Low-head Barriers Research Articles

Assessment of the viable effects of structural reconstruction of habitats in a mountain stream: A long-term study

The Carpathians are biodiversity hotspots in Europe. However, transformations resulting from human activity have affected stream catchments where forest management is carried out. Numerous anti-rubble weirs and other low-head barriers disrupt the continuity of many streams. New requirements for human interference in stream channels were formulated. Measures are urgently needed to restore the ecological continuity and mosaicism of semi-natural habitats in mountain streams. Recently, small streams have been restored in mountainous areas. To assess the viable effects of stream restoration, analyses of in-stream habitats and fish communities, which are excellent bioindicators, were performed. This study aimed to assess the effects of structural reconstruction of habitats in a mountain stream (Eastern Carpathians, Poland). The analyses were based on a long-term assessment of fish responses to barrier removal and habitat changes following stream restoration measures. The research was conducted from 2009 to 2022. Our study indicates numerous positive and viable effects of using boulder ramps as technical solutions for restoring small mountain streams. Restored ecological continuity and in-stream habitats provide fish with adequate feeding grounds, spawning grounds, and fry-growth places. On the other hand, we indicated that the assessment of the effects of mountain stream restoration should be carried out over a long-term period, in three stages after the completion of work: early, medium, and late. This may allow for the most reliable assessment of the effects of restoration measures and the early detection of the need for corrections. It is also necessary to monitor possible changes that may occur in new hydrotechnical objects, such as boulder ramps, due to natural hydro-morphodynamic processes in the stream bed.

Test of a Screw-Style Fish Lift for Introducing Migratory Fish into a Selective Fish Passage Device

Barriers are an effective mechanism for managing invasive species, such as sea lamprey in the Laurentian Great Lakes but are detrimental because they limit the migration of desirable, native species. Fish passage technologies that selectively pass desirable species while blocking undesirable species are needed. Optical sorting tools, combined with newly developed computer learning algorithms, could be used to identify invasive species from high-resolution imagery and potentially isolate them from an assortment of the Great Lakes fishes. Many existing barriers lack fishways, and optical sorting may require fish to be dewatered for image capture. The Archimedes screw, a device originating from 234 BCE, offers the potential to continuously lift fish and water over low-head barriers or into an optical sorting device. To test the efficacy of an Archimedes screw and fish lifting to capture and pass Great Lakes fishes, we built a field-scale prototype and installed it at the Cheboygan Dam, Michigan in the USA in 2021. The fish lift safely transported 704 fish (688 of which were suckers (Catostomidae)) in 11 days. The passage of the suckers through the fish lift increased with the water temperature and attraction flow. There were no observed injuries in the transported fish or mortalities in a subset of suckers held post-transport.

Noise Generation and Acoustic Impact of Free Surface Hydropower Machines: Focus on Water Wheels and Emerging Challenges.

The noise generated by free surface hydropower machines, e.g., water wheels, has led to complaints and to restrictions in their operation in urban areas. This problem generally occurs when water wheels are not well designed and are installed without expertise. Despite the relevance of the problem, and the growing interest in the use of water wheels at existing low head barriers, the acoustic impact of water wheels has not yet been properly addressed by the scientific community. Therefore, in this manuscript, the importance of the problem and the related scientific challenges are discussed, supported by case studies and theoretical considerations. A literature review on the topic is carried out, although little information is available in the scientific domain. The aim of this work is to increase the awareness on this problem, in order to stimulate future research and to suggest useful guidelines for future water wheel projects, thereby increasing the water wheel potential and reducing noise disturbance for people.

Short-Term Effects of Low-Head Barrier Removals on Fish Communities and Habitats

Barrier removal is increasingly being seen as the optimal solution to restore lotic habitat and fish communities, however, evidence of its efficacy is often limited to single sites or catchments. This study used a before–after methodology to examine the short-term (average, 541 days) effects of low-head (0.1–2.9 m) barrier removal at 22 sites distributed across Denmark and northern England on fish density, community, and river habitat responses. Following barrier removal, changes in the aquatic habitat were observed, such that the area immediately upstream of the former barrier location became shallower, with larger substrate and faster flow conditions. The reinstatement of this habitat was especially valuable in Danish streams, where these habitat features are rare, due to the naturally low gradients. Across all 22 sites fish species richness and diversity was similar before and after removal of barriers, likely because of the short study timescale (1–2 years). Across all sites combined, there was an increase in total fish density following barrier removal. A large increase in salmonid (Salmo trutta and Salmo salar) densities following barrier removal occurred at 7 out of 12 Danish sites. No similar response in salmonid density was observed at any of the UK sites which were mostly characterized by high channel gradients and short ponded zones. Two UK barrier removal sites showed marked increases in density of non-salmonid fish species. This study suggests that the removal of low-head barriers can be an effective method of restoring lotic habitats, and can lead to positive changes in fish density in the former ponded zone. The short-term effect of small barrier removal on the fish community is more variable and its effectiveness is likely to be determined by wider riverine processes.

