EDP Sciences logo
Submit your paper
Search
Menu
All issues No 425 (2024) Knowl. Manag. Aquat. Ecosyst., 425 (2024) 23 Full HTML
Issue
Knowl. Manag. Aquat. Ecosyst.
Number 425, 2024
Riparian ecology and management
Article Number 23
Number of page(s) 14
DOI https://doi.org/10.1051/kmae/2024021
Published online 09 December 2024

© J.D. Champkin et al., Published by EDP Sciences 2024

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License CC-BY-ND (https://creativecommons.org/licenses/by-nd/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. If you remix, transform, or build upon the material, you may not distribute the modified material.

1 Introduction

In recent decades, in response to widespread river degradation, restoration initiatives involving in-stream habitat modifications as well as channel re-profiling and repositioning have become widespread in European and North American rivers (Brun, 2015; Friberg et al., 2016). River restoration initiatives have sought to increase the hydrological connectivity, geomorphological complexity and biological richness of aquatic ecosystems, in turn aiming to ameliorate the effects of past river regulation (Brun, 2015; Deffner and Haase, 2018). In Europe, river restoration has been widely recognized as an integral tool (Schmitt et al., 2018; Deffner and Haase, 2018) in the European Union (EU) Water Framework Directive (WFD). This directive has the aim of returning rivers and streams to ‘Good Ecological Status’ (European Commission, 2015a), which more than half of the surface waters in the EU failed to reach in 2012 and 2018. As implemented in the WFD, fishes are often used as indicators of ecological status (Solimini et al., 2006). Consequently, several studies have assessed the response of fishes to a variety of measures implemented to restore both river systems (e.g. Haase et al., 2013; Lorenz et al., 2013; Smith, 2013) and adjacent floodplain habitats (Stoltefaut et al., 2024). However, evidence for improvements in fish communities following such measures is variable and inconsistent, with some studies reporting positive responses and others minimal beneficial change (e.g. Lorenz et al., 2013; Thompson et al., 2018; Sinclair et al., 2023).

A fish species for which population uplift is urgently needed and river restoration may have an important role for conservation purposes is the critically endangered European eel Anguilla anguilla − a catadromous fish of prominent ecological and economic value (Pike et al., 2020). European eels represent important prey for a range of piscivorous birds and mammals (Knights, 2003; Almeida et al., 2012). The European eel has also been exploited by humans as a food source for centuries, forming the foundation of large commercial fisheries (Feunteun, 2002; Bevacqua et al., 2007; Dekker, 2019) and an aquaculture industry of global significance (Nielsen and Prouzet, 1998). However, the status of the European eel stock is critical, with recruitment undergoing a strong decline since the 1980s, after which it has remained at a very low level (ICES, 2023). This decline has likely had ecological consequences for other species at both lower (prey) and higher (predator) trophic levels. Moreover, there are important economic implications of European eel decline since more than 25,000 people in Europe depend upon eel fisheries and aquaculture for a substantial proportion of their income (Dekker, 2003, 2004). Therefore, a Council Regulation (EU 1100/2007) established a framework for the recovery of European eel stocks and the species was added to CITES Annex II (Ciccotti et al., 2012). Since 2008, the European eel has also been classified globally as critically endangered by the IUCN Red List of Threatened Species (Pike et al., 2020), and more recently, using IUCN criteria, as threatened with extinction in the UK at both regional (Britain) and national (England, Scotland and Wales) levels (Nunn et al., 2023).

Partly due to the complex life cycle of the European eel (Wright et al., 2022), there are several different factors attributed to its decline (Feunteun, 2002; van Ginneken and Maes, 2005; Friedland et al., 2007). These include oceanographic changes (Westerberg, 1998; Kettle et al., 2011), such as declines in Atlantic Ocean productivity (Desaunay and Guerault, 1997; Dekker, 1998) and changes in the Gulf Stream current, which threaten the transport of larvae to the European continent (White and Knights, 1994; Knights et al., 1996). Additionally, several continental human-related factors may have also contributed to the species' decline, particularly artificial barriers (e.g. sea defences, weirs, sluices, watermills and hydroelectric dams: Solomon and Beach, 2004; McCarthy et al., 2008). These may prevent both upstream and downstream migration (Moriarty and Dekker, 1997; Acou et al., 2008) and in some cases cause high mortality (Winter et al., 2006; Brown et al., 2007). Further, human-induced degradation of freshwater habitats including channelization, water abstraction, eutrophication and pollution also negatively affect the European eel (Feunteun, 2002; Starkie, 2003). Therefore, the International Council for the Exploration of the Seas (ICES, 2023) has recently added a strong conservation element in its advice on fishing opportunities for the European eel, stating that all species' habitats should be restored, including connectivity but also chemical, physical and biological features.

To ameliorate the European eel decline, member states of the EU have implemented national Eel Management Plans (EMPs: Aprahamian et al., 2007; Bilotta et al., 2011) aimed at facilitating increased escapement of adults to the Sargasso Sea (European Commission, 2014). Conservation work has centred on mitigating the impacts of barriers to the upstream migration of juveniles through the implementation of eel passes (Knights and White, 1998; Solomon and Beach 2004; Piper et al., 2023). However, several EMPs have also included initiatives to restock freshwater systems with juveniles (Moriarty and Dekker, 1997; Feunteun, 2002; Bevacqua et al., 2015), although the effectiveness of this measure remains unknown (e.g. Haubrock et al., 2019; Rohtla et al., 2021). While river restoration does feature in several regional EMPs (Couldrick et al., 2011), assessments of the effects of habitat improvement strategies on European eel populations are still widely lacking, despite habitat degradation being cited as one of the primary causes of the species' decline (Feunteun, 2002). It is of great importance, therefore, that river restoration implications for the European eel are more widely assessed.

The River Glaven in England (UK) was the subject of an extensive two-phase river restoration initiative involving embankment removal to reconnect the river with its flood plain, followed by re-meandering (Champkin et al., 2018). A previous study evaluated the impact of restoration on general fish communities at Hunworth (Champkin et al., 2018), whereas the current study focuses, in more detail, on European eel density, size structure (length, weight), movement and growth (Objective 1), as well as on the consequences of restoration for the species (Objective 2). To address Objective 1, changes in European eel density and size structure were assessed over the study period in the control reach, with spring and annual growth as well as movement between reaches described. To address Objective 2, a Before‐After‐Control‐Impact (BACI) study was undertaken, comprising one survey prior to embankment removal (pre-restoration: 2009), two surveys following embankment removal but prior to re-meandering (post-embankment removal: 2009–2010) and four surveys subsequently (2011–2014: post-re-meandering). We hypothesized that physical changes in substrata, channel character and meso-habitat resulting from the restoration would have affected European eel growth rates and demographic variables.

2 Methods

2.1 Study area

The River Glaven is a small chalk stream in North Norfolk, eastern England, UK. It rises from headwaters in the Bodham area (52.90° N, 1.13° E) and is 17 km long. It initially flows in a south-westerly direction prior to a sharp turn at Hunworth, after which it continues northwards, discharging into the North Sea at Blakeney Point (Fig. 1). The Glaven River flows over Upper Cretaceous chalk bedrock in turn overlain by chalk-rich sandy till and glaciogenic sand and gravels (Clilverd et al., 2013, 2016). Accordingly, much of the river is classified as chalk stream (Pawley, 2008) − a river type of high conservation importance that is highly vulnerable to human-induced degradation (Bowes et al., 2005). The River Glaven catchment area covers 115 km2 and consists of diverse land use, predominantly arable land, ancient and other deciduous woodland, coniferous plantations (upper river) and grazing meadows (middle and lower reaches).

