River connectivity restoration for upstream-migrating European river lamprey: The efficacy of two horizontally-mounted studded tile designs

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 ( n released = 133), and also compared the passage performance between a SDT lane and a dual-density studded tile (DDT) lane ( n released = 115) at a sloping weir, using PIT telemetry. No passage was recorded ( n attempted = 89) at the 2-m wide SDT lane, but 61.6% ( n passed/ 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% ( n passed/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.


| INTRODUCTION
River fragmentation has led to large declines in the abundance of many aquatic species (Richter, Braun, Mendelson, & Master, 1997). A major contributor to fragmentation within riverine habitats is the construction of cross-channel structures, such as dams and weirs (Rosenberg, McCully, & Pringle, 2000). These have been largely installed and maintained for societal reasons, including for hydropower generation, gauging river height, for irrigation and the creation of reservoirs for water supply to urban areas. Their presence as crosschannel structures alters river morphology and hinders the natural movement of aquatic fauna (Radinger & Wolter, 2015;Reidy-Liermann, Nilsson, Robertson, & Ng, 2012).
However, this is often not possible for societal reasons, and fishways are increasingly installed to enable fish movements whilst still maintaining the function of the structure (Silva et al., 2018). Many fishway designs are costly and vary in their effectiveness at both attracting and passing target and non-target fish species (Bunt, Castro-Santos, & Haro, 2012;Noonan, Grant, & Jackson, 2012).
Therefore, more cost-effective solutions are being explored.
Research into the use of studded and bristle substrates as a lowcost solution for fish passage, to be retrofitted to sloping weirs or installed on ramps, has increased globally (Baker & Boubee, 2006;Kerr, Karageorgopoulous, & Kemp, 2015; Montali-Ashworth, Vowles, de Almeida, & Kemp, 2020;Rooney, Wightman, O'Conchuir, & King, 2015;Tummers, Kerr, O'Brien, Kemp, & Lucas, 2018;Vowles, Don, Karageorgopoulous, & Kemp, 2017). They are designed to disturb the flow of water and to provide a physical structure in the form of studs/ bristles for fish, particularly those with anguilliform movement, to use as lateral body support and afford forward propulsion through pushingoff the studs/bristles (D'Aguiar, 2011;Rooney et al., 2015). As such, horizontally-mounted studded tiles (where tiles are mounted flat so that the studs point upwards) are being increasingly recommended as either a mitigation measure for Petromyzontiformes passage at weirs (Rooney et al., 2015;Tummers et al., 2018;Vowles et al., 2017) or for selective removal of invasive Great Lakes sea lamprey (Petromyzon marinus; Hume, Lucas, Reinhardt, Hrodey, & Wagner, 2020). Nevertheless there remains limited knowledge regarding the efficacy of studded media, including the optimal configuration, size and spacing of studs for target species. The utility of studded ramps to restore habitat connectivity for European river lamprey (Lampetra fluviatilis; hereafter referred to as river lamprey) has rarely been tested and remains poorly understood (Tummers et al., 2018;Vowles et al., 2017). River lamprey and sea lamprey are of conservation importance in several countries . In Europe, under the EU Habitats and Species Directive, these species are designated conservation features for many Natura 2000 protected areas (Special Areas of Conservation [SACs] in the United Kingdom and Ireland). Provision of adequate migration passage solutions for native migratory lampreys is, therefore, a global priority in lamprey conservation .
Field trials using single-density studded tiles (SDTs; Figure 1a) suggested they were moderately effective for passing sub-adult river lamprey at a sloping weir (passage efficiency, 25.6% ;Tummers et al., 2018), when compared to an adjacent non-tiled control section of the weir and a Larinier fishway (passage efficiency of 8.6 and 1.5%, respectively). However, for a semelparous, migratory species, as all lampreys are, this is an inadequate passage efficiency (a passage efficiency target exceeding 90% has been recommended for native diadromous fishes including lampreys; Lucas & Baras, 2001;Lucas, Bubb, Jang, Ha, & Masters, 2009). As a result, Tummers et al. (2018) recommended increasing the contiguous area, and proportion, of weir face covered by studded tiles, with the expectation that overall passage rates would be increased through (a) greater access opportunity, and/or (b) greater lateral continuity of the passage route. In comparison, observations during laboratory trials of dual-density studded tiles (DDTs; Figure 1b), originally designed to facilitate upstream European eel (Anguilla anguilla) passage when vertically-mounted (where tiles are mounted on their side with the studded surface directed sideways, often towards and against another surface such as a wall), showed a 14.1-23.9% passage efficiency for river lamprey under varying flow conditions at a model sloping weir when horizontally-mounted (Vowles et al., 2017). Although this is lower than the passage efficiency observed by Tummers et al. (2018) for SDTs, DDTs have not been tested in the field. Along with this, recent research from Hume et al. (2020) using a similar quincunx "5-dice" stud configuration in a mesocosm experiment, but with greater stud spacing for larger Great Lakes sea lamprey, demonstrated approximately 98% passage efficiency. Therefore, field-based assessment of different stud configurations, including DDTs, is needed, as there may be potential for DDTs to provide a more effective passage option for river lamprey at sloping weirs under field conditions. The aims of this study were to (a) quantify river lamprey passage after expanding a SDT lane at a sloping weir from 1 to 2-m wide as suggested by Tummers et al. (2018), and (b) compare the efficacy of two available studded tile designs (DDT and SDT) at enabling river lamprey to pass upstream of the weir by replacing a 1-m wide section of the SDT tile lane with a 1-m wide DDT lane at a sloping weir (thereby creating two adjacent lanes of different tile designs).
Our hypotheses were that (a) more river lamprey would be detected succeeding in passage as a result of increasing the width of SDT substrate available, and (b) more river lamprey would succeed in passing the weir using the DDT lane rather than the SDT lane, reflecting differences in sensitivity to alternative stud configurations.  (Foulds & Lucas, 2013;Lucas et al., 2009). River lamprey, and sea lamprey, are designated features of the Yorkshire Derwent SAC and the Humber SAC, but both areas are recorded as being F I G U R E 2 Map of the study site at Buttercrambe gauging weir. Antennas (A1-A4) are shown on the Near Wing-Wall (NWW)/dual-density studded tile (DDT) lane and the Away from Wing-Wall (AWW)/single-density studded tile (SDT) lane. The turbine intake is bounded by vertical screens to prevent entrainment of juvenile and adult river lamprey in unfavourable condition for river and sea lamprey, largely due to barriers restricting their access to suitable habitat (Birnie-Gauvin et al., 2017).
Buttercrambe gauging weir is owned by the Environment Agency and was originally built for flow-gauging, but now provides a water head for Aldby Park hydropower plant which has been active since September 2017 (see Tummers et al., 2018). Over 98% of Derwent lamprey spawning habitat is upstream of Buttercrambe (Lucas et al., 2009)