3-D Numerical Study of a Bottom Ramp Fish Passage Using Smoothed Particle Hydrodynamics

Worldwide, the overwhelming number of man-made barriers in fluvial systems has been identified as one of the major causes of the reported staggering average declines of migratory fish. Fish passages have been shown to help mitigate such problems. Close-to-nature types of fish passages, such as bottom ramps, bypass channels, and fish ramps can be used to minimize the impact of artificial steep drops (e.g., weirs) on the migration of aquatic fauna, especially in cases of low-head barriers. This study focuses on the characterization of the flow pattern in a bottom ramp. A 3-D numerical model based on the meshless smoothed particle hydrodynamics (SPH) method was successfully validated and then employed for the simulation of turbulent free-surface flow in a straight channel with complex geometry. The effects of bed roughness, channel slope, and flow rate were quantified in terms of flow depth, velocity fields, and area‒velocity ratios. During the study, several new tools were developed, leading to new functionalities in pre-processing, solver, and post-processing which increase the applicability of DualSPHysics in the field of eco-hydraulics.

Fish passage developments for small-bodied tropical fish: field case-studies lead to technology improvements

In tropical coastal lowland rivers, there are frequently biodiverse upstream migrations of post-larval and juvenile diadromous fish from estuaries into freshwater. Tidal barriers completely block major migratory pathways for these fish and have contributed to major losses of freshwater biodiversity. In northern Australia, early efforts to improve tropical river fish passage with salmonid-style fishways completely failed. Since the mid-1990s, low gradient vertical-slot and rock fishways improved fish passage but the smallest and most abundant fish (i.e. from 10 to 100 mm long) often still failed to ascend. Since the mid-2000s, there was a paradigm shift in hydraulic design criteria for new fishways, with a renewed focus on: (i) low turbulence, (ii) maximized roughness and hydraulic boundary layers to optimise fish ascent. We used a combined methodology, firstly developing a conceptual model of fish movement to inform fishway design criteria, secondly tabulating past and present fishway design criteria, and thirdly conducting a series of brief field case-studies, at tidal barriers in tropical rivers for new technical and rock fishways. Our objective was to evaluate the success of these new designs for passage of very small (from 9 mm long) diadromous fish on low head barriers (i.e. <3 m high). We conclude that while there have been improvements in passage of small-bodied fish at tidal barriers further experimental work is still needed to test and refine current ecohydraulic fishway design criteria.

Win, win, win: Low cost baffle fish pass provides improved passage efficiency, reduced passage time and broadened passage flows over a low-head weir

The number of low-head barriers to fish migration far outweighs the number of large magnitude barriers and thus the cumulative negative impact on fish communities could also be far greater. Removal of man-made obstructions to fish migration is the most beneficial mitigation measure for reconnecting fragmented rivers but is not always possible and thus fish passes must be constructed. Given the large number of low-head barriers, cheap but effective fish passes must be identified. This study measured passage of brown trout (Salmo trutta L.) at a low-head gauging weir on Eshton Beck, England, before and after low cost baffle (LCB) fish pass construction using passive integrated transponder (PIT) telemetry. The LCB fish pass significantly improved overall passage efficiency from a maximum of 64% to 91%. There was a significant decrease in delay at the obstruction after the LCB fish pass was constructed and fish passed on a greater range of flows (0.08–5.39 m3 s−1) in comparison to before (0.56–1.92 m3 s−1). Fish ascended the fish pass through the low velocity channel (gaps in the baffles) as well as over the baffles, though a higher proportion were detected ascending over baffles at higher flows. It was therefore concluded that similar low-head structures should incorporate this style of fish pass to improve longitudinal connectivity for brown trout and other species with similar passage capabilities.

