1. Many human activities in and on rivers cause the loss of freshwater biodiversity, especially fish, which now are one of the most endangered vertebrate groups. River fragmentation caused by the construction of dams is one of the main threats to fish species. In Spain, which has the highest number of dams per square kilometre in the world, more than half of all fish species are threatened by these constructions. The government has initiated the National Strategy for River Restoration, a plan to restore rivers and preserve their inhabitants, which includes the removal of dams.2. An information search and query was conducted to determine if fish monitoring was performed before and after dam removal, and the result was negative. Therefore, an assessment of the effects of dam removal on fish communities at a large spatial scale was not possible.Instead, an analysis was carried out to measure the effects of dam removal on river connectivity using a geometric network.3. The analysis of river connectivity improvement showed that 66% of removed dams had one or more dams less than 5 km away. The removal of dams increased the connected river length by an average of 6.4 km per dam removed, with the range varying between 1.04 km and 9.48 km, depending on the river basin. 4. These results show that, although monitoring programmes are strongly recommended after restoration actions, they are not usually performed. This is a wasted opportunity to gather large datasets to understand better the effects of human actions on fish communities and on rivers.5. River connectivity results may reflect a demolition strategy based more on economic and social opportunism rather than on ecological considerations. It is strongly recommended that dam removal plans should be based on ecological selection methods to achieve greater river improvements with less investment.

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Barriers to longitudinal river connectivity: review of impacts, study methods and management for Iberian fish conservationRiver connectivity is essential for the correct functioning of freshwater ecosystems at all scales. However, it has not received the necessary attention by researchers, managers and policymakers until recent years. In this review, we recap the state of knowledge in river connectivity and its applications to conservation. We describe the particular characteristics of river connectivity and summarise the effects of its interruption in different freshwater ecosystem elements. We then focus on the effects of the lack of segment connectivity in fish species and review the different methods developed to study it. The application of connectivity in freshwater fish conservation areas is also reviewed, which highlights the lack of studies on this subject. Finally, connectivity restoration is studied. The review addresses these topics in a general way and then focus on the Iberian Peninsula. The Iberian Peninsula is an interesting place to study river connectivity because it has one of the highest numbers of dams per square kilometre and a large number of endemic and endangered freshwater fish species. Despite the high number of fish species affected by water extraction and damming, river connectivity and its effect in Iberian freshwater fish populations have not been well studied. A small number of studies analyse the effect of small dams in nearby fish communities, but large-scale impact assessments are scarce. More connectivity analyses are needed to improve freshwater ecosystem conservation strategies. We conclude addressing some gaps in the knowledge of fragmentation and research opportunities in river connectivity and conservation

The signal crayfish Pacifastacus leniusculus Dana, 1852 is a successful invasive species in the Iberian Peninsula. This is of particular conservation concern, as fish fauna is highly endemic and also threatened within this ecoregion. The aim of this study was to analyze behavioral interactions and trophic overlap between signal crayfish and native fishes in Iberian rivers (northern Spain). Video cameras were used to record fish “dominance/evasion” after spatial encounters with crayfish. Diet composition and isotopic signatures (δ13C and δ15N) were compared to evaluate the niche overlap. Fish were dominant in 25% of the encounters with juvenile crayfish, whereas this percentage was only 4% with adult crayfish. Observations showed a high fish “evasion” response for Pyrenean stone loach Barbatula quignardi (Bǎcescu-Meşter, 1967) (>30%). Dietary results showed a high trophic overlap between signal crayfish with the pelagic Pyrenean minnow Phoxinus bigerri Kottelat, 2007 and the benthic loach. However, the isotopic niche overlap was low, with brown trout Salmo trutta L., 1758 showing the highest area (only 0.1 ‰2). Overall, our findings suggest that interferences may occur with native species for food (i.e., benthic invertebrates). Consequently, measures should be applied to control invasive crayfish in Iberian rivers.