INTRODUCTION<\/h2>\n
River environments were historically dynamic systems characterised by natural meandering and highly diverse channel morphology. However, increasing anthropogenic pressure has resulted in\u00a0up to 95\u00a0% of\u00a0aquatic ecosystems worldwide being directly affected or intensively used by humans\u00a0[1], making them some of\u00a0the\u00a0most degraded habitats in\u00a0existence [2, 3]. This transformation \u2013 including channel straightening, culverting, and the\u00a0construction of\u00a0weirs and sills \u2013 has in\u00a0many cases turned formerly diverse watercourses into uniform channels disconnected from their floodplains. At the\u00a0same time, the\u00a0structure and functioning of\u00a0aquatic ecosystems, and hence their ecological status, have deteriorated. According to the\u00a0European\u00a0Environment Agency, more than\u00a0half of\u00a0Europe\u2019s\u00a0rivers are in\u00a0poor ecological condition\u00a0[4]. The\u00a0situation is similar in\u00a0the\u00a0Czech Republic, where engineering modifications have impaired the\u00a0ecological status of\u00a0tens of\u00a0thousands of\u00a0kilometres of\u00a0watercourses\u00a0[3].<\/p>\n
Fragmentation is a\u00a0major problem affecting the\u00a0river network in\u00a0the\u00a0Czech Republic. Thousands of\u00a0transverse structures are currently registered, more than\u00a06,000 of\u00a0which exceed 1\u00a0m in\u00a0height and have no fish pass [5, 6]. These barriers fundamentally affect native ecosystems and have considerable impacts on fish communities, among other biota [5, 7]. The\u00a0loss of\u00a0suitable spawning grounds and refuges, together with habitat fragmentation, has particularly severe effects on rheophilic fish species, resulting in\u00a0reduced abundance and biomass as well as lower species diversity [8\u201310]. However, populations of\u00a0diadromous fish species, such as Atlantic salmon (Salmo salar<\/em>) and European\u00a0eel (Anguilla anguilla<\/em>), are affected most severely, as their life cycles involve reproductive migrations between freshwater rivers and the\u00a0marine environment. In\u00a0the\u00a0Czech Republic, these species are currently entirely dependent on artificial stocking.<\/p>\n Both the\u00a0Rolava and Oh\u0159e underwent extensive channel modifications in\u00a0the\u00a0past, closely associated with the\u00a0industrial development of\u00a0the\u00a0Ore Mountains. The\u00a0earliest wooden and stone sills were constructed as far back as the\u00a016th century to serve hammer mills and other mills, particularly around Nejdek. The\u00a0work of\u00a0naturalist Anton\u00edn Fri\u010d, who conducted an\u00a0extensive survey of\u00a0the\u00a0Oh\u0159e River basin, provides important evidence of\u00a0the\u00a0condition of\u00a0the\u00a0river in\u00a0the\u00a019th century. His findings indicate that, although various weirs already existed on the\u00a0Oh\u0159e at that time, the\u00a0river remained passable to migrating salmon except for structures on its lower reaches at Terez\u00edn and Doksany\u00a0[11].<\/p>\n Both rivers underwent major transformation during the\u00a01880s, at the\u00a0height of\u00a0industrialisation, when the\u00a0Oh\u0159e and its tributary, the\u00a0Rolava, began\u00a0to be systematically exploited for modern urban\u00a0infrastructure and industry. In\u00a0Karlovy Vary, the\u00a0city purchased a\u00a0mill at Tuhnice on the\u00a0Oh\u0159e in\u00a01880, commissioning its first waterworks there two years later and constructing the\u00a0Tuhnice weir. On the\u00a0Rolava, an\u00a0industrial complex was established at Ryb\u00e1\u0159e in\u00a01885 to serve the\u00a0Knoll porcelain\u00a0factory, which entailed the\u00a0construction of\u00a0further engineered sills within\u00a0the\u00a0channel. These interventions significantly reduced fish passage in\u00a0both rivers.<\/p>\n The\u00a0number, type and height of\u00a0individual transverse barriers are key factors when assessing fish passage. Brown trout (Salmo trutta<\/em>) are capable of\u00a0negotiating obstacles approximately 60 cm high, and under ideal conditions even up to 100 cm, whereas a\u00a0sill just 50 cm high represents a\u00a0virtually impassable barrier to most other native fish species [7, 12]. Smaller or benthic species, such as Eurasian\u00a0minnow (Phoxinus phoxinus<\/em>) and European\u00a0bullhead (Cottus gobio<\/em>), are even more sensitive in\u00a0this respect. For these species, even very low transverse structures of\u00a0approximately 5\u201310 cm can\u00a0restrict passage [7, 12].<\/p>\n Existing transverse barriers can be made passable in several ways, with fish passes among the most commonly used solutions. Fish passes have a long\u00a0history in\u00a0the\u00a0Czech lands: they were constructed on the\u00a0Oh\u0159e near Terez\u00edn as early as 1885, during the\u00a0time of\u00a0Anton\u00edn Fri\u010d\u00a0[13]. Their effectiveness depends on numerous factors, including how readily fish can\u00a0locate the\u00a0entrance, flow velocity within\u00a0the\u00a0pass, the\u00a0overall gradient, water depth and the\u00a0dimensions of\u00a0the\u00a0internal chambers\u00a0[7]. If these characteristics do not correspond to the\u00a0biological requirements and swimming abilities of\u00a0particular species, the\u00a0result is selective passage, whereby the\u00a0structure remains a\u00a0barrier to some of\u00a0the\u00a0fish community [7, 12]. Ensuring free fish migration should therefore be a\u00a0priority, not least in\u00a0order to meet the\u00a0objectives of\u00a0the\u00a0Water Framework Directive\u00a0[14].<\/p>\n This study aims to assess fish passage along the\u00a0Rolava and compare commonly used methodological approaches. A\u00a0further objective is to predict the\u00a0ecological effects of\u00a0restoration measures, specifically the\u00a0effects of\u00a0restoring river continuity in\u00a0the\u00a0Rolava and Oh\u0159e on species composition and biodiversity.<\/p>\n The\u00a0study area is located in\u00a0the\u00a0northern part of\u00a0the\u00a0Karlovy Vary Region and comprises the\u00a0Rolava River and the\u00a0adjoining section of\u00a0the\u00a0Oh\u0159e. The\u00a0Rolava (36.7 km long; catchment area 137.3 km\u00b2) rises in\u00a0the\u00a0Ore Mountains at an\u00a0elevation of\u00a0920.8\u00a0m above sea level. Its average gradient is approximately 1.5\u00a0%, although it exceeds 2\u00a0% in\u00a0the\u00a0middle reaches. To provide flood protection for built-up areas in\u00a0Nejdek, Nov\u00e1 Role and Karlovy Vary, the\u00a0channel was engineered and stabilised with retaining walls and embankments during the\u00a01960s and 1970s. The\u00a0steep gradient and anthropogenic modifications to the\u00a0river necessitated the\u00a0construction of\u00a0transverse structures (weirs and sills). These barriers reduce the\u00a0overall gradient but also fragment the\u00a0river network and impede longitudinal fish migration. In\u00a0Karlovy Vary, the\u00a0Rolava flows into the\u00a0Oh\u0159e at river km 171.6; the\u00a0historical significance of\u00a0the\u00a0latter is reflected in\u00a0its Celtic name, Agara, meaning \u2018salmon river\u2019.