Development of a fish leaping framework for low-head barriers

Leaping is an indispensable part of the upstream spawning migration of a fish species. The natural barriers replaced with artificial dams and obstacles can obstruct the leaping process and destruct the life cycle of fish species, causing their extinction in extreme scenarios. To help design and improve the artificial barriers, many studies have been conducted to model the leaping success of fish species. However, generic results were scarcely obtained to be extended for a wide range of barriers. The main reasons can be identified as the lack of thorough understanding of the interaction between fish locomotion and water flow regime upstream of the investigated barriers. Hence, the aim of this study is to propose a leaping framework compatible with a diverse range of fish species and barriers. This framework includes a detailed hydraulic sub-model as well as locomotion model capable of tracing fish in both water and air environments. The functionality of the proposed framework is further discussed using a selected case study.

Not just for adults! Evaluating the performance of multiple fish passage designs at low-head barriers for the upstream movement of juvenile and adult trout Salmo trutta

Longitudinal connectivity in salmonid streams is vital for juvenile as well as adult fish, yet most upstream passage studies consider only larger adults. Upstream passage of juvenile and adult brown trout Salmo trutta at low-head (<3m) structures on two River Ribble tributaries (NW England) was investigated using Passive Integrated Transponder (PIT) telemetry during summer-autumn 2013 and 2014. The efficiency of a Servais low-cost baffle (LCB) fish pass was evaluated for the first time, along with two pool-weir (PW) passes, an embedded rock ramp (ERR) and an open culvert (C), the latter a man-made structure within predicted swim speed, acting as an experimental control. We used a combination of naturally migrating trout and displacement experiments. Resident fish were displaced from above to below structures, utilising their homing instinct to instigate their ascent of the structure, with up to 91% of displaced trout attempting to pass. Approximately 30% of parr morphotype trout released at their capture locations attempted to pass upstream of structures in both streams. Passage efficiencies of up to 82% for the LCB pass design were similar to the PW (up to 79%) and better than the ERR (71%), but below that for C (96%–100%). Significant differences occurred between fish passes in time to passage, and number of attempts to pass, with all but PW1 having significantly longer time to passage than the control culvert. Median time to passage at PW2 decreased almost fifty fold between 2013 and 2014, following modification to equalise step heights at the structure. Logistic regression demonstrated a strong body-length effect on passage success at passes, with 50% probability of successful passage (82–132mm) varying, but not significantly, between passes. We conclude that small trout, including juveniles, can and do exhibit functionally significant upstream movement and that greater consideration should be given of their passage needs as well as for large, adult trout.

Evaluating the effectiveness of a Larinier super active baffle fish pass for European river lamprey Lampetra fluviatilis before and after modification with wall-mounted studded tiles

To help achieve effective longitudinal river connectivity, evaluation of the efficacy of fishway use by upstream-migrating fishes is needed. Larinier super active baffle (SAB) fishways are relatively cheap retrofit fish passes, suitable for low-head barriers, widespread in Europe and the most commonly fitted technical pass in Britain. Their suitability for non-salmonids, however, is poorly quantified. The efficacy of a 15% gradient SAB fishway and effects of flow regime and water temperature were tested for European river lamprey (Lampetra fluviatilis) using passive integrated transponder (PIT) and acoustic telemetry at a Crump weir on the River Derwent, Northeast England. In migration season 2013–14, over a wide range of flows, 90.1% of 350 tagged lamprey entered the fishway. One fish (0.3%) of those that entered the pass ascended successfully, even though measured water velocity was within laboratory-measured performance conditions for this species. Of 29 acoustic-tagged lamprey that visited the weir over the same period, four (13.8%) ascended it directly, during elevated flows. These data suggest that high turbulence and/or the physical characteristics of baffles may inhibit lamprey ascent of the pass. In migration season 2014–15, we tested the effect of adding studded modular plastic tiles adjacent to the fishway wall, employing PIT antennas separately interrogating the entrance and exit of each of the main fishway and tile routes. 85.8% (169) of 197 tagged lamprey entered the fishway, of which 72/169 (42.6%) entered the tile entrance. Passage efficiency of entrants was 7.1% (12/169), all of which used the tiled route. Reduced local flow velocity in combination with increased availability of resting habitat within the tiles may have facilitated increased passage. Although fishway passage efficiency increased after placement of the modular tiles, it remained half that measured for direct weir passage, both of which are inadequate for connectivity restoration. Quantitative tests of studded tiles placed on the sloping downstream weir face by comparison to control conditions may be more effective and are needed. This study demonstrates the importance of carrying out full-scale field tests to supplement possible solutions developed under laboratory conditions.