The River Glaven catchment is of high conservation interest because it supports several species listed under Annex II of the EU Habitats Directive (92/43/EEC). These include white-clawed crayfish Austropotamobius pallipes, European bullhead Cottus gobio, brook lamprey Lampetra planeri and European otter Lutra lutra (European Commission 2015b), in addition to brown trout Salmo trutta and the European eel, which are UK priority species (Joint Nature Conservation Committee, 2014). Nevertheless, many reaches of the River Glaven have suffered a long history of human-induced alteration. These include straightening and relocation of the channel, interruption of longitudinal connectivity through the introduction of mills and weirs, and removal of bankside trees and large in-channel wood pieces (Clilverd et al., 2013;Harwood et al., 2022). Further, as for many lowland European rivers, the River Glaven is degraded by nutrient and fine-sediment pollution associated with upper catchment farmland and the influence of sewage treatment facilities.

The study area comprised the Hunworth meadow river restoration reach immediately downstream of Hunworth Bridge (52.882152° N, 1.0658938° E) and an unmodified control reach just above the bridge (boundary: from 52.882035° N, 1.0661432° E to 52.880925° N, 1.0673284° E) (Fig. 1). The two study reaches are situated around 8 km upstream of the Glaven's tidal limit at Cley at an altitude of around 20 m. Four major barriers (three water mills and one tidal sluice) are present below the study reaches which are in themselves separated by a small Environment Agency gauging weir (Harwood et al., 2022). This isolates the fish populations of either reach to some extent, although not during periods of elevated discharge.

thumbnail Fig. 1

The River Glaven at Hunworth (North Norfolk, eastern England, UK). Both study reaches are shown as is Thornage Mill, downstream of the restoration reach, and the Environment Agency gauging station between the two study reaches. The geographical extent of the River Glaven catchment and its position in the UK are shown in the inset. Image courtesy of H. Clilverd (UCL).

2.2 The Hunworth Meadow restoration

Prior to restoration, the River Glaven was constrained by embankments along the entire length of the Hunworth meadow, which ranged from 0.4 to 1.1 m above the meadow surface (Clilverd et al., 2016). These embankments entirely eliminated flooding at the site. The restoration reach is a 400 × 100 m grazed grassland bordered by a minor road to the south and coniferous woodland and arable land to the north. The river at the site is both alkaline (pH 7.7–8.0) and eutrophic, with phosphate and nitrate nitrogen concentrations of <0.05 mg l−1 and 5.8–7.5 mg l−1 respectively (Clilverd et al., 2013).

The restoration was undertaken in two phases. The first phase was conducted during March 2009 and involved the removal of embankments to reconnect the river with the flood plain (Fig. S1). Then, in August 2010, the river was re-meandered to create a more sinuous channel, with pools and riffles incorporated into the design (Fig. S1). During this process, six backwaters (ranging in length from 3 to 18 m) were also created from remnants of the old river channel and were not in-filled (Sayer, 2014). For the most part, river banks were left to natural plant colonization, although limited planting was undertaken with small patches of locally sourced manna grass Glyceria maxima (from the study reach) used to assist bank stabilization.

The re-meandering increased channel length of the restoration reach from 370 to 430 m and decreased mean channel width by approximately 0.5 m (from ≈3.2 ± 0.4 to ≈ 2.7 ± 0.5 m). This resulted in an overall increase in channel surface area of 407 m2 (from 1549 to 1956 m). Mean water depth did not change significantly following restoration, but the dominant substrate changed from silt/sand to gravel, which increased by >13% between 2009 and 2012. Restoration also increased meso-habitat diversity, with a greater number of deeper pools present at the restoration reach. In the control stretch, mean depth declined by ≈ 23% (from 24.1 ± 2.2 cm in 2009 to 18.4 ± 1.5 cm in 2012) likely as result of reduced discharge, but no change occurred in substrate composition. In addition, the incidence of riffle meso-habitats declined, while runs increased. The prevalence of glides or pools remained unchanged in the control stretch between 2009 and 2012 (Champkin et al., 2018).

2.3 Eel sampling and tagging

The eel population was sampled along the restoration and control reaches of the River Glaven over the course of seven successive sampling events to encompass geomorphological and hydrological conditions prior to and following the two-phase re-naturation scheme. Both reaches were initially sampled in February or March 2009 (pre-restoration) and then subsequently in June of the same year, following embankment removal. Both reaches were sampled again in June 2010, prior to re-meandering works (post-embankment removal), and then over consecutive days in each subsequent May or June for four years after the restoration was completed, most recently in June 2014 (post-re-meandering). The restoration (downstream) reach was always sampled first to avoid disturbance-induced turbid water (due to sediment resuspension while electro-fishing) from the control (upstream) reach making downstream fishing more challenging.

On each sampling date, the reach to be sampled was isolated using upstream and downstream stop nets (8 mm mesh). Each reach was then electro-fished using a 230 V Electracatch control box, 50 Hz Pulsed Direct Current, 2 m twin-tailed cathode and two persons fishing, each with a 300 mm ringed anode and hand net. This was performed in successive removals style (DeLury, 1951), based on three complete runs through the 370 and 160 m restoration and control reaches, respectively, with approximately the same fishing effort (in terms of time and people involved) put into each. All captured fishes were quickly transferred to large, aerated tanks adjacent to the river, identified to species level, with their abundance recorded (Champkin et al., 2018). European eel density was calculated as abundance, estimated using the general weighted k-pass estimator (Carle and Strub, 1978) in the removal function of package FSA (Ogle, 2017) for R v. 4.3.3 (R Core Team, 2024), divided by the area surveyed.

All captured fish were anaesthetized on the bankside in a solution of 2-phenoxyethanol (2 ml l−1), measured for total length (TL, nearest mm) and then weighed (precision 0.1 g). To allow recapture inferences and calculations of growth rate, fish captured from both reaches during the pre-restoration (March 2009) and post-embankment removal sampling (June 2009) were tagged utilizing two distinct tagging methods, depending on body length. Fish larger than 250 mm TL had a passive integrated transponder (PIT) tag inserted into the peritoneal cavity, specifically to lie in the posterior third of this region. Fish < 250 mm TL were injected with visible implant elastomer (VIE) tags, with different colours and bodily locations used to allow for distinctive identification. Following the completion of three electrofishing runs, and once they had fully recovered from the effects of the anaesthesia, all fish were returned to the reach from which they were captured. During successive sampling events, all fish were anaesthetized and scanned with a PIT reader to identify recaptured individuals; if no PIT tag could be detected, then a VI light was used to illuminate the VIE tag. Any untagged fish captured post-embankment removal were also tagged using the methods described above. The numbers of fish caught, tagged and recaptured per each time period are shown in Table 1. All work was carried out under a UK Home Office Project Licence (PPL 80/2302).

During the re-meandering works in August 2010 and prior to the old river channel being backfilled, a ‘fish rescue’ was carried out by a private contract team using the aforementioned sampling approach in order to temporarily remove and relocate fishes. However, these data are derived from short reaches of the existing channel only; as such, they are not representative and were excluded from all population demography analyses. However, tagged fish recorded during this operation were opportunistically used for growth rate calculations (Tab. 1).

Table 1

Numbers of European eels Anguilla anguilla in the River Glaven at Hunworth (north Norfolk, eastern England, UK) caught, tagged and recaptured during the study. n.a. = not applicable.