<\/p>\n According to the\u00a0classification of\u00a0A. Fri\u010d [15, 16], the\u00a0Rolava belongs to the\u00a0trout-to-grayling zone owing to its steep gradient, rapid flow and cold, highly oxygenated water. By contrast, the\u00a0Oh\u0159e represents the\u00a0barbel zone, characterised by a\u00a0gentler gradient, slower flow and higher water temperatures with naturally lower oxygen concentrations. While the\u00a0Rolava provides habitat for cold-water salmonids, the\u00a0Oh\u0159e is characterised by fish species that prefer deeper, warmer and more nutrient-rich environments, such as Barbus\u2013Chondrostoma communities<\/p>\n To determine changes in\u00a0the\u00a0fish community of\u00a0the\u00a0Rolava and the\u00a0adjoining section of\u00a0the\u00a0Oh\u0159e, the\u00a0available scientific literature and publications were reviewed. Given the\u00a0limited amount of\u00a0published information, fisheries statistics from the\u00a0Czech Anglers Union (catch records and stocking records) for the\u00a0Rolava 1A, Rolava 1B, Rolava 2, Oh\u0159e 13 and Oh\u0159e 14 fishing grounds were also included in\u00a0the\u00a0assessment. Although these data have certain\u00a0limitations, they can\u00a0provide an\u00a0important basis for interpreting long-term changes in\u00a0the\u00a0fish community.<\/p>\n As historical sources and statistics generally lack precise data on population abundance or size structure, these records were evaluated qualitatively in\u00a0terms of\u00a0species presence or absence. This provided a\u00a0basic comparative framework for assessing long-term changes in\u00a0species diversity within\u00a0the\u00a0study area.<\/p>\n The current status of the fish community was determined by field monitoring focused on the lower reaches of the Rolava, from Nov\u00e1 Role to its confluence with the Oh\u0159e in Karlovy Vary. Monitoring also covered the immediate\u00a0vicinity of\u00a0the\u00a0confluence, which is particularly important in\u00a0terms of\u00a0fish migration. Surveys were conducted at eight sites on 18\u201319\u00a0May 2022, 27\u00a0June 2023, 17\u201318\u00a0April 2024 and 20\u00a0May 2025. Six sites were located on the\u00a0Rolava (a\u00a0100\u00a0m section downstream of\u00a0each transverse barrier), with a\u00a0further two 100\u00a0m sites on the\u00a0Oh\u0159e in\u00a0the\u00a0vicinity of\u00a0the\u00a0confluence (Fig.\u00a01<\/em>).<\/p>\n Fish were sampled by electrofishing in\u00a0accordance with applicable Czech legislation. Continuous wading electrofishing, as described in\u00a0detail in\u00a0[2], was used for sampling. Fish were captured using an\u00a0ELT 60-IIH petrol-powered backpack electrofishing unit (300\u2013500 V, pulsed DC; Hans Grassl GmbH, Sch\u00f6nau am K\u00f6nigssee), equipped with a\u00a02\u00a0m pole with a\u00a0circular anode 30 cm in\u00a0diameter and a\u00a0mesh size of\u00a04\u00a0mm. Each section was fished twice consecutively, with the\u00a0transverse structure in\u00a0the\u00a0watercourse serving as the\u00a0boundary at which sampling ended. All captured fish were subsequently identified to species. Standard length (SL) and total length (TL) were measured for each individual to the\u00a0nearest 1\u00a0mm.<\/span><\/p>\n Several methodological approaches were used to provide a comprehensive assessment of fish passage along the Rolava. The initial assessment was\u00a0based on data from the\u00a0Passportization of\u00a0watercourses, followed by biological monitoring using radiotelemetry. Finally, data from fish sampling conducted downstream of\u00a0each barrier were used as an\u00a0additional basis for assessing fish passage.<\/p>\n The\u00a0first stage of\u00a0the\u00a0analysis used data from the\u00a0mapping of\u00a0watercourses and wetlands contained in\u00a0the\u00a0Passportization of\u00a0watercourses\u00a0[17], which records the\u00a0height of\u00a0each barrier and whether a\u00a0fish pass is present or absent. The\u00a0barriers between the\u00a0confluence with the\u00a0Oh\u0159e and Nov\u00e1 Role were subsequently also assessed directly by the\u00a0assessors.<\/p>\n In\u00a0the\u00a0next stage, biological monitoring of\u00a0fish migration using radiotelemetry was conducted from 30\u00a0September 2021 to 11\u00a0October 2022 at two restored sills on the\u00a0Rolava (river km 8.8 and 9.2) in\u00a0Nov\u00e1 Role. A\u00a0total of\u00a056\u00a0brown trout were tagged. Guidelines for assessing fish passes\u00a0[18] recommend monitoring one or two indicator species by telemetry in\u00a0small watercourses within\u00a0the\u00a0trout zone. However, previous findings\u00a0[19] showed that brown trout was the\u00a0only species occurring in\u00a0sufficient numbers and of\u00a0a\u00a0suitable size in\u00a0the\u00a0section concerned. Adult fish (n = 40) were fitted with NTF-6-2 transmitters (Lotek, Newmarket, Canada), while juveniles (n = 16) were fitted with NTF-5-2 transmitters (Lotek, Newmarket, Canada). The\u00a0transmitters were programmed to frequencies of\u00a0138.30 MHz and 138.42 MHz, with pulse intervals of\u00a05.0\u20135.5 s. Fish were collected by electrofishing directly within\u00a0the\u00a0study area on 29\u00a0September 2021. To stimulate upstream migration, some tagged individuals were translocated to a\u00a0different sector. This enabled their ability to negotiate barriers to be tested by exploiting the\u00a0homing effect, i.e. their tendency to return to their original home range (see Fig.\u00a02<\/em> for further details).<\/p>\n Fish movements and successful passage of\u00a0the\u00a0barriers were monitored using a\u00a0combination of\u00a0two radiotelemetry methods. Mobile telemetry using an\u00a0SRX 800 receiver (Lotek, Newmarket, Canada) and a\u00a060CSx GPS receiver (Garmin, Kansas, USA) was conducted at regular monthly intervals to determine the\u00a0current positions of\u00a0individuals in\u00a0the\u00a0sections upstream and downstream of\u00a0the\u00a0monitored sills. This enabled detailed mapping of\u00a0fish movements throughout the\u00a0study reach. Continuous recording of\u00a0migration through the\u00a0restored sections was provided by automatic stationary ATS telemetry using Lotek SRX 800D and SRX DL receivers (Lotek, Newmarket, Canada). These stations were equipped with two antennas directed upstream and downstream of\u00a0the\u00a0sills, enabling fish passage through the\u00a0monitored structures to be accurately detected.<\/p>\n Two parameters were monitored when assessing fish-pass performance: passage efficiency (the\u00a0proportion of\u00a0fish that successfully traversed the\u00a0entire fish pass relative to those that merely entered it) and overall migration success (the\u00a0proportion of\u00a0successful migrants relative to all tagged fish). The\u00a0number of\u00a0tagged fish was adjusted using an\u00a0overall coefficient of\u00a0K\u00a0= 0.17, which accounts for transmitter loss, fish mortality and the\u00a0detection accuracy of\u00a0the\u00a0ATS systems\u00a0[18]. All individuals detected downstream of\u00a0the\u00a0barrier were included in\u00a0the\u00a0analysis of\u00a0migration success, comprising both released fish and individuals that had migrated naturally to the\u00a0site from upstream or downstream. The\u00a0direction and timing of\u00a0migratory behaviour were also recorded during monitoring.<\/p>\n The\u00a0resulting data were subsequently analysed statistically. Differences in\u00a0successful passage of\u00a0the\u00a0lower and upper sills were compared using Fisher\u2019s\u00a0exact test\u00a0[20]. Differences in\u00a0movement rates between seasons were assessed using repeated-measures analysis of\u00a0variance (repeated-measures ANOVA)\u00a0[20] or a\u00a0linear mixed-effects model (LMM)\u00a0[21]. Subsequent pairwise comparisons between individual seasons (post-hoc analysis) were performed using paired t-tests with Bonferroni correction of\u00a0the\u00a0significance level\u00a0[20].