To what extent do human-altered landscapes retain population connectivity? Historical changes in gene flow of wetland fish Pungitius pungitius.

Understanding how human-altered landscapes affect population connectivity is valuable for conservation planning. Natural connectivity among wetlands, which is maintained by floods, is disappearing owing to farmland expansion. Using genetic data, we assessed historical changes in the population connectivity of the ninespine stickleback within a human-altered wetland system. We predicted that: (i) the contemporary gene flow maintained by the artificial watercourse network may be restricted to a smaller spatial scale compared with the gene flow preceding alteration, and (ii) the contemporary gene flow is dominated by the downstream direction owing to the construction of low-head barriers. We evaluated the potential source population in both timescales. Seventeen studied populations were grouped into four genetically different clusters, and we estimated the migration rates among these clusters. Contemporary migration was restricted to between neighbouring clusters, although a directional change was not detected. Furthermore, we consistently found the same potential source cluster, from past to present, characterized by large amounts of remnant habitats connected by artificial watercourses. These findings highlight that: (i) artificial connectivity can sustain the short-distance connectivity of the ninespine stickleback, which contributes to maintaining the potential source populations; however, (ii) population connectivity throughout the landscape has been prevented by agricultural developments.

Behavioural response of larval sea lamprey (Petromyzon marinus) in a laboratory environment to potential damage-released chemical alarm cues

Sea lampreys (Petromyzon marinus L., 1758) invaded the Great Lakes early in the 20th century and have caused economic and ecological damage to native fish species. The integrated sea lamprey control program involves low-head barrier dams, lampricides, and trapping. The search for low cost and less toxic alternatives to lampricides could involve the use of repellents in the form of chemical alarm cues. The objective of this study was to determine whether larval sea lamprey showed a behavioural response when exposed to damage-released chemical alarm cues by increasing their swimming time, rate of direction changes, or rate of escape attempts in an artificial stream channel experiment. Larval sea lampreys were exposed to conspecific larval sea lamprey extract, heterospecific swordtail (Xiphophorus hellerii Heckel, 1848) extract, or a distilled water control. The larvae increased their rate of escape attempts after exposure to both swordtail and larval lamprey extracts and their rate of direction changes after exposure to sea lamprey extract. However, larvae did not increase their swimming time in response to any experimental stimuli. This is the first study to suggest that larval sea lamprey respond to potential chemosensory risk assessment cues.

Use of chemosensory cues as repellents for sea lamprey: Potential directions for population management

Sea lamprey invaded the Great Lakes in the early 20th century and caused an abrupt decline in the population densities of several native fish species. The integrated management of this invasive species is composed of chemical (lampricide) applications, low-head barrier dams, adult trapping and sterile male release. Recently, there has been an increased emphasis on the development of control methods alternative to lampricide applications. We propose as an alternative-control method the use of chemosensory cues as repellents for sea lamprey population management. Based on the available evidence at this time, we suggest that injury-released chemical alarm cues show promise as repellents for sea lamprey and further research should be directed at determining whether sea lamprey show an avoidance response to these types of chemosensory cues. From a management perspective, these chemosensory cues could be used to restrict sea lamprey access to spawning grounds. Repellents could also be used together with attractants like sex pheromones to manipulate sea lamprey behavior, similar to the “push–pull” strategies utilized with insect pests.

Effects of low-head barriers on stream fishes: taxonomic affiliations and morphological correlates of sensitive species

Low-head barriers used in the control of parasitic sea lamprey (Petromyzon marinus) in the basin of the Laurentian Great Lakes can alter the richness and composition of nontarget fishes in tributary streams. Identification of taxa sensitive to these barriers is an important step toward mitigating these effects. Upstream–downstream distributions of fishes in 24 pairs of barrier and reference streams from throughout the basin were estimated using electrofishing surveys. For 48 common species from 34 genera and 12 taxonomic families, 8–19 species, 5–16 genera, and 2–7 families showed evidence of being sensitive to barriers, with the variation in number depending on the statistical measure applied. Barriers did not differentially affect species from certain genera or families, nor did they affect species of certain body form. Therefore, taxonomic affiliation and swimming morphology are not useful for predicting sensitivity to barriers for fishes that co-occurred with sea lampreys but were not sampled adequately by our survey. Our estimates of sensitivity will help fisheries managers make sound, defensible decisions regarding the construction, modification (for fish passage), and removal of small, in-stream barriers.