2.4 Data analysis

Annual growth rates were based upon a fish individual's increase in length between June 2009 and its most recent capture, divided by the duration of the time interval between capture in days. The resulting values for daily growth rate (mm day−1) were then multiplied by 365 to calculate annual (mm year−1) growth rates. For fish not captured in June 2009, calculations of annual growth rates were based upon their original length in February or March 2009 and their most recent capture. Growth increments for a spring period (the approximate three-month period between the initial two sampling events in February or March 2009 and in June 2009) were also calculated, where recaptures allowed.

Several tagged fish had ascended the small weir separating the upstream and downstream reaches. Consequently, the data were separated into three classes: individuals tagged and subsequently always recaptured in the restoration reach (sedentary group 1); individuals tagged and always recaptured in the control reach (sedentary group 2); and individuals captured at least once in each reach (mobile group). Growth rates of all individuals recaptured prior to major restoration work were included when describing mean annual and spring growth rates in the Glaven, but for comparison between reaches, growth rates of all individuals that were captured consecutively in the same reach at least twice were considered.

Changes in European eel density (per 100 m2), length and weight over time were assessed only at the control reach using linear models given potential effects of restoration on observed trends at the restoration reach, with Time period (1–7; see Tab. 1) used as a continuous explanatory variable (Objective 1). Length and weight were log-transformed prior to being fitted as response variables to comply with the linear regression assumptions of normality and homoscedasticity of model residuals. ANOVA was used to compare annual and spring growth rates of individuals between the restoration and control reaches. Annual growth rates were also compared between mobile and sedentary individuals as well as VIE-tagged and PIT-tagged individuals using ANOVA to see if there were any differences in growth between different groups.

Impact of restoration on European eel density (per 100 m2), length and weight were assessed using a BACI design, with one sampling event pre-restoration, two post-embankment removal, and four following re-meandering (Objective 2). Due to the lack of tagged fish captured at least twice following the re-naturation (n = 3 over both reaches: Table 1), it was not possible to assess impact of restoration on eel growth rates. In the BACI design, a statistically significant effect is identified if a change in any of the response variables is detected at the restoration reach following intervention relative to the control reach (Schwarz, 2014), so that the parameter of interest is the Reach × Treatment period interaction. Given small sample sizes and lack of replicates, a randomization approach was used to test for impact of restoration on European eel density. Abundance data were bootstrapped from the normal distribution using mean and standard deviation values from the Carle and Strub model, within the stratification Reach, Treatment and Time period (10,000 iterations). These bootstrapped values were then transformed into densities and averaged per Treatment and Reach. Differences between the restoration and control reaches was calculated for each Treatment, and resulting boxplots were compared for overlap to determine if there was any statistically significant difference (α = 0.05) between different treatments. To test for impact of restoration on European eel length and weight, linear models were fitted, with log-transformed eel length and weight used as response variables and Reach (restoration vs control reach), Treatment (pre-restoration vs post-embankment removal vs post-re-meandering) and their interaction used as fixed effects.

Linear models were fitted using package lme4 (Bates et al., 2015) of R and evaluated by an F-test with the Anova function in the car package (Fox and Weisberg, 2011). When testing for effect of Time period, as well as restoration (Reach × Treatment) on European eel length and weight, fish ID was not fitted as a random effect on the intercept given the small overall proportion of tagged and recaptured eel individuals (Tab. 1) and thus any effect of autocorrelation was considered negligible. Marginal effect plots were constructed in R package ggplot2 (Wickham, 2009). Model residuals were tested against the assumptions of normality, homogeneity, and independence using standard graphical validations (Figs. S2–S6; Zuur et al., 2009).

3 Results

The density of European eel showed a pronounced decrease between 2009 and 2014 in the control reach (F1,5 = 44.31, P < 0.05), with mean density decreasing by 1.5 individuals 100 m−2 (Fig. 2). The size structure of the population in the control reach also changed, with both the length (F1,152 = 11.65, P < 0.05) and weight of individual fish (F1,152 = 16.46, P < 0.05) showing increases of 11.5 mm year−1 and 6.2 g year−1, on average, respectively between 2009 and 2014 (Fig. 2).

Of the 59 recaptured individuals, the majority (73%) showed high site fidelity and were classified as sedentary, with some individuals being captured at the same reach three to four times during the study period. Sixteen individuals were observed to move from one reach to another and were thus classified as mobile. Most of these mobile individuals (75%) moved upstream. Two mobile individuals undertook multiple trips over the weir from one reach to another and back to the initial reach.

Spring and annual growth rates of all individuals recaptured prior to restoration ranged from −5 to 36 mm year−1 (mean ± SE = 7.2 ± 2.0 mm year−1) and −5 to 104 mm year−1 (mean ± SE = 35.8 ± 2.5 mm year−1), respectively. One individual exhibited a negative annual growth rate, three individuals had negative spring growth rates, and three individuals did not grow over the spring period. When comparing all VIE-tagged and PIT-tagged individuals, no difference was found in annual growth rates (F1,55 = 2.98, P = 0.09). Similarly, mobile (mean ± SE = 34.9 ± 3.8 mm year−1) and sedentary individuals (mean ± SE = 34.1 ± 2.3 mm year−1) exhibited similar growth rates (F1,55 = 0.47, P = 0.50). Mean annual growth of sedentary eels was 35% greater in individuals residing at the restoration reach (37.56 ± 3.00 mm year−1) than at the control reach (27.90 ± 3.68 mm year−1), albeit marginally not statistically significant (F1, 33 = 3.60, P = 0.06; Fig. 3). There was no difference in the spring growth rate of eels between the two reaches (F1, 24 = 0.48, P > 0.05; Fig. 3). Both spring and annual growth rates of individuals at either reach varied widely, and several individuals resident in the restoration reach increased in length by over 30 mm between March and June 2009, while others showed very little or no change in length over the same period.

There was no difference in European eel density between the control and restoration reaches post-embankment removal, but density had slightly increased in the restoration reach compared to the control reach following re-meandering, although was not statistically significant when compared to the pre-restoration phase (Fig. 4). Habitat improvement also had no effect on European eel length or weight as indicated by a lack of significant BACI interaction term, i.e. Reach × Treatment (Tab. 2; Figs 5 and 6).

thumbnail Fig. 2

Marginal effects from linear models showing significant changes in density, length, and weight of European eel Anguilla anguilla during the study period in the control reach. Error bars represent 95% confidence intervals of the estimated effects.
thumbnail Fig. 3

Spring growth (March–June 2009) and annual growth rate of two populations of European eel resident in the two adjacent reaches of the Hunworth restoration reach, calculated using length data of tagged and recaptured individuals between 2009 and 2012. Boxes represent all values between the 25th and 75th quartiles and show the median (—) and mean (•) for each category. Box plot whiskers represent minimum and maximum calculated growth increments for each population. Outliers (+) are also shown. n = number of individuals from which growth rate was calculated.
thumbnail Fig. 4

Boxplots showing changes in density of European eel in the restoration and control reaches of the River Glaven during the study period following a randomization approach. Top plot: boxplot showing bootstrapped density values during pre-restoration (1), post-embankment removal (2–3) and post-re-meandering (4–7). Bottom left plot: boxplot showing averaged bootstrapped densities per Treatment and Reach, where a = pre-restoration, b = post-embankment removal, and c = post-re-meandering. Bottom right plot: boxplot showing difference in bootstrapped densities between restoration and control reach per Treatment. Dark black horizontal line represents 50% of the data, light black horizonal lines represent 25% and 75% of the data (Interquartile Range − IQR) and light black vertical lines minimun and maximum data values in comparison to IQR. Dark black dots represent outliers.
Table 2

