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The common occurrence of dams is a major threat to riverine fish communities globally and a common remediation technique for fragmented fish communities is fishway construction. Demonstrating the effectiveness of this technique on impacted species is required to rationalise or improve management practices. This study investigates the effects of the installation of a fishway complex on the catadromous common galaxias (Galaxias maculatus) population at a catchment scale. We used electrofishing in a before-after-control-impact design to investigate the effects of the installation of a rock-ramp and a vertical-slot fishway complex on this species. This study demonstrated an increase in the relative abundance of common galaxias of 7.6–28.1 times and a shift to higher proportions of small fish in tributaries upstream of the fishway complex (as far as 90 km upstream). These findings provide strong evidence for the efficacy of the installation of fishways to increase the number of catadromous fishes in rivers where fish migrations have been blocked by dams. [ABSTRACT FROM AUTHOR]

Abstract Dams commonly restrict fish movements in large rivers but can also help curtail the spread of invasive species, such as invasive bigheaded carps (Hypophthalmichthys spp). To determine how dams in the upper Mississippi River (UMR) affect large-scale invasive and native fish migrations, we tracked American paddlefish (Polyodon spathula) and bigheaded carp across > 600 river km (rkm) and 16 navigation locks and dams (LD) of the UMR during 2 years with contrasting water levels. In 2022, a low-water year, both native paddlefish and invasive bigheaded carp had low passage rates (4% and 0.6% respectively) through LD15, a movement bottleneck being studied for invasive carp control. In contrast, flooding in 2023 led to open-river conditions across multiple dams simultaneously, allowing 53% of paddlefish and 46% of bigheaded carp detected in Pool 16 to move upstream through LD15. Bigheaded carp passed upstream through LD15 rapidly (μ = 32 rkm per day) a maximum of 381 rkm, whereas paddlefish moved an average of 9 upstream rkm per day (maximum of 337 rkm). Our results can inform managers examining trade-offs between actions that enhance native fish passage or deter movements of invasive species. This understanding is critical because current climate change models project increases in flooding events like that observed during 2023.

Dams commonly restrict fish movements in large rivers but can also help curtail the spread of invasive species, such as invasive bigheaded carps (Hypophthalmichthys spp). To determine how dams in the upper Mississippi River (UMR) affect large-scale invasive and native fish migrations, we tracked American paddlefish (Polyodon spathula) and bigheaded carp across > 600 river km (rkm) and 16 navigation locks and dams (LD) of the UMR during 2 years with contrasting water levels. In 2022, a low-water year, both native paddlefish and invasive bigheaded carp had low passage rates (4% and 0.6% respectively) through LD15, a movement bottleneck being studied for invasive carp control. In contrast, flooding in 2023 led to open-river conditions across multiple dams simultaneously, allowing 53% of paddlefish and 46% of bigheaded carp detected in Pool 16 to move upstream through LD15. Bigheaded carp passed upstream through LD15 rapidly (μ = 32 rkm per day) a maximum of 381 rkm, whereas paddlefish moved an average of 9 upstream rkm per day (maximum of 337 rkm). Our results can inform managers examining trade-offs between actions that enhance native fish passage or deter movements of invasive species. This understanding is critical because current climate change models project increases in flooding events like that observed during 2023. [ABSTRACT FROM AUTHOR]

The challenge of managing aquatic connectivity in a changing climate is exacerbated in the presence of additional anthropogenic stressors, social factors, and economic drivers. Here we discuss these issues in the context of structural and functional connectivity for aquatic biodiversity, specifically fish, in both the freshwater and marine realms. We posit that adaptive management strategies that consider shifting baselines and the socio‐ecological implications of climate change will be required to achieve management objectives. The role of renewable energy expansion, particularly hydropower, is critically examined for its impact on connectivity. We advocate for strategic spatial planning that incorporates nature‐positive solutions, ensuring climate mitigation efforts are harmonized with biodiversity conservation. We underscore the urgency of integrating robust scientific modelling with stakeholder values to define clear, adaptive management objectives. Finally, we call for innovative monitoring and predictive decision‐making tools to navigate the uncertainties inherent in a changing climate, with the goal of ensuring the resilience and sustainability of aquatic ecosystems. [ABSTRACT FROM AUTHOR]

In temperate rivers, where environmental conditions vary seasonally, many fishes migrate among summer, spawning, and winter habitats. Dams disrupt these migrations, limiting access to habitat and potentially affecting populations. Bigmouth Buffalo (Ictiobus cyprinellus) is a species of fish with at-risk populations in central Canada. The impact of dams on the extent of Bigmouth Buffalo migration and the overlap between summer and winter home ranges is unknown. Here, we assessed the migratory history of 80 Bigmouth Buffalo tagged with acoustic transmitters in the Red River (USA and Canada), a large binational waterway regulated by semi-passable dams. We sought to understand when and why Bigmouth Buffalo migrate, and how river use varies seasonally. Following more than 6 years of data collection, we found that the degree and probability of overlap between winter and summer home ranges varied by river section between barriers. Importantly, overlap was lowest in the longest continuous river section where well-defined migratory behaviours were observed. The results of this study reveal previously unknown details about Bigmouth Buffalo migration, demonstrate the consequences of river fragmentation on geographic space use, and highlight the importance of river connectivity to fish migration.

