Electric‐fishing surveys on the River Tale (Devon) for the River ......Otter Beaver Trial (ROBT)...

Electric‐fishing surveys on the River Tale (Devon) for the River Otter

Beaver Trial ‐ 2019

Author: Dr. Andrew Vowles ([email protected])

Reviewed by: Prof. Paul Kemp ([email protected]) and Dr. Jim Kerr ([email protected])

Date: September 2019

International Centre for Ecohydraulics Research, Faculty of Engineering and Physical Sciences, Boldrewood Innovation Campus, Burgess Road, University of Southampton, Hampshire, UK, SO16 7QF.

I

Executive Summary

Wild‐living Eurasian beaver (Castor fiber) have been the focus of research and monitoring in the

Otter Catchment (Devon, England) since the beginning of the River Otter Beaver Trial in 2015. As

part of this monitoring, the International Centre for Ecohydraulics Research (University of

Southampton) conducted quantitative electric‐fishing surveys on the River Tale (at Danescroft) in

2016, 2017 and 2019 in response to beaver dam construction. This report details the results of the

2019 survey.

In August 2019 a control / impact design was employed. Two control reaches were surveyed that

were upstream and downstream and not in close proximity to the only beaver dam on the main‐

stem of the river. Two modified (“impacted”) reaches were also surveyed. The first was the beaver

pool, a reach that was impounded by the beaver dam. The second was immediately downstream of

an old beaver dam that collapsed in winter 2017. Reaches were surveyed using a multiple‐pass

method between stop nets. Physical habitat (depth, wetted width and dominant substrate type) was

also measured.

The beaver dam increased depth, wetted width and promoted fine sediment deposition within the

beaver pool. Downstream of the old beaver dam, depth was generally shallow, flow swift and the

channel narrow. In total, seven species of fish were captured, with the combined sample from the

four electric‐fishing reaches consisting of 37.0% minnow (Phoxinus phoxinus), 26.2% bullhead

(Cottus gobio), 17.9% stone loach (Barbatula barbatula), 11.4% brown trout (Salmo trutta), 5.1%

lamprey (Lampetra spp.), 2.3% European eel (Anguilla anguilla) and 0.1% three‐spined stickleback

(Gasterosteus aculeatus). The only unique species (i.e. found in only one reach) was the stickleback

in the beaver pool.

When standardised by surface area, fish density was highest downstream of the old beaver dam

(1.93 m‐2). When standardised by river length, fish density was highest in the beaver pool (8.60 fish

m‐1). Fish biomass was highest in the beaver pool, independent of standardisation method (10.10 g

m‐2 and 66.76 g m‐1). For brown trout, density was highest downstream of the old beaver dam but

biomass was highest in the beaver pool. The size of trout in the beaver pool was also generally larger

compared to other reaches. Smaller, juvenile trout associating with riffle habitat, while larger fish

occupy pool habitat with woody debris is supported in the literature. Density and biomass of

lamprey, minnow and stickleback was also highest in the beaver pool, suggesting this to be suitable

habitat for these species / life‐stages (e.g. juvenile lamprey live in silt beds for numerous years). The

density, biomass and size of bullhead was greatest downstream of the old beaver dam, in a reach

where the physical habitat closely matches that which this species tends to occupy (moderate

velocities and coarse substrates). Their density and biomass was lowest in the beaver pool, where

the physical habitat is less suitable. Eel were generally rare, but density and biomass were highest in

the upstream control reach (a finding consistent with that of the 2017 survey), and included

numerous small specimens. This indicates that the current beaver dam is not a complete barrier to

the upstream movement of this catadromous species. How much of a barrier to upstream

movement the dam was for eel, or any other species was not quantified during this survey.

II

Acknowledgements

Thanks to Helen Currie and Lewis Dolman from the International Centre for Ecohydraulics Research

(University of Southampton) who assisted in the physical habitat and electric‐fishing surveys.

III

Table of Contents

Executive Summary………………………………………………………………………………………………………………………......I

Acknowledgements…………………………………………………………………………………………………………………………..II

Table of Contents…………………………………………………………………………………………………………………………….III

Introduction………………………………………………………………………………………………………………………………………1

Methods……………………………………………………………………………………………………………………………………………2

Results………………………………………………………………………………………………………………………………………………5

Discussion……………………………………………………………………………………………………………………………………….10

Conclusions……………………………………………………………………………………………………………………………………..11

References……………………………………………………………………………………………………………………………………...12

Appendix A………………………………………………………………………………………………………………………………………13

Appendix B………………………………………………………………………………………………………………………………………14

Appendix C………………………………………………………………………………………………………………………………………16

Appendix D……………………………………………………………………………………………………………………………………..17

1

Introduction

Following local extinction approximately 400 years ago, there have been numerous drivers behind