Statistical results of ANOVA applied to linear model testing for changes in European eel length and weight in the two reaches of the River Glaven pre-restoration, post-embankment removal, and post-re-meandering (Treatment). * Denotes significant effect at α = 0.05.

thumbnail Fig. 5

Plots showing changes in the length of European eel in the restoration and control reaches of the River Glaven during the study period. Top plot: line plot showing mean length with 95% confidence intervals in restoration and control reaches during pre-restoration (1), post-embankment removal (2–3) and post-re-meandering (4–7). Emanating vertical dotted lines denote embankment removal and re-meandering. Bottom plot: marginal effects plot from the linear model showing no significant impact of interaction of Reach and Treatment on eel length, where a = pre-restoration, b = post-embankment removal, and c = post-re-meandering. The error bars represent 95% confidence intervals of the estimated effects.
thumbnail Fig. 6

Plots showing changes in the weight of European eel in the restoration and control reaches of the River Glaven during the study period. Top plot: line plot showing the mean weight with 95% confidence intervals in restoration and control reaches during pre-restoration (1), post-embankment removal (2–3) and post-re-meandering (4–7). Emanating vertical dotted lines denote embankment removal and re-meandering. Bottom plot: marginal effects plot from the linear model showing no significant impact of interaction of Reach and Treatment on eel weight, where a = pre-restoration, b = post-embankment removal, and c = post-re-meandering. The error bars represent 95% confidence intervals of the estimated effects.

4 Discussion

Habitat improvement measures have been identified as important to European eel conservation in many catchments under regional EMPs (Couldrick et al., 2011). Despite this, our study is one of the first to assess the local effects of a typical river restoration initiative on the European eel population in detail (see Champkin et al., 2018).

The European eel population in the control reach of the River Glaven declined over the study period, with a concurrent increase in the length and weight of individuals. This likely relates to more general trends in European eel escapement and recruitment over the study period. Specifically, decreasing European eel density in the River Glaven can be linked to the overall decline in recruitment observed across the species' distribution range. A northward shift in the position of the Gulf Stream was associated with reductions in elver catches in most European catchments throughout the 1980s and early 1990s (White and Knights, 1994). Therefore, climate change-induced oceanographic changes with consequent shortages of available food resources (Bonhommeau et al., 2008) may have decreased recruitment of European eel larvae to the East Anglian coast (Chang et al., 2019) and hence the River Glaven. At a local river scale, barriers to upstream migration can represent one of the main factors determining the demography and density of European eel populations (White and Knights, 1997). Very few man-made barriers in British rivers are completely impassable by the European eel, and consequently the proportion of freshwater habitat rendered inaccessible by such structures (<5%) is much less than in other European countries where hydropower stations and large dams prevail (Moriarty and Dekker, 1997). Nevertheless, river flows for most lowland British rivers are interrupted by numerous mills, weirs, sluices and tidal gates, which can prevent the complete colonization of habitats by European eel (Moriarty and Dekker, 1997; Lasne and Laffaille, 2008).

European eels tend to migrate progressively further upstream throughout their development, such that their mean age and size are generally greater in upper catchment areas, while abundance is accordingly lower (Feunteun et al., 2003;Couldrick et al., 2011). Evidence from recaptured individuals in this study suggests that many eels lead a relatively sedentary lifestyle and show high site fidelity in the River Glaven prior to maturation, with some individuals being recaptured in the same short reach up to three years after initially being tagged. Other tagging and telemetry studies that have tracked movements in the freshwater stage of catadromous Anguillidae species in small streams and marsh ecosystems have demonstrated similar behaviour (Baras et al., 1998; Laffaille et al., 2005; Ovidio et al., 2013). Many tagged individuals recaptured in this study exceeded 350 mm at which size a proportion of an eel population (i.e. males) is known to mature and begin seaward migration (Tesch, 2003). This suggests that eels could have vacated the river system over the survey period, but did not.

The growth rates of yellow-stage European eels are typically highly variable between individuals and among different water bodies (Moriarty, 2003), being dependent on factors such as habitat characteristics, water temperature, population density and food availability (Tesch, 2003). This concurs with the present findings in which calculated annual growth rates varied extensively between individuals. Nevertheless, the mean annual growth rate of 36 mm year−1 was consistent with values calculated for European eel populations in other British river systems: e.g. 38 mm year−1 in the upper River Thames (Naismith and Knights, 1993), 31 mm year−1 in the River Frome, southern England (Mann and Blackburn, 1991), 33 mm year−1 in the Barrow River, Northern Ireland (Moriarty, 1983), and 16–28 mm year−1 in the lower River Severn, western England (Aprahamian, 2000). Higher annual and spring growth rates were observed for European eels at the restoration reach, although this was not statistically significant.

In this study, no significant response of the European eel population to habitat restoration was detected during the study period. This could suggest that increases in meso-habitat heterogeneity were not sufficient to initiate a change in the size structure or density of the European eel population, that other factors may be more important, or that the period monitored after the restoration measure was not sufficiently long (Didham et al., 2020). Although European eels inhabit a great diversity of freshwater meso-habitats (Ovidio et al., 2013), they are typically closely associated with cryptic areas comprising soft (silt) substrate, high aquatic macrophyte cover and crevices between boulders, roots or beneath undercut banks in which they seek refuge from predators and bright light (Ovidio et al., 2013). Recently, in a study of European eel distribution and habitat preferences in Swedish coastal rivers and streams, it was concluded that restoration efforts aimed at enhancing European eel populations should focus on the lower reaches of larger rivers (where European eels are more abundant) with stony substrates (Degerman et al., 2019). However, in the River Glaven itself, Harwood et al. (2022) did not find a strong positive relationship between European eel occurrence and coarse substrates, instead showing that presence of eels was significantly benefited by overhanging branches and in-stream large wood. This may be due to the vastly different bed structure of the Glaven compared to a typical rocky Swedish stream, where large boulders are present. Given the association between large wood habitat and European eel occurrence in the wider River Glaven catchment, it is possible that habitat modifications were not beneficial for European eel because they failed to provide suitable refuge habitat. Both before and after restoration, overhanging trees were rare and the restoration section entirely lacks tree root and in-stream large wood habitat. In the context of Eurasian otter recovery in the catchment for which European eel is a key diet item (Almeida et al., 2012), absence of sufficient predation-refuges in fallen trees and associated debris dams may be of critical importance.

As they grow, European eels demonstrate ontogenetic shifts in meso-habitat preference in fresh water (Laffaile et al., 2003, 2004). Smaller individuals (<300 mm) typically inhabit shallower regions with a greater density of aquatic macrophytes and deeper silt substrate, whereas larger individuals are most commonly associated with slow-flowing, deeper water courses (Lafaille et al., 2004; Couldrick et al., 2011) that have less macrophyte cover (Lafaille et al., 2003, 2004). These findings suggest that, to preserve and promote freshwater habitats for European eel, a heterogeneous patchwork of habitats must be maintained throughout a catchment. Thus, it cannot be concluded that the Hunworth restoration will not be beneficial for European eel when considered at the whole-catchment scale. In addition, the lack of significant changes in eel density and size structure could be an artefact of small sample size, lack of replicates and unbalanced design, which could have hampered the detection of positive effects on the eel population. Future studies are therefore required that incorporate an adequate experimental design to see if the patterns observed immediately following restoration are upheld in the longer term. This is especially pertinent given the return of trees to the banks of the restoration reach in the present day (C. Sayer, pers. obs).

