The 11th Pacific Symposium on Flow Visualization and Image Processing
1-3 December 2017,Kumamoto, JAPANEFFECTS OF SLOPES AND SPACES OF PROTRUSIONS IN FISH LADDER FOR EELS ON
MIGRATION RATE OF ANGUILLA JAPONICA
K. Onitsuka1, J. Akiyama1 and K. Izumi1
1

Department of Civil and Architectural Engineering, Kyushu Institute of Technology
1-1 Sensuicho, Tobata-Ku, Kitakyushu 804-8550 Japan

ABSTRACTIn recent year, populations of Anguilla japonica havebeen decreasing sharply. This decrease is partly because itis estimated to be overfishing of Anguilla japonica, thedeterioration of the river environment and so on.
Migrating fishes are impeded by weirs and dams in theriver. Fishways have been constructed to swimming fish
like Plecoglossus altivelis altivelis in order to solve thisproblem. On the other hand, it is thought that special
fishway is necessary for demersal fish like eels.
Furthermore, studies on fish ladder for eels haven’t been
conducted. In the present study, an investigation on
migration rate of Anguilla japonica was conducted under
the condition that slope of fish ladder for eels and spacesof protrusions are changed. It was found that migration
rates decreased with the increase of slopes and spaces ofprotrusions. Besides, Anguilla japonica migrated while
using protrusions by serpentine movement.
INTRODUCTION
Anguilla japonica is a group of fish that belong to theorder Anguilliformes family Anguilidae. They are
demersal fish, so they have a poor swimming ability
compared to swimming fish. Some of studies concerning
their biology and farming has been advanced recently.
Especially, Tsukamoto et al. (2011) made a great
discovery that spawning ground of Anguilla japonica is in
the west sea of Mariana Islands. However, there are many
unsolved questions with respect to migrating process or
swimming characteristics.
Japanese people eat many eels year after year. On theother hand, population of Anguilla japonica has been
decreasing in recent years. Santos et al. (2016) pointed
that the reasons of the population decreasing are
overfishing of Anguilla japonica, the deterioration of theriver environment and so on. Even among them, there areproblems that weirs and dams in the river impede
migration of fish. Fishways have been constructed to slove
this problems. However, many fishways are pool-and-weirfishway to swimming fish. Therefore, special fishways arenecessary for demersal fish like eels. In Japan, fish ladders
for eels are still very few. In addition, studies on fish
ladder for eels haven’t been conducted. Fish ladder foreels (Anguilla anguilla) are used by setting brushes androughness in bottom surface in Europe and America (seeClay, C.H., 1994, Solomon, D.J. & Beach, M.H., 2004,
Legault, A., 1992). However, little is known about thebest structure of fish ladders for eels. In the present study,
an investigation on migration rates of Anguilla japonica
was conducted under the condition that slopes of fish
ladder for eels and spaces of protrusions are changed.

EXPERIMENTAL DEVICE AND METHODS
Figure 1 shows an experimental device consisting of
two tanks and a fish ladder. This tank length ( Lx ) was 0.6m,
width ( Bz ) was 0.4m, and height ( H y ) was 0.3m. Fishladder length ( L ) was 0.8m, width ( B ) was 0.2m, andheight of side wall (Δ h ) was 0.05m. This fish ladder ispainted gray. Fish ladder width (B) was determinedreferencing fish ladder for European eels (Anguillaanguilla).
Table 1 shows the experimental case. Quantity of flow
( Q ) providing in the upstream tank is 80 (ml/s), and thewater depth (h) is 0.2m by setting displacement indownstream tank. Figure 2 shows an arrangement of
protrusions. These protrusions are column made ofpolyvinyl chloride. The diameter of protrusions is 30mm
and the height is 35mm. In this study, spaces of protrusions(d) were set to three patterns of 10, 20 and 30 (mm). Slopsof fish ladder (θ) were set to three patterns of 15°, 30° and
45°. The left bank of the upper reach in this fish ladder isthe origin position. x axis is taken in streamwise direction. z
axis is taken in the transverse direction. The water depth in
this fish ladder was about 5mm in all cases. Moreover, thewater temperature was about 20°C in all cases.
Figure 3 shows young eels used in this experiment.
Averaged body length ( B ) is about 200mm. Young eels
migrate the river positively. The number of young eelsused to experiment was 20. They were inserted indownstream tank. After it is confirmed that Anguillajaponica settled down, this migrating experiment was
conducted for 30 minutes. Trajectory of Anguilla japonica
is recorded with the digital video camera. Number of pixelof the digital video camera is 1440×1080, and recording
speed is 30fps. The swimming positions of Anguillajaponica were obtained with the aid of the digital video
camera. Besides, the number of eels migrating was counted.
L

