The 11th Pacific Symposium on Flow Visualization and Image Processing
1-3 December 2017,Kumamoto, JAPANEFFECTS OF VEGETATION DENSITY ON SWIMMING BEHAVIOR OFZACCO PLATYPUS
K. Onitsuka1, J. Akiyama1 and K. Joji1
1

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

ABSTRACT
It is important to secure the rest area for fish. Fish has
the ordinary and dark muscle. When fish uses ordinary
muscle, fish gets tired. In such a situation, fish needs a rest.
In this study, vegetation density in open-channel ischanged. The trajectories of Zacco platypus wereobserved. It was found that Zacco platypus utilized theslow velocity area as rest space. Staying in vegetation area
increases with an increase of vegetation density. An
approach rate to vegetation area rises with an increase ofvegetation density. However, it was not found that clear
relationship with a migration rate of Zacco platypus andvegetation density.
1. INTRODUCTION
Nakamura (1985) showed the condition that is able tolive the fish in the river is securing of flow discharge,
securing of water quality, security of the bait, theprotection from a natural enemy, security of the spawning
ground, the security of paths of excursion, and the securityof the place of refuge. In this condition security of theplace is thought that is classified in two from theviewpoint of muscle fatigue and scale at time. The muscleof the fish has the dark muscle and the ordinary muscle.
Usually the fish uses the dark muscle which does notaccumulate for fatigue, and the fish does not want to use
the ordinary muscle which fatigue is accumulated. Fatigueis accumulated when swimming speed is over the 2-4times of the flow velocity divided by the body of fish, sothe fish often needs the rest. In this case, rest time ispresumed that within few seconds to few minutes.
Furthermore, it is necessary to evacuate when fish met thepredation risk and met to velocity more than the burst
swimming speed at the time of flood. In this case fishevacuates regardless of accumulation of muscle fatigue.
Further, the condition that the fish leaves the place ofrefuge is when natural enemy leaves or the flood goesaway, therefore the fish stayed in the place of refuge forthe long time.
Many examples of study exist about the fish is using
the place of refuge. Takamizu et al. (2007) changed
vegetation density sistematically and released Tribolodon
hakonensis. It was found that the number of fish
approaching decreased with vegetation density increases.
Azuma et al. (1999) showed that Tribolodon hakonensis
which flowed down by flood evacuated to vegetation or
wand. However, little is known about the relation ofvegetation density or hydraulic functions and swimmingbehavior.

In this study, simulated vegetation density in openchannel was changed to make clear effects of vegetation
density on the swimming behavior of Zacco platypus.
2. MATERIALS AND METHODS
Figure 1 shows the open-channel. Open-channel was
designed like following. The pool length was 4.0m andwidth Lz was 0.8m. x axis is taken in the direction offlowing. y axis is taken in the vertical direction frombottom. z axis is taken in the direction of the crossing.
Case name was defined as shown in Table 1. The spaces ofthe round columns d were set to five patterns within therange from 4 to 20cm. Spaces of the round columns wasdefined as the distance between the center of the spaces of
the round columns. The area was defined as vegetation area.
Figure. 2 shows the layout drawing of the round columns invegetation area of D-04 as an example. 10 Zacco platypus
averaged body length BL is 70mm were used in eachexperiment. The recording has been carried out for 5
conditions. In this way, the experiment is performed 5times in each case. The flow velocity divided by average
length of Zacco platypus was fixed 10 in each case. The
number of pixel of the digital video camera is 1440×1080,
and recording speed is 30fps. Experiment time is 1minute ineach case. After the experiment, the picture of swimming
behavior of Zacco platypus was captured at 0.5s interval.
Approach rate, migration rate, and staying time forvegetation area were calculated with the captured picturesand coordinated plot software. Three components of flow
velocity were measured. Average of flow velocity ( U , V ,W ) of each direction axis ( x , y , z ) and synthetic flow
velocity was calculated after measuring. When flow
velocity is measured, Zacco platypus is not put into theopen-channel.
3. RESULTS AND DISCUSSION
3.1. Flow condition
Figure 3 shows vector figures of flow velocity VV forcase D-04, D-12 and D-20. Flow velocity in vegetation
area and the down stream decreases with spaces of theround columns narrow. It was found that flow velocity of
the left bank side is fast and right bank side is slow in eachcase. Figure 4 shows mean flow velocity V in vegetation
area for case D-04, D-12 and D-20. Mean flow velocity invegetation area decreases with the spaces of the roundcolumns narrow. It is thought that fatigue accumulates
because fish uses ordinary muscle which fatigue
accumulates in the area where the flow velocity divided

