In-stream habitat availability for river dolphins in response to flow: Use of ecological integrity to manage river flows

Population decline and extinction risk of river dolphins are primarily associated with flow alteration. Previous studies predominantly highlighted maintenance of adequate flow for low water seasons when habitats contract and the risk of local extinction escalates. Although river dolphins are sensitive to reduction in river flow, no studies quantify the relationships between flow and ecology of river dolphins to mitigate the potential adverse impacts of flow alteration. We quantify the relationships between flow and the ecology of river cetaceans concerning Ganges River dolphins (GRD; Platanista gangetica gangetica) usable area availability (AWS) for the low water season at wider flows (50–575 m3/s) at finer spatial and temporal scales. This study reveals that distribution of area usable to GRD is highly regulated by the adequate flow and river attributes (velocity and depth) interactions that likely offer energetically efficient modes of locomotion to GRD, suggesting the hydro-physical environment as a major determinant of river dolphin distribution and abundance. Flow and AWS relationships indicate that the flow during the dry season negatively contributed to AWS, whereas that of pre-monsoon maximized the AWS, suggesting that modifying flow regimes does alter in-stream habitats at varying spatial scales and may influence life-history strategies. Substantial fragmentation in suitable pool availability and loss of longitudinal connectivity exhibited by dry season flow suggested a higher risk of adverse biological effects during the dry season, which may reduce population viability by reducing survivorship and reproduction failure. Owing to river dolphins’ dependence on the attribute of freshwater flow, they can be expected to be more affected by flow regulations as interactive effects. Considering the seasonal effects and changes in the availability of usable areas by flow alteration, adopting effective habitat retention plans by water-based development projects appears critical to avoid further ecological risks in aquatic species conservation. Identifying priority riverscapes for river cetaceans and prioritizing investment opportunities is an essential first step towards effective riverine cetacean conservation.

Fishways as Downstream Routes in Small Hydropower Plants: Experiences with a Potamodromous Cyprinid

Fish need to move upstream and downstream through rivers to complete their life cycles. Despite the fact that fishways are the most commonly applied solution to recover longitudinal connectivity, they are not considered viable for downstream migration. Therefore, alternative facilities are recommended to facilitate downstream migration. However, a few recent studies have disagreed with this general assumption, showing the potential for bidirectional movements. This study advances our understanding of the potential of fishways for downstream migration by studying their efficiency in a run-of-the-river hydropower plant in the Duero River (Spain). To achieve this, downstream movements of the Iberian barbel (n = 299) were monitored in a stepped fishway for two years with passive integrated transponder (PIT)-tag technology, considering the effect of fish origin and release zone. The results showed that 24.9% of barbels descended through the fishway, with the origin and release zone affecting the fishway location. In addition, downstream movements were observed throughout the whole year, except in winter. The study concludes that, under specific scenarios, fishways could act as safe alternative routes for downstream migration.

Analysis of fishways in the Middle and Lower Jinsha River Basin (China)

<p>This study was carried out in the middle and lower reaches of the Jinsha River in southwest China, which represents the upper Yangtze River. Hydraulic structures (14 cascade hydropower stations) are planned and/or constructed in this system, which is considered as largest hydropower base. We aim to summarize appropriate measures to restore the riverine continuum in the middle and lower reaches of the Jinsha River, where high-head cascade hydropower dams are located or planned.</p><p>We distributed a questionnaire to Chinese researchers in the related fields (scientists, hydropower operators and NGOs in China, n = 60). According to the responses, fishways, fish lift, fish lock, trap-and-truck system as well as fish hatcheries (artificial breeding) are recognized to ensure passing respectively preserving fish in the Jinsha River basin.</p><p>A longitudinal connectivity assessment of the study area revealed a severely disturbed continuity status. Based on the biological analyses of the demands of the target fish species and review of fish pass technologies, a vertical-slot fishway is proposed.</p><p>Considering the dam heights and the geographical conditions, it is recommended to combine the vertical-slot fishway with these alternatives to achieve a higher efficiency in passing fish as well as to recover the river continuity towards regional sustainable development.</p>

Effect of small impediments on channel morphology – intermittent rivers in Brazilian drylands.