the reintroduction of the Eurasian beaver (Castor fiber) to the UK. Principal among these are the

desire to “rewild” the country with native species extirpated by humans and to restore ecological

function to freshwater and wetland habitats (Halley & Rosell, 2002; Law et al. 2017). Beavers modify

freshwater habitats, most dramatically through the construction of dams that raise and stabilise

water levels with the intension of maintaining a submerged lodge entrance that reduces exposure to

predation and assists foraging. Constructed from felled vegetation, stones and mud, beaver dams

create localised lentic habitat in areas that are predominantly lotic. Physically, this creates deeper,

slower flowing patches within river systems, increasing habitat heterogeneity which can benefit

freshwater biodiversity (Stringer & Gaywood, 2016; Law et al. 2019). Fine sediment and nutrient

deposition is also promoted upstream of beaver dams, with cleaner water being discharged below,

influencing sediment and nutrient dynamics (Puttock et al. 2017). Consequently, beavers exert an

influence over biological, physical and chemical characteristics of rivers.

The impact of beavers on fish can be both positive and negative. For example, in the meta‐analysis

conducted by Kemp et al. (2012), 184 benefits compared to 119 costs were identified in the scientific

literature. The interactions between impacts can be complex and are likely to vary between species.

While water impounded by beaver dams may provide rearing habitat and refuge from adverse flows

and predators, they also inundate spawning gravels which are subsequently covered with fine

sediment and may impede longitudinal fish movements (for reviews on the impact of beavers on

fish, see Collen & Gibson, 2001 and Kemp et al. 2012). Beaver dams are also more dynamic than

anthropogenic structures, driven partly by fluctuating hydrological regimes which may damage

beaver dams and result in partial or complete collapse. The dynamic nature of beaver dams means

that any impacts to fish are likely to vary both spatially and temporally. Consequently, impacts of

beaver reintroductions on fish communities should be monitored over a number years, particularly

in locations where dams are constructed.

In England, a trial reintroduction of wild‐living Eurasian beavers commenced in the River Otter

Catchment (Devon) in 2015 (Elliott et al. 2017). Devon Wildlife Trust (DWT) on behalf of the River

Otter Beaver Trial (ROBT) are tasked with coordinating research and monitoring activities relating to

the trial reintroduction. As part of this monitoring, the International Centre for Ecohydraulics

Research (ICER; University of Southampton) have conducted a series of electric‐fishing surveys on

the River Tale, a tributary of the River Otter, at a site where beavers have been occupying and

damming for approximately four years. The first dam on the River Tale was constructed in spring

2016. In response to this, ICER conducted an electric‐fishing survey in October 2016 to assess the

fish community at sites modified and unmodified by the beaver dam. This dam partially breached in

winter 2016/17 and the fish communities were re‐surveyed in July 2017. For results from these first

two surveys, see Vowles and Kemp (2018). In winter 2017 this dam collapsed under high flow.

Following this, another beaver dam was constructed approximately 80 m upstream of the old beaver

dam site and has been in place for over a year. This report details the results of a third survey of the

fish community that was completed in August 2019. The aim of the survey was to collect

quantitative information on the fish community at sites modified and unmodified by the new beaver

dam, and immediately downstream of the old beaver dam.

2

Methodology

Surveys were conducted on the River Tale, a tributary of the River Otter (Devon, UK), between 6 and

9 August 2019. Despite some heavy rain showers during the survey period, daily river discharge on

the 6 (0.11 m3 s‐1), 7 (0.11 m3 s‐1) and 8 (0.28 m3 s‐1) August remained low and visibility good. The

increase in discharge on the 8 August reflected prolonged heavy rain that commence in the evening

after fieldwork had ceased. The heavy rain continued overnight resulting in higher (0.47 m3 s‐1)

discharge and levels of turbidity on 9 August.

Electric‐fishing surveys

Surveys were completed by electric‐fishing upstream through four stop netted reaches using a

multiple‐pass method. The reaches were approximately 30 m in river length and consisted of an

upstream and downstream control, beaver pool and an area immediately downstream of an old

beaver dam (Figure 1). The control reaches were at the upstream and downstream extent of the 2

km section of river at Danescroft (owned by Neil and Nicola Hart). They were selected as areas not in

close proximity to the beaver dam and were in a similar location to electric‐fishing reaches that were

surveyed in October 2016 and July 2017 (Appendix A). The beaver pool was a reach (approx. 120 m)

upstream and as close to a current beaver dam as was feasible to sample by wading, and where the

flow was impounded. The final reach was immediately downstream of a beaver dam that collapsed

in Autumn 2017 and in the same location as a reach surveyed in July 2017 (which at the time was

immediately downstream of the beaver dam).