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

vegetation area・・・・・・・・・・・・・・・・・・

Bv  0.4m

0.6

top-view

・ measurement point

Lx  2.0m
Figure 1. Experimental device

・・・・・・・・・zLz  0.8m

V (m/s)0.3

x0.15

Table 1. Experimental case
case nameequal distance(cm)

D-04
4D-08
8velocity U m BL (1/s)

D-121210

D-16
16

D-20
20041220d (cm)Figure 4. Mean flow velocity

D-04

Figure 2. Layout drawing of the round columns
S-04

0.60

1.0

VV (m/s)
0.48

z/Lz

0.42

Figure 5. Approaching rate

0.36

0.5

0.30
0.24

0.25

0.18
0.12
0.06

00

0.25

0.5(a)

x/Lx

1.0

0

D-04

S-12

0.60

1.0

VV (m/s)
0.48

z/Lz

0.42
0.36

0.5

0.30
0.24

0.25

0.18
0.12
0.06

00

0.25

0.5(b)

x/Lx

1.00D-12

S-20

0.60

1.0

VV (m/s)
0.48

z/Lz

0.42
0.36

0.5

0.30
0.24

0.25

0.18
0.12
0.06

000.250.5x/Lx

1.00(c) D-20
Figure 3. The vector figure of flow velocity
by averaged body length of Zacco platypus is 10.
Therefore, it is thought that Zacco platypus swam invegetation area and the down stream to avoid
accumulation of fatigue.

3.2. Approaching rate
The approaching rate of Zacco platypus is defined asfollows:
Approaching rate
Number of fish approached to vegetation area n(1)Number of fish that used to experiment N
Figure 5 shows the approaching rate for vegetation area ofZacco platypus in each case. It is thought that the fishtakes a rest in the state that is lower than cruising speed
without the accumulation of fatigue when using the dark
muscle. Generally, the cruising speed V fC is defined asfollows:

V fC / BL = 2～4(2)the approaching rate in case D-04 reaches near 80 % andthe approaching rate decreases with spaces of the roundcolumns spread. In Figure 3 and Figure 4, flow velocity is
restrained around 0.10～0.24(m/s) in vegetation area. Thisvelocity divided by average length of Zacco platypus isless than 4. Zacco platypus swims in slow velocity area toavoid accumulation of fatigue because Zacco platypus
accumulates fatigue when flow velocity divided byaverage length of Zacco platypus is 10. Therefore, Zacco
platypus can swim with the cruising speed V fC that has noaccumulation of fatigue. It follows from that Zacco
platypus approaches to vegetation area to rest
3.3. Swimming trajectory
Figure 6 shows swimming position that were given foreach 0.5 second in each case. Zacco platypus avoids fastThe 11th Pacific Symposium on Flow Visualization and Image Processing
1-3 December 2017,Kumamoto, JAPANflow direction

flow velocity area in left bank side in all case. Zacco
platypus swims in vegetation area when the spaces of theround columns were narrow in D-04, D-08, while thenumber of Zacco platypus in vegetation area decreases in
D-12, D-16 and D-20 in comparison with D-04, D-08.
Zacco platypus staying in down stream of vegetation area
moves to vegetaion area with the spaces of the roundcolumns narrow. Therefore, it is clear that Zacco platypus
uses the spaces of the round columns as a rest area when
the spaces of the round columns narrow and flow velocity
in the spaces of the round columns decreases.
Figure 7 shows an example of the swimming
trajectory of Zacco platypus every 0.5 second in any 1 minwith each case. Considering D-04 and D-08 shown light
blue line and yellow line, Zacco platypus turns frequently
and stays in vegetation area when the spaces of the roundcolumns narrow. On the other hand, considering D-12, D16 and D-20 shown green line, red line and blue line, thetendency that Zacco platypus migrates and go to main
flow area is shown. Therefore, it is shown that Zacco
platypus uses vegetation area as a rest area when thespaces of the round columns is small and do not likevegetation area with spaces of the round columns spread.
It is thought that Zacco platypus likes the slow velocity
area to avoid using ordinary muscle.

vegetation area

1z / Lz
0.6

0.4

0.2

00

0.2

0.4

(a)

0.6

x / Lx

1D-04

flow direction

vegetation area

1z / Lz

0.6
0.4
0.2
00

0.2

0.4

(b)

x / Lx

0.6

D-08
vegetation area

flow direction

11z / Lz

0.6

0.4

0.2

3.4. Fish school radius
Fish school radius was calculated from the Eq. 3 thatIshikawa (2000) defined.
NR   rii 1000.2(c)N(3)

