially the most significant pressure affecting European rivers. With the projected number of barriers exceeding 1 million for the EU alone, the quantification of river network connectivity is of extreme practical importance. The Nature Restoration Regulation aims to restore connectivity by increasing the length of free-flowing rivers by 25,000 km across Europe. In this work, we quantify connectivity for European river basins, quantify connectivity affectations and account for ecological status while projecting potential restoration outcomes. Methodology: We used RivTool and RivConnect to manage data across European river basins and to quantify connectivity using graph-based metrics. For this, we validated natural and artificial barrier position and used European-wide datasets for navigation routes, railways and highways to account for additional connectivity impacts. Results: River connectivity in Europe is severely impaired by barrier placement and infrastructure development. The integration of ecological status shows a generalized decrease in potential connectivity. Conclusions: River connectivity quantification needs to be an integrative process and must account for natural and artificial connectivity as well as habitat quality and quantity. It is clear that single-use measures are ineffective. If we enhance connectivity without improving ecological status, the outcomes will lack effectiveness, as will the reverse solution. Basin-wide, data-diverse approaches to connectivity enhancement planning should be the norm and not the exception. Nevertheless, this is, to our knowledge, the first integrated assessment of river network connectivity of European river basins that integrates natural and artificial barriers, additional connectivity affections and ecological quality.

Introduction: Fish species represent 25% of all vertebrates across the globe and are one of the most threatened animal groups. At least 40% of the fish fauna occurs in rivers for part of their life cycle. European rivers are home to more than 600 fish species, while also being some of the most impaired and altered ecosystems. Objective: The objective was to assess the vulnerability of freshwater fish communities in European river basins. Methodology: Using RivTool and the CCM2 database, we developed the River Restoration Units (R2Us), a set of spatial units that takes into account river network functioning and allows a higher spatial discretisation than river basins. We developed RivFish, a database about the presence of native freshwater-dependent fish in 1556 Europeans river basins. For this, we collected data from 77 references and validated synonyms and scientific names for 667 species. We used the latest International Union for Conservation of Nature (IUCN) Red List assessment to define species distributions in European rivers. After intersecting with the R2U layer, we curated and validated species names and spatial occurrence using RivFish. To map the vulnerability of freshwater fish communities, we used the Habitats Directive (HD) and the IUCN datasets. These consider a distinct number of species and assess conservation status differently: the HD evaluated 165 species, while the IUCN evaluated 516 species. The HD data allowed calculating the composite indicator of Conservation Status, whereas the IUCN data enabled calculating the vulnerability index. Results: Both ana-lyses show higher richness in central Europe, particularly in the Danube basin. Spatially, both highlight southern Europe as the area where fish communities have the highest vulnerability. However, the HD analysis also indicates the Danube and the western Atlantic basins as having high vulnerability. The IUCN analysis shows the Anatolian and Mediterranean biogeographical regions as those with the highest vulnerability values. Conclusions: Southern Europe’s higher vulnerability is likely associated with restricted distribution ranges and high levels of endemicity in Mediterranean fish communities. Overall, these findings improve current knowledge and show that input data may be key to effort allocation towards the management and conservation of European freshwater fish communities.

European rivers are home to more than 600 native freshwater‐dependent fish species, with nearly half of them raising some level of conservation concern. Tackling this decline requires prioritising efforts towards the most vulnerable communities, at a spatial scale aligned with river restoration. This study investigated fish vulnerability in European sub‐catchments considering the most recent IUCN assessment and species richness. Overall, the most vulnerable fish communities are mainly found in the Mediterranean and Anatolian bioregions. Future studies should address how these vulnerability patterns can be associated with the current pressures acting on rivers to draw effective measures for freshwater fish conservation.

Anthropogenic barriers fragment Portuguese rivers, threatening endemic freshwater fish communities. This study compiled national inventories and peer-reviewed research (2002–2024) to quantify fishway implementation, evolution and typology, while evaluating fish performance from published research. One hundred fishways built between 1950 and 2024 were recorded, half of which were constructed after the implementation of the Water Framework Directive in Portugal (29 Dec 2005), tripling the annual construction rate. Fishways were found to be associated mainly with weirs (46%) and small hydropower plants (44%), with typology being dominated by the pool-type design (67%), nature-like facilities (18%), fish locks and combined systems (6% each), fish lifts (2%) and a single eel pass. Forty scientific contributions addressed fishway effectiveness; three-quarters dealt with pool-type facilities, while 12.5% and 10% focused on nature-like fishways and lifts, respectively. Experimental and field studies highlighted species-specific hydraulic preferences, the benefits of vertical slot and multislot configurations, and the potential of retrofitting fishways with macro-rugosities (i.e., fixed structural elements placed on the bottom) to improve non-salmonid fish passage. However, low attraction efficiency, limited multi-season monitoring and risks of aiding invasive species remain a concern. Research needs are proposed, including the refinement of species-specific hydrodynamic criteria, and the development of standardized efficiency metrics and of selective passage solutions, to advance fishway performance under Mediterranean hydrological constraints.