RESULTS AND DISCUSSION
Migration rates of Anguilla japonica
Migration rates of Anguilla japonica were calculated
as the following equation (1).
Migration rate
Number of fish that migrated in upstream tank n(1)=
Number of fish that used to experiment N (= 20)
Figure 4 shows migration rates ( n N ) in each space of
protrusions (d). Migration rate is the maximum in the cased10-15 (d=10mm, θ=15°). Migration rates have decreased
with the increase of spaces of protrusions (d). In addition,
migration rates have tendency to decrease with theThe 11th Pacific Symposium on Flow Visualization and Image Processing
1-3 December 2017,Kumamoto, JAPAN

L
Flow

B

Δh

θ
Hy

h
Bz
Lx

Figure 1. Experimental device
Table 1. Experimental cased (mm)

θ(degree)30°
d10-30d20-30d30-3015°
d10-15d20-15d30-15

10
20
30

1

45°
d10-45d20-45d30-45Figure 2. An arrangement of protrusions
Figure 3. Young eels used in this experimentmigration rate

1.2

Anguilla japonica

n
N

:

0.8

15

30

15
‹

45

Anguilla japonica
:

N c1
N
0.8

30
‹
45
‹

0.6

challenge rate

30

15

15
‹

45

30
‹
45
‹

0.6

0.4
0.4

0.2

0.2
0

0

10mm
10

10mm

20 d (mm) 30mm
30Figure 4. Migration rate
0.7

θ = 15°d:

10(mm)

20(mm)

30(mm)

0.7

Anguilla japonica

n0.6
ah
N0.5c

d:

10(mm)

θ = 30°nah
0.6

20(mm)

Nc
0.5

0

0
0
0

0.2

0.2

0.2

0.1

0.1

0.1

250

350 H450
c (mm) 550(a) θ=15°20(mm)

0

0.3
0

150

10(mm)

0
0.4

0
0.3

50

θ = 45°d:30(mm)

0.3

0Anguilla japonica

30(mm)

00.4

0.4

20 d (mm) 30mm
30Figure 5. Challenge rate

0.7

Anguilla japonica

n0.6
ah
N
c
0.5

20mm

10

20mm

0

0
50

150

250

350 H 450
c (mm) 550(b) θ=30°
Figure 6. The Frequency distribution of arrival heights

increase of slopes (θ) in each space of protrusions.
Accordingly, it was found that young eels migrate easily
in the case that the space of protrusions is d=10mm andthe slope is θ=15° in the range of this study.

50

150

250

350 H450
c (mm) 550(c) θ=45°

Challenge rates and arrival heights
The number of eels challenging migration ( N c ) is
number of Anguilla japonica entering this fish ladder from

The 11th Pacific Symposium on Flow Visualization and Image Processing
1-3 December 2017,Kumamoto, JAPAN
d10-15

80

080

B (m)
60

60
B (m)

040
.1

0.40
1

20
0.15

0.152000.20 0
0

51.25

0.2

102.5

0.4

153.75

L (m)

205

0.8

d10-300.20 0
0

51.25

0.2

(a) d10-15
d10-45

0

0.401

0.401

20
0.15

20
0.15

48.75

0.2

97.5

0.4

146.25

L (m)

195

0.8

0.20 0
0

51.25

0.2

102.5

0.4

153.75

205

0.8

L (m)

(d) d20-15
d20-30d20-45

0

80

B (m)
60

60
B (m)

040
.1

040
.1

20
0.15

20
0.15

0

51.25

102.5

153.75

205

0.2

0.4

L (m)

0.8

0.20 0
0

51.25

0.2

(e) d20-30

102.5

0.4

153.75

0205
.8

L (m)