<p>Land use on rural drylands has as its occupation process developing a hydric security and transportations infrastructure system. Dryland tropical systems present fluvial hydrological regime controlled by precipitation inputs, with few or none springs, showing ephemeral and intermittent rivers. Floodway crossings are a widespread infrastructure, on the countryside road network, to cross small creeks, especially intermittent and ephemeral rivers during the rainy seasons. Floodways are concrete or rock block structure, with or without culverts, that allows the river flow goes through or over it. They are part of a set of small longitudinal impediments, like small earth dams and check-dams, and can significantly impact the connectivity, notably by the high density of these impediments on rural areas. This research analysed the effect of floodways crossing on longitudinal connectivity of intermittent small rivers, focusing on morphological and sedimentological impacts in Brazilian Dryland. We analysed four floodways crossing with culverts installed on sandbed intermittent rivers, with upstream catchment are from 10 Km² to 130 Km². The analyses were based on rainfall data, lateral and longitudinal topographic profiles, generate by UAV surveys; and sedimentological samples of upstream and downstream of each structure. The Effective Catchment Area (ECA) was the first step to understand that several dams, and other longitudinal disconnect elements, decrease the ECA sharply, from 2 Km² to 38 Km² of the floodways analysed. Consequently, it affects the magnitude and frequency of water and especially sediment that reaches the floodway crossings. The results reveal the increase of upstream local base level, affecting 500 to 1000 meters, and coarse sediment retention, which is 1.7 to 3.6 times the standard percentage of very coarse sand and gravel. The retained sediment can be re-worked (reconnect) by extreme rainfall/discharges events, recurrence 0,22/year, and when the silting surface reaches the culvert level. The evolution of the upstream silting process is controlled not only by construction age but also by ECA spatiality and changes, and frequency and magnitude of rainfall/discharges events. The results discussion enabled developing an evolution model based on four stages: Installation, Adaptation, Coexistence, and Silting up. The Installation stage is the building process that locally deconfigures the channel morphology and, sometimes, inserts unfamiliar materials on channels. The Adaptation Stage starts with the first flow events that recreate a channel morphology but affected by the floodway, with the beginning of enhanced upstream sedimentation and downstream erosion. The Coexistence stage the disconnectivity effect is evident with the upstream sedimentation moving upstream. The downstream erosion creates a pool, expanding the floodway/riverbed height gap, and progressively increasing the vertical incision downstream. Lastly, when the sedimentation reaches the culvert level or the floodway, sediment retention decreases, and most of the transported sediment overpass the impediment. The frequency and magnitude of flow events control the time to progress through each stage, remembering the ECA analysis importance over space and time. This proposed model that still on initial development stage can help the integrative environmental management on areas impacted by widespread small longitudinal impediments.</p>

The European eel Anguilla anguillain Cyprus - Investigating the role of freshwaters for eel conservation using integrated monitoring methods

The European eel (Anguilla anguilla) is a catadromous fish species, with population trends presenting significant declines over the last four decades. These declines throughout their range have resulted in their classification as ‘critically endangered’ by the IUCN. In addition, the European Union has implemented specific legislation surrounding A. anguilla, requiring member states to develop eel management plans [The EC Eel Regulation (1100/2007)]. Aimed to facilitate increased recruitment, these regulations state >40% of historic eel biomass should be allowed safe passage between inland waters and the sea. Cyprus however, applied and were granted an exemption from this, on the basis that there are no rivers on the island of suitable habitat and flow regimes which naturally host A. anguilla (2009/310/EC). Following this decision, recent findings have suggested that historically eels were more widespread in Cyprus than previously recognised. Indeed, a study by Zogaris et al. (2012) indicated that eels are likely the island’s most widespread native species. Cyprus’ freshwater lotic systems are dominated by intermittent rivers and streams, however the natural state and fish populations of these systems are poorly understood. The freshwaters of the island are now heavily fragmented, and with an estimated 108 dams retaining water are host to one of the highest densities of dam reservoirs in Europe. These interruptions to longitudinal connectivity lead to degraded freshwater systems in the lowlands, but despite this the island does have perennial freshwaters, particularly in the western side of the island and at higher elevations. If A. anguilla are indeed present in inland Cyprus, one key deterministic factor on their survival could be access to perennial summer refugia. Here, multiple monitoring methods were applied to build knowledge on present day eel distribution in Cyprus. By increasing knowledge regarding distribution, we can re-evaluate whether conservation initiatives are in fact justified and worthwhile. In 2020 environmental DNA metabarcoding was applied, 130 samples were taken across 26 freshwater sites to provide an up-to-date snapshot of eel distribution. In addition to this, temporal trends were considered based on an island wide fish monitoring programme spanning 2009 - 2019 which predominantly used electric fishing. Overall the results suggest that A. anguilla is widespread in western lowland Cyprus; 11/26 study sites (31/130 samples) tested positive for eel using eDNA metabarcoding, while eels were captured in 61/299 surveys (355 individuals) over the 10 year fish monitoring programme. The trends in eel distribution are highly concordant across methods, although not all sites were monitored with both methods (Fig. 1). These data indicate widespread eel recruitment in lowland freshwaters, but a lack of eels at higher elevation perennial areas. Lowland perennial areas are few and far between, however results here suggest they are abundant in A. anguilla. While higher elevation areas had higher overall freshwater fish species richness suggesting good habitat quality, the lack of the migratory eel in upper reaches indicates that barriers including dam structures may be preventing access for such migratory fish species. Environmental DNA detected eels in intermittent outlet flows of dams, but only in 2/9 reservoirs surveyed. We provide evidence for present day widespread eel recruitment in Cyprus’ inland freshwaters, however highlight the need to increase connectivity to inland perennial systems in order to enhance survival of this critically endangered species at its eastern most range.