5 Conclusion

Whilst river restoration initiatives can be beneficial to fish assemblages (Hohausová and Jurajda, 2005; Summers et al., 2008; Lorenz et al., 2013), many studies show that positive responses of fish populations to such approaches are not assured (e.g. Smith, 2013; Sinclair et al., 2023). Previous studies recommend larger-scale rehabilitation efforts to boost fish density and diversity, advising a focus on catchment-scale improvements such as barrier removal and enhanced connectivity, and addressing water quality issues including eutrophication, sedimentation and pollution for ecosystem recovery (Champkin et al., 2018). This includes small coastal streams, which may have an important, but overlooked, role in conservation strategies for European eel (Copp et al., 2021). Given its small size, the River Glaven could be more important as a conduit for eel upstream and downstream passage, rather than as permanent habitat (Harwood et al., 2022). Other studies have also concluded that the restoration and protection of wider wetland ecosystems, which are among the most important freshwater macro-habitats for European eels (Laffaille et al., 2004), would be more beneficial than river restoration (Eybert et al., 1998). In the River Glaven, it is known that outfall sluices from old artificial lakes act as considerable barriers for European eels, with substantial aggregations observed for elvers downstream of these structures (Harwood et al., 2022). Thus, future eel conservation work might be best focused on addressing wider-scale river-to-lake European eel passage issues. Due to its complex life cycle, the European eel is threatened by different factors, from the global to the local scale, and these interactions make it difficult to assess the effectiveness of small-scale conservation projects (Bevacqua et al., 2015). Reducing fishing pressure (Lyach, 2022), tackling the illegal trade in glass eels (Richards et al., 2020) and implementing habitat restoration plans (as here), may not yield significant contributions to the restoration of fish stocks if they are not integrated into a cohesive management strategy (Ciccotti et al., 2012). In this case of the Hunworth restoration reach, further investigation is required to establish whether European eel and other fishes will derive benefits from the river restoration initiative described herein over the longer term.

Acknowledgments

We thank the Stody Estate for access to the study site. A special thanks to Dr Stephen D. Gregory for providing valuable feedback on this manuscript. The Wild Trout Trust, Norfolk Wildlife Trust, Natural England, R. D. Hey and the River Glaven Conservation Group facilitated the river rehabilitation work. The study was partly funded by the Department for Environment, Food, and Rural Affairs (Defra).

Supplementary Material

Fig. S1. Restoration reach of the River Glaven at Hunworth (north Norfolk, UK): (a) in January 2009, prior to the renaturation project; (b) after removal of embankments in March 2009; and (c) in December 2010, after recreation of meanders in August.

Fig. S2. Diagnostic plots suggesting that the assumptions of normally distributed, homoscedastic and independent residuals of linear model testing for effect of Time period on eel density at the control reach were not severely violated.

Fig. S3. Diagnostic plots suggesting that the assumptions of normally distributed, homoscedastic and independent residuals of linear model testing for effect of Time period on eel length at the control reach were not severely violated.

Fig. S4. Diagnostic plots suggesting that the assumptions of normally distributed, homoscedastic and independent residuals of linear model testing for effect of Time period on eel weight at the control reach were not severely violated.

Fig. S5. Diagnostic plots suggesting that the assumptions of normally distributed, homoscedastic and independent residuals of linear model testing for effect of interaction of Reach and Treatment on eel length were not severely violated.

Fig. S6. Diagnostic plots suggesting that the assumptions of normally distributed, homoscedastic and independent residuals of linear model testing for effect of interaction of Reach and Treatment on eel weight were not severely violated.