Reversing the negative impacts that anthropogenic habitat fragmentation has on animal movement is a key goal in the management of landscapes and conservation of species globally. Accurate assessment of measures to remediate habitat fragmentation, such as fish passage solutions in rivers, are imperative but are particularly challenging for territorial species, which are less likely to leave their existing home range, or populations composed of both migratory and resident individuals (i.e., partial migration). This investigation quantified the movements of translocated (captured upstream of the impediment and released downstream) and non‐translocated (captured and released downstream of the impediment) riverine brown trout (Salmo trutta L.), a species known to perform a homing movement, through a fish pass using passive integrated transponder (PIT) telemetry. A significantly higher proportion of translocated fish approached, entered, and passed (on a wider range of flows) compared to non‐translocated fish, consistent with the theory that motivation is a key driver in fish pass use. Translocated fish that entered the pass were significantly larger than those that approached but did not enter, presumably due to physiological capability. Translocated fish were a more reliable indicator of the fish passage solution effectiveness than non‐translocated fish. Our findings hence imply that many fish passage solutions globally, and potentially measures to remediate habitat fragmentation for other taxa, may have been mistakenly assessed for unmotivated animals. Studying both non‐translocated and translocated fish is recommended to provide more accurate and cost‐effective fish passage solution assessments. [ABSTRACT FROM AUTHOR]

More than a century of impoundments in the Penobscot River, Maine, USA, has contributed to population declines in migratory fish in the system. A decade of change, research, and monitoring has revealed direct and indirect ways that dams have influenced the river habitat, connectivity for migratory fish, and the food web. The removal of two main-stem dams (in 2012 and 2013) and bolstering of fish passage have been part of coordinated restoration efforts in the watershed. Integral to this undertaking was support for short- and long-term monitoring and research that included physical habitat, fish passage, and broad scale ecological assessments. Herein we discuss the seven interconnected and complex ways that dams have affected the Penobscot River ecosystem, particularly for migratory fish. These include familiar influences ascribed to dams: i) impaired access to habitat, ii) injury and mortality, and iii) delays of migration. Other ecological influences are less studied and more subtle: iv) facilitation of predation, v) community shifts, and vi) demographic shifts. Lastly, dams result in vii) a loss of ecosystem services that would otherwise be intact in an unimpounded system. We draw on both direct examples from the Penobscot River and broader information to characterize how impoundments have transformed this ecosystem for more than a century. Recent dam removals and mitigation efforts have reestablished some of these ecological functions.

Irrigation infrastructure expansion threatens the diversity of freshwater fish worldwide. Irrigation infrastructure creates migration barriers which can block access to important nursery, feeding and spawning habitat. Lao PDR is a landlocked country situated within the Lower Mekong River Basin where there is a substantial dependency on rice and fish for food, income and livelihoods. The country is experiencing an unprecedented boom in irrigation infrastructure investment, with modernisation programs being implemented in every province. Despite significant investment in infrastructure upgrades, and the potential impact on freshwater fish, little consideration has been given to fish passage solutions. In 2008, we commenced a fish passage program in Lao PDR. The intent of this case study is to outline the pivotal elements of the program of knowledge development and transfer, in the context of river connectivity and fisheries management in Lao PDR. We also highlight challenges in international research in development and lessons learned.

American shad (Alosa sapidissima) are native to the east coast of North America from the St. Johns River, Florida, to the St. Lawrence River region in Canada. Since the 1800s, dams have reduced access to spawning habitat. To assess the impact of dams, we estimated the historically accessed spawning habitat in coastal rivers (485,618 river segments with 21,113 current dams) based on (i) width, (ii) distance from seawater, and (iii) slope (to exclude natural barriers to migration) combined with local knowledge. Estimated habitat available prior to dam construction (2,752 km2) was 41% greater than current fully accessible habitat (1,639 km2). River-specific population models were developed using habitat estimates and latitudinally appropriate life history parameters (e.g., size at age, maturity, iteroparity). Estimated coast-wide annual production potential was 69.1 million spawners compared with a dammed scenario (41.8 million spawners). Even with optimistic fish passage performance assumed for all dams (even if passage is completely absent), the dam-imposed deficit was alleviated by fewer than 3 million spawners. We estimate that in rivers modeled without dams, 98,000 metric tons of marine sourced biomass and nutrients were annually delivered, 60% of which was retained through carcasses, gametes and metabolic waste. Damming is estimated to have reduced this by more than one third. Based on our results, dams represent a significant and acute constraint to the population and, with other human impacts, reduce the fishery potential and ecological services attributed to the species.

Many rivers are heavily fragmented, resulting from anthropogenic cross‐channel structures. Cost‐effective solutions are needed to restore habitat connectivity for migratory fishes, including those of conservation concern, such as the European river lamprey (Lampetra fluviatilis). Studded material is becoming increasingly used as a low‐cost retrofit solution for lamprey passage at sloping weirs, although little is known about the efficacy of the material or what stud arrangements may be most effective. This study tested whether expanding a single‐density studded tile (SDT) lane from 1 to 2‐m width increased passage success (nreleased = 133), and also compared the passage performance between a SDT lane and a dual‐density studded tile (DDT) lane (nreleased = 115) at a sloping weir, using PIT telemetry. No passage was recorded (nattempted = 89) at the 2‐m wide SDT lane, but 61.6% (npassed/attempted = 53/86) passed using DDT/SDT lane combination. However, increased passage efficiency was likely a result of high river flow (Q2.0‐Q30.6) during DDT/SDT comparison versus low (Q8.3‐Q88.5) while the 2‐m wide SDT lane was employed. There was no evidence that passage occurred using solely one stud configuration. It is, therefore, hypothesised that passage of river lamprey at weirs is more dependent on flow regime than the provision of either stud configuration. However, with 46.1% (npassed/released = 53/115) of those released during DDT/SDT comparison passing on the instrumented section (10.5% of weir face), the provision of studded tiles may aid in lamprey passage at high flows, presumably as the tiles generate a low‐velocity boundary layer that can be utilised as lamprey swim above the studs. [ABSTRACT FROM AUTHOR]