Vegetation and Geomorphic Connectivity in Mountain Fluvial Systems

Rivers are complex biophysical systems, constantly adjusting to a suite of changing governing conditions, including vegetation cover within their basins. This review seeks to: (i) highlight the crucial role that vegetation’s influence on the efficiency of clastic material fluxes (geomorphic connectivity) plays in defining mountain fluvial landscape’s behavior; and (ii) identify key challenges which hinder progress in the understanding of this subject. To this end, a selective literature review is carried out to illustrate the pervasiveness of the plants’ effects on geomorphic fluxes within channel networks (longitudinal connectivity), as well as between channels and the broader landscape (lateral connectivity). Taken together, the reviewed evidence lends support to the thesis that vegetation-connectivity linkages play a central role in regulating geomorphic behavior of mountain fluvial systems. The manuscript is concluded by a brief discussion of the need for the integration of mechanistic research into the local feedbacks between plants and sediment fluxes with basin-scale research that considers emergent phenomena.

Multislot Fishway Improves Entrance Performance and Fish Transit Time over Vertical Slots

Pool-type fishways have been increasingly tested to improve fish passage performance and minimize migration delays. Designing cost-effective fishways is essential for a trade-off between water uses and successful longitudinal connectivity restoration. The multislot fishway (MSF) concept, which operates with 30–50% lower discharge than a vertical slot fishway (VSF), was recently developed. This study assessed and compared the entrance performance (entrance time; entry efficiency) and transit times of two cyprinids for VSFs and MSFs. Four configurations, with the same structural characteristics (slope; water depth; head drop; pool width and depth), operating with different discharges (Q), were tested (VSF 1: Q = 112 L·s−1; VSF 2: Q = 80 L·s−1; MSF 1: Q = 58 L·s−1; MSF 2: Q = 37 L·s−1). Hydrodynamics characterization was performed using a numerical model. Results showed that entry efficiency was higher in MSFs than in VSFs, while entrance time and transit time were overall lower. Numerical modelling revealed that velocities were around 30% lower in MSFs, and turbulence could reach a difference of around 70% in Reynolds shear stress and 50% in turbulent kinetic energy. Overall, MSFs can be considered as a cost-effective fishway solution that can balance the trade-offs between divergent interests in water uses.

A cascade of dams affects fish spatial distributions and functional groups of local assemblages in a subtropical river

Dams reduce the longitudinal connectivity of rivers and thereby disrupt fish migration and the spatial distribution of species, impacts that remain poorly studied for some Neotropical rivers from mega-diverse basins. We investigated the spatial distribution of fish species with different trophic and movement/reproductive/size characteristics to assess how functional groups have responded to a cascade of dams on the Uruguai River in southern Brazil. Fish abundance, biomass, and species composition were evaluated at eight locations along the longitudinal gradient. The fish assemblage in the upper stretch was mainly characterized by small and medium-sized species at higher trophic levels, whereas the sites located furthest downstream displayed more medium and large-sized species, including many carnivorous species. Species with high fecundity, seasonal migrants, and catfishes with internal fertilization were common in the river´s middle and lower reaches. Detritivorous species dominated areas distant from the dams. Overall, functional diversity of local fish assemblages was greater in lower reaches. The cascade of dams has impacted the distribution of functional groups of local fish assemblages of Uruguai River. The alteration of functional groups in upper reaches of the river has potential consequences for ecosystem processes and services, such as nutrient cycling and fisheries.