| Tile lanes
Two studded tile designs were used in this study ( Figure 1). The DDTs (identical to those described by Vowles et al., 2017;Berry and Escott Engineering, UK) measured 0.50 × 0.50 m and consisted of 48 large (spaced 55 mm on rows and 29 mm on diagonals at stud base) and 77 small (spaced 30 mm on rows and 17 mm on diagonals at stud base), 55 mm high, blunt-ended studs ( Figure 1b). The small studs occupy approximately 33% of the tile, and the large studs approximately 67%. Each stud row is offset from the previous, resulting in a stud arrangement resembling a quincunx "5-dice" configuration. The size and spacing between the DDT studs was designed to fit the observed range of wavelengths from serpentine locomotion of juvenile European eel, and so modifications to the DDTs, suggested by the environmental regulator, were carried out to adapt the tiles for the larger river lamprey adults (Tummers et al., 2018). The SDTs (identical to those described by Tummers et al., 2018; Berry and Escott Engineering, UK) were created by removing the small studs and every second row of larger studs from the DDTs. As a results, the SDTs measured 0.50 × 0.34 m, with 24 large (spaced 68 mm on rows and 88 mm on diagonals at the stud base), 55 mm high, blunt ended studs ( Figure 1a). This stud arrangement resembles a square "4-dice" configuration.  Antennas were all connected to a single reader box (Oregon RFID, Oregon) with a four-port multiplexer which was synchronised to interrogate each antenna alternately to reduce interference due to their close proximity to one another (approximately 4 reads per second per antenna). The PIT antenna array was powered by a 110 Ah 12 V leisure battery that was trickle charged from 240 V mains power via a linear supply battery charger.