The seasonal pattern and the composition of downstream drift of fish eggs and larvae through the fish ladder of the Porto Primavera dam, located in the Upper Paraná River, Brazil, were studied. Additionally, the condition of individual larvae with regard to injuries was examined. Samples were collected monthly over three reproductive seasons at upper and lower pools at the twilight (nightfall and dawn). Ichthyoplankton densities were low, especially for eggs, and the downstream passage of fish eggs and larvae through the fish ladder was negatively correlated to water residence time of the reservoir. Highest larvae densities occurred between December and March, with highest densities in February. The ichthyoplankton abundance was similar between sites and times of day, with the predominance of partially damaged larvae (60%). Seventeen taxa were identified, especially non-migratory species. Long-distance migratory species represented less than 1% of the total. Since many migratory species have found favorable habitats for spawning and development in the tributaries of Porto Primavera Reservoir, the dispersion of larvae from these free-flowing tributaries located far from dam through reservoir may not be beneficial for their survival. At the same time, despite the low density and diversity of migratory species larvae drifting downstream from the dam, this fish ladder may have dispersive potential as long as the reservoir displays a shorter water residence time. [ABSTRACT FROM AUTHOR]

Effective fishways are required for restoring fish migrations and reversing worldwide declines in freshwater fish while making sustainable use of water resources. Mitigation of barrier effects at high-head dams and weirs is often impeded by poor fishway performance and high costs. Improved and less-costly designs are urgently needed. Our innovative tube fishway concept combines established fishways techniques with aquaculture's pumping methods and fish-behaviour insights for safe upstream fish passage. We experimented with scaled-down fishway designs using juvenile Australian bass (Percalates novemaculeata). An experimental horizontal-cylinder design successfully combined volitional-passage functions of existing fishways with non-volitional transfer using pumped water. Three key principles of fish behaviour in fishways led to design improvements: disturbed fish often seek refuge at depth; fishes' escape reactions strongly motivate swimming into flows; and curved structures can reduce delays. In nine trials of the best fishway design, 44 of 45 Australian bass passed within 50 min. cycles. The tube fishway concept offers potential for effective upstream fish passage at new and existing barriers >∼2 m high, with low construction and operation costs and capacity to operate in variable flow regimes. Further development is proceeding with larger-scale laboratory trials, an innovative pumping system and more species. [ABSTRACT FROM AUTHOR]

Catadromous European eel (Anguilla anguilla) are a critically endangered fish species due in part to in-river anthropogenic barriers (e.g., pumping stations, weirs, hydropower facilities). European legislation stipulates that safe downstream passage must be provided at hazardous intakes. Where present, gravity sluices have the potential to act as safe and low-cost downstream passage for seaward migrating silver eels at pumping station, but operational changes are required. This study used catchment-wide and fine-scale acoustic telemetry to investigate if operational changes (OC) at a pumping station (PS) with a co-located gravity sluice (GS) facilitated safe downstream passage for silver European eels. Specifically, night-time pump operations were ceased, river levels prior to sluicing were elevated and the GS was opened during key eel migration windows, i.e., at night during the new moon phase in autumn. No tagged eels passed through any pumps and the majority (2018 = 87.5%, 2020 = 88.9%) that approached the PS during OC passed downstream through the GS. Most eels approached during the first period of night sluicing after release (2018 = 73.9% and 2020 = 76.5%) and passed downstream during the first sluice event they experienced at the PS (2018 = 66.7% and 2020 = 75.0%). During the final approach prior to passage, very few retreats back upstream occurred at a median (IQR) distance of 34 (7.25) m from the GS and were predominantly a short distance (1–8 m). Overall, OC at a PS with a GS are considered a win-win-win, despite opening the sluice for <3% of the study period, given safe downstream passage was maximised, the financial benefits of sluicing water (∼£14,670 in direct operational costs over two years) and the relative ease of implementation. • No pump operation and gravity sluice opened at night during eel migration studied • No acoustic tagged (n = 62) eels passed through pumps during operational changes • Gravity sluice attraction, entrance and passage efficiencies were all high (>87.5%) • Most eels quickly passed via the sluice during new moon phase of the lunar cycle • Sluicing was easily implemented and provided considerable operational cost savings [ABSTRACT FROM AUTHOR]