To electric‐fish, a pulsed DC current was produced from an Easyfisher control box powered by a 2.5

HP Honda generator. One team member operated the anode and moved in an upstream direction,

zigzagging across the river. Up to two other team members followed and aimed to catch all fish with

hand‐nets. There was approximately 30 minutes between each of the three electric‐fishing passes

which were completed per reach, allowing time for fine sediment to sufficiently settle and ensure a

good level of visibility was maintained. Despite this, visibility was poor when electric‐fishing the

upstream control reach on 9 August due to a period of heavy over‐night rain. This lower visibility

may have reduced the number of fish captured, and should be taken into consideration when

interpreting the data for this reach. Once captured, fish were held in aerated river water before

being counted, identified to species, measured (to the nearest mm) and weighed (to the nearest 0.1

g). All fish were returned to the electric‐fishing reach from which they were captured. Mortality was

low (<2%) and restricted to minnow.

Physical habitat surveys

Physical habitat characteristics were quantified for each electric‐fishing reach. Parameters were

recorded at five approximately equidistant points along transects which spanned the width of the

river at 5 m longitudinal intervals. Water depth (cm) was measured using a metre rule, while

dominant substrate type was visually estimated as silt (< 0.0063 cm), sand (0.0063 – 0.2 cm), gravel

(0.2 – 1.6 cm), pebble (1.6 – 6.4 cm) or cobble (6.4 – 25.6 cm). The surface area for each reach was

calculated using the mean wetted width (recorded at each transect) and the distance between the

up‐ and down‐stream stop nets.

3

Figure 1. Danescroft, a site on the River Tale (Devon, UK), where multiple pass electric‐fishing was conducted

along four approximately 30 m reaches in August 2019 to quantify fish communities inhabiting an area

occupied by the Eurasian beaver (Castor fiber).

Data analysis

To estimate the effectiveness of electric‐fishing between reaches, capture efficiency (Ec) was

calculated for each species for each reach as:

(1)

Sources: Esri, HERE, DeLorme, Intermap, incremeUSGS, FAO, NPS, NRCAN, GeoBase, IGN, KadasEsri Japan, METI, Esri China (Hong Kong), swisstoOpenStreetMap contributors, and the GIS User Co

¯0 37.5 75 150 Metres

Reach 4:

Downstream

control

Reach 3: Downstream of old

beaver dam

Reach 2: Beaver pool

Reach 1: Upstream control

Current beaver dam

Site of old

beaver dam

4

where T is the total number of fish caught in a reach, k is the number of electric‐fishing passes per

reach, N is the maximum likelihood population estimate and X is an intermediate statistic used in the

calculation of N.

X was calculated as:

∑ (2)

where i is the electric‐fishing pass number and Ci is the number of fish caught in the ith sample.

N was calculated as:

∏

1.0 (3)

where n is the smallest number satisfying equation 3. As estimating N is an iterative process, the

initial value of n used was T and was then increased by 1 until equation 3 was satisfied (Lockwood &

Schneider, 2000).

An Ec for each reach was calculated as the average Ec values for each species. Ec was similar between

electric‐fishing reaches and was 0.46, 0.42, 0.40 and 0.48 for the upstream control, beaver pool, old

beaver dam, and downstream control, respectively. Ec values for each species per electric‐fishing

reach can be found in Appendix B.

To quantify the similarity in the fish community composition between electric‐fishing reaches, a

Bray‐Curtis Index, taking into account fish species and abundance at each electric‐fishing reach was

calculated as:

1 ∑

∑ (4)

where is the abundance of a species at one electric‐fishing reach and the abundance at

another. When expressed as a percentage, a value of 100% reflects sites which have identical fish

species composition.

Fish density and biomass were standardised by both surface area (e.g. fish m‐2) and river length (e.g.

fish m‐1). Both methods of standardisation are reported for total density and biomass per electric‐

fishing reach. As the nature of differences for each species per reach were similar independent of

method used, only values standardised by surface area are reported at species level (for comparison

between methods, see Appendix C).

As the assumption of normality and homogeneity of variance, as indicated by Shapiro‐Wilk and

Levenes Tests respectively, were violated by most data, Kruskal‐Wallis tests were used to identify

differences in depth and fish size (for each species) between electric‐fishing reaches. A Dunn’s Test

with Bonferroni correction was used for pairwise comparisons on significant results.

5

Results

Physical habitat

Median water depth differed between electric‐fishing reach (X2 = 69.53, d.f. = 3, p < 0.001; Figure 2).

‘Reach 2: Beaver pool’ was deeper than ‘Reach 1: Upstream control’ (p < 0.001), ‘Reach 3: old

beaver dam site’ (p < 0.001) and ‘Reach 4: Downstream control’ (p < 0.001). Reach 3 was also

shallower than reach 4 (p < 0.05).