Access here

References

  • Acou A, Laffaille P, Legault A, Feunteun E. 2008. Migration pattern of silver eel (Anguilla anguilla, L.) in an obstructed river system. Ecol Freshw Fish 17: 432–442. [CrossRef] [Google Scholar]
  • Almeida D, Copp GH, Masson L, Miranda R, Murai M, Sayer CD. 2012. Changes in the diet of a recovering Eurasian otter population between the 1970s and 2010. Aquat Conserv 22: 26–35. [CrossRef] [MathSciNet] [Google Scholar]
  • Aprahamian MW. 2000. The growth rate of eel in tributaries of the lower River Severn, England, and its relationship with stock size. J Fish Biol 56: 223–227. [CrossRef] [Google Scholar]
  • Aprahamian MW, Walker AM, Williams B, Bark A, Knights B. 2007. On the application of models of European eel (Anguilla anguilla) production and escapement to the development of Eel Management Plans: the River Severn. ICES J Mar Sci 64: 1472–1482. [CrossRef] [Google Scholar]
  • Baras E, Jeandrain D, Serouge B, Philippart JC. 1998. Seasonal variations in time and space utilization by radio-tagged yellow eels Anguilla anguilla (L.) in a small stream. In Lagardère JP, Anras, M.L.B., Claireaux, G. (Eds.), Advances in invertebrates and fish telemetry (pp. 187–198). Developments in Hydrobiology Vol. 130. Dordrecht: Springer. [CrossRef] [Google Scholar]
  • Bates D, Maechler M, Bolker B, Walker S. 2015. Fitting linear mixed-effects models using lme4. J Stat Softw 67: 1–48. [Google Scholar]
  • Bevacqua D, Melia P, Crivelli AJ, Gatto M, De Leo GA. 2007. Multi-objective assessment of conservation measures for the European eel (Anguilla anguilla): an application to the Camargue lagoons. ICES J Mar Sci 64, 1483–1490. [CrossRef] [Google Scholar]
  • Bevacqua D, Melià P, Gatto M, De Leo GA. 2015. A global viability assessment of the European eel. Global Change Biol 21: 3323–3335. [CrossRef] [PubMed] [Google Scholar]
  • Bilotta GS, Sibley P, Hateley J, Don A. 2011. The decline of the European eel Anguilla anguilla: quantifying and managing escapement to support conservation. J Fish Biol 78: 23–38. [CrossRef] [PubMed] [Google Scholar]
  • Bonhommeau S, Chassot E, Planque B, Rivot E, Knap AH, Le Pape O. 2008. Impact of climate on eel populations of the Northern Hemisphere. Mar Ecol Prog Ser 373: 71–80. [CrossRef] [Google Scholar]
  • Bowes MJ, Leach DV, House WA. 2005. Seasonal nutrient dynamics in a chalk stream: the River Frome, Dorset, UK. Sci Total Environ 336: 225–241. [CrossRef] [PubMed] [Google Scholar]
  • Brown L, Boubee J, Haro A. 2007. Behaviour and fate of downstream migrating eels at hydroelectric power station intakes, in 6th International Symposium on Ecohydraulics, Christchurch Convention Centre: New Zealand, 18–23 Feb 2007. [Google Scholar]
  • Brun A. 2015. The “Renaturation” of urban rivers: the case of the St Charles River in Quebec. In Grafton Q, Daniell K, Nauges C, Rinaudo JD, Chan N. (Eds.) Understanding and managing urban water in transition (pp. 527–548). Global Issues in Water Policy Vol. 15. Dordrecht: Springer. [CrossRef] [Google Scholar]
  • Carle FL, Strub MR. 1978. A new method for estimating population size from removal data. Biometrics 34: 621–630. [CrossRef] [Google Scholar]
  • Champkin JD, Copp GH, Sayer CD, Clilverd H, George L, Vilizzi L, Godard MJ, Clarke J, Walker AM. 2018. Responses of fishes and lampreys to the re-creation of meanders in a small English chalk stream. River Res Appl 34: 34–43. [CrossRef] [Google Scholar]
  • Chang YLK, Miyazawa Y, Miller MJ, Tsukamoto K. 2019. Influence of ocean circulation and the Kuroshio large meander on the 2018 Japanese eel recruitment season. PloS ONE 14: e0223262. [CrossRef] [PubMed] [Google Scholar]
  • Ciccotti E, Leone C, Bevacqua D, De Leo G, Tancioni L, Capoccioni F. 2012. Integrating habitat restoration and fisheries management: a small-scale case-study to support eel conservation at the global scale. Knowl Manag Aquatic Ecosyst 407: 04. [CrossRef] [EDP Sciences] [Google Scholar]
  • Clilverd HM, Thompson JR, Heppell CM, Sayer CD, Axmacher JC. 2013. River-floodplain hydrology of an embanked lowland chalk river and initial response to embankment removal. Hydrolog Sci J 58: 627–650. [CrossRef] [Google Scholar]
  • Clilverd HM, Thompson JR, Heppell CM, Sayer CD, Axmacher JC. 2016. Coupled hydrological/hydraulic modelling of river restoration impacts and floodplain hydrodynamics. River Res Appl 32: 1927–1948. [CrossRef] [Google Scholar]
  • Copp GH, Daverat F, Bašić T. 2021. The potential contribution of small coastal streams to the conservation of declining and threatened diadromous fishes, especially the European eel. River Res Appl 37: 111–115. [CrossRef] [Google Scholar]
  • Couldrick P, Walker M, Maltby AM. 2011. Habitat improvement strategy for the European eel (Anguilla anguilla), in 2011 Association of Rivers Trusts, July 2011. [Google Scholar]
  • Deffner J, Haase P. 2018. The societal relevance of river restoration. Ecol Soc 23: 35. [CrossRef] [Google Scholar]
  • Degerman E, Tamario C, Watz J, Nilsson PA, Calles O. 2019. Occurrence and habitat use of European eel (Anguilla anguilla) in running waters: lessons for improved monitoring, habitat restoration and stocking. Aquat Ecol 53: 639–650. [CrossRef] [Google Scholar]
  • Dekker W. 1998. Long-term trends in the glasseels immigrating at Den Oever, the Netherlands. Bulletin français de la pêche et de la pisciculture 349: 199–214. [CrossRef] [EDP Sciences] [Google Scholar]
  • Dekker W. 2003. Status of the European eel stock and fisheries. In Aida K, Tsukamoto, K. & Yamauchi, K. (Eds.), Eel biology (pp. 237–254). Tokyo: Springer-Verlag. [CrossRef] [Google Scholar]
  • Dekker W. 2004. Slipping through our hands. Population dynamics of the European eel. PhD Thesis, University of Amsterdam. [Google Scholar]
  • Dekker W. 2019. The history of commercial fisheries for European eel commenced only a century ago. Fish Manag Ecol 26: 6–19. [CrossRef] [Google Scholar]
  • DeLury DB. 1951. On the planning of experiments for the estimation of fish populations. J Fish Res Board Canada 8b: 281–307. [CrossRef] [Google Scholar]
  • Desaunay Y, Guerault D. 1997. Seasonal and long-term changes in biometrics of eel larvae: a possible relationship between recruitment variation and North Atlantic ecosystem productivity. J Fish Biol 51: 317–339. [CrossRef] [Google Scholar]
  • Didham RK, Basset Y, Collins CM, Leather SR, Littlewood NA, Menz MH, Müller J, Packer L, Saunders ME, Schönrogge K, Stewart AJ. 2020. Interpreting insect declines: seven challenges and a way forward. Insect Conserv Divers 13: 103–114. [CrossRef] [Google Scholar]
  • European Commission. 2014. Eel: National management plans. http://ec.europa.eu/fisheries/marine_species/wild_species/eel/management_plans/index_en.htm [Google Scholar]
  • European Commission. 2015a. Water Framework Directive − Ecological status and intercalibration. http://ec.europa.eu/environment/water/water-framework/objectives/status_en.htm [Google Scholar]
  • European Commission. 2015b. Environment: Species protected under the Habitats Directive. http://ec.europa.eu/environment/nature/conservation/species/habitats_dir_en.htm [Google Scholar]
  • Eybert MC, Bernard JY, Constant P, Feunteun E, Hédin J, Questiau S. 1998. Marsh meadow restoration in the brière marsh (France): evolution of flora, birds and fishes. Gibier Faune Sauvage 15: 987–1016. [Google Scholar]
  • Feunteun E. 2002. Management and restoration of European eel population (Anguilla anguilla): an impossible bargain. Ecol Eng 18: 575–591. [CrossRef] [Google Scholar]
  • Feunteun E, Laffaille P, Robinet T, Briand C, Baisez A, Olivier J-M., Acou A. 2003. A review of upstream migration and movements in inland waters by anguillid eels: toward a general theory. In Aida K, Tsukamoto, K. & Yamauchi, K. (Eds.) Eel biology (pp. 191–213). Tokyo: Springer-Verlag. [CrossRef] [Google Scholar]
  • Fox J, Weisberg S. 2011. An {R} companion to applied regression. Second Edition. Thousand Oaks CA: Sage. [Google Scholar]
  • Friberg N, Angelopoulos NV, Buijse AD, Cowx IG, Kail J, Moe TF, Moir H, O'Hare MT, Verdonschot PFM, Wolter C. 2016. Effective river restoration in the 21st century: from trial and error to novel evidence-based approaches. In Advances in ecological research (Vol. 55, pp. 535–611). Academic Press. [CrossRef] [Google Scholar]
  • Friedland KD, Miller MJ, Knights B. 2007. Oceanic changes in the Sargasso Sea and declines in recruitment of the European eel. ICES J Mar Sci 64: 519–530. [CrossRef] [Google Scholar]
  • Haase P, Hering D, Jähnig SC, Lorenz AW, Sundermann A. 2013. The impact of hydromorphological restoration on river ecological status: a comparison of fish, benthic invertebrates and macrophytes. Hydrobiologia 704: 475–488. [CrossRef] [Google Scholar]
  • Harwood AJ, Perrow MR, Sayer CD, Piper AT, Berridge RJ, Patmore IR, Emson D, Cooper G. 2022. Catchment‐scale distribution, abundance, habitat use, and movements of European eel (Anguilla anguilla L.) in a small UK river: implications for conservation management. Aquat Conserv 32: 797–816. [CrossRef] [Google Scholar]
  • Haubrock PJ, Balzani P, Criado A, Inghilesi AF, Tricarico E, Monteoliva AP. 2019. Predicting the effects of reintroducing a native predator (European eel, Anguilla anguilla) into a freshwater community dominated by alien species using a multidisciplinary approach. Manag Biol Invas 10: 171–191. [Google Scholar]
  • Hohausová E, Jurajda P. 2005. Restoration of a river backwater and its influence on fish assemblage. Czech J Anim Sci 50: 473–482. [CrossRef] [Google Scholar]
  • ICES. 2023. European eel (Anguilla anguilla) throughout its natural range. In Report of the ICES. [Google Scholar]