Abstract Passage performance of upstream-migrating lamprey (Lampetra fluviatilis) was compared between sections of a Crump flow-gauging weir with and without studded tiles, and at a bottom-baffle fishway, on the River Derwent, England. The effects of microhydropower operation on attraction to multiple routes were also studied. Studded tiles were fixed horizontally on the weir face near the right bank, forming a 1 m wide treatment route, neighboured by a tileless control route. A bottom-baffle fishway was present at the right bank, alongside the hydropower tailrace. Two further weir-face control routes at the left bank, in combination with those on the right side, enabled comparison of lamprey attraction relative to the weir flow. Downstream and upstream ends of the right-hand weir-face routes, and of the fishway, downstream ends of the left-hand weir face routes, and the entrance of the hydropower tailrace area were instrumented with PIT antennas (n = 9 total). Of 395 PIT-tagged lamprey, released 0.52 rkm downstream of the weir on 10 separate dates in early winter 2017 (turbine on for 21/43 days of study period), 363 (91.9%) were detected by at least one of the antennas (median [IQR] minimum delay at weir: 15.0 [7.4–21.4] days). All lamprey detected at the left-bank antennas (attraction efficiency AE: 255/395 [64.6%]) were also detected elsewhere. The fishway was ineffective (AE: 343/395 [86.8%]; passage efficiency PE: 5/343 [1.5%]). While lamprey were more attracted towards the control relative to the adjacent tiled route, a higher number of fish traversed the weir using the latter (AE tiled route: 172/395 [43.5%]; PE tiled route: 44/172 [25.6%]; AE control route: 257/395 [65.1%]; PE control route: 22/257 [8.6%]). Lamprey were attracted towards the right half of the channel when the turbine was running, as only n = 88/4190 (2.1% of total attempts) detections were made at the two left-bank control antennas in the turbine-on condition, compared to 2775/13,029 (21.3%) at the same two antennas when the turbine was off. While improved passage efficiency was achieved using surface-mounted studded tiles, further in situ evaluations are needed to optimize their performance. [ABSTRACT FROM AUTHOR]

Humans and freshwater ecosystems have a long history of cohabitation. Today, nearly all major rivers of the world have an in-stream structure which changes water flow, substrate composition, vegetation, and fish assemblage composition. The realization of these effects and their subsequent impacts on population sustainability and conservation has led to a collective effort aimed to find ways to mitigate these impacts. Barrier removal has recently received greater interest as a potential solution to restore river connectivity, and reestablish high quality habitats, suitable for feeding, refuge and spawning of fish. In the present study, we present thirty years of data from electrofishing surveys obtained at two sites, both prior to and following the removal of a small-scale hydropower dam in Central Jutland, Denmark. We demonstrate that the dam removal has led to a dramatic increase in trout density, especially in young of the year. Surprisingly, we found that this increase was not just upstream of the barrier, where the ponded zone previously was, but also downstream of the barrier, despite little changes in habitat in that area. These findings suggest that barrier removal may be the soundest conservation option to reinstate fish population productivity. [ABSTRACT FROM AUTHOR]

Fisheries managers need effective methods to limit the spread of invasive round goby Neogobius melanostomus in North America. Elevating carbon dioxide (CO2) in water at pinch points of rivers (e.g., inside locks) is one approach showing potential to deter the passage of invasive fishes, such as bigheaded carps Hypophthalmichthys spp., but the effectiveness of this method to alter round goby behavior has not been determined. The goal for this study was to determine CO2 concentrations that alter round goby behavior across a range of water temperatures. Free-swimming avoidance (voluntary response) and loss of equilibrium (involuntary response) were quantified by exposing round goby to increasing CO2 concentrations at 5, 15, and 25 °C using a shuttle box choice arena and static tank. Water chemistry was measured concurrent with behavioral endpoints and showed that round goby avoided a threshold of 99-169 mg/L CO2 (79,000-178,000 μatm) and lost equilibrium at 197-280 mg/L CO2 (163,000-303,000 μatm). Approximately 50% lower CO2 concentrations were found to modify behavior at 5 °C relative to 25 °C, suggesting greater effectiveness at lower water temperatures. We conclude that CO2 modified round goby behavior and concentrations determined in this study are intended to guide field testing of CO2 as an invasive fish deterrent. [ABSTRACT FROM AUTHOR]

Network theory offers new perspective on movement data by evaluating the relationships between animal movements (links) and detection locations (nodes) in spatially complex systems, including human-altered landscapes., We applied network analyses to intra- and interspecific movement patterns in the migration behaviour and dam passage success of two anadromous fish species, Pacific lamprey Entosphenus tridentatus Gairdner and Chinook salmon Oncorhynchus tshawytscha Walbaum, when moving through a large multifishway hydroelectric project (Bonneville Dam, USA)., Network analyses revealed greater variation in movement for Pacific lamprey compared with Chinook salmon. Salmon that passed the dam had networks consisting of more direct passage routes with fewer overall movements compared with lamprey that passed the dam. Lamprey that did not pass the dam exhibited a wide range of behaviours, from approaching only one fishway site to testing all possible passage routes. Accounting for the time spent in the network improved the ability to detect biological differences in network structure for lamprey that did and did not pass the dam., The movement patterns likely resulted from different behavioural responses to complex environmental and internal factors affecting a philopatric species (Chinook salmon) vs. a non-philopatric species (Pacific lamprey) when moving through an engineered environment designed primarily for salmon., Synthesis and applications. Our case study highlights the potential for network analyses to link questions of basic movement ecology with monitoring of movement and behaviour in human-altered landscapes. Network analyses can thus serve as a valuable tool for describing movement and behaviour in the face of environmental change and assessing the effectiveness of mitigation efforts at spatially complex obstacles to animal movement. [ABSTRACT FROM AUTHOR]

