Effect of total dissolved gas supersaturation on the passage behavior of silver carp (Hypophthalmichthys molitrix) and ya-fish (Schizothorax prenanti) through an experimental vertical slot fishway.

Total dissolved gas (TDG) supersaturation caused by flood discharge water poses a threat to vital activities such as migration, foraging, and evasion in fish species upstream of the Yangtze River, which may impair the ability of fish to pass through fishways during the migration period, causing poor utilization of fishways. Previous studies have shown that TDG supersaturation reduces the critical and burst swimming abilities of fish, suggesting potential adverse effects on swimming performance. However, studies focusing on the impact of TDG on fish swimming behavior in experimental vertical-slot fishways remain scarce. Therefore, in this study, silver carp (Hypophthalmichthys molitrix) and ya-fish (Schizothorax prenanti) were used as the study species, and comparative passage experiments were carried out in an experimental vertical slot fishway to systematically analyze the effects of TDG supersaturation on their passage behavior. The passage success of the silver carp was 57%, 39%, 26%, and 27% at TDG levels of 100%, 110%, 120%, and 130%, respectively. Passage success of ya-fish was 73%, 37%, 31%, and 35% at TDG concentrations of 100%, 110%, 120%, and 130%, respectively. The passage time for both species increased significantly with increasing TDG levels. Furthermore, the passage routes of silver carp changed significantly compared to the control group, whereas the passage routes of ya-fish changed insignificantly. High levels of TDG supersaturation (≥120%) also contributed to a higher mortality rate of ya-fish passing through the vertical slot fishway. The research results provide valuable data on the influence of TDG supersaturation on fish movement behavior responses in experimental vertical slot fishways, offering a reference for the design of fishways and the formulation of reservoir operation schemes.

A step forward in fishway engineering: Validation and implementation of advanced algorithms for effective stepped fishway design, modeling, and retrofitting.

Stepped fishways are the main alternative to allow fish migration in river obstacles. Their design is a multidisciplinary process, where civil engineering meets biology. This can bias the fishway design towards one discipline, which may cause low efficiencies or inadequate solutions. Likewise, it is often challenging to incorporate new discoveries into well-established design principles. To solve these problems, we have developed a novel tool named "Escalas". Escalas is a multipurpose platform for the assisted design, 1D simulation, assessment, and correction of stepped fishways. Escalas architecture allows fishway assessment during different hydraulic scenarios in the river (i.e., different water levels and discharges in the river), automatic dimensioning considering fish's physical needs, the study of any type of stepped fishway, to test solutions for malfunctioning or to assess fishway retrofitting. This is achieved by a modular variable definition during fishway design or definition, which allows multiple combinations of connections within and/or between cross-walls and independent discharge equation definition. This work aims to introduce Escalas to the research and engineering community, describe its algorithms, and show and validate its performance by its use in real and practical cases. Among others, results demonstrate how the tool can reproduce uniform and non-uniform performances on stepped fishways and allows fishway retrofitting to make hydraulic conditions compatible with fish usage during different river scenarios. Therefore, this work represents a step forward in the fishway engineering discipline by applying methods of engineering informatics and providing a technical and scientific base to make engineering decision-making more reliable and accessible as well as to incorporate new advances in fishway research into the engineering design process.

Vertical Slot Fishways: Incremental Knowledge to Define the Best Solution.

River artificial fragmentation is arguably the most imperilling threat for freshwater-dependent fish species. Fish need to be able to freely move along river networks as not only spawning grounds but also refuge and feeding areas may be spatially and temporally separated. This incapacity of free displacement may result in genetic depletion of some populations, density reduction and even community changes, which may in turn affect how meta-community balances are regulated, potentially resulting in functional resilience reduction and ecosystem processes' malfunction. Fishways are the most common and widely used method to improve connectivity for fish species. These structures allow fish to negotiate full barriers, thus reducing their connectivity impairment. Among all technical fishway types, vertical slot fishways (VSF) are considered to be the best solution, as they remain operational even with fluctuating water discharges and allow fish to negotiate each cross-wall at their desired depth. In the present study, we collected both published and original data on fish experiments within VSF, to address two questions, (1) What variables affect fish passage during experimental fishway studies? and (2) What is the best VSF configuration? We used Bayesian Generalized Mixed Models accounting for random effects of non-controlled factors, limiting inherent data dependencies, that may influence the model outcome. Results highlight that fish size, regardless of the species, is a good predictor of fishway negotiation success. Generally, multiple slot fishways with one orifice proved to be the best solution. Future work should be focused on small-sized fish to further improve the design of holistic fishways.