  • Joint Nature Conservation Committee. 2014. UK BAP priority fish species (excluding purely marine species). http://jncc.defra.gov.uk/page-5164 [Google Scholar]
  • Kettle AJ, Asbjørn Vøllestad L, Wibig J. 2011. Where once the eel and the elephant were together: decline of the European eel because of changing hydrology in southwest Europe and northwest Africa? Fish Fish 12: 380–411. [CrossRef] [Google Scholar]
  • Knights B. 2003. Enhancing stocks of European eel, Anguilla anguilla, to benefit bittern, Botaurus stellaris. In Cowx IG (Ed.), Interactions between fish and birds: Implications for management (pp. 298–313). Oxford, UK: Blackwell Science Ltd. [CrossRef] [Google Scholar]
  • Knights B, White EM. 1998. Enhancing immigration and recruitment of eels: the use of passes and associated trapping systems. Fish Manag Ecol 5: 459–471. [CrossRef] [Google Scholar]
  • Knights B, White E, Naismith IA. 1996. Stock assessment of European eel, Anguilla anguilla (L.) In Cowx, I.G. (Ed.) Stock assessment in inland fisheries (pp. 431–447). Oxford, UK: Fishing News Books. [Google Scholar]
  • Laffaille P, Feunteun E, Baisez A, Robinet T, Acou A, Legault A, Lek S. 2003. Spatial organisation of European eel (Anguilla anguilla L.) in a small catchment. Ecol Freshw Fish 12: 254–264. [Google Scholar]
  • Laffaille P, Baisez A, Rigaud C, Feunteun E. 2004. Habitat preferences of different European eel size classes in a reclaimed marsh: a contribution to species and ecosystem conservation. Wetlands 24: 642–651. [CrossRef] [Google Scholar]
  • Laffaille P, Acou A, Guillouet J. 2005. The yellow European eel (Anguilla anguilla L.) may adopt a sedentary lifestyle in inland freshwaters. Ecol Freshw Fish 14: 191–196. [CrossRef] [Google Scholar]
  • Lasne E, Laffaille P. 2008. Analysis of distribution patterns of yellow European eels in the Loire catchment using logistic models based on presence-absence of different size classes. Ecol Freshw Fish 17: 30–37. [CrossRef] [Google Scholar]
  • Lorenz AW, Stoll S, Sundermann A, Haase P. 2013. Do adult and YOY fish benefit from river restoration measures? Ecol Eng 61: 174–181. [CrossRef] [Google Scholar]
  • Lyach R. 2022. The effect of fishery management on the yield of the critically endangered European eel Anguilla anguilla in mesotrophic rivers and streams in Central Europe. Fishes 7: 42. [CrossRef] [Google Scholar]
  • Mann RHK, Blackburn JH. 1991. The biology of the eel Anguilla anguilla (L. ) in an English chalk stream and interactions with juvenile trout Salmo trutta L. and salmon Salmo salar L. Hydrobiologia 218: 65–76. [Google Scholar]
  • McCarthy TK, Frankiewicz P, Cullen P, Blaszkowski M, O'Connor W, Doherty D. 2008. Long-term effects of hydropower installations and associated river regulation on River Shannon eel populations: mitigation and management. Hydrobiologia 609: 109–124. [CrossRef] [Google Scholar]
  • Moriarty C. 1983. Age determination and growth of eels, Anguilla anguilla (L.). J Fish Biol 23: 257–264. [CrossRef] [Google Scholar]
  • Moriarty C. 2003. The yellow eel. In Aida K, Tsukamoto, K. & Yamauchi, K. (Eds.), Eel biology (pp. 89–105). Tokyo: Springer-Verlag. [CrossRef] [Google Scholar]
  • Moriarty C, Dekker W. 1997 Management of European eel fisheries. Second report of EC Concerted Action AIR A94-1939, Irish Fisheries Bulletin No. 15. [Google Scholar]
  • Naismith IA, Knights B. 1993. The distribution, density and growth of the European eel, Anguilla anguilla, in the freshwater catchment of the River Thames. J Fish Biol 42: 217–226. [CrossRef] [Google Scholar]
  • Nielsen T, Prouzet P. 1998. Capture-based aquaculture of the wild European eel (Anguilla anguilla). In Lovatelli A, Holthus, P.F. (Eds.), Capture-based aquaculture. Global overview (pp. 141–168). FAO Fisheries Technical Paper No. 508, Rome, Italy. [Google Scholar]
  • Nunn AD, Ainsworth RF, Walton S, Bean CW, Hatton-Ellis TW, Brown A, Evans R, Atterborne A, Ottewell D, Noble RAA. 2023. Extinction risks and threats facing the freshwater fishes of Britain. Aquat Conserv 33: 1460–1476. [CrossRef] [Google Scholar]
  • Ogle DH. 2017. FSA: Fisheries Stock Analysis. R package version 0.8. 14. [Google Scholar]
  • Ovidio M, Seredynski AL, Philippart JC, Matondo BN. 2013. A bit of quiet between the migrations: the resting life of the European eel during their freshwater growth phase in a small stream. Aquat Ecol 47: 291–301. [CrossRef] [Google Scholar]
  • Pawley SM. 2008. The Glaven Valley (Glandford Quarry) (TG 055415). In Candy I, Lee R, Harrison A.M. (Eds.), The Quaternary of northern East Anglia, Devon, England (pp. 192–203). Quaternary Research Association. [Google Scholar]
  • Pike C, Crook V, Gollock M. 2020. Anguilla anguilla. The IUCN Red List of Threatened Species 2020: eT60344A152845178. [Google Scholar]
  • Piper AT, Rosewarne PJ, Pike C, Wright RM. 2023. The eel ascending: the influence of lateral slope, climbing substrate and flow rate on eel pass performance. Fishes 8: 612. [CrossRef] [Google Scholar]
  • R Core Team. 2024. R: A language and environment for statistical computing. Vienna, Austria: R Foundation for Statistical Computing. https://www.R-project.org/ [Google Scholar]
  • Richards JL, Sheng V, Yi CW, Ying CL, Ting NS, Sadovy Y, Baker D. 2020. Prevalence of critically endangered European eel (Anguilla anguilla) in Hong Kong supermarkets. Sci Adv 6: eaay0317. [CrossRef] [PubMed] [Google Scholar]
  • Rohtla M, Silm M, Tulonen J, Paiste P, Wickström H, Kielman-Schmitt M, Kooijman E, Vaino V, Eschbaum R, Saks L, Verliin A. 2021. Conservation restocking of the imperilled European eel does not necessarily equal conservation. ICES J Mar Sci 78: 101–111. [CrossRef] [Google Scholar]
  • Sayer CD. 2014. Conservation of aquatic landscapes: ponds, lakes, and rivers as integrated systems. Water 1: 573–585. [Google Scholar]
  • Schmitt K, Schäffer M, Koop J, Symmank L. 2018. River bank stabilisation by bioengineering: potentials for ecological diversity. J Appl Water Eng Res 6: 262–273. [Google Scholar]
  • Schwarz CJ. 2014. Analysis of BACI experiments. Course Notes for Beginning and Intermediate Statistics http://www.stat.sfu.ca/∼cschwarz/CourseNotes [Google Scholar]
  • Sinclair JS, Mademann JA, Haubrock PJ, Haase P. 2023. Primarily neutral effects of river restoration on macroinvertebrates, macrophytes, and fishes after a decade of monitoring. Restorat Ecol 31: e13840. [CrossRef] [Google Scholar]
  • Smith MA. 2013. Outcomes of river renaturation on instream hydraulics and fish communities. PhD Thesis, University of Hull. [Google Scholar]
  • Solimini AG, Cardoso AC, Heiskanen A. (Eds.) 2006. Indicators and methods for the ecological status assessment under the Water Framework Directive: Linkages between chemical and biological quality of surface waters. European Commission: Joint Research Centre. [Google Scholar]
  • Solomon DJ, Beach MH. 2004. Fish pass design for eel and elver (Anguilla anguilla). Environment Agency R&D Technical Report W2-070/TR, Bristol, UK. [Google Scholar]
  • Starkie A. 2003. Management issues relating to the European eel, Anguilla anguilla. Fish Manag Ecol 10: 361–364. [CrossRef] [Google Scholar]
  • Stoltefaut T, Haubrock PJ, Welti EA, Baker NJ, Haase P. 2024. A long-term case study indicates improvements in floodplain biodiversity after river restoration. Ecol Eng 198: 107143. [CrossRef] [Google Scholar]
  • Summers DW, Giles N, Stubbing DN. 2008. Renaturation of brown trout, Salmo trutta, habitat damaged by riparian grazing in an English chalkstream. Fish Manag Ecol 15: 231–240. [CrossRef] [Google Scholar]
  • Tesch FW. 2003. The Eel. Fifth ed. Oxford: Blackwell Publishing. [Google Scholar]
  • Thompson MSA, Brooks SJ, Sayer CD, Woodward G, Axmacher JC, Perkins DM, Gray C. 2018. Large woody debris "rewilding" rapidly restores biodiversity in riverine food webs. J Appl Ecol 55: 895–904. [CrossRef] [Google Scholar]
  • van Ginneken VJT, Maes GE. 2005. The European eel (Anguilla anguilla, Linnaeus), its lifecycle, evolution and reproduction: a literature review. Rev Fish Biol Fish 15: 367–398. [CrossRef] [Google Scholar]
  • Westerberg H. 1998. Oceanographic aspects of the recruitment of eels to the Baltic Sea. Bulletin français de la pêche et de la pisciculture 349: 177–185. [CrossRef] [EDP Sciences] [Google Scholar]
  • White EM, Knights B. 1994. Elver and eel stock assessment in the Severn and Avon. National Rivers Authority, R&D Project Record 256/13/ST. Bristol, UK. [Google Scholar]
  • White EM, Knights B. 1997. Dynamics of upstream migration of the European eel, Anguilla anguilla L., in the Rivers Severn and Avon, England, with special references to the effects of man-made barriers. Fish Manag Ecol 4: 311–324. [CrossRef] [Google Scholar]
  • Wickham H. 2009. ggplot2. Elegant Graphics for Data Analysis. New York: Springer. [Google Scholar]
  • Winter HV, Jansen HM, Bruijs MCM. 2006. Assessing the impact of hydropower and fisheries on downstream migrating silver eel, Anguilla anguilla, by telemetry in the River Meuse. Ecol Freshw Fish 15: 221–228. [CrossRef] [Google Scholar]
  • Wright RM, Piper AT, Aarestrup K, Azevedo JMN, Cowan G, Don A, Gollock M, Rodriguez Ramallo S, Velterop R, Walker A, Westerberg H. 2022. First direct evidence of adult European eels migrating to their breeding place in the Sargasso Sea. Sci Rep 12: 15362. [CrossRef] [PubMed] [Google Scholar]
  • Zuur A, Ieno E, Walker N, Saveliey A, Smith G. 2009. Mixed effects models and extensions in ecology with R. New York: Springer. [CrossRef] [MathSciNet] [Google Scholar]