| Passive integrated transponder antenna array
The PIT antennas were tested prior to river lamprey release, as well as during each site visit, by manually passing a PIT tag over the PIT antennas. The detection range was found to be approximately 0.3 m horizontal to the antenna plane (the normal orientation for tagged river lamprey swimming over the weir). Three of the four PIT antennas were operational throughout the 2018 study period. A1 suffered damage on December 19, 2018 and was subsequently not operational for the remainder of the 2018 study period (operational for 57.9% of the study period; A1 was repaired for the 2019 study period). However, the last time a river lamprey was detected on any antenna in the 2018 study period was January 2, 2019, suggesting that A1 was operational for 77.6% of the period with river lamprey movement, although there is a chance that river lamprey could have attempted passage again on A1 after this period and consequently not been detected. All PIT antennas were operational throughout the 2019 study period.

| River lamprey capture, transport and tagging
River lamprey were captured using a combination of Netlon and Apollo II type lamprey traps in the tidal Yorkshire Ouse, as a result of low catch per unit effort for river lamprey in the River Derwent (Jang & Lucas, 2005). This methodology has previously been shown not to affect subsequent post-release behaviour (Lucas et al., 2009) and Ouse/Derwent river lamprey are from the same population (Bracken, Hoelzel, Hume, & Lucas, 2015). Traps were checked weekly, and all river lamprey removed on a given day were placed in a sealed transport container (85 L bucket with clip-on lid, filled to approximately 50-60 L) with continuously aerated river water gathered from the Ouse. River lamprey were then transported to Buttercrambe (approximately 26 km by road; travel time approximately 30 min), for tagging and release. River lamprey were sedated in a solution of river water and buffered tricaine methanesulphonate (MS-222; 0.1 g/L) before being measured in length (mm) and weight (g). Individuals longer than 300 mm were selected for tagging. A HDX PIT tag (Oregon RFID, 3.65 × 32 mm, 0.8 g in air) was inserted into the body cavity via a 3-4 mm incision made on the ventral side of each river lamprey.
Incisions were not closed using either sutures or glue. Previous laboratory studies by one of the authors adopting the tagging method described above found no PIT tag loss in a sample of 60 tagged lamprey over a period of 5 months (M. Lucas, unpublished). River lamprey were then placed in a container with aerated river water until they

| Environmental data collection
Data for river discharge (m 3 /s) and river height (m) from downstream of the weir were obtained directly from Buttercrambe gauging weir.
Discharge was gauged every 15 min from an ultrasonic flow meter, and river height from an ultrasonic gauge approximately 2 m downstream from the weir. Historic daily mean discharge data were downloaded from the National River Flow Archive for Buttercrambe gauging weir for the period September 1973 to January 2020 in order to generate flow exceedance values (Q x ).

| Statistical analyses
The proportion of river lamprey attempting to pass the weir via the tiled lanes was calculated as the number of river lamprey detected on any PIT antenna divided by the total number of river lamprey released. Passage efficiency for each study year at the NWW or DDT route (2018/2019, respectively) and the AWW or SDT route (2018/2019, respectively) was calculated as the number of river lamprey that were detected on A3 or A4 divided by the number of attempting river lamprey detected on A1 or A2, respectively. For those which had completed passage of the weir and that were detected on A1/A2 before being detected on A3/A4, the time from first detection to passage (the time difference between the first detection on A1/A2 and the first detection on A3/A4) and the passage duration (the time difference between the last detection on A1/A2 and the first on A3/A4) was calculated.
The number of attempts made by a river lamprey, that was detected on A1/A2, until its first successful passage (first detection on A3/A4) was calculated. New attempts were considered to have been made if the time difference between two subsequent detections on A1/A2 was equal to or greater than 240 s. This was determined by calculating the time interval between all detections and identifying T A B L E 1 The number, length (mm) and weight (g) of river lamprey tagged per date, and the number of those tagged that were also detected attempting passage at the studded tile sections of Buttercrambe weir