River catchments worldwide are heavily fragmented by anthropogenic barriers, reducing their longitudinal connectivity and contributing to the decline of migratory fish populations. Direct impacts of individual barriers on migratory fish are well-established, but barrier impacts on onward migration are poorly understood, despite their relevance to evidence-based, catchment-scale, management of threatened species. This study investigated the upstream spawning migration of 352 acoustic tagged river lamprey (Lampetra fluviatilis), translocated upstream of two key barriers (R2: n = 60 & 59; R3: n = 59 & 52) compared to a control group (R1: n = 61 & 59), across two contrasting (dry and wet, n = 180 and 172) years in the River Yorkshire Ouse, England, to reveal the impact of barriers on the onward migration of upstream migrating fish. Release further upstream increased the degree of catchment penetration, with median distance upstream of R1 56.1% and 68.6% greater for lamprey released at R2 and R3 respectively. Median delays at the two downstream-most main river barriers by the control group were 23.8 and 5.4 days (2018/19) and 9.3 and 11.4 days (2019/20). However, impacts of delay were only observed on the time to reach spawning habitat, time to reach final assumed spawning location and speed of movement in one upper catchment tributary during 2019/20 whilst they were only observed on time to reach spawning habitat during 2018/19 and on assumed spawning location distance during 2019/20 in the other. Ultimately, limited impacts of delay at barriers on onward fish migration post-passage were observed but median catchment penetration was increased with consecutive release upstream. This study demonstrated the importance of a true understanding of barrier impacts to inform catchment-wide planning, evidence vital for management worldwide. Although the findings of this study do support the use of trap and transport as a measure to remediate barrier impacts on migration, fish passage engineering improvements or barrier removal, at structures shown to be the most inhibiting to fish migration should be considered the best and most sustainable option to improve barrier passage. [Display omitted] • The impact of two river barriers on onward fish migration compared to controls. • Anadromous river lamprey (n = 352) tracked in two contrasting years (dry and wet). • Translocation past downstream-most barriers increased catchment penetration. • Migration delays at downstream-most barriers increased during dry conditions. • Limited impacts of delay on onward fish migration post-passage were observed. [ABSTRACT FROM AUTHOR]

This study demonstrated that juvenile (glass) eels used a specific substrate (eel tiles) to circumvent a model Crump weir under an experimental setting. Upstream passage efficiency was 0 and 67% for the unmodified (no studded eel tiles on the downstream face; control) and modified (with studded eel tiles on the downstream face; treatment) set-ups, respectively, and was greater for a small (59%) compared to large (41%) stud configuration. Eels were active and motivated to ascend the weir during both control and treatment set-ups. Approach and attempt rates were elevated during the first few minutes of the treatment compared to control trials. Eels were edge-oriented under both set-ups and ascended the weir through the tiles during single burst swimming events (reaching estimated speeds of 68.5 cm·s-1). Eel tiles may provide a cost-effective solution for mitigating impacts of anthropogenic barriers to juvenile eel migration. Further research is required to determine passage efficiencies under higher flows, for a greater size range of eel, and for other migratory anguilliform fish (e.g. lamprey, Lampretra spp. and Petromyzon marinus L.). The performance of eel tiles should be validated through robust field studies. [ABSTRACT FROM AUTHOR]

This paper describes a series of experiments designed to measure the effect of exposure to a full-scale, vertical axis hydrokinetic turbine on downstream migrating juvenile Atlantic salmon ( N = 175) and upstream migrating adult American shad ( N = 208). Controlled studies were performed in a large-scale, open-channel flume, and all individuals approached the turbine under volitional control. No injuries were observed, and there was no measurable increase in mortality associated with turbine passage. Exposure to the turbine elicited behavioral responses from both species, however, with salmon passing primarily over the downrunning blades. Shad movement was impeded by the device, as indicated by fewer attempts of shorter duration and reduced distance of ascent up the flume. More work should be performed in both laboratory and field conditions to determine to what extent these effects are likely to influence free-swimming fish. [ABSTRACT FROM AUTHOR]

Construction of instream barriers, preventing fish from accessing spawning, nursery and feeding habitat, is a major issue impacting fisheries sustainability throughout the world. Since European settlement, development in the Murray-Darling Basin for irrigation and potable water supplies has led to the construction of over 10,000 barriers to fish movement. The Native Fish Strategy listed fish passage as a major driving action and was proactive in progressing cost-effective solutions to help inform large-scale rehabilitation programmes. The strategy identified a list of high-priority barriers for mitigation works based on feedback from jurisdictional agencies. Research initiatives were then implemented, with measurable outcomes, to help address key knowledge gaps. Research demonstrated that a project to restore passage to the Murray River main channel was meeting all ecological and engineering objectives. Follow-on work identified low-cost mechanisms to improve the effectiveness of existing fishways without compromising ecological functionality. The Native Fish Strategy was also explicit in addressing fish passage issues at irrigation infrastructure and wetland regulators. Work to minimise these impacts included quantifying the scale of irrigation-associated infrastructure and also optimising screen designs to be retrofitted to pump systems to prevent fish entrainment. Options to enhance lateral movement were also identified. The objective of this study is to summarise the fish passage issues progressed by the Native Fish Strategy to develop basin-wide solutions to enhance fish passage over the long term. [ABSTRACT FROM AUTHOR]