(f) d20-45
d30-15800d30-30

080

60
B (m)

B (m)60

0.40
1

0.140

20
0.15

0.20 000.8

d20-15(c) d10-45
0

80

0.2 0
0

205

L (m)

0

B (m)
60

0

153.75

80

60
B (m)

0

0.4

(b) d10-30

80

0.20

102.5

0.1520

51.25

0.2

102.5

0.4

153.75

L (m)

205

0.8

0.20
00

51.25

0.2

(g) d30-15

102.5

0.4

0205
.8

L (m)

153.75(h) d30-30
Figure 7. Migration routes of Anguilla japonica

the downstream tank. Challenge rates of Anguilla japonica
were calculated as the following equation (2).
Challenge rate
Number of fish that challenged N c=(2)
Number of fish that used to experiment N (= 20)
Figure 5 shows challenge rates ( N c N ) in each space of
protrusions (d). Challenge rate was the maximum in thecase d10-15 (d=10mm, θ=15°). Challenge rates have atendency to decrease with the decrease of spaces ofprotrusions (d). In addition, challenge rates in smaller
slopes are higher than that in larger slopes. Referring to
Figure 4, challenge rates are higher than migration rates inall cases. This means that Anguilla japonica challenges
migration whether they succeed in migrating or not.

Anguilla japonica is distinguished in each slope. The
number of their arrival heights in each case is n . Besides,
arrival heights ( H c ) are counted in every 50mm range fromthe lower ends of slopes in this fish ladder. Figure 6(a)~(c)show frequency distribution of arrival heights ( H c ). The
Frequencies of arrival heights have dispersion in theminimum slope (θ=15°). This trend is especiallyremarkable in the case of d=10mm. However, it was
confirmed that the frequency distribution of arrival heights
has a high frequency in low value with the increase ofslopes. Therefore, Anguilla japonica challenges migrationin the highest slope (θ=45°). On the other hand, their arrival
heights ( H c ) declined in comparison to lower slopes.
ahThe 11th Pacific Symposium on Flow Visualization and Image Processing
1-3 December 2017,Kumamoto, JAPAN
d:

0.16

10(mm)

20 (mm)

30 (mm)Anguilla japonica

Vmr
0.14
BL

0.12
(1/s)
0.1
0.08
0.06
0.04
0.02
0

45
‹
30
30‹ θ (deg .) 45

15
15‹

Figure 8. Averaged migrating speed of Anguilla japonica
1

1

Anguilla japonica

1

Anguilla japonica

Anguilla japonica

mr

n
0.8
N
mr

n
45
0.8
N

0.6

0.6
15

15
0.6

30

30

15

0.4

45

0.4

45

30

0.2

0.2

0.2

n
0.8
N

d=10mm : 15

mr

30

45

:

mr

15

d=20mm

30

1

1.2

1.4

1.6

S mr
1.8 L st

2

30

mr

0.4

0

0

d=30mm : 15

mr

1

1.2

1.4

1.6

S mr1.8L st

0

2

1

1.2

1.4

1.6

S mr1.8 L st2(a) d=10mm(b) d=20mm(c) d=30mm
Figure 9. The Frequency distribution of relations between sinuosity of Anguilla japonica and slopes
Migration routes of Anguilla japonica
Figure 7(a)~(h) show migration routes of Anguillajaponica in every cases. It wasn’t confirmed that there are agreat difference in each case. However, it was found that
Anguilla japonica migrated while using protrusions byserpentine movement in all cases.
Migrating speed of Anguilla japonica
V B is the value that the averaged migrating speed( Vmr ) was divided by averaged body length of AnguillamrL

japonica ( BL ). Figure 8 shows the averaged migrating
speed of Anguilla japonica in every slopes. The amount ofchanges in the case d=10mm is small. In addition, theaveraged migrating speed ( V B ) has a tendency toincrease with the increase of slopes in the same space of
protrusions (d). Besides, the averaged migrating speed( V B ) has a tendency to increase with the increase of
spaces of protrusions in the same slope of fish ladder foreels (θ). Consequently, it is assumed that the increase offlow velocity has an impact on the increase of migrating
speeds with the increase of slopes (θ). In addition, Anguillajaponica had difficulty in using protrusions with theincrease of spaces of protrusions (d).
mr

mr

L

L

Sinuosity of Anguilla japonica
The length of the straight lines ( Lst ) is the length
between entering points of Anguilla japonica in the entryof this fish ladder (x=0.8m) and arriving points in the exit