<\/p>\n The final key approach to assessing fish passage used data on the current structure of the fish community, evaluated on the basis of species composition by comparing sections upstream and downstream of individual barriers. The input data comprised standardised abundances (CPUE; individuals\/100 m)\u00a0for 23 fish species from 32 sampling events at eight sites between 2022 and 2025. To reduce the influence of highly dominant species, the data were square-root transformed prior to analysis. Similarities in community composition among individual sampling events and sectors were visualised using non-metric multidimensional scaling (NMDS) ordination based on Bray\u2013Curtis dissimilarity [22]. The ordination plot included 95% confidence ellipses for individual sectors and the positions of key species.<\/p>\n The\u00a0statistical significance of\u00a0differences in\u00a0species composition among sites and sectors was tested using permutational multivariate analysis of\u00a0variance (PERMANOVA) with 999 permutations. SIMPER (similarity percentages) analysis was used to identify the\u00a0species contributing most to the\u00a0observed differences between sections\u00a0[23]. All statistical analyses were performed using R\u00a0[24].<\/p>\n Historical records from the\u00a019th century report the\u00a0occurrence of\u00a020 fish species in\u00a0the\u00a0study area\u00a0[25], or 28 species according to\u00a0[26]. These earliest surveys comprise almost exclusively native fish species, although 19th-century records make no mention of\u00a0rainbow trout (Oncorhynchus mykiss), despite evidence that the\u00a0species was already being stocked in\u00a0the\u00a0Rolava in\u00a01887\u00a0[27]. Studies dating from the\u00a0late 20th century\u00a0[28] recorded a\u00a0total of\u00a037 species in\u00a0the\u00a0catchment. Among native species, pike-perch (Sander lucioperca<\/em>) and ide (Leuciscus idus<\/em>) appear for the\u00a0first time, although the\u00a0newly recorded taxa are predominantly non-native species, including brook trout (Salvelinus fontinalis<\/em>), brown bullhead (Ameiurus nebulosus<\/em>), largemouth black bass (Micropterus salmoides<\/em>), silver carp (Hypophthalmichthys molitrix), bighead carp (Hypophthalmichthys nobilis<\/em>) and common nase (Chondrostoma nasus<\/em>), which is non-native to the\u00a0Oh\u0159e catchment.<\/p>\n The highest species diversity in the area was reported by Mat\u011bj\u016f a Holub [29], who recorded as many as 43 fish species. Newly recorded species included non-native Siberian sturgeon (Acipenser baerii<\/em>) and invasive pumpkinseed (Lepomis gibbosus<\/em>), as well as the native crucian carp (Carassius carassius<\/em>) and sunbleak (Leucaspius delineatus<\/em>). However, a more detailed description\u00a0of\u00a0the\u00a0authors\u2019 local records shows that most occurrences of\u00a0these newly reported species were in\u00a0private ponds or isolated pools, and that their occurrence in\u00a0the\u00a0Rolava and Oh\u0159e is very rare. Anglers\u2019 records provide valuable information on the\u00a0occurrence of\u00a0certain\u00a0species. Analysis of\u00a0catch and stocking records from the\u00a0Czech Anglers Union shows that 23 fish species are regularly caught in\u00a0the\u00a0Oh\u0159e and Rolava. However, these statistics primarily cover species of\u00a0commercial or recreational importance and contain\u00a0no data on smaller species such as European\u00a0bullhead (Cottus gobio<\/em>), stone loach (Barbatula barbatula<\/em>) and Eurasian\u00a0minnow (Phoxinus phoxinus<\/em>).<\/p>\n Our monitoring conducted between 2022 and 2025 recorded the\u00a0presence of\u00a023 fish species. A\u00a0detailed overview of\u00a0species occurrence in\u00a0the\u00a0individual studies is provided in\u00a0Tab.\u00a01<\/em>.<\/p>\n Analysis of\u00a0the\u00a0Pasportization of\u00a0watercourses\u00a0[17], followed by our own field assessment of\u00a0the\u00a0study reach (from the\u00a0confluence with the\u00a0Oh\u0159e to Nov\u00e1 Role), showed a\u00a0high degree of\u00a0anthropogenic fragmentation of\u00a0the\u00a0Rolava river network. A\u00a0total of\u00a041 transverse structures (weirs, sills, ramps, etc.) were identified along the\u00a0Rolava, disrupting its longitudinal continuity. Detailed analysis showed that 25 of\u00a0these barriers have a\u00a0head difference of\u00a0more than\u00a00.6\u00a0m. At present, only eight of\u00a0these structures (32\u00a0%) are equipped with a\u00a0fish pass.<\/p>\n The main barriers to fish migration within the study reach are two large weirs in Karlovy Vary that lack fish passes. The first is Weir No. 1 \u2013 Ryb\u00e1\u0159e (river km 1.1), which represents the most problematic point in terms of river continuity, as it\u00a0isolates the\u00a0entire Rolava from the\u00a0Oh\u0159e. It is a\u00a0fixed weir fitted with a\u00a0movable gate, with a\u00a0total height exceeding 1.5\u00a0m (see Fig.\u00a03<\/em> for further details).<\/p>\n The\u00a0second migration barrier is the\u00a0weir at Star\u00e1 Role (river km 4.6). This vertical structure, more than\u00a01.5\u00a0m high, consists of\u00a0large boulders set in\u00a0concrete (Fig.\u00a04<\/em>).<\/p>\n In\u00a0addition to these two main\u00a0barriers, there are three smaller transverse structures within\u00a0Karlovy Vary, with head differences of\u00a00.2\u20130.3\u00a0m. The\u00a0structural condition of\u00a0these three structures is currently partly degraded due to localised erosion of\u00a0the\u00a0structure or missing concrete infill (Fig.\u00a05<\/em>).<\/p>\n Upstream of\u00a0the\u00a0weir at Star\u00e1 Role (river km 4.6) lies a\u00a0section where two sills in\u00a0Nov\u00e1 Role were restored in\u00a02020. These works restored fish passage at the\u00a0structures at river km 8.8 and 9.2 and were subsequently followed by restoration of\u00a0the\u00a0structure at river km 8.4. The\u00a0original impassable migration barriers, which exceeded 1.5\u00a0m in\u00a0height, were converted into ramp-type fish passes (Fig.\u00a06<\/em>).<\/p>\n Restoration of\u00a0these three structures in\u00a0Nov\u00e1 Role opened approximately 10 km of\u00a0the\u00a0river to fish passage. Subsequent radiotelemetry monitoring confirmed successful fish passage through this reach. Passage efficiency through the\u00a0fish passes themselves ranged from 88 to 100\u00a0%, while overall migration success ranged from 66 to 90\u00a0% (Tab.\u00a02<\/span><\/em>).<\/span><\/p>\n Of\u00a0the\u00a040 tagged adult brown trout, 16 were confirmed to have passed the\u00a0lower weir and 14 the\u00a0upper weir. Among the\u00a016 juvenile trout, five individuals migrated across the\u00a0lower weir, one of\u00a0which also successfully passed the\u00a0upper weir. Statistical analysis, however, found no difference in\u00a0the\u00a0passability of\u00a0the\u00a0two monitored weirs for either adult or juvenile fish (p = 0.4669).<\/span><\/p>\n Fish activity was highest during the\u00a0first 14 days after release in\u00a0both age groups (t = 13.531, p < 0.001), with movement during this period predominantly upstream. In\u00a0adults, this direction of\u00a0movement was closely associated with a\u00a0pronounced homing effect, i.e. an\u00a0attempt to return to their original territory. This proved to be a\u00a0key motivating factor, with adult trout translocated downstream of\u00a0the\u00a0barriers showing overall return success of\u00a075\u2013100\u00a0%. Success was directly affected by translocation distance: fish moved two sectors downstream achieved 75% return success, whereas those moved one sector achieved 100%. By contrast, fish released at their capture site remained within\u00a0their sector.