Few studies have been conducted to evaluate the effectiveness of nature-like fishways, particularly in low gradient warmwater streams with diverse fish communities. We evaluated a nature-like fishway that was installed to facilitate upstream passage at a low head dam on Indian Creek near Spencerville, Ontario, Canada. A passive integrated transponder ( PIT) array was used to quantify attraction and passage efficiency for 391 PIT tagged warmwater fish, represented by seven species. Attraction efficiency for the three most common species, common shiner ( Luxilus cornutus), creek chub ( Semotilus atromaculatus) and white sucker ( Catostomus commersonii), was 63.3%, 83.7% and 65.6%, respectively, and passage efficiencies were 5.1%, 38.4% and 25%, respectively. Creek chub were able to locate the fishway in less time than white sucker and common shiner; however, took longer to successfully pass. Manipulation of creek chub release locations was used to separate issues of attraction and passage and revealed that passage efficiency was highest (76.2%) for those released within the fishway and intermediate for those released at the entrance (42.1%). This multispecies fishway improved stream connectivity, but additional work is needed to fine tune its configuration. Similar projects that engage stakeholders in nature-like fishway construction are a promising approach for the thousands of small dams that occur on low gradient streams around the globe, but those studies should incorporate a biological evaluation to ensure that attraction and passage efficiency are optimised. [ABSTRACT FROM AUTHOR]

Abstract: Avoidance of abrupt accelerations of flow exhibited by downstream migrating fish at screens used to divert them, or at fishway entrances, can cause delay and adversely impact efficiency. The use of alternative stimuli to attract fish and mask the unwanted deterrent effects associated with velocity gradients is of interest to those working in fish passage engineering. The influence of a continuous light source on the downstream movement of brown trout (Salmo trutta) as they encountered accelerating velocities created by a constricted channel in an experimental flume under three discharge regimes was assessed. It was predicted that: (1) in the absence of a light source, behavioural responses typical of downstream moving salmonids would be elicited on encountering velocity gradients, and that these responses would be initiated at some threshold spatial velocity gradient relative to body length and (2) light would act as an attractant and mask the deterrent effects of a velocity gradient and thus reduce delay. Typical avoidance behaviours, e.g. rheotactic switches in orientation or retreating upstream before re-approaching a velocity gradient, were common. The spatial velocity gradient threshold at which a response was initiated when dark was similar (ca. 0.4cms−1 cm−1) independent of discharge. Fish responded farther upstream at a lower spatial velocity gradient threshold (ca. 0.2cms−1 cm−1) in the presence of both mechanosensory and visual cues when light. Contrary to the second prediction, downstream movement was further delayed by the addition of a light stimulus. The findings support an alternate hypothesis, that responsiveness (avoidance) can be enhanced when multimodal stimuli are presented. [Copyright &y& Elsevier]

ABSTRACT Fish passage through an experimental vertical-slot fishway was assessed at a floodplain regulator on the Mekong River in Central Laos between April and July 2009. Experiments were conducted to investigate the influence of fishway floor slope (1v:15h or 1v:7.5h) on fish passage success with a view to developing a series of optimal design criteria for the construction of vertical-slot fishways at other barriers to fish passage in the Lower Mekong Basin. A total of 14 661 fish from 73 species were captured during the experiments. Catches were dominated by riverine (white) ( n = 51; 69% of total) and floodplain (black) species ( n = 15; 20%) which represented 19 families in total. The work demonstrated that fish were actively attempting upstream passage from the Mekong River to an adjacent floodplain and displayed strong migratory behaviour during river level rises. Migratory activity was greatest during sharp rises in water level but reduced substantially when river level fell. Fish community composition varied greatly among the two fishway floor slopes and the control group. More fish species were collected from control samples, but the most fish and species were collected when the fishway was configured on a moderate hydraulic slope (1v:15h). A range of size classes were also collected from control and moderate-slope groups, but steeper-gradient catches were dominated by larger fish. This study demonstrated that vertical-slot fishways could provide passage for a biodiverse fish community where fish move laterally onto floodplains. The construction of fishways which consider the local fish ecology and hydrology may therefore represent a valuable management tool to help restore important movement pathways for tropical freshwater fish. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The effects of barriers to migration by adult salmon smolt have been well studied, but effects to migration of juvenile salmon during freshwater residency have not been evaluated thoroughly. We tested for differences in the relative abundance of juvenile Pacific salmon upstream and downstream of road-crossing culverts in 10 streams, 5 with moderate (upland) and 5 with low (wetland) slope. We compared stream width, bed slope, and substrate in natural channels and culverts; measured culvert water velocity during base flow; and obtained maximum and minimum culvert velocities using the flow time of dissolved solutes. We compared flow velocities to swimming speeds of rearing juvenile salmon to evaluate potential passage of fish through culverts. In upland streams with spawning adults, catch-per-unit-trap (CPUT) of juvenile coho salmon was often 2 to 3× greater above than below culverts. In these reaches, downstream migration of salmon fry from spawning locations may be limited by culvert flow velocities that exceed burst swimming speeds of juveniles. Where 2 upland crossing structures were replaced with culverts that replicated natural channel slopes, widths, and substrate, flow velocities decreased and differences in coho salmon CPUT were eliminated. In wetland streams with adult spawning reduced or absent above the crossing, CPUT of juveniles was greater below than above culverts where the outlet was perched. Thus, culverts can influence upstream and downstream fish movement and altered the distribution of rearing fish within streams. [ABSTRACT FROM AUTHOR]