Figure 2. Water depth (cm) in four reaches of the River Tale that were surveyed by electric‐fishing in August

2019. The horizontal lines represent the median value, and boxes define the 25th and 75th percentile. The

whiskers represent maximum and minimum values, excluding outliers. Outliers (> 1.5 X the interquartile range)

are shown as dots.

Dominant substrate type in reach 1, 3, and 4 was gravel, compared to silt in reach 2 (Table 1).

Fish community

The total number of fish collected from the four electric‐fishing reaches was 748 and consisted of

37.0% minnow, 26.2% bullhead, 17.9% stone loach, 11.4% brown trout, 5.1% lamprey, 2.3%

European eel and 0.1% stickleback. Total fish abundance was similar in ‘Reach 1: Upstream control’,

‘Reach 3: old beaver dam site’ and ‘Reach 4: Downstream control’, but approximately 37% higher in

‘Reach 2: Beaver pool’ (Figure 3). Bullhead were the most abundant species in reach 1, 3 and 4, in

comparison to minnow in reach 2 (Figure 3). European eel were rare in most reaches, with 70.6%

captured in reach 1, and none in reach 2. Three‐spined stickleback were also rare, with just one

individual captured (in reach 2). The remaining five species (brown trout, lamprey, bullhead, stone

loach and minnow) were found in all electric‐fishing reaches, although for lamprey, most were in

reach 2 (92.1%). The fish community composition was similar (≥78.9%) in reach 1, 3 and 4 (Table 2).

6

The fish community composition in reach 2 had low levels of similarity (<50%) when compared to

the other three reaches (Table 2).

Table 1. Dominant substrate type, classified according to the Wentworth grainsize scale, within four electric‐

fishing reaches of the River Tale (Devon) in August 2019.

Substrate type

Percent of transect survey points



Reach 3: Old beaver dam site

Reach 4: Downstream control

Silt (<0.0063 cm

5.7 57.1 11.4 11.4

Sand (0.0063 – 0.2 cm)

11.4 0.0 14.3 5.7

Gravel (0.2 – 1.6 cm)

40.0 11.4 37.1 71.4

Pebble (1.6 – 6.4 cm)

31.4 17.1 31.4 8.6

Cobble (6.4 – 25.6 cm)

11.4 14.3 5.7 2.9

Figure 3. Total abundance of fish within four electric‐fishing reaches sampled at a site on the River Tale

(Devon, UK) occupied by Eurasian beaver (Castor fiber). Bars are stratified by fish species.

When standardised by surface area, the total density of fish was greatest in reach 3 (1.93 fish m‐2),

reach 1 (1.83 fish m‐2), reach 2 (1.30 fish m‐2) and then reach 4 (1.26 fish m‐2). When standardised by

river length, density was greatest in reach 2 (8.60 fish m‐1), reach 1 (6.00 fish m‐1), reach 3 (5.68 fish

7

m‐1) and then reach 4 (4.62 fish m‐1). Total biomass was greatest in reach 2 (10.10 g m‐2 and 66.76 g

m‐1), reach 1 (8.58 g m‐2 and 28.15 g m‐1), reach 4 (7.63 g m‐2 and 27.94 g m‐1) and then reach 3 (7.27

g m‐2 and 21.46 g m‐1), irrespective of standardisation method. Although species‐specific density and

biomass varied between reaches, the general pattern was similar independent of standardisation

method (see Appendix C). Reach 1 contained the highest density of European eel and stone loach;

reach 2 contained the highest density of lamprey, minnow and stickleback; reach 3 contained the

highest density of brown trout and bullhead (Table 3). Reach 4 did not contain the highest density of

any species. As with density, reach 1 contained the highest biomass of European eel and stone loach,

and reach 3 the highest biomass of bullhead. Reach 2 contained the highest biomass for the

remaining species (brown trout, lamprey, minnow and stickleback; Table 3).

Table 2. Bray‐Curtis Index showing levels of similarity in the fish community composition between the four

electric‐fishing reaches surveyed on the River Tale (Devon) in August 2019. A value of 100% indicates identical

fish community composition.

Upstream control Beaver pool Old dam site Downstream control

Upstream control 100.0

Beaver pool 48.8 100.0

Old dam site 81.8 44.7 100.0

Downstream control 78.9 49.9 86.6 100.0

The size (mm) of minnow (X2 = 7.13, d.f. = 3, p = 0.068) and stone loach (X2 = 6.49, d.f. = 3, p = 0.090)

did not differ between electric‐fishing reaches. Brown trout (X2 = 21.05, d.f. = 3, p < 0.001) and

bullhead (X2 = 23.22, d.f. = 3, p < 0.001) did differ in size between reaches. Trout were larger in reach

2 compared to reach 3 (Figure 4a), and bullhead were larger in reach 3 compared to reach 1 and 4

(Figure 4b). No statistical tests were performed for European eel, lamprey or stickleback due to low

numbers caught, however, the length – frequency distributions of all species in each electric‐fishing

reach can be seen in see Appendix D.

