Blackwell Publishing AsiaMelbourne, AustraliaFISFisheries Science0919 92682006 Blackwell Science Asia Pty LtdJune 2006723485493Original ArticleControlling DO levels in net pen water
S Srithongouthai
et al.
FISHERIES SCIENCE
2006; 72: 485–493
Control of dissolved oxygen levels of water in net pens for
fish farming by a microscopic bubble generating system
Sarawut SRITHONGOUTHAI,1 Akira ENDO,2 Akihiro INOUE,3 Kyoko KINOSHITA,3
Miho YOSHIOKA,3 Ayako SATO,3 Takaaki IWASAKI,2 Ichiro TESHIBA,2 Hisatsune NASHIKI,2
Daigo HAMA1 AND Hiroaki TSUTSUMI3*
1
Keiten Co., Ltd., Hondo, Kumamoto 863-0044, 2Tashizen Techno Work Co., Ltd., Ishihara, Kumamoto 8618046, and 3Faculty of Environmental and Symbiotic Sciences, Prefectural University of Kumamoto, Tsukide,
Kumamoto 862-8502, Japan
ABSTRACT: A microscopic bubble generating system (MBGS) has been developed to control dissolved oxygen (DO) levels suitable for fish farming. The MBGS has been tested to confirm its capability in net pens. Water conditions in a fish farm were monitored every two hours from June to October
2004 by setting an online vertical profiling system (OVPS) close to the net pen. DO in the net pen
water decreased to physiologically stressful levels for the fish during the night (4.84–5.51 mg/L), while
the DO was kept in saturated conditions during the day, due to oxygen supply from phytoplankton. The
MBGS was operated from the evening to the morning of the next day for 16 h, to successfully create
DO-saturated conditions in the net pen water at night. By using microscopic bubbles during the warm
seasons, DO levels in the net pen water could be improved to a level suitable for fish farming. However, the low DO levels (<5.0 mg/L) of the bottom water occasionally extended to the net pen layers,
despite the supply of microscopic bubbles to the water. To maintain the DO of the net pen water at
levels suitable for fish farming, DO supply to the net pen water and the bottom water needs to be
increased, and the organically enriched sediment just below the net pens needs to be treated.
KEY WORDS: dissolved oxygen, fish farming, microscopic bubble, online vertical profiling
system, red sea bream.
INTRODUCTION
Fish farming using net pens started in some
Japanese bays in the early 1970s. It soon became
popular in enclosed coves and bays sheltered from
waves and wind along the Japanese coast. Fish
farming has been introduced to the coastal areas
of countries throughout the world.1 In Japan, the
annual total yield from fish farming reached
approximately 27 200 t/year in 2003.2 However,
this type of cultivated fishery encounters problems because fish are reared densely in net pens
with limited space, consuming a large amount of
food and discharging a large amount of waste outside the net pens in the form of feces and food
residues,3–9 while water exchange around the fish
farms is restricted because of their location in
*Corresponding author: Tel: 81-96-383-2929.
Fax: 81-96-384-6756. Email: [email protected]
Received 16 May 2005. Accepted 17 November 2005.
enclosed coves and bays. On the sea floor just
below the net pens, organically enriched sediment
is already deposited thickly and it deteriorates to
extremely anaerobic conditions, producing high
levels of toxic hydrogen sulfide to the benthic
organisms and causing oxygen deficiency of the
bottom water during summer.10,11 Such deterioration of the benthic environment due to the occurrence of hypoxic waters often depresses the
benthic ecosystem, forming simpler and poorer
macrobenthic communities.12 Severe macrobenthic faunal change of this kind occurred in a
cove with fish farms using net pens in Amakusa,
Kumamoto, Japan, only five years after the start of
culture, and the benthic communities already suffered from the occurrence of hypoxic waters during the summer following the production of high
levels of hydrogen sulfide in the sediment.10 The
decline of the metabolic activities of the benthic
communities that are able to decompose organic
particles sinking from the net pens should accelerate further deposition of the organic particles on
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the sea floor just below the net pens. Deposition of
organic particles discharged from the net pens
accelerates further deposition of the organic
matter.
Net pens for fish culture are usually 8–10 m deep.
It is likely that the seawater inside the net pens is
affected by dissolved oxygen (DO) deficiency in the
bottom water through vertical exchange of the
water. Water DO levels are critical to cultured fish.
For example, red sea bream stops feeding activities
when the DO is less than 4 mg/L.13 The decrease
in the DO concentration of the water depresses
growth of cultured fish and decreases the production efficiency of the fish culture. Since fish farms
started on the Japanese coast in the 1970s, there
have been several measures to prevent the deposition of organic particles and to keep the bottom
conditions healthy, including sand cover over
the organically enriched sediment, dredging, and
treatment with chemical and bacterial agents to
promote decomposition of organic matter in
the organically enriched sediment.14–16 Recently,
Ruangdej and Fukami17 solved anoxic conditions
in bottom water by supplying dim light to stimulate photosynthetic activity in the laboratory. However, none of these measures has proven effective
and efficient.