<\/span><\/p>\n In\u00a0terms of\u00a0seasonality, movement was greatest in\u00a0autumn and activity lowest in\u00a0summer. Autumn migration differed significantly from spring (t = 4.405, p\u00a0< 0.001), summer (t = 5.375, p < 0.001) and winter activity (t = \u22124.126, p < 0.001). The\u00a0high return success and the\u00a0ability of\u00a0juvenile fish to negotiate both barriers confirm that both weirs are passable. Movement within\u00a0this reach therefore depends primarily on the\u00a0fishes\u2019 current motivation.<\/span><\/p>\n Fish passage was also assessed by sampling fish below individual barriers. This ichthyological survey enabled detailed assessment of\u00a0the\u00a0species and size structure of\u00a0fish communities at sites immediately downstream of\u00a0each barrier. Comparison of\u00a0species composition and ichthyofaunal abundance among these sites revealed statistically significant differences (F = 8.7939; p\u00a0<\u00a00.001). These marked differences in\u00a0community distribution were evident both between the\u00a0main\u00a0stem of\u00a0the\u00a0Oh\u0159e and among the\u00a0reaches isolated by weirs. SIMPER analysis showed that five fish species accounted for more than\u00a081\u00a0% of\u00a0the\u00a0overall variability in\u00a0the\u00a0data. Brown trout made the\u00a0largest contribution to differences among communities (35.0\u00a0%), followed by chub\u00a0(16.9\u00a0%), European\u00a0bullhead (15.2\u00a0%), stone loach (8.2\u00a0%) and gudgeon (6.3\u00a0%). The\u00a0mean\u00a0relative abundance of\u00a0individual species (%) (\u00b1 standard deviation) is summarised in\u00a0detail in\u00a0Tab.\u00a03<\/span><\/em>.<\/span><\/p>\n The\u00a0Oh\u0159e sites (below\/above the\u00a0confluence) exhibited high species diversity. Chub (Squalius cephalus<\/span><\/em>) dominated (30.3 \u00b1 16.3\u00a0% and 38.9 \u00b1 6.2\u00a0%, respectively), together with European\u00a0bullhead (Cottus gobio<\/span><\/em>) (24.9 \u00b1 16.5\u00a0% and 18.3\u00a0\u00b1\u00a05.0\u00a0%, respectively) and common dace (Leuciscus leuciscus<\/span><\/em>). These were the\u00a0only two sites in\u00a0the\u00a0entire study reach where common barbel (Barbus barbus<\/span><\/em>) occurred, while the\u00a0Oh\u0159e site above the\u00a0confluence also contained an\u00a0isolated occurrence of\u00a0the\u00a0critically endangered European\u00a0brook lamprey (Lampetra planeri<\/span><\/em>) (0.2 \u00b1 0.3\u00a0%).<\/span><\/p>\n Upstream along the\u00a0Rolava, the\u00a0diversity of\u00a0rheophilic cyprinid species declined, accompanied by an\u00a0increasing prevalence of\u00a0salmonids. Weir No.\u00a01\u00a0\u2013 Ryb\u00e1\u0159e represented a\u00a0transition zone where European\u00a0bullhead remained dominant (39.1 \u00b1 12.7\u00a0%), but brown trout (27.9 \u00b1 11.2\u00a0%) and stone loach (12.7 \u00b1 6.8\u00a0%) already accounted for substantial proportions of\u00a0the\u00a0community. Eurasian\u00a0minnow was recorded only at this site (0.3 \u00b1 0.6\u00a0%). At Weirs Nos. 2\u20136, the\u00a0community was completely dominated by brown trout, whose mean\u00a0relative abundance progressively increased from 56.2 \u00b1 9.6\u00a0% (Weir No.\u00a02) to a\u00a0maximum of\u00a081.1\u00a0\u00b1\u00a010.7\u00a0% (Weir No. 6). Stone loach formed a\u00a0consistent component of\u00a0the\u00a0community, reaching its highest relative abundance at Weir\u00a0No.\u00a02\u00a0(20.3\u00a0\u00b1\u00a04.0\u00a0%). European\u00a0grayling (Thymallus thymallus<\/span><\/em>) was also well represented, particularly at Weirs Nos. 2 and 3, although its relative abundance declined sharply at the\u00a0uppermost structures.<\/span><\/p>\n Isolated occurrences of\u00a0eurytopic species were recorded at selected weir sites, specifically pike-perch (Weirs Nos. 3 and 5), Prussian\u00a0carp (Weirs Nos.\u00a04 and 6) and tench (Tinca tinca<\/span><\/em>) (most weir sites). Their mean\u00a0relative abundance, however, did not exceed 1.4\u00a0% in\u00a0any case and was accompanied by a\u00a0high standard deviation, indicating sporadic occurrence in\u00a0these reaches. Northern pike (Esox lucius<\/span><\/em>) was recorded exclusively at Weir No. 3. The\u00a0observed distribution and overall structure of\u00a0fish communities at the\u00a0individual sites were further evaluated using multivariate statistical analysis. The\u00a0resulting NMDS ordination (Fig.\u00a07<\/span><\/em>) had a\u00a0stress value of\u00a00.127, indicating a\u00a0very good representation of\u00a0the\u00a0data in\u00a0two-dimensional space and allowing reliable interpretation.<\/span><\/p>\n The ordination plot confirms a clear separation of Sector 1 (downstream of Weir No. 1 in the Ryb\u00e1\u0159e area of Karlovy Vary) from the reaches upstream\u00a0<\/span>of\u00a0this barrier. The\u00a0red ellipse clearly delineates the\u00a0distinctive Sector 1 community (Oh\u0159e sites below\/above the\u00a0confluence and Weir No. 1), characterised by protected rheophilic species such as European\u00a0brook lamprey, European\u00a0bullhead and ide. Sectors 2 (Weirs Nos. 2\u20135) and 3 (Weir No. 6), by contrast, show high similarity in\u00a0species composition and considerable spatial overlap. On the\u00a0right-hand side of\u00a0the\u00a0plot are species indicative of\u00a0wash-in\u00a0from surrounding standing waters (e.g. tench and Prussian\u00a0carp), which supplement the\u00a0otherwise less species-rich rheophilic fish community upstream of\u00a0the\u00a0barriers.<\/span><\/p>\n When interpreting long-term changes in\u00a0the\u00a0ichthyofauna, it is necessary to consider the\u00a0methodological limitations and specific characteristics of\u00a0the\u00a0individual information sources, which directly account for differences in\u00a0the\u00a0total numbers of\u00a0species recorded. The\u00a0relatively low number of\u00a020 species recorded in\u00a0the\u00a0first half of\u00a0the\u00a019th century\u00a0[25] was undoubtedly influenced by the\u00a0state of\u00a0taxonomy at the\u00a0time, when some closely related taxa, such as freshwater bream (Abramis brama<\/em>) and white bream (Blicca bjoerkna<\/em>), were not routinely distinguished in\u00a0the\u00a0literature\u00a0[28]. Overall species richness in\u00a0the\u00a0past was therefore very likely higher than\u00a0contemporary sources suggest.<\/p>\n Lower species richness is also apparent in\u00a0the\u00a0statistics of\u00a0the\u00a0Czech Anglers Union (\u010cRS). Angling records are primarily biased towards species of\u00a0commercial and recreational interest, which explains the\u00a0complete absence of\u00a0data on small species such as European\u00a0bullhead, stone loach and Eurasian\u00a0minnow. Despite this bias, however, angling statistics represent a\u00a0highly valuable data source that effectively complements electrofishing, which itself is selective. This selectivity is particularly evident for more mobile species such as common carp (Cyprinus carpio<\/em>), silver carp and asp (Leuciscus aspius<\/em>), as well as species inhabiting deeper parts of\u00a0rivers, such as Wels catfish (Silurus glanis<\/em>), whose abundance tends to be substantially underestimated. This is also confirmed by our monitoring results: these species are absent from our records despite their documented occurrence in\u00a0the\u00a0study area. In\u00a0such cases, recreational anglers\u2019 catch records and \u010cRS fish-stocking statistics provide an\u00a0invaluable indication of\u00a0the\u00a0actual presence of\u00a0these taxa in\u00a0the\u00a0river and important context for distinguishing naturally reproducing populations from those dependent on artificial stocking.