Cite this article as: Champkin JD, Bašić T, Haubrock PJ, Balzani P, Sayer CD, George LK, Godard MJ, Vilizzi L., Copp GH. 2024. Short-term growth, movement and response of European eel Anguilla anguilla to re-meandering of a small English chalk stream. Knowl. Manag. Aquat. Ecosyst., 425. 23.

All Tables

Table 1

Numbers of European eels Anguilla anguilla in the River Glaven at Hunworth (north Norfolk, eastern England, UK) caught, tagged and recaptured during the study. n.a. = not applicable.

In the text
Table 2

Statistical results of ANOVA applied to linear model testing for changes in European eel length and weight in the two reaches of the River Glaven pre-restoration, post-embankment removal, and post-re-meandering (Treatment). * Denotes significant effect at α = 0.05.

In the text

All Figures

thumbnail Fig. 1

The River Glaven at Hunworth (North Norfolk, eastern England, UK). Both study reaches are shown as is Thornage Mill, downstream of the restoration reach, and the Environment Agency gauging station between the two study reaches. The geographical extent of the River Glaven catchment and its position in the UK are shown in the inset. Image courtesy of H. Clilverd (UCL).
In the text
thumbnail Fig. 2

Marginal effects from linear models showing significant changes in density, length, and weight of European eel Anguilla anguilla during the study period in the control reach. Error bars represent 95% confidence intervals of the estimated effects.
In the text
thumbnail Fig. 3

Spring growth (March–June 2009) and annual growth rate of two populations of European eel resident in the two adjacent reaches of the Hunworth restoration reach, calculated using length data of tagged and recaptured individuals between 2009 and 2012. Boxes represent all values between the 25th and 75th quartiles and show the median (—) and mean (•) for each category. Box plot whiskers represent minimum and maximum calculated growth increments for each population. Outliers (+) are also shown. n = number of individuals from which growth rate was calculated.
In the text
thumbnail Fig. 4

Boxplots showing changes in density of European eel in the restoration and control reaches of the River Glaven during the study period following a randomization approach. Top plot: boxplot showing bootstrapped density values during pre-restoration (1), post-embankment removal (2–3) and post-re-meandering (4–7). Bottom left plot: boxplot showing averaged bootstrapped densities per Treatment and Reach, where a = pre-restoration, b = post-embankment removal, and c = post-re-meandering. Bottom right plot: boxplot showing difference in bootstrapped densities between restoration and control reach per Treatment. Dark black horizontal line represents 50% of the data, light black horizonal lines represent 25% and 75% of the data (Interquartile Range − IQR) and light black vertical lines minimun and maximum data values in comparison to IQR. Dark black dots represent outliers.
In the text
thumbnail Fig. 5

Plots showing changes in the length of European eel in the restoration and control reaches of the River Glaven during the study period. Top plot: line plot showing mean length with 95% confidence intervals in restoration and control reaches during pre-restoration (1), post-embankment removal (2–3) and post-re-meandering (4–7). Emanating vertical dotted lines denote embankment removal and re-meandering. Bottom plot: marginal effects plot from the linear model showing no significant impact of interaction of Reach and Treatment on eel length, where a = pre-restoration, b = post-embankment removal, and c = post-re-meandering. The error bars represent 95% confidence intervals of the estimated effects.
In the text
thumbnail Fig. 6

Plots showing changes in the weight of European eel in the restoration and control reaches of the River Glaven during the study period. Top plot: line plot showing the mean weight with 95% confidence intervals in restoration and control reaches during pre-restoration (1), post-embankment removal (2–3) and post-re-meandering (4–7). Emanating vertical dotted lines denote embankment removal and re-meandering. Bottom plot: marginal effects plot from the linear model showing no significant impact of interaction of Reach and Treatment on eel weight, where a = pre-restoration, b = post-embankment removal, and c = post-re-meandering. The error bars represent 95% confidence intervals of the estimated effects.
In the text

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.