| RESULTS
A total of 248 river lamprey (n 2018 = 133; n 2019 = 115) were tagged and released downstream of Buttercrambe weir (Table 1) Table 2) for the first complete passage success per river lamprey (n = 38; 15 river lamprey removed from analysis for being detected on A3/A4 before A1/A2). Lane fidelity could not be calculated for 2018 due to no river lamprey being detected on A3/A4. In 2019, the passage efficiency for those that remained in the DDT and SDT lanes were 52.0% (n A1 = 25, n A3 = 13) and 23.1% (n A2 = 13, n A4 = 3), respectively, suggesting that passage at DDT tiles and/or near to the wing-wall might be more efficient. Overall, 31 river lamprey (36.0% of the 86 that attempted) were first detected succeeding in passage on A3, and 22 (25.6% of the 86 that attempted) on A4, and these were not significantly different (Chi-square test, χ 2 1 = 1.53, p = .22). In both 2018 and 2019, significantly more passage attempts were made when the weir was drowned out (n 2018 = 260; n 2019 = 305) than when it was not (n 2018 = 151; Chi-Squared test: χ 2 1 = 28.9, p < .001; n 2019 = 6; Chi-Squared test: χ 2 1 = 287.4, p < .001; Figure 4). Eightyfive of the 86 river lamprey that were recorded attempting passage in 2019 were first detected when the weir was drowned out, and all 53 successful passages occurred when the weir was drowned out. as experienced for parts of the Experiment 1 study period in 2018, especially during the first 3 weeks), not only was the downstream weir edge completely exposed generating a vertical step up to 0.2 m high that river lamprey would have to overcome, but there was also little water flowing over the tiles themselves.