<\/p>\n The\u00a0lower species diversity recorded in\u00a0our study is probably attributable to a\u00a0combination of\u00a0its smaller spatial extent and the\u00a0methodology used. Unlike study\u00a0[29], which combined field surveys with records from the\u00a0Nature Conservation Agency of\u00a0the\u00a0Czech Republic (NCA CR) database covering long periods, our study relied solely on our own electrofishing surveys. Study\u00a0[29] also included specific isolated habitats such as private fishponds and pools. Species such as Siberian\u00a0sturgeon (Acipenser baerii<\/span><\/em>) and sunbleak (Leucaspius delineatus<\/span><\/em>) are associated with these habitats and only rarely enter the\u00a0flowing channels of\u00a0the\u00a0Oh\u0159e or Rolava, for example during fishpond harvesting.<\/span><\/p>\n Long-term changes in\u00a0the\u00a0community are closely associated with the\u00a0increasing occurrence of\u00a0non-native fish species, a\u00a0trend that has been clearly apparent in\u00a0the\u00a0study area since the\u00a020th century. Whereas non-native species were virtually absent from 19th-century historical surveys and the\u00a0river community consisted exclusively of\u00a0native species, non-native taxa are now considerably more frequent and have become a\u00a0permanent component of\u00a0species richness in\u00a0the\u00a0catchment. This upward trend was initially driven by deliberate introductions of\u00a0species valued for commercial or recreational purposes, such as grass carp, silver and bighead carp, brook trout and rainbow trout, whose populations in\u00a0the\u00a0river continue to be maintained by \u010cRS stocking. In\u00a0addition to these management interventions, however, recent data also show an\u00a0increasing occurrence of\u00a0spontaneously spreading invasive taxa included on the\u00a0so-called blacklist, such as Prussian\u00a0carp, topmouth gudgeon and pumpkinseed. The\u00a0gradual shift in\u00a0species composition from an\u00a0entirely native community towards a\u00a0system in\u00a0which non-native species are increasingly common thus clearly reflects long-term anthropogenic pressure and changes in\u00a0the\u00a0character of\u00a0the\u00a0river habitat as a\u00a0whole.<\/span><\/p>\n The\u00a0high degree of\u00a0fragmentation of\u00a0the\u00a0Rolava represents a\u00a0major constraint on the\u00a0functioning of\u00a0the\u00a0river ecosystem. A\u00a0total of\u00a041 barriers currently occur along a\u00a0river only 37 km long. A\u00a0head difference exceeding 0.6\u00a0m is generally considered a\u00a0critical threshold that most native fish species are unable to negotiate. On the\u00a0Rolava, 25 of\u00a0these barriers exceed 0.6\u00a0m, representing a\u00a0critical degree of\u00a0obstruction for most migrating fish. This unfavourable situation is further compounded by the\u00a0fact that only eight of\u00a0these high barriers are currently equipped with fish passes.<\/p>\n Despite the\u00a0clear need to address this fragmentation, socioeconomic constraints must be taken into account when designing specific restoration measures. Weirs No. 1 \u2013 Ryb\u00e1\u0159e (river km 1.1) and No. 5 \u2013 Star\u00e1 Role (river km 4.6) must therefore be regarded as barriers whose complete removal is not currently feasible. Both structures stabilise the\u00a0river gradient and maintain\u00a0the\u00a0impounded water levels required for the\u00a0operation of\u00a0small hydropower plants, while the\u00a0Ryb\u00e1\u0159e weir also provides an\u00a0important source of\u00a0non-potable water for the\u00a0local swimming pool. Complete restoration of\u00a0river continuity through removal of\u00a0these structures is therefore not realistic. Longitudinal fish passage in\u00a0the\u00a0lower Rolava can, however, be restored through engineering measures that are planned for both sites in\u00a0the\u00a0near future.<\/p>\n At Weir No. 1 in\u00a0Ryb\u00e1\u0159e, a\u00a0pool-type fish pass with boulder obstacles is planned on the\u00a0right bank in\u00a0the\u00a0immediate vicinity of\u00a0the\u00a0small hydropower plant. This location is optimal because migrating fish will be naturally attracted towards the\u00a0pass by the\u00a0strong attraction flow discharged from the\u00a0power plant turbines. Weir No. 5 in\u00a0Star\u00e1 Role is expected to undergo major modification this year. The\u00a0planned conversion of\u00a0the\u00a0vertical concrete structure into a\u00a0boulder ramp will create a\u00a0rough bed with sufficient water depth, enabling safe passage for a\u00a0wide range of\u00a0aquatic organisms.<\/p>\n By contrast, given their height (< 0.3 m), the remaining three smaller barriers located between the two large structures do not represent a constraint for rheophilic species with good jumping and swimming abilities, such as brown trout and European grayling. Moreover, their partly degraded structural condition has created areas of reduced flow, local cavities and zones of lower water velocity within the structures. It can therefore be assumed that, under normal flow conditions, these sills are also passable to fish species with limited swimming ability, such as European bullhead and Eurasian minnow. However, the actual degree of fish passage at these smaller, damaged structures will need to be confirmed by further study. In terms of prioritising remedial measures on the Rolava, it therefore makes sense to focus primarily\u00a0on restoring fish passage at the\u00a0two major Karlovy Vary weirs (Nos. 1 and 5), which currently constitute complete migration barriers.<\/span><\/p>\n Assessment of\u00a0fish passage in\u00a0rivers is fundamental to understanding the\u00a0effects of\u00a0river-network fragmentation on fish communities and their biodiversity. This principle is closely linked to the\u00a0requirements of\u00a0the\u00a0Water Framework Directive\u00a0[14], which does not require active monitoring of\u00a0passage at individual barriers but does require assessment of\u00a0the\u00a0longitudinal continuity of\u00a0the\u00a0water body as a\u00a0whole. River-network continuity is a\u00a0key supporting element in\u00a0determining the\u00a0ecological status of\u00a0water bodies. Selecting an\u00a0appropriate method for assessing fish passage is therefore essential for obtaining valid data that can\u00a0support the\u00a0effective design of\u00a0remedial measures or functional fish passes in\u00a0accordance with European\u00a0standards. The\u00a0following analysis compares telemetry and ichthyological sampling as two approaches that differ in\u00a0their level of\u00a0detail, practical requirements and potential for interpretative bias.<\/span><\/p>\n Telemetry is a\u00a0highly precise tool for detailed analysis of\u00a0individual behaviour. It can\u00a0identify the\u00a0specific barriers at which fish are stopped, document relatively precisely the\u00a0time required to negotiate a\u00a0migration barrier, and relate this activity to prevailing environmental conditions such as temperature and discharge. In\u00a0terms of\u00a0habitat connectivity, telemetry provides unique data on the\u00a0dynamics of\u00a0movement between key habitats. In\u00a0our study, telemetry confirmed fish passage at the\u00a0two restored barriers at river km 8.8 and 9.2, clearly demonstrating that both adult and juvenile brown trout were able to negotiate these fish passes successfully. Telemetry also allowed us to determine the\u00a0speed of\u00a0these movements and showed that fish generally returned to their home territory immediately after release or within\u00a0a\u00a0few days (Fig.