This study of Murray cod ( Maccullochella peelii peelii) movements in a large lowland river in south-eastern Australia indicated that the species was not sedentary, but undertook complex movements that followed a seasonal pattern. While there were sedentary periods with limited home ranges and high site fidelity, Murray cod also under took larger movements for considerable portions of the year coinciding with its spawning schedule. This generally comprised movements (up to 130 km) from a home location in late winter and early spring to a new upstream position, followed by a rapid downstream migration typically back to the same river reach. Timing of movements was not synchronous amongst individuals and variation in the scale of movements was observed between individuals, fish size, original location and years. [ABSTRACT FROM AUTHOR]

Government (Fisheries and Oceans Canada) and industry (Abitibi Consolidated Company of Canada) have been working cooperatively to protect and enhance wild Atlantic salmon populations on the Exploits River, insular Newfoundland, Canada. Since the mid-1960s, enhancement programmes and construction of fish passage facilities at natural and man-made barriers in the watershed have expanded the range and increased the returning adult population from 1200 fish (circa 1960) to 22,000 fish, on average. Since 1997, attention has been paid to improving passage and survival of downstream migrating smolts and kelts at two hydroelectric facilities at Grand Falls-Windsor and Bishops Falls. At Grand Falls-Windsor a floating louver and bypass system was installed in a power canal and extensive biological, hydraulic, and modelling studies have been conducted to assess, modify, and optimize fish passage. At Bishops Falls, a retrofitted surface spill bypass system was installed in an existing spill gate and similar studies have been conducted to improve passage success. Studies have also been conducted on the upstream migrating adults at these facilities and associated fishways, using conventional and physiological telemetry, to assess tailrace attraction and residency, and the relative energy cost of upstream migration to bypass the hydro plants and fishways. This article reviews the results of these various studies to demonstrate how cooperative work has improved passage of anadromous salmon in coexistence with hydroelectric development. [ABSTRACT FROM AUTHOR]

The diel variation in boat electrofishing catches was investigated on the lower Murray River (south-eastern Australia), downstream of three low-level weirs. Fish assemblage composition did not differ among three sites, but differed substantially between day and night. Significantly more small-bodied species ( e.g. Australian smelt Retropinna semoni, flyspecked hardyhead Craterocephalus stercusmuscarum, bony herring Nematalosa erebi and Murray rainbowfish Melanotaenia fluviatilis) and some large-bodied predators ( e.g. golden perch Macquaria ambigua) were caught during daytime samples. Significantly more flatheaded gudgeon Phylipnodon grandiceps, carp gudgeon Hypseleotris spp. and perch Perca fluviatilis were collected at night. The size composition of the fish assemblage also changed over the diel cycle. Significantly smaller goldfish Carassius auratus, flyspecked hardyhead, bony herring and Australian smelt were sampled during the day, while generally smaller individuals of golden perch and the common carp Cyprinus carpio were caught at night. These findings suggest that sampling during both day and night is required to adequately characterize riverine fish assemblages in the context of fish migration studies. [ABSTRACT FROM AUTHOR]

1. The keystone piscivore northern pike Esox lucius can structure fish communities, and models predicting pike-focused connectivity will be important for management of many waters. 2. We explored the ability of pike to colonize upstream locations and modelled presence–absence in lakes based on landscape features derived from maps. An upstream connectivity model (UC model) was generated using data from 87 lakes. We validated the UC model with retrospective whole-lake experiments involving introductions ( n = 49) and extirpations (by rotenone) of pike ( n = 96), as well as with the natural distribution of pike in lakes ( n = 1365) within 26 drainage basin networks in northern Sweden. 3. The UC model predicted the incidence of pike in lakes with stream-connections with 95·4% accuracy, based mainly on a single variable, SV5max, that measures the minimum distance found between 5 m elevation intervals (= maximum stream slope) along watercourses from nearest downstream source of potential immigrants. Recolonizations of pike in rotenone lakes generated a near-identical classification tree, as in the UC model. The classification accuracy of pike presence in the external validation procedure ranged from 88·7 to 98·7% between different drainage basins. Predictions of pike absence were not as accurate, due possibly to undetected introductions, but still lead to 86·6% overall accuracy of the external validation. Most lakes lacking pike, but misclassified as having pike based on low SV5max, were isolated from downstream sources of pike by subsurface streamflow through bouldery areas (SSB). 4. Synthesis and applications. The variable SV5max provide managers with a tool for revealing the location and severity of natural dispersal barriers to pike (and logically also barriers to other species with equivalent or less dispersal capacity). Because presented models only require map-based information, and have high predictive power, they may have the potential to be of fundamental use in predicting distribution of freshwater fish. These predictions may provide the means for prioritizing in risk assessment and control programmes to combat pike invasions, as well as contribute to determining a reference state of species incidence in specific lakes. Our results also point towards a possibility that, even where stream slope is low, long-term effective barriers may be designed that mimic natural SSB. [ABSTRACT FROM AUTHOR]

Salmonid fishways have been used in many countries for non-salmonid fishes, including Australia, but generally with poor results. Trapping the entrance and exit of a 1:9 gradient salmonid fishway on the Murray River confirmed very poor passage of native fish, with <1% of the most abundant species ascending. Fifty years of fish passage monitoring showed the numbers of three native species declining by 95–100% and non-native fish becoming dominant. Fishways are now being designed for native fish and being quantitatively assessed, but daily flow management also needs to be addressed. The ecological model for passage of potamodromous fishes has changed from passing adults of a few species to one that incorporates the whole fish community, specifically: immature fish of large-bodied species that dominate numbers migrating upstream; a diverse range of movement strategies; and small-bodied species, crustaceans and low numbers of less-mobile species. [ABSTRACT FROM AUTHOR]