A case study of habitat compensation to ameliorate impacts of hydroelectric development: effectiveness of re‐watering and habitat enhancement of an intermittent flood overflow channel

Development of the Rose Blanche River, insular Newfoundland, Canada, for hydroelectricity resulted in destruction of fluvial habitat and habitat compensation was required to achieve ‘no net loss’(NNL) of habitat productive capacity. The preferred compensation alternative involved modification of a 1·2 km long natural high flow, flood bypass channel, wetted only during peak snow melt events. The channel consisted of 99·6 units (100 m2) of habitat and was modified, with hydraulic control structures, to ensure a constant regulated flow. Physical enhancement included addition of spawning gravels, bank stabilization, protection dykes to prevent flooding and installation of low head barriers to create pools. A 3 year study (2000–2002) was undertaken to assess: (1) habitat stability in the channel, (2) re‐population of the compensatory channel, (3) biological characteristics of fishes utilizing the channel and (4) movement and migration between the compensatory channel and the main river. Study results indicated evolution of meso‐habitat characteristics due to geomorphological and hydrological factors, with distribution of spawning gravels and increased input of organic matter. The channel was utilized preferentially by brook trout Salvelinus fontinalis, with a mix of size and age classes, indicating habitat use by all life stages. Total fish biomass over 3 years increased in the compensation channel while it decreased in the river main stem. Young‐of‐the‐year density was strong for both brook trout and Atlantic salmon Salmo salar in 2001 and 2002 suggesting good spawning and incubation conditions. Year class strength was less in the main river indicating differential (improved) survival in the compensatory habitat. Tag returns provided little evidence of site fidelity for both species suggesting considerable movement within the channel and possibly between the main river and channel. Habitat compensation effectiveness, in terms of NNL, determined that fish production in the compensatory channel achieved 69, 92 and 128% of lost production in 2000, 2001 and 2002, respectively. Fish production increased each year after development and NNL was achieved by the third year. The NNL was achieved in the compensatory channel which contained 100 units as opposed to the 570 units destroyed, a 1 : 5·7 ratio. The results of the study indicated that both habitat area and a measure of habitat productive capacity need to be considered in planning and assessing habitat compensation projects.

Low-head Sea Lamprey Barrier Effects on Stream Habitat and Fish Communities in the Great Lakes Basin

Low-head barriers are used to block adult sea lamprey ( Petromyzon marinus) from upstream spawning habitat. However, these barriers may impact stream fish communities through restriction of fish movement and habitat alteration. During the summer of 1996, the fish community and habitat conditions in twenty-four stream pairs were sampled across the Great Lakes basin. Seven of these stream pairs were re-sampled in 1997. Each pair consisted of a barrier stream with a low-head barrier and a reference stream without a low-head barrier. On average, barrier streams were significantly deeper (df = 179, P = 0.0018) and wider (df = 179, P = 0.0236) than reference streams, but temperature and substrate were similar (df = 183, P = 0.9027; df = 179, P = 0.999). Barrier streams contained approximately four more fish species on average than reference streams. However, streams with lowhead barriers showed a greater upstream decline in species richness compared to reference streams with a net loss of 2.4 species. Barrier streams also showed a peak in richness directly downstream of the barriers, indicating that these barriers block fish movement upstream. Using Sørenson's similarity index (based on presence/absence), a comparison of fish community assemblages above and below low-head barriers was not significantly different than upstream and downstream sites on reference streams (n = 96, P > 0.05), implying they have relatively little effect on overall fish assemblage composition. Differences in the frequency of occurrence and abundance between barrier and reference streams was apparent for some species, suggesting their sensitivity to barriers.

Low-Head Barrier Dams Restrict the Movements of Fishes in Two Lake Ontario Streams

The Great Lakes Fishery Commission (GLFC) is considering greater use of low-head barrier dams on stream tributaries of the Laurentian Great Lakes to control populations of sea lampreys Petromyzon marinus. The impact of these barriers on nontarget fishes is not known. A mark–recapture study on four Lake Ontario streams examined movements of fishes in streams with (barrier) and without (reference) low-head barriers. A significantly lower proportion of fishes moved across a real barrier on barrier streams than across a hypothetical barrier on reference streams (0.15 versus 0.50, respectively). The impact of the barriers on movement was more pronounced in spring and fall than in summer. However, the likelihood of fishes moving versus not moving between sample segments on either side of a barrier location (but not across the barrier) did not differ significantly between barrier and reference streams. The upstream (longitudinal) decline in species richness was greater for barrier streams than for reference streams in each season. At both interspecific and intraspecific levels, mean total lengths of fish traversing real barriers were significantly greater than the mean total lengths of fish traversing hypothetical barriers. Our findings demonstrate that low-head barriers restrict the movements of some fishes and suggest this restriction affects assemblage structure above the barrier.