\u00a08<\/span><\/em>). It also enabled detailed monitoring of\u00a0migratory behaviour over the\u00a0broader temporal context of\u00a0an\u00a0entire year. Although movement frequency was highest shortly after release, continuous year-round monitoring provided essential information on whether trout also actively used other parts of\u00a0the\u00a0river throughout the\u00a0year. This confirmed that fish actively use different parts of\u00a0the\u00a0river according to their seasonal requirements, something that could not have been detected without continuous monitoring.<\/span><\/p>\n In\u00a0addition to monitoring these larger-scale movements, telemetry allows detailed mapping of\u00a0local movements by individual fish, helping to identify particular parts of\u00a0the\u00a0river that are attractive to them. Continuous monitoring of\u00a0fish presence at a\u00a0given location makes it possible to identify the\u00a0microhabitats they preferentially use. Such information is highly valuable for future management and remedial measures, providing an\u00a0important basis for the\u00a0optimal design of\u00a0river restoration, particularly in\u00a0heavily modified urban\u00a0reaches where appropriate channel configuration and the\u00a0availability of\u00a0refuges are essential for maintaining fish communities.<\/span><\/p>\n The disadvantages of telemetry, however, include its high cost, the need for surgical implantation and, above all, constraints on transmitter weight. Ideally, transmitter mass should not exceed 2 % of the fish\u2019s body mass. Although the smallest commercially available transmitters weigh approximately 0.24 g,\u00a0<\/span>theoretically allowing individuals weighing as little as 12 g to be tagged, their battery life is then only a\u00a0few days. Such a\u00a0short period is entirely inadequate for long-term monitoring, particularly of\u00a0species that are not highly mobile. Telemetry is therefore inherently unable to reliably establish whether a\u00a0reach is passable to smaller fish species such as European\u00a0bullhead, Eurasian\u00a0minnow and stone loach, for which long-term monitoring using this method is impractical. Other monitoring methods capable of\u00a0assessing whether barriers can\u00a0be negotiated are therefore necessary when evaluating connectivity for smaller species. Regular ichthyological sampling upstream and downstream of\u00a0a\u00a0barrier can\u00a0play a\u00a0key role, with the\u00a0presence or absence of\u00a0these small species providing important evidence as to whether they are actually able to pass the\u00a0barrier.<\/span><\/p>\n By contrast, ichthyological sampling upstream and downstream of\u00a0barriers provides broader ecological insight at lower cost. It can\u00a0identify barriers from the\u00a0presence or absence of\u00a0particular taxa, detect invasive species, and reveal the\u00a0age or size structure of\u00a0populations. Using this method, our monitoring identified a\u00a0total of\u00a023 fish and lamprey species. These included protected and ecologically valuable species (European\u00a0bullhead, Eurasian\u00a0minnow, burbot, ide and European\u00a0brook lamprey), as well as non-native or invasive taxa (Prussian\u00a0carp, three-spined stickleback, rainbow trout and topmouth gudgeon). These records provide valuable information for designing future remedial measures to protect threatened species while also allowing the\u00a0risk of\u00a0non-native species spreading through the\u00a0catchment to be assessed.<\/span><\/p>\n Detailed assessment of\u00a0size structure also enabled us to determine the\u00a0current status of\u00a0individual populations and establish whether they were reproducing naturally within\u00a0the\u00a0study area. Successful natural reproduction in\u00a0the\u00a0Rolava was confirmed for brown trout, stone loach and European\u00a0bullhead. In\u00a0the\u00a0Oh\u0159e, natural reproduction was demonstrated for, among others, all three Leuciscus\/Squalius<\/span> species \u2013 chub, ide and common dace \u2013 as well as common barbel, roach, three-spined stickleback and topmouth gudgeon.<\/span><\/p>\n The main limitations of fish sampling include more frequent direct disturbance of the river environment and the risk of injury to fish during capture and handling, particularly larger individuals; in rare cases, mortality may also occur. The efficiency and selectivity of electrofishing are also strongly influenced by numerous environmental factors, including water conductivity, channel width and depth, channel complexity, flow velocity and the nature of the bed substrate. Sampling provides no continuous record of the movements of individual\u00a0<\/span>fish over time and therefore offers only a\u00a0snapshot at the\u00a0time of\u00a0sampling; it cannot establish whether a\u00a0fish previously negotiated a\u00a0particular barrier and subsequently returned to its original home range. Finally, the\u00a0interpretation of\u00a0sampling results may be substantially confounded in\u00a0fisheries subject to artificial stocking, where the\u00a0presence of\u00a0stocked individuals can\u00a0mask the\u00a0true passability or otherwise of\u00a0the\u00a0river for a\u00a0particular species.<\/span><\/p>\n The\u00a0two methods used to assess fish migration therefore have their own specific strengths and limitations. Combining ichthyological sampling with radiotelemetry, however, provides a\u00a0comprehensive and robust tool for objectively assessing river connectivity and the\u00a0functioning of\u00a0aquatic ecosystems, with telemetry supplying the\u00a0individual and temporal detail that sampling lacks. Data on the\u00a0distribution and abundance of\u00a0individual species provide direct indications of\u00a0which barriers act as selective filters and which are completely impassable to particular taxa or size classes. Field sampling therefore provides an\u00a0essential complement to telemetry data, revealing the\u00a0actual long-term effects of\u00a0barriers on the\u00a0natural structure of\u00a0fish populations in\u00a0the\u00a0affected reaches of\u00a0the\u00a0Rolava.<\/span><\/p>\n Restoring longitudinal river continuity and reconnecting the\u00a0Rolava with the\u00a0Oh\u0159e will represent a\u00a0major ecological change that is likely to influence the\u00a0future composition of\u00a0the\u00a0local fish community. It will also be important to determine how natural reproduction of\u00a0salmonids responds to this restored connection, as successful natural reproduction is a\u00a0key prerequisite for reversing the\u00a0negative trend in\u00a0their populations\u00a0[5].<\/span><\/p>\n The\u00a0first part of\u00a0the\u00a0prediction is based on planned engineering works on the\u00a0Rolava, specifically the\u00a0restoration of\u00a0fish passage at two key migration barriers. At Star\u00e1 Role, the\u00a0existing Weir No. 5 will be replaced by a\u00a0boulder ramp, but the\u00a0crucial step will be the\u00a0construction of\u00a0a\u00a0fish pass at Weir No. 1 in\u00a0Ryb\u00e1\u0159e\u00a0[30]. This weir currently constitutes an\u00a0impassable barrier to fish. Restoring passage will reopen the\u00a0migration route from the\u00a0Oh\u0159e and should lead to recolonisation of\u00a0the\u00a0Rolava by native species such as European\u00a0bullhead, Eurasian\u00a0minnow and European\u00a0brook lamprey. These ecologically demanding species require high-quality, clean\u00a0water and commonly occur as far upstream as the\u00a0upper parts of\u00a0the\u00a0trout zone; they can\u00a0therefore be expected gradually to recolonise the\u00a0Rolava upstream as far as the\u00a0next impassable barrier. The\u00a0spread of\u00a0European\u00a0brook lamprey, however, will be limited by the\u00a0availability of\u00a0low-flow areas with fine muddy sediments, where its larvae (ammocoetes) spend the\u00a0first years of\u00a0their lives. Burbot (Lota lota<\/span><\/em>) could also return to the\u00a0Rolava. It is another characteristic species of\u00a0the\u00a0trout zone and is regularly stocked by the\u00a0Czech Anglers Union in\u00a0the\u00a0Oh\u0159e in\u00a0Karlovy Vary. Once fish passage has been restored, the\u00a0Rolava should again\u00a0provide suitable and characteristic habitat for the\u00a0species.