Migrating fish traversing velocity barriers are often forced to swim at speeds greater than their maximum sustained speed (Ums). Failure to select an appropriate swim speed under these conditions can prevent fish from successfully negotiating otherwise passable barriers. I propose a new model of a distance-maximizing strategy for fishes traversing velocity barriers, derived from the relationships between swim speed and fatigue time in both prolonged and sprint modes. The model predicts that fish will maximize traversed distance by swimming at a constant groundspeed against a range of flow velocities, and this groundspeed is equal to the negative inverse of the slope of the swim speed-fatigue time relationship for each mode. At a predictable flow velocity, they should switch from the optimal groundspeed for prolonged mode to that for sprint mode. Data from six migratory fish species (anadromous clupeids: American shad Alosa sapidissima, alewife A. pseudoharengus and blueback herring A. aestivalis; amphidromous: striped bass Morone saxatilis; and potomodromous species: walleye (previously known as Stizostedion vitrium) and white sucker Catostomus commersonii) were used to explore the ability of fish to approximate the predicted distance-maximizing behaviors, as well as the consequences of deviating from the optima. Fish volitionally sprinted up an open-channel flume against fixed flow velocities of 1.5-4.5 m s-1, providing data on swim speeds and fatigue times, as well as their groundspeeds. Only anadromous clupeids selected the appropriate distance-maximizing groundspeed at both prolonged and sprint modes. The other three species maintained groundspeeds appropriate to the prolonged mode, even when they should have switched to the sprint optima. Because of this, these species failed to maximize distance of ascent. The observed behavioral variability has important implications both for distributional limits and fishway design. [ABSTRACT FROM AUTHOR]

Abstract Migratory bull trout (Salvelinus confluentus) historically spawned in tributaries of the Clark Fork River, Montana and inhabited Lake Pend Oreille as subadult and adult fish. However, in 1952 Cabinet Gorge Dam was constructed without fish passage facilities disrupting the connectivity of this system. Since the construction of this dam, bull trout populations in upstream tributaries have been in decline. Each year adult bull trout return to the base of Cabinet Gorge Dam when most migratory bull trout begin their spawning migration. However, the origin of these fish is uncertain. We used eight microsatellite loci to compare bull trout collected at the base of Cabinet Gorge Dam to fish sampled from both above and further downstream from the dam. Our data indicate that Cabinet Gorge bull trout are most likely individuals that hatched in above-dam tributaries, reared in Lake Pend Oreille, and could not return to their natal tributaries to spawn. This suggests that the risk of outbreeding depression associated with passing adults over dams in the Clark Fork system is minimal compared to the potential genetic and demographic benefits to populations located above the dams. [ABSTRACT FROM AUTHOR]

In an attempt to develop an efficient acoustic fish fence, we have designed an infrasound source able to generate large nearfield particle acceleration. The source generates water movements by means of two symmetrical pistons in an air-filled cylinder with 21 cm bore. The pistons are driven by eccentric coupling to an electric motor, with 5 cm p.p. amplitude. The piston movements are 180° out of phase. The piston reaction forces are thus opposed, leading to vibration free operation. The submergible infrasound source is operated freely suspended in the water mass. The emitted sound frequency is 11.8 Hz. The particle acceleration is about 0.01 m s-2 at a distance of 3 m, corresponding to the threshold intensity for deterring effects of infrasound on Atlantic salmon smolts. The sound source was employed to test the effect of intense infrasound on migrating European silver eels. Fish confined in a tank displayed startle behaviour and prolonged stress reactions, telemetrically monitored as tachycardia, in response to intense infrasound. The field tests were carried out in the River Imsa. A trap that catches all the descending eels is installed near the river mouth. The trap was separated in four equal sections. During the periods with infrasound exposure, the proportion of silver eels entering the section closest to the sound source was reduced to 43% of the control value. In the section closest to the opposite river bank, infrasound increased the proportion of trapped eels to 144% of the control values. This shift of the migrating eels away from the infrasound source was highly significant. [ABSTRACT FROM AUTHOR]

More than two thirds of large rivers worldwide are fragmented, threatening freshwater biodiversity, river integrity, and the services that freshwater ecosystems provide for human populations around the globe. In an effort to alleviate the impacts of barriers, engineered solutions have been developed, though with somewhat underwhelming results. River restoration, especially dam removal, is viewed as the optimal option though seldom the go-to approach. In this study, we evaluated the effects of a large restoration project (pseudo dam removal) in River Kolding, Southern Jutland, Denmark, via a before-after-control-impact (BACI) approach. Using a large dataset of electrofishing data from 74 sites (including downstream unaffected sites, reconnected sites and upstream regulated sites), we found that habitat connectivity was restored successfully, with a large increase in young-of-the-year brown trout (Salmo trutta) at reconnected sites, reaching similar densities to downstream (non-affected) sites. We further observed a decrease in length at reconnected sites, suggesting that natural spawning and rearing habitats were successfully restored too. • We use a BACI approach to evaluate the effects of a pseudo dam removal that reconnected one of two affected tributaries. • We found an immediate increase in YOY brown trout following the restoration project. • Dam removal is an efficient method to restore river connectivity and habitat suitability. [ABSTRACT FROM AUTHOR]