(x=0m). The length of actual migrating paths ( S mr ) is thelength of actual migrating routes of Anguilla japonica.
The sinuosity of Anguilla japonica ( Smr Lst ) is defined as
the following equation (3).
Length of migrating routes S mr
Sinuosity=(3)
Length of straight lines Lst
Accordingly, it means that the closer the sinuosity( Smr Lst ) is 1, the less serpentine Anguilla japonica
becomes and the more linear they migrate.
Figure 9(a)~(c) show the frequency distribution ofrelations between sinuosity of Anguilla japonica andslopes in every spaces of protrusions. The dispersion of
sinuosities ( Smr Lst ) has been decreasing with the increaseof slopes (θ) in all cases. Furthermore, the sinuosity has atendency to be small. Therefore, it was found that
Anguilla japonica migrated in the condition that theyhardly bend their body with the increase of slopes (θ).
Considering Figure 8, it became the clear that thesinuosity ( Smr Lst ) has decreased and the averaged
migrating speed ( V B ) has increased with the increaseof slopes (θ).
mr

L

CONCLUSION
In this study, an investigation on migration rates ofyoung eels (Anguilla japonica) was conducted under thecondition that slopes of fish ladder for eels and spaces ofprotrusions are changed. As a result, it was found that
following.

The 11th Pacific Symposium on Flow Visualization and Image Processing
1-3 December 2017,Kumamoto, JAPAN(1) Anguilla japonica migrates easily in the case thatspaces of protrusions is d=10mm and the slope is
θ=15° in the range of this experimental cases.
(2) Challenge rates have decreased with the increase ofslopes of fish ladder for eels (θ) and spaces ofprotrusions (d).
(3) Anguilla japonica migrated while using protrusions
by serpentine movement in all cases.
(4) It was confirmed that the sinuosity ( Smr Lst ) has
decreased and the averaged migrating speed ( V B )has increased with the increase of slopes (θ).
This experiment of fish ladder for eels was conductedin a constant quantity of flow. It is necessary to investigate
the quantity of flow, the diameter of protrusions, shapes
and materials to realize appropriate fish ladder for eels in
the future.
mr

NOMENCLATURE
L length, m
B width, mH height, m
Δh height of side wall, mQ quantity of flow, ml/s
h depth of water, md space between protrusions and others, mmθ slope of fish ladder for eels, degree
B L averaged body length, mm
N number of fish that used to experiment (=20)
n number of fish that migrated in upstream tankS actual migrating path length of Anguilla japonica
Subscripts
x x direction or downflow direction
y y direction

z
c
st
mr

z direction or transverse direction
challenge to migration
straight line
migrating route of Anguilla japonica

ACKNOWLEDGEMENTS
This study was supported by Grants-in-aid for Scientific
Research (17K06580) when the study conducts. Finally, theauthors are grateful to Marusho Corporation for providing
eels treated in this paper.

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Mochioka, N., Miller, M.J., Aoyama, J., Kimura, S.,
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Oya, M., Watanabe, T., Hata, K., Ijiri, S., Kazeto, Y.,
Nomura, K. and Tanaka, H. (2011): “Oceanic
spawning ecology of freshwater eels in the western
North Pacific”. Nature Communications, Vol. 2, pp.
1-9.
(2) Santos, J.M., Rivaes, R., Oliveira, J. and Ferreira, T.
(2016): “Improving yellow eel upstream movementswith fish lifts”. Journal of Ecohydraulics, Vol. 1, pp.
50-61.
(3) Clay, C.H. (1994): Design of fishways and other fish
facilities, CRC Press, pp. 121-127.
(4) Solomon, D.J. and Beach, M.H. (2004): “Fish passdesign for eel and elver (Anguilla anguilla)”. R&D
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(5) Legault, A. (1992): “Étude de quelques facteurs desèlectivitè de passes à anguilles”. Bull. Fr. Pêche
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