Multispecies fishways in a Mediterranean river: Contributions as migration corridors and compensatory habitat for fish.

River connectivity is essential for the resilience of fish assemblages and populations and is a priority goal to reach good ecological status for river systems. Increasing knowledge on the functionality of restoration tools such as fishways is relevant for future management strategies. The present two-year assessment showed clear ecological contributions of different types of multispecies fishways in the fish assemblage of a strongly modified Mediterranean-type river. Just after their implementation, early and extended use by dominant river-resident fish of both naturelike and technical fishways were observed. All fishways were used in different seasons, especially during the migratory periods by potamodromous cyprinids, suggesting a possible use as migration corridors. Fishways also may provide compensatory habitats for small and juvenile individuals throughout the annual cycles, mostly for rheophilic fish inside nature-like bypasses and for limnophilics inside technical types. Fluvial habitat characteristics and lower flow variability inside the fishways could favour their role as a fish refuge, mainly to juveniles of cyprinids, in heavily regulated rivers where large flow fluctuations occurred. Nature-like fishways could be a better option to function as a compensatory habitat for rheophilic cyprinids in Mediterranean-type Rivers, even more because their use by large nonnative limnophilics seems to be very scarce. However, technical fishways could offer the opportunity to establish control traps of some nonnative fish, which could be of interest to reduce the risk of spreading invasive fish. Therefore, fish ecology and local hydrology should drive the decision between the types to implement. The obtained information on the ecological functionality of multispecies fishways should be considered for applying successful river restorations that are demanded by water and wildlife management schemes (e.g., the European Water Framework Directive).

A Step to Smart Fishways: An Autonomous Obstruction Detection System Using Hydraulic Modeling and Sensor Networks.

Stepped fishways are structures that allow the free movement of fish in transversal obstacles in rivers. However, the lack of or incorrect maintenance may deviate them from this objective. To handle this problem, this research work presents a novel low-cost sensor network that combines fishway hydraulics with neural networks programmed in Python (Keras + TensorFlow), generating the first autonomous obstruction/malfunction detection system for stepped fishways. The system is based on a network of custom-made ultrasonic water level nodes that transmit data and alarms remotely and in real-time. Its performance was assessed in a field study case as well as offline, considering the influence of the number of sensing nodes and obstruction dimensions. Results show that the proposed system can detect malfunctions and that allows monitoring of the hydraulic performance of the fishway. Consequently, it optimizes the timing of maintenance on fishways and, thus, has the potential of automatizing and reducing the cost of these operations as well as augmenting the service of these structures. Therefore, this novel tool is a step forward to achieve smart fishway management and to increase their operability.

Evaluating the Ecological Benefits of Management Actions to Complement Environmental Flows in River Systems.

Globally, many river systems are under stress due to overconsumption of water. Governments have responded with programmes to deliver environmental water to improve environmental outcomes. Although such programmes are essential, they may not be sufficient to achieve all desired environmental outcomes. The benefits of environmental water allocation may be improved using 'complementary measures', which are non-flow-based actions, such as infrastructure works, vegetation management and pest control. The value of complementary measures is recognised globally, but their ecological benefits are rarely well understood, either because there is limited experience with their application, or the importance of context- and location-specific factors make it difficult to generalise benefits. In this study, we developed an approach to evaluate complementary measures at different levels of detail as a mechanism to aid decision-making. For systems that require a rapid, high-level evaluation, we propose a score-based multi-criteria benefit assessment module. If more ecological detail is necessary, we outline a method based on conceptual models, expert elicitation and probability assessment. These results are used to populate a cumulative benefit assessment tool. The tool evaluates the benefits of proposed measures in the wider context by including variables such as flow, dependence on ongoing maintenance and additional ecological values. We illustrate our approach through application to the Murray-Darling Basin, Australia. As many water recovery programmes mature into their evaluation phases, there is an increasing need to evaluate the ecological benefits of including complementary measures in the toolkit available to policy makers.

Low light intensities increase avoidance behaviour of diurnal fish species: implications for use of road culverts by fish.