<\/span><\/p>\n Gudgeon (Gobio gobio<\/span><\/em>) and rheophilic cyprinids such as common dace and chub can\u00a0initially be expected to occur mainly in\u00a0the\u00a0lower Rolava. However, ongoing climate change, resulting in\u00a0warmer river water and lower precipitation, could gradually shift this ecological boundary upstream, potentially allowing these species to colonise the\u00a0currently cooler submontane reaches in\u00a0the\u00a0future. By contrast, a\u00a0substantial expansion of\u00a0non-native fish species such as three-spined stickleback (Gasterosteus aculeatus<\/span><\/em>), Prussian\u00a0carp and topmouth gudgeon (Pseudorasbora parva<\/span><\/em>) is not expected. Given the\u00a0biological requirements of\u00a0these limnophilic taxa, restoration of\u00a0passage at the\u00a0lower barriers is likely to allow them to penetrate only the\u00a0lowermost, anthropogenically modified urban\u00a0reach of\u00a0the\u00a0Rolava in\u00a0Karlovy Vary.<\/span><\/p>\n The\u00a0second part of\u00a0the\u00a0prediction concerns the\u00a0development of\u00a0salmonid populations and is based on our ichthyological surveys conducted in\u00a0the\u00a0Rolava between 2022 and 2025. These surveys were carried out mainly in\u00a0late spring and early summer, with a\u00a0total of\u00a032 sampling surveys conducted in\u00a0the\u00a0Rolava and Oh\u0159e. In\u00a0the\u00a0final year of\u00a0the\u00a0project, we recorded a\u00a0marked increase in\u00a0the\u00a0abundance of\u00a0brown trout fry in\u00a0the\u00a0upper part of\u00a0the\u00a0study reach near Weirs Nos. 5 and 6, reaching tens to hundreds of\u00a0individuals per 100\u00a0m of\u00a0river. The\u00a0captured individuals were approximately 30\u00a0mm long. According to the\u00a0stocking plan, the\u00a0Czech Anglers Union (\u010cRS) routinely stocks the\u00a0Rolava 1 A\u00a0fishery with one-year-old trout or larger stocking fish at least 100\u00a0mm in\u00a0length. Given this pronounced size difference, it is clear that the\u00a0individuals recorded in\u00a0our survey originated from natural spawning. This contrasts markedly with previous years, when fry were either entirely absent or recorded only in\u00a0very low numbers\u00a0[19].<\/span><\/p>\n Given the\u00a0isolated nature of\u00a0this finding, it is not yet clear whether the\u00a0high abundance of\u00a0fry was primarily attributable to restored fish passage at the\u00a0structures at river km 8.8 and 9.2 or to the\u00a0particular conditions prevailing that year. Further monitoring of\u00a0the\u00a0ichthyofauna at the\u00a0site will be necessary to confirm any lasting benefits of\u00a0restoring fish passage. Nevertheless, the\u00a0finding is an\u00a0important indication of\u00a0the\u00a0river\u2019s\u00a0potential for natural reproduction. Supporting natural reproduction appears to be a\u00a0key means of\u00a0achieving stable populations with a\u00a0balanced age structure. Czech Anglers Union data (Fig.\u00a09<\/span><\/em>) show a\u00a0long-term decline in\u00a0catches of\u00a0both brown trout and European\u00a0grayling despite intensive artificial stocking, which has been unable to compensate effectively for this decline. Although precise stocking records are available only from 2017 onwards, according to information provided by Ing. H. Dort, a\u00a0fisheries officer of\u00a0the\u00a0regional association, stocking also took place in\u00a0earlier years at approximately 1,500 grayling and 2,000 trout per year. The\u00a0causes of\u00a0this negative trend are complex and include predation pressure, illegal fishing and general habitat degradation, a\u00a0phenomenon observed in\u00a0most trout fisheries in\u00a0the\u00a0Czech Republic. Restoring fish passage is therefore one of\u00a0the\u00a0prerequisites for conserving local populations, but it must go hand in\u00a0hand with measures addressing other environmental pressures.<\/span><\/p>\n In\u00a0recent years, the\u00a0Rolava has become a\u00a0key focus of\u00a0efforts to restore river continuity in\u00a0the\u00a0Karlovy Vary Region. The\u00a0principal aim is to restore fish passage, which is essential for maintaining stable populations of\u00a0rheophilic fish species. River restoration builds directly on the\u00a0historical legacy of\u00a0naturalist Anton\u00edn Fri\u010d, who recognised the\u00a0need for unobstructed migration routes in\u00a0Czech river basins as early as the\u00a0second half of\u00a0the\u00a019th century.<\/span><\/p>\n The combination of impassable structures in the lower and upper reaches divides the Rolava into a series of isolated sections, substantially reducing\u00a0<\/span>the\u00a0ecological potential of\u00a0the\u00a0entire catchment, preventing fish from using their natural migration routes and hindering compliance with legislative requirements for the\u00a0biological integrity of\u00a0aquatic ecosystems. Removing barriers on the\u00a0Rolava therefore opens the\u00a0way to restoration of\u00a0the\u00a0entire river ecosystem, the\u00a0return of\u00a0many threatened and rare species, and the\u00a0development of\u00a0near-natural communities of\u00a0fish and other aquatic organisms.<\/span><\/p>\n Analysis of\u00a0changes in\u00a0the\u00a0ichthyofauna reveals an\u00a0interesting trend: the\u00a0total number of\u00a0species recorded during our monitoring (23 species) remains very similar to that reported in\u00a019th-century historical records (20\u201328 species)<\/span>. However, the\u00a0composition of\u00a0the\u00a0community has changed substantially. Extreme fragmentation of\u00a0the\u00a0river by 41 transverse barriers has resulted in\u00a0the\u00a0disappearance of\u00a0some sensitive native species from the\u00a0Rolava. Over time, these have been replaced by non-native or invasive species that are more tolerant of\u00a0environmental degradation.<\/span><\/p>\n The\u00a0successful restoration of\u00a0three structures in\u00a0Nov\u00e1 Role in\u00a02020 demonstrated that restoring fish passage is both technically and biologically feasible. Radiotelemetry clearly showed that the\u00a0fish passes constructed are fully functional, at least for brown trout, and allow natural movement of\u00a0fish along the\u00a0river. The\u00a0high passage success at the\u00a0restored structures therefore provides encouraging evidence for the\u00a0planned restoration of\u00a0fish passage at the\u00a0critical barriers in\u00a0Ryb\u00e1\u0159e and Star\u00e1 Role, an\u00a0essential prerequisite for the\u00a0long-term sustainability of\u00a0the\u00a0local fish community.<\/span><\/p>\n Monitoring conducted between 2022 and 2025, particularly in\u00a0the\u00a0final year, indicates substantial natural reproduction in\u00a0the\u00a0Rolava. Whereas brown trout fry were scarce in\u00a0previous years, the\u00a0most recent data confirm that the\u00a0river is capable of\u00a0providing suitable conditions for successful natural spawning.