Figure 8 shows frequency distribution of the value inwhich the fish school radius R divided by the averaged
body length of Zacco platypus BL in each case. It is0.6

x / Lx

1D-12

flow direction

120.4

vegetation areaz / Lz

0.6

0.4

0.2

defined that R is fish school radius, ri is distance
between centroid of fish school and each test fish, and N
is total population in each fish school. The fish school
radius R tends to increase with the spaces of the roundcolumns narrow. Considering Figure 6, Zacco platypus
swims around down stream area of the vegetation area inD-12, D-16 and D-20 which spaces of the round columns
area are wide. Fish school radius R is large and Zacco
platypus swims without crowding after enteringvegetation area in D-04 and D-08 which the spaces of theround columns are small. Furthermore, considering Figure
8, frequency distribution of the value in which the fishschool radius concentrates on a small value. Thus, it isconfirmed that Zacco platypus forms fish school.
Therefore, Zacco platypus forms fish school and swims inthe down stream area of the vegetation area when thespaces of the round columns is small.
3.5. Average staying timeFigure 9 shows the averaged staying time per one
Zacco platypus T in vegetation area in each case. Theaverage staying time increases with the spaces of theround columns narrow. Considering Figure 7, it is thoughtthat this is because Zacco platypus swims little by little in

00

0.2

0.4(d)

0.6

x / Lx

1D-16

flow direction

vegetation area

1z / Lz

0.6

0.4

0.2

00

0.2

0.4

0.6

x / Lx1(e) D-20
Figure 6. Swimming position
the vegetation area with the spaces of the round columns
narrow.
3.6. Migration rate
The migration rate of Zacco platypus is defined asfollows:
Migration rate

Number of fish that migrated n m
Number of fish that approached to vegetation area n(4)The 11th Pacific Symposium on Flow Visualization and Image Processing
1-3 December 2017,Kumamoto, JAPANvegetation area

flow direction

1z / Lz

D-04
D-08

0.6

D-12
D-16

0.4

D-20

0.2

initial position of fish

000.2

0.4

x / Lx

0.6

1

Figure 7. Swimming trajectory
151T (s)

nnmnN

90.6

60.4

30.2

D-04

0.3

D-08

n R BnLNR NBL

D-12
D-16

0.2

D-20

migration rate

0.1

0005

1015 R

20

Figure 8. Frequency distribution ofthis fish school radius

04BL8121620 d (cm)Figure 9. Averaged staying time

Figure 10 shows the migration rate of Zacco platypus
which approached to vegetation area. The clear tendencyof the relationship between migration rate of Zacco
platypus and the spaces of the round columns was notconfirmed. Takamizu et al. (2007) changed density ofgroyne and observed the swimming behavior of Zacco
platypus. However, the relationship between density ofgroyne and the migration was not elucidated. Therefore,
the existence of the round columns is ineffective inpromoting migration of Zacco platypus. However, it isthought that the round columns produce effect as the rest
area
4. CONCLUSION
In this study, simulated vegetation density in openchannel is changed to make clear effects of vegetation
density on the swimming behavior of Zacco platypus. As
a result, the following have been understood.
(1) Zacco platypus used vegetation area as a rest area
when spaces of vegetation were from 4 to 8cm.
(2) Zacco platypus stayed in vegetation area whendensity of vegetation was high, while Zacco platypus
stayed at downstream side of vegetation area whendensity of vegetation was low.
(3) The relation of migration rate of Zacco platypus
which entered vegetation area and vegetation density
was not confirmed.
NOMENCLATURE
L length, m
B width, md spaces between the round columns and others, cm

48121620d (cm)Figure 10. Migration rate

B L averaged body length, mm
VV flow velocity, m/s
V mean flow velocity in vegetation area, m/s
V fc the cruising speed, m/s
R fish school radius, mmri distance between centroid of fish school and each testfish, mm
T the average staying time per one Zacco platypus, sACKNOWLEDGEMENTS
This study is supported by Grants-in-aid for Scientific
Research (17K06580) when the study conducts.
REFERENCES(1) S. Nakamura. (1995): Topics about fishway,Sankaidou.
(2) K. Takamizu, T. Kurihara, M. Aoki, F. Uchiyama, Y.
Fukui. (2007): “Hydraulic functions of the pile dikeand its impact on fish behaviour in the vicinity”.
Journal of Hydraulic Engineering, JSCE, Vol. 51, pp.
12731278.
(3) N. Azuma, S. Kamoshita, W. Sawara, Y. Seki, K.
Watanabe. (1999): “Behaviour of Japanese dace
Leuciscus
hakonensis
andMasu
salmonOncorhynchus masou during flood period”.
Environmental systems research, Vol. 27, pp.
793798.
(4) M, Ishikawa. (2000): “An Experimental Study on
Evaluation of Fish Schooling Behavior Properties”.
Advances in river engineering, Vol.6, pp.101-106.