8

Figure 4a. Fork length (mm) of brown trout and b. total length (mm) of bullhead captured in the four electric‐

fishing reaches surveyed on the River Tale (Devon) in August 2019. The horizontal lines represent the median

value, and boxes define the 25th and 75th percentile. The whiskers represent maximum and minimum values,

excluding outliers. Outliers (> 1.5 X the interquartile range) are shown as dots. Significant differences are

highlighted with horizontal lines above boxplots.

9

Table 3. Summary statistics for six fish species captured when electric‐fishing four reaches of the River Tale (Devon, England) in August 2019 at a site where Eurasian

beaver (Castor fiber) have constructed a dam. Density and biomass values are standardised by surface area (m‐2). In addition, one three‐spined stickleback (Gasterosteus

aculeatus) was captured in ‘Reach 2: Beaver pool’ and was 36mm in length and 0.1g in weight. Stickleback were not found in any of the other reaches.

Electric-fishing reach

Brown trout (Salmo trutta) Bullhead (Cottus gobio)

Number caught

Fork length [mean ± SD (range), mm]

Weight [mean ± SD (range), g]

Density (fish m-2)

Biomass (g m-2)

Number caught

Total length [mean ± SD (range), mm]


Density (fish m-2)

Biomass (g m-2)

1: Upstream control

11 129 ± 59 (57 - 238)

35.4 ± 46.9 (2.1 - 165.1)

0.12 4.10 63 41 ± 12 (24 - 66)

1.1 ± 1.1 (0.1 – 3.8)

0.66 0.73

2: Beaver pool 24 152 ± 57 (59 - 277)

57.6 ± 53.9 (2.2 – 222.2)

0.12 6.91

10 47 ± 13 (30 - 65)

1.5 ± 1.2 (0.1 – 3.8)

0.05 0.08

3: Old beaver dam site

33 83 ± 33 (60 - 207)

11.3 ± 30.0 (2.3 – 103.0)

0.39 4.47

64 48 ± 10 (24 - 70)

1.6 ± 1.0 (0.1 – 4.8)

0.77 1.18

4: Downstream control

17 124 ± 55 (53 - 211)

35.4 ± 37.5 (1.9 – 110.8)

0.14 4.97 59 39 ± 12 (21 - 70)

1.0 ± 1.1 (0.1 – 4.9)

0.49 0.48

European eel (Anguilla anguilla) Lamprey (Lampetra spp.)

Number caught



Density (fish m-2)

Biomass (g m-2)

Number caught



Density (fish m-2)

Biomass (g m-2)

1: Upstream control

12 177 ± 48 (84 - 248)

10.1 ± 7.1 (0.5 – 21.3)

0.13 1.27 5 111 ± 32 (68 - 141)

2.6 ± 1.8 (0.7 – 4.8)

0.05 0.14

2: Beaver pool 0 N/A N/A 0.00 0.00

31 148 ± 13 (108 - 172)

6.5 ± 1.8 (2.1 – 10.9)

0.16 1.01


4 134 ± 34 (95 – 175)

3.9 ± 2.9 (1.3 – 7.7)

0.05 0.19 1 57

0.4 0.01 0.00


1 339 72.1 0.01 0.59 1 82 1.2 0.01 0.01

Minnow (Phoxinus phoxinus) Stone loach (Barbatula barbatula) Number caught

Fork length [mean ± SD (range), mm]


Density (fish m-2)

Biomass (g m-2)

Number caught



Density (fish m-2)

Biomass (g m-2)

1: Upstream control

36 51 ± 9 (42 - 80)

1.5 ± 1.3 (0.4 – 6.1)

0.38 0.56 47 74 ± 6 (63 - 95)

3.6 ± 1.1 (1.1 – 6.4)

0.49 1.79

2: Beaver pool 150 50 ± 14 (20 - 92)

1.8 ± 2.0 (0.1 – 11.1)

0.75 1.36

44 70 ± 6 (52 - 82)

3.4 ± 1.4 (1.2 – 7.8)

0.22 0.75


37 50 ± 6 (42 - 69)

1.4 ± 0.7 (0.7 – 3.8)

0.44 0.62 22 72 ± 6 (58 - 83)

3.1 ± 0.9 (0.9 – 4.9)

0.26 0.82


54 55 ± 15 (13 - 85)

2.4 ± 1.9 (0.1 – 7.5)

0.45 1.05 21 72 ± 5 (65 - 85)