<\/span><\/p>\n These findings are particularly significant in\u00a0the\u00a0context of\u00a0the\u00a0general decline in\u00a0the\u00a0abundance of\u00a0rheophilic fish species in\u00a0recent years, which even intensive artificial stocking has been unable to compensate for effectively. This trend is also reflected in\u00a0declining catches of\u00a0brown trout and European\u00a0grayling in\u00a0the\u00a0Rolava 1A fishery. The\u00a0results indicate that where fish are provided with adequate connectivity, natural reproduction is not only possible but can\u00a0become one of\u00a0the\u00a0principal means of\u00a0maintaining stable populations with a\u00a0balanced age structure.<\/span><\/p>\n This work was financially supported by project No. SS02030027, Water systems and water management in\u00a0the\u00a0Czech Republic under climate change conditions (Water Centre), under the\u00a0Environment for Life programme of\u00a0the\u00a0Technology Agency of\u00a0the\u00a0Czech Republic.<\/span><\/span><\/em><\/p>\n The\u00a0authors thank Oh\u0159e Basin\u00a0Authority, state enterprise, for providing valuable background materials. Thanks are also due to the\u00a0Czech Anglers Union \u2013 West Bohemian\u00a0Regional Association, in\u00a0particular Ing. Hynek Dort, and to colleagues Patrik Slanina and V\u00edt B\u0159ich\u00e1\u010dek for their assistance with fish sampling. Finally, the\u00a0authors thank Ing. Ji\u0159\u00ed Musil, Ph.D., and MVDr. Jana Kvi\u010derov\u00e1, Ph.D., for their expert assistance and cooperation.<\/span><\/span><\/em><\/p>\n The\u00a0Czech version of\u00a0this article was peer-reviewed, the\u00a0English version was translated from the\u00a0Czech original by Environmental Translation Ltd.<\/span><\/p>\n","protected":false},"excerpt":{"rendered":" The study examines the effects of river network fragmentation on populations of rheophilic fish species in the Rolava and Oh\u0159e river basins, with particular emphasis on assessing fish passage and the effectiveness of restoration measures. An analysis of the river inventory indicates a high degree of anthropogenic disturbance in the Rolava; of the 41 transverse structures identified, more than half constitute critical barriers over 0.6 m high that most native fish species are unable to overcome. <\/p>\n","protected":false},"author":8,"featured_media":40583,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":"","_members_access_role":[],"_members_access_error":""},"categories":[94,87],"tags":[3121,4244,4242,4243,2553,2959],"coauthors":[4213,4214,4215,803,4216],"class_list":["post-40584","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-current-issue","category-hydrochemistry-radioecology-microbiology","tag-biodiversity","tag-fish-passes","tag-fragmentation","tag-ichthyofauna","tag-ohre-river-basin","tag-restoration"],"acf":[],"_links":{"self":[{"href":"https:\/\/www.vtei.cz\/en\/wp-json\/wp\/v2\/posts\/40584","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.vtei.cz\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.vtei.cz\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.vtei.cz\/en\/wp-json\/wp\/v2\/users\/8"}],"replies":[{"embeddable":true,"href":"https:\/\/www.vtei.cz\/en\/wp-json\/wp\/v2\/comments?post=40584"}],"version-history":[{"count":13,"href":"https:\/\/www.vtei.cz\/en\/wp-json\/wp\/v2\/posts\/40584\/revisions"}],"predecessor-version":[{"id":40965,"href":"https:\/\/www.vtei.cz\/en\/wp-json\/wp\/v2\/posts\/40584\/revisions\/40965"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.vtei.cz\/en\/wp-json\/wp\/v2\/media\/40583"}],"wp:attachment":[{"href":"https:\/\/www.vtei.cz\/en\/wp-json\/wp\/v2\/media?parent=40584"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.vtei.cz\/en\/wp-json\/wp\/v2\/categories?post=40584"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.vtei.cz\/en\/wp-json\/wp\/v2\/tags?post=40584"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.vtei.cz\/en\/wp-json\/wp\/v2\/coauthors?post=40584"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}MATERIALS AND METHODS<\/h2>\n
Study area<\/h3>\n
Fish community composition<\/h3>\n
![\"\"](\"https:\/\/www.vtei.cz\/wp-content\/uploads\/2026\/10\/Bouse-fig-1-2.jpg\")
<\/a>\nFig.\u00a01. Geographical distribution of eight monitored sites on the lower reaches of\u00a0the\u00a0Rolava river and the adjacent reach of the Oh\u0159e river<\/h6>\n
Assessment of\u00a0fish passage<\/h3>\n
<\/a>\nFig.\u00a02. Division of the study area into sectors and the release of fish within individual sectors of the Rolava river; arrows indicate whether fish were released into the same sector in which they were captured or translocated to a different sector to encourage upstream migration of tagged individuals across migration barriers (homing effect)<\/h6>\n
RESULTS<\/h2>\n
Fish community composition<\/h3>\n
Tab.\u00a01. Occurrence of\u00a0fish species in\u00a0the\u00a0Oh\u0159e River basin\u00a0in\u00a0the\u00a0Karlovy Vary area<\/h5>\n
<\/a>\nAssessment of\u00a0fish passage<\/h3>\n
<\/a><\/h3>\nFig.\u00a03. Current condition of\u00a0the\u00a0fixed weir with a\u00a0movable gate in\u00a0Ryb\u00e1\u0159e at river km 1.1 (photo: E. Bou\u0161e)<\/h6>\n
<\/a>\nFig.\u00a04. Weir in Star\u00e1 Role at river km 4.6, constructed of boulders embedded in concrete (photo: E. Bou\u0161e)<\/h6>\n
<\/a>\nFig.\u00a05. Weir in\u00a0Karlovy Vary at river km 1.5 \u2013 an\u00a0example of\u00a0a\u00a0migration barrier with\u00a0a\u00a0damaged structure (photo: E. Bou\u0161e)<\/h6>\n
<\/a>\nFig.\u00a06. Restored weir in\u00a0Nov\u00e1 Role at river km 9.2 (photo: E. Bou\u0161e)<\/h6>\n
Tab. 2. Quantitative assessment of fish\u2013pass efficiency on the Rolava river<\/h5>\n
<\/a>\nTab.\u00a03. Relative abundance of\u00a0ichthyofauna in individual river sections over a\u00a04-year period, expressed in\u00a0% \u00b1 standard deviation<\/h5>\n
<\/a>\n<\/a>\nFig.\u00a07. NMDS ordination plot showing similarity in the species composition of\u00a0ichthyofauna at the monitored sites (Sectors 1\u20133); stress = 0.127<\/h6>\n
DISCUSSION<\/h2>\n
Fish community composition<\/h3>\n
Assessment of\u00a0fish passage<\/h3>\n
Comparison of\u00a0methods \u2013 radiotelemetry vs. fish sampling at barriers<\/h3>\n
<\/a>\nFig.\u00a08. Movement of adult brown trout ID 17 following translocation from Sector\u00a03 to\u00a0Sector 1 and subsequent homing: released in Sector 1 in September, detected in\u00a0Sector 2 in October, and returned to its original home range in Sector 3 by November<\/h6>\n
Predicted future development of\u00a0the\u00a0fish community in\u00a0the\u00a0Rolava<\/h3>\n
<\/a>\nFig. 9. Annual catch and stocking data for brown trout (Salmo trutta m. fario<\/em>) and grayling (Thymallus thymallus<\/em>) in Rolava 1A angling district (data source: West Bohemian Territorial Union of the Czech Anglers Union [\u010cRS])<\/h6>\n
CONCLUSION<\/h2>\n
<\/a>\nGudgeon (Gobio gobio<\/em>), a typical representative of fish from fast-flowing sections with sandy or gravelly bottoms (photo: M. Barankiewicz)<\/h6>\n
<\/a>\nMeasurement of a captured brown trout (Salmo trutta m. fario<\/em>) during a field fish stock survey (photo: E. Bou\u0161e)<\/h6>\n
<\/a>\nStone loach (Barbatula barbatula<\/em>), an inconspicuous yet significant bottom-dweller of our trout and grayling streams (photo: E. Bou\u0161e)<\/h6>\n
<\/a>\nBrown trout (Salmo trutta m. fario<\/em>) \u2013 an iconic species of clean, cold, and well-oxygenated waters (photo: M. Barankiewicz)<\/h6>\n
<\/a>\nBrook lamprey (Lampetra planeri<\/em>) \u2013 a rare and strictly protected species that is a sensitive indicator of water purity and the natural state of the river (photo: M. Barankiewicz)<\/h6>\n
Acknowledgements<\/h3>\n