| DISCUSSION
Restoring habitat connectivity for migratory fishes is important for allowing lifecycle completion, dispersal, gene flow and contribution to natural ecosystem processes (Lucas & Baras, 2001;Reidy-Liermann et al., 2012). Extensive research and development has been carried out on the design and installation of effective fish passage solutions for economically important species, such as salmonids (Bunt et al., 2012;Noonan et al., 2012). However, management practices for those species that have been less valued (e.g., lampreys) often incorporate less costly solutions, frequently because existing conventional fishway designs are often found to be ineffective for non-target species such as lampreys (Foulds & Lucas, 2013). As shown by Tummers et al. (2018) and the present study (a combined 3 years of research), the use of the relatively cheaper horizontally-mounted studded tiles (less than 10% of the cost of a conventional engineered fishway) for attempting to re-establish river connectivity for river lamprey has, to date, been rather ineffective, with passage efficiency in both studies of much less than the 90% target for a diadromous migratory fish (Lucas & Baras, 2001). However, this does not indicate that a studded ramp passage solution for river lamprey need be ineffective if researched from a "first principles" perspective of what makes a passage route attractive and effective. Hume et al. (2020) have demonstrated that a 45 studded ramp exceeding 1 m in height could deliver a passage efficiency of 98% for Great lakes sea lamprey, suggesting that studded ramps with the right design can be effective for upstream lamprey passage.
The proportion of river lamprey released that were recorded attempting passage during this study was slightly lower than in the previous years of study at the same weir (2019:74.8%; 2018:66.9%; 2017:91.9% [Tummers et al., 2018];2014:85.8%;2013:90.1% [Tummers et al., 2016). This reduction may in part be due to some river lamprey moving downstream post-release instead of continuing their upstream migration (Foulds & Lucas, 2013), but may also be due to the reduced and different areas of the weir-fishway infrastructure instrumented with PIT antennas across all studies. River lamprey, like many fish that migrate upstream, are attracted to areas of greater flow, and so are more likely to be detected attempting passage at a co-located fishway and turbine tailrace (Dodd et al., 2018;Tummers et al., 2018). In the previous studies at the same site, the Larinier fishway ( Figure 2) was instrumented with PIT antennas, which may have attracted a greater proportion of river lamprey than only 2 m of the weir face, but was not instrumented in the present study due to its poor passage efficiency (0.3-7.1%; Tummers et al., 2016Tummers et al., , 2018. It is, therefore, likely that more lamprey than were detected in this study attempted passage via the Larinier fishway route, but their success would have been limited. However, as there were similar proportions of first detections of river lamprey on both the NWW/DDT and AWW/SDT lanes, it is unlikely that the greater attraction flow from the Larinier fishway and turbine tailrace played a role in the decision of which lane to use. The passage efficiency across the two experiments contrasted drastically. Where no river lamprey were recorded passing the weir during 2018 (although it may be that lamprey passed the weir via a non-instrumented route), 61.6% of river lamprey attempting passage in 2019 succeeded in passing the weir. This is the highest reported passage efficiency for river lamprey using horizontally-mounted studded tiles in the field (e.g., 25.6% in Tummers et al., 2018), and sug-  (Kemp, Russon, Vowles, & Lucas, 2011;Tummers et al., 2016). Despite the greater passage efficiency, tiles in the current designs still do not provide adequate passage for river lamprey, as with over 98% of Derwent river lamprey spawning habitat located upstream of Buttercrambe weir (Lucas et al., 2009), a passage success (of those attempting) of at least 90% is a necessary target (Lucas & Baras, 2001). In conjunction with the lower than ideal passage success that the tiles provide, the tiles did not appear to alleviate delays to migration, with median delays (from first detection on A1/A2 to first detection on A3/A4) of 3 days being observed. Delays to migration may increase predation pressures on migratory fish populations (Schwinn, Baktoft, Aarestrup, Lucas, & Koed, 2018), and evidence of river lamprey predation at this site in terms of river lamprey remains adjacent to PIT tags found on the river banks have been observed throughout the study periods (A Lothian, pers. obs.) Although neither SDTs nor DDTs appear to function adequately as retroactively-fitted passage solutions for river lamprey, the provisions of such engineered solutions, like studded tiles, enables some passage facility during periods of high flows. Despite only approximately 10.5% of the weir width (2 m of the 19 m wide Buttercrambe weir) being instrumented with PIT antennas, 46.1% of the released river lamprey in 2019 were detected succeeding in passing via that route, suggesting that the studded tiles might provide additional aid. We hypothesise that this is through surface roughening which produces a low-velocity layer above the tile that river lamprey can utilise while burst-swimming over the tiles (Kerr et al., 2015;Vowles et al., 2017;Watson, Goodrich, Cramp, Gordos, & Franklin, 2018). This requires a flow over the tiles deep enough to enable this behaviour, and would explain why the tiles were ineffective during the lower flow conditions of 2018. Further to this, river lamprey may be able to attach directly to the tile between the studs (if stud spacing allows) and utilise areas of further reduced velocity to rest during passage attempts (Kerr et al., 2015;Vowles et al., 2017).
It may be that the stud arrangements in the current study are limiting river lamprey to passing over the tiles and not travelling within the stud spacing. Hume et al. (2020), showed that plastic substrate with taller and wider studs, and a greater stud spacing in a quincunx "5-dice" arrangement, were highly effective (approximately 98% passage efficiency) at enabling ascent of Great Lakes sea lamprey (more similar in size to European river lamprey than European sea lamprey) when a low flow was passed over the studded material (depth of water between studs approximately 69.2 mm at a velocity approximately 0.2 m/s) which were also set at a steep angle (45 from horizontal). In the Hume et al. (2020) study, the Great Lakes sea lamprey were observed swimming within the studded matrix, potentially made possible by the wider stud spacing and alternating stud positions. Therefore, studded tiles may prove to be an effective solution for restoring habitat connectivity for river lamprey, but further research into the optimal stud arrangement and size which enables river lamprey to either swim through them or above them in a variety of flow conditions is needed. We recommend that the next avenue for research on studded tile design for river lamprey should incorporate a wider stud spacing in a quincunx "5-dice" arrangement, similar to that used by Hume et al. (2020).
In conclusion, although neither the SDT nor the DDT designs appear to be adequate for facilitating the necessary passage efficiency target (90%) for upstream migrating river lamprey, horizontallystudded tiles show promise if designed correctly, and thus more research is required to produce an optimal design considering stud size, spacing and arrangement. Currently, the SDT and DDT designs do not enable passage under low flow conditions, and therefore fail to meet legislative standards for providing adequate fish passage across a range of environmental conditions (Armstrong et al., 2010). However, in their current form, these horizontally-mounted studded tile designs may provide sufficient surface roughening when fully submerged to establish an effective, low-velocity boundary layer which river lamprey could utilise while burst swimming.