3.1 ± 0.8 (1.9 – 4.4)

0.17 0.53

10

Discussion

The beaver dam on the River Tale affected the physical characteristic of the river. In the beaver pool,

a reach where flow was impounded by the dam, water velocity was visibly slow and deeper in

comparison to the upstream and downstream control, and the reach immediately downstream of an

old beaver dam. The reach downstream of the old beaver dam was generally shallow and swift

flowing, characteristic of riffle habitat. The dominant substrate type in the beaver pool was silt,

indicating that the increase and decrease in water depth and velocity, respectively, has promoted

fine sediment deposition. In contrast, the remaining three reaches were dominated by gravel. The

effect of the beaver dam on the physical characteristics is broadly consistent with results from July

2017, when the site was last surveyed. Then, a beaver dam (which has since collapsed) also

increased depth, reduced velocity and promoted fine sediment deposition.

Total fish abundance was similar immediately downstream of the old beaver dam (161) and in the

control reaches (upstream: 174, downstream: 153). Total abundance in the beaver pool was

approximately 37% higher than the other three reaches. Although there was a notable reduction in

bullhead in the beaver pool, the number of minnow and lamprey was markedly greater in

comparison to the other reaches. Furthermore, although the beaver pool did not contain European

eel, it was the only site to support a unique species, the three‐spined stickleback. In fact, three‐

spined stickleback have not previously been found in any reaches surveyed by ICER on the River Tale

(i.e. in October 2016 or July 2017). Such differences in the fish community composition were

reflected in the Bray‐Curtis Similarity comparisons, where the beaver pool was least similar when

compared to the other reaches. The differences in fish community composition are likely a reflection

of the differing physical characteristics created upstream of the beaver dam.

When standardised by surface area, the electric‐fishing reach immediately downstream of the old

beaver dam contained the highest total density of fish. However, when standardised by river length,

density was greatest in the beaver pool. Total fish biomass was also highest in the beaver pool,

independent of standardisation method, indicating that this reach contained more and/ or larger

fish. The differences that were observed in terms of fish species abundance between electric‐fishing

reaches are also reflected in the standardised metrics (density and biomass per m‐2 or per m‐1; see

Appendix C). For brown trout, density and biomass were highest in the reach immediately

downstream of the old beaver dam and in the beaver pool, respectively. This suggests that the

shallow, swift flowing conditions near the old beaver dam provided good habitat for juvenile trout

which were abundant, whereas the deep, slow flow in the beaver pool was suitable habitat for larger

salmonids. This general trend supports literature from similar river systems where juvenile trout

were strongly associated with gravel riffles and larger trout with the surrounding pools and large

woody debris (Langford et al. 2012). For bullhead, density, biomass and average size was greatest

immediately downstream of the old beaver dam. The physical habitat in this reach closely matches

that which bullhead tend to occupy; moderate velocities and coarse substrates that provide

adequate shelter and spawning sites (Tomlinson & Perrow, 2003). The density and biomass for

lamprey, minnow and three‐spined stickleback was greatest in the beaver pool. For lamprey, this is

logical given the habitat requirements of juveniles, which burrow into silt beds (Maitland, 2003).

Although minnow and stickleback are found in a wide variety of aquatic habitats, they have a

tendency to form aggregations and are typical of moderate to slow flowing environments. The

density and biomass for the remaining species (stone loach and European eel) was highest in the

upstream control site, which provided a mixture of shallow ‐ swift and deep ‐ slow flow and a high

proportion of coarse (pebble / cobble) substrate.

11

Continued beaver activity and the dynamic nature of beaver dams has modified the channel of the

River Tale at Danescroft over a number of years. In July 2017, the only beaver dam on the main‐stem

of the river was partially breached, and subsequently collapsed in winter 2017. A new dam, now the

only one on the main‐stem of the Rive Tale, was constructed approximately 80 m upstream and is

more substantial than the former. It has a greater head drop and the impounded reach is larger.

Despite this, there are similarities in the fisheries data between the 2017 and 2019 electric‐fishing

surveys. Firstly, the density and biomass of bullhead was highest in reach 3 during both years. This

was the site immediately downstream of the beaver dam (2017) and immediately downstream of

the old beaver dam (2019) and an area that has maintained good riffle habitat. Similarly, bullhead

density and biomass was lowest in the beaver pool during both surveys, a reach where the physical

characteristics are less well suited to this species. European eel were rare in most reaches but

relatively abundant in the upstream control, with 84.2% and 70.6% found in this reach during 2017

and 2019, respectively. The coarse substrate, which includes some large boulders and tree roots

towards the left‐hand bank, likely provides refuge and ambush locations for this cryptic species.