Evaluation of Atlantic salmon parr responses to habitat improvement structures in an experimental channel in Newfoundland, Canada

Distributional patterns and microhabitat selection of Atlantic salmon (Salmo salar) parr were investigated in relation to habitat improvement structures in a controlled flow experiment channel at Noel Paul's Brook, Newfoundland. The channel consisted of six replicates, each containing three randomly arranged treatments. Each replicate included a control treatment with no habitat modification, a mid-channel treatment with a boulder cluster and low-head barrier dam, and a stream bank treatment with undercut banks and wing deflectors. The influence of size class, density, discharge and diurnal/nocturnal differences on microhabitat selection were evaluated. Results showed that the mid-channel treatment did not serve its purpose at lower discharges (0.032–0.063 m3 s−1), and as a result was not the treatment of choice. However, as the discharge increased (0.13 m3 s−1), more salmon took up residence in this treatment. In all experiments, greater depths were selected in the stream bank treatment, and salmon parr in the mid-channel treatment consistently selected positions closer to cover. Larger parr preferred greater depths and were found closer to the improvement structures. Benthic and drifting food availability were also estimated, and results showed that ‘funnelling effects’ of the drift were created near the structures. This study indicates that these structures have the potential to create favourable feeding sites, and provide the necessary physical characteristics required by salmon parr. © 1998 John Wiley & Sons, Ltd.

Abundance of Spawning Pacific Salmon in Two Lake Superior Streams, 1981–1987

We examined, by mark and recapture procedures, the abundance of Pacific salmon entering two Lake Superior tributaries to spawn between 1981 and 1987. In 1987, abundance of pink salmon in the Carp and Pancake rivers declined to 50 and 25%, respectively, of that observed in 1981. In contrast, abundance estimates and catch records indicated that coho and chinook salmon had increased since 1981. The construction of a lowhead barrier dam on the Carp River in 1983/84 allowed passage of coho and chinook but precluded passage by pink salmon. Construction of the barrier dam did not appear to be the major cause for changes in abundance of pink salmon. Most mature pink salmon (>50% of total abundance) entered the spawning streams during a 2-week period following either onshore wind or a heavy rainfall. We submit that it is premature to suggest a lake-wide decline in pink salmon abundance.

The effect of mouthpieces on the flow of water through a submerged short pipe

To help achieve effective longitudinal river connectivity, evaluation of the efficacy of fishway use by upstream-migrating fishes is needed. Larinier super active baffle (SAB) fishways are relatively cheap retrofit fish passes, suitable for low-head barriers, widespread in Europe and the most commonly fitted technical pass in Britain. Their suitability for non-salmonids, however, is poorly quantified. The efficacy of a 15% gradient SAB fishway and effects of flow regime and water temperature were tested for European river lamprey (Lampetra fluviatilis) using passive integrated transponder (PIT) and acoustic telemetry at a Crump weir on the River Derwent, Northeast England. In migration season 2013–14, over a wide range of flows, 90.1% of 350 tagged lamprey entered the fishway. One fish (0.3%) of those that entered the pass ascended successfully, even though measured water velocity was within laboratory-measured performance conditions for this species. Of 29 acoustic-tagged lamprey that visited the weir over the same period, four (13.8%) ascended it directly, during elevated flows. These data suggest that high turbulence and/or the physical characteristics of baffles may inhibit lamprey ascent of the pass. In migration season 2014–15, we tested the effect of adding studded modular plastic tiles adjacent to the fishway wall, employing PIT antennas separately interrogating the entrance and exit of each of the main fishway and tile routes. 85.8% (169) of 197 tagged lamprey entered the fishway, of which 72/169 (42.6%) entered the tile entrance. Passage efficiency of entrants was 7.1% (12/169), all of which used the tiled route. Reduced local flow velocity in combination with increased availability of resting habitat within the tiles may have facilitated increased passage. Although fishway passage efficiency increased after placement of the modular tiles, it remained half that measured for direct weir passage, both of which are inadequate for connectivity restoration. Quantitative tests of studded tiles placed on the sloping downstream weir face by comparison to control conditions may be more effective and are needed. This study demonstrates the importance of carrying out full-scale field tests to supplement possible solutions developed under laboratory conditions.