Inadequately designed culverts can be physical barriers to fish passage if they increase the velocity of water flow in the environment, alter natural turbulence patterns or fail to provide adequate water depth. They may also act as behavioural barriers to fish passage if they affect the willingness of fish species to enter or pass through the structure due to altered ambient light conditions. To understand how reduced light intensity might affect fish behaviour in culverts, the authors performed a behavioural choice experiment quantifying the amount of time individual fish spent in dark and illuminated areas of a controlled experimental channel. They found that behavioural responses were largely reflective of the species' diel activity patterns; the diurnal species Craterocephalus stercusmuscarum and Retropinna semoni preferred illuminated regions, whereas the nocturnal/crepuscular Macquaria novemaculeata preferred the darkened region of the channel. Bidyanus bidyanus were strongly rheotactic, and their behaviour was influenced more by water flow direction than ambient light level. The authors then determined that a threshold light intensity of only c. 100-200 lx (cf. midday sunlight c. 100,000 lx) was required to overcome the behavioural barrier in c. 70% of the diurnally active C. stercusmuscarum and R. semoni tested. When these values were placed into an environmental context, 15 road-crossing (3.4-7.0 m long) box (c. 1 m × 1 m, height × width) and pipe (c. 1 m diameter) culverts sampled in Brisbane, Australia, recorded light intensities in the centre of the structure that were below the threshold for C. stercusmuscarum and R. semoni movement and could potentially be a barrier to their passage through the structure. Attention is required to better understand the impacts of low light intensity in culverts on fish passage and to prioritize restoration.

Schizothorax prenanti swimming behavior in response to different flow patterns in vertical slot fishways with different slot positions.

Fish swimming behavior is a critical parameter for the design of vertical slot fishways. Although Schizothorax prenanti is a significant reproductive potamodromous migratory fish species in the upper reaches of the rivers in Southwest China, its swimming behavior has not been extensively researched. Therefore, in this study, a comparative experiment was conducted in vertical slot fishways to analyze the behavior of Schizothorax prenanti in response to different flow patterns, with respect to different slot positions. The experimental model was designed with a length scale of 1:4, and a method for selecting the appropriate fish size in the scaled physical model was proposed. Based on these experiments, it was found that the typical upstream trajectories of Schizothorax prenanti are traceable to the sidewall of the pool, which are characterized by low velocity and turbulent kinetic energy (TKE). The hydraulic variables exhibited an asymmetric distribution within the vertical slot, and the fish were found to pass through the area with the lowest velocity and TKE. A flow pattern with a guide wall length-to-pool width ratio of P/B = 0.25, in which Schizothorax prenanti can immediately find the sidewall, is suitable for fish migration. Therefore, this is the recommended value for the construction of effective vertical-slot fishway structures.

Upstream dam impacts on gilded catfish Brachyplatystoma rousseauxii (Siluriformes: Pimelodidae) in the Bolivian Amazon

This paper explores the effects of two run-of-river dams (Jirau and Santo Antônio) built in cascade in the middle Madeira River (Brazil) on the interruption of long-distance migration routes of the gilded catfish (Brachyplatystoma rousseauxii). A participative monitoring system was set up to compare capture by commercial fishers approximately 1500 km upstream of the dams in Bolivia, before (1998-2007) and after (2015-2017) dam closure. A significant decrease in gilded catfish catches and in catch per unit effort was observed after dam closure, whereas no significant difference in mean weight was found. Back-estimation of age suggests that the few individuals remaining after dam closure in 2009 are a mixture of old homers that returned upstream before dam closure, and residents hatched after dam closure and trapped in the upper Madeira. Unless fishways in the Madeira River dams improve their efficiency, the gilded catfish might become rare and in danger of regional extinction in the upper Madeira basin in the next few years, negatively affecting river fisheries.(AU)

Este estudio explora el impacto de las represas en cascada Jirau y Santo Antônio en el curso medio del río Madeira (Brasil) sobre la interrupción de la ruta de migración del bagre dorado (Brachyplatystoma rousseauxii). Se estableció un sistema de monitoreo participativo para comparar la captura de dorado por pescadores comerciales en Bolivia, aproximadamente 1 500 km río arriba de las represas, antes (1998-2007) y después (2015-2017) de su cierre. Se observó una disminución significativa en las capturas y en la captura por unidad de esfuerzo de dorado después del cierre de ambas represas. No hubo diferencias significativas en el peso promedio antes y después del cierre. La estimación de la edad de los pocos individuos capturados arriba de las represas sugiere que se trata de una mezcla de individuos adultos que han podido pasar las represas antes de su cierre en 2009, y residentes nacidos después del cierre de las represas, atrapados en el alto Madeira. Si los sistemas de traspaso de peces implementados en las represas del rio Madeira no mejoran su eficiencia, el dorado podría volverse escaso y en peligro de extinción regional en la cuenca alta del Madeira en los próximos años, afectando negativamente la pesca de esta especie.(AU)