Furthermore, the results suggest that the current beaver dam on the River Tale was not a complete

barrier to the upstream movements of this catadromous species. However, how much of a barrier to

upstream movement the beaver dam presents was not quantified for eel, or any other species,

during this survey.

Despite some similarities in the electric‐fishing data between 2017 and 2019 surveys, there are also

some differences. Most notably, the density and biomass of lamprey and minnow was low in the

beaver pool in comparison to most sites in 2017, but highest in August 2019. It may be that the more

substantial (or well‐established) impoundment has created larger silt deposits for lamprey to inhabit

in comparison to the beaver pool in 2017, which was smaller and partially breached. In 2017 it was

noted that there appeared to be little submerged woody material within the beaver pool for smaller

fish, such as minnow, to seek shelter within. Although not quantified in 2019, woody material within

the beaver pool was abundant and may have provided shelter for minnow, enabling more to occupy

this area while reducing predation risk (e.g. from larger trout).

Conclusions

Beaver dams on the River Tale (Devon) increase water depth, decrease water velocity and promote

fine sediment deposition immediately upstream. In August 2019, the total abundance of fish in the

beaver pool was approximately 37% higher than an upstream and downstream control, and a reach

immediately downstream of the old beaver dam. Biomass, but not density of brown trout was

highest in the beaver pool, indicating this to be suitable habitat for larger individuals. Density of

trout was greatest in a long riffle that characterised the site immediately downstream of the old

beaver dam, and provided good habitat for juvenile trout. The beaver pool also provided the best

habitat for lamprey and minnow, as indicated by the highest density and biomass values for these

species in this reach. The beaver activity on the River Tale alters physical habitat within the

immediate vicinity in a way that benefits certain fish species and life‐stages but not others.

12

References

Collen, P. & Gibson, R.J. (2001). The general ecology of beavers (Castor spp.), as related to their

influence on stream ecosystem and riparian habitats, and the subsequent effects on fish – a review.

Reviews in Fish Biology and Fisheries 10, 439‐461.

Elliott, M., Blythe, C., Brazier, R.E., Burgess, P., King, S., Puttock, A. & Turner, C. (2017). Beavers –

Nature’s water engineers. Devon Wildlife Trust

Halley, D.J. & Rosell, F. (2002). The beaver’s reconquest of Eurasia: status, population development

and management of a conservation success. Mammal Review 32, 153‐178.

Kemp, P.S., Worthington, P.A., Langford, T.E.L., Tree, A.R..J & Gaywood, M.J. (2012). Qualitative and

quantitative effects of reintroduced beavers on stream fish. Fish and Fisheries 13, 158‐181.

Langford, T.E.L., Langford, J. & Hawkins, S.J. (2012). Conflicting effects of woody debris on stream

fish populations: implications for management. Freshwater Biology 57, 1096‐1111.

Law, A., Gaywood, M.J., Jones, K.C., Ramsey, P. & Willby, N.J. (2017). Using ecosystem engineers as

tools in habitat restoration and rewilding: beaver and wetlands. Science of the Total Environment

605‐606, 1021‐1030.

Law, A., Levanoni, O., Foster, G., Ecke, F. & Willby, N.J. (2019). Are beavers a solution to the

freshwater biodiversity crisis? Diversity and Distributions doi: 10.111/ddi.12978.

Lockwood, R.N. & Schneider, J.C. (2000). Stream fish population estimates by mark recapture and

depletion methods. In Schneider, J.C (ed.). Manual if fisheries survey methods II: with period

updates. Michigan Department of Natural Resources, Fisheries Special Report 25, Ann Arbor.

Maitland, P.S. (2003). Ecology of the River, Brook and Sea Lamprey. Conserving Natura 2000 Rivers

Ecology Series No. 5. English Nature, Peterborough (UK).

Puttock, A., Graham, H.A., Cunliffe, A.M., Elliott, M. & Brazier, R.E. (2017). Eurasian beaver activity

increases water storage, attenuates flow and mitigates diffuse pollution from intensively‐managed

grasslands. Science of the Total Environment 576, 430‐443.

Stringer, A.P. & Gaywood, M.J. (2016). The impacts of beavers Castor spp. on biodiversity and the

ecological basis for their reintroduction to Scotland, UK. Mammal Review 46: 270‐283.

Tomlinson, M.L. & Perrow, M.R. (2003). Ecology of the Bullhead. Conserving Natura 2000 Rivers

Ecology Series No. 4. English Nature, Peterborough (UK).

Vowles, A.S. & Kemp, P.S. (2018). Electric‐fishing surveys for the River Otter (Devon) Beaver Trial.

University of Southampton Report for Devon Wild Life Trust.

13

Appendix A: Reaches electric‐fished in 2016 and 2017

A.1. A site on the River Tale (Devon, UK) where multiple pass electric‐fishing was conducted along

three approximately 50 m reaches in October 2016 (a.) and four approximately 25 m reaches in July

2017 (b.) to determine the impact of a beaver dam on the fish community.

Sources: Esri, HERE, DeLorme, Intermap, increment P Corp., GEBCO,USGS, FAO, NPS, NRCAN, GeoBase, IGN, Kadaster NL, OrdnanceSurvey, Esri Japan, METI, Esri China (Hong Kong), swisstopo,MapmyIndia, © OpenStreetMap contributors, and the GIS UserCommunity

0 75 15037.5 Meters

¯

Sources: Esri, HERE, DeLorme, Intermap, increment P Corp., GEBCO,USGS, FAO, NPS, NRCAN, GeoBase, IGN, Kadaster NL, OrdnanceSurvey, Esri Japan, METI, Esri China (Hong Kong), swisstopo,MapmyIndia, © OpenStreetMap contributors, and the GIS UserCommunity

0 75 15037.5 Meters

¯

River Tale

Reach 1 (upstream control)

¯

Reach 2 (impounded)

Reach 3 (downstream control)

Beaver dam

Directio

n of flo

w

Reach 1 (upstream control)

0 37.5 75 150 Metres

Reach 2 (impounded)

Reach 3 (downstream

impacted)

Reach 4

(downstream

control)

a. b.

14

Appendix B – Number of fish captured, per species, in the River Tale in 2019

with an estimate of population size and electric‐fishing capture efficiency

Brown trout (Salmo trutta)



Reach 3: Old beaver dam


Total catch 11 24 33 17

Population estimate 12 25 34 17

Capture efficiency 0.50 0.62 0.65 0.81

Bullhead (Cottus gobio)








European eel (Anguilla anguilla)






Population estimate 14 N/A 5 N/A

Capture efficiency 0.52 N/A 0.31 N/A

Lamprey (Lampetra spp.)






Population estimate 5 42 N/A N/A

Capture efficiency 0.56 0.34 N/A N/A

Stone loach (Barbatula barbatula)








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Minnow (Phoxinus phoxinus)








Three‐spined stickleback (Gasterosteus aculeatus)





Total catch 0 1 0 0

Population estimate N/A N/A N/A N/A

Capture efficiency N/A N/A N/A N/A

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Appendix C – Comparisons between density and biomass values standardised

by surface area and river length, per species.

Species Reach Density (fish m‐2) Density (fish m‐1) Biomass (g m‐2) Biomass (g m‐1)

Brown trout

1. Upstream control 0.12 0.38 4.10 13.43

2. Beaver pool 0.12 0.79 6.91 45.57

3. Old beaver dam 0.39 1.61 4.47 13.17

4. Downstream Control 0.14 0.51 4.97 18.18

Bullhead


2. Beaver pool 0.05 0.33 0.08 0.51

3. Old beaver dam 0.77 2.26 1.18 3.49


European eel


2. Beaver pool 0.00 0.00 0.00 0.00

3. Old beaver dam 0.05 0.14 0.19 0.55


Lamprey


2. Beaver pool 0.16 1.02 1.01 6.65

3. Old beaver dam 0.01 0.04 0.00 0.01


Minnow


2. Beaver pool 0.75 4.96 1.36 9.00

3. Old beaver dam 0.44 1.31 0.62 1.82


Stone loach


2. Beaver pool 0.22 1.45 0.75 4.94

3. Old beaver dam 0.26 0.78 0.82 2.41


Stickleback


2. Beaver pool 0.005 0.033 0.001 0.003

3. Old beaver dam 0.000 0.000 0.000 0.000


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Appendix D: Fish Length – frequency distributions

D.1. Length – frequency distributions of brown trout (Salmo trutta) in four electric‐fishing reaches on the River

Tale, surveyed on August 2019. The bin size for length is 10 mm.

D.2. Length – frequency distributions of minnow (Phoxinus phoxinus) in four electric‐fishing reaches on the

River Tale, surveyed on August 2019. The bin size for length is 5 mm.

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D.3. Length – frequency distributions of stone loach (Barbatula barbatula) in four electric‐fishing reaches on

the River Tale, surveyed on August 2019. The bin size for length is 5 mm.

D.4. Length – frequency distributions of bullhead (Cottus gobio) in four electric‐fishing reaches on the River


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D.5. Length – frequency distributions of lamprey (Lampetra spp.) in four electric‐fishing reaches on the River


D.6. Length – frequency distributions of European eel (Anguilla anguilla) in three electric‐fishing reaches on

the River Tale, surveyed on August 2019. The ‘Beaver pool’ is not shown as no eel were captured in this reach.

The bin size for length is 20 mm.

Electric‐fishing surveys on the River Tale (Devon) for the River ......Otter Beaver Trial (ROBT)...

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