The aim of this study is to create healthy water
conditions in the fish farm with a device that
generates microscopic bubbles of 5–40 µm in
diameter. These bubbles have an extremely high
dissolving efficiency in water.18–20 The device may
offer an advantage over conventional bubbling
devices in adding dissolved oxygen into the water
efficiently, and in reducing costs and energy consumption for aeration of the water. A microscopic
bubble generating system (MBGS) was set in an
S Srithongouthai et al.
enclosed bay (Kusuura Bay, Amakusa, Kyushu,
western Japan), and experiments were conducted
to control the DO levels of the water in the net
pens. Water quality of the fish farm was monitored
with an online vertical profiling system (OVPS).
Daily fluctuations of DO in the water column in the
fish farm are reported, and the effectiveness of the
MBGS to control DO levels of the water in the fish
farm is discussed.
MATERIALS AND METHODS
Study area and the fish farm operation
Kusuura Bay is an enclosed bay (11.4 km2 in area,
approximately 4 km wide and 3.5 km long) located
between Amakusa Kamishima Island and Amakusa
Shimoshima Island, on the west coast of Kyushu,
western Japan (130°13′E, 32°23′N). This bay is in a
transitional area between Ariake Bay, connected
through a small channel at the northern end and
the Yatsushiro Sea through an opening of approximately 400 m at the southern end (Fig. 1). The
water depth at the study site varied between 16
and 20 m. Annual water temperature ranged
12.9–28.8°C, and salinity varied between 19.0 and
34.0 psu in 2003 (unpubl. data).
Fish farms with a total area of 0.28 km2 were first
established at the center of the bay in 1975. In
recent years, five companies have operated fish
culture in the bay, rearing red sea bream Pagrus
major and yellowtail tuna Seriola quinqueradiata
in net pens 12 m × 12 m × 8 m deep, and producing
approximately 2000 t of fish per year. A water quality monitoring station (Stn 1) was established close
to the red sea bream farming (Fig. 1).
Fig. 1 Study area, and location
of the fish farm and an experimental site (Stn 1).
FISHERIES SCIENCE
Controlling DO levels in net pen water
487
nanotech.co.jp/ads) by the AquaTrack System
(YSI/Nanotech), using the packet communication
system of NTTDoCoMo (Tokyo, Japan) (DoPa).
Power is supplied from a battery equipped with
solar panels. This system was set on a raft of 8 m2
area beside the net pens at Stn 1 in the fish farm.
Online Vertical Profiling System
The online vertical profiling system (OVPS) is a set
of instruments to monitor the real-time vertical
profile of the water. The OVPS consists of an acoustic Doppler profiler (SonTek/YSI, ADP, Yellow
Spring, OH, USA), a vertical profiling system (YSI/
Nanotech, Kawasaki, Japan), AquaTrack System
(YSI/Nanotech), and solar panels (Fig. 2a). The
ADP is a 100-m rated acoustic vector-averaging
instrument that measures the vertical profiles of
current speed and direction, using the acoustic frequency shift obtained from reflections from particles in the water. The vertical profiling system has
an automatic winch and a multiparameter sonde
(6600EDS, YSI/Nanotech) that is capable of measuring salinity, temperature, depth and DO in the
water. This sonde has a wiper to clean the DO sensor for long-term measurements. The automatic
winch lowers the sonde with a cable at 1 m depth
intervals from the surface water to 2 m above
the seafloor every two hours. All measurements
are transmitted to the website (http://www.
Microscopic Bubble Generating System
The microscopic bubble generating system
(MBGS) has an electric power generator, a switchboard, an air compressor, an air-flow controller
and head injectors (Tashizen Techno Works, Kumamoto, Japan). In this study, the four modules
except the head injectors were set on the raft with
the OVPS at Stn 1, and each of four head injectors
was put in the water at the center of a net pen. The
head injector releases microscopic bubbles of 5–
40 µm diameter (Fig. 3) with 400 W of electric supply from the power generator and air supply from
the air compressor on the raft. This system can
work for approximately 16 h before refueling.
(b)
Radio antenna
(a)
Electric power
generator
Solar panel
Radio antenna
Air
compressor
Switchboard
Air-flow
controller
Automatic
winch
Data
transceiver
Battery
Data
transceiver
ADP
Multi-sonde
Head injector
An online vertical profiling system (OVPS)
(c)
Fig. 2 Schematic drawings of (a)
the online vertical profiling system
(OVPS) and (b) microscopic bubble
generating system (MBGS) developed in this study. (c) Photograph
of the OVPS and the MBGS set on a
raft beside the net pens at Stn 1 in
the fish farm.
A microscopic bubble generating system
(MBGS)
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(a)
S Srithongouthai et al.
assessed in the evening (16:00–22:00 h) on 8 June 8
2004 before bubbles were added to the water, and
after adding bubbles on 9 June 2004. Each head
injector released air bubbles at 0.5 L/min flow rate.
Experiment 3: Long-term monitoring of the
water quality in the fish farm with the MBGS
(b)
A head injector was set at 5 m depth in two neighboring net pens each located at the north side and
south side of the raft with the OVPS and the MBGS
at Stn 1. Vertical profiles of DO were assessed every
two hours from 28 June−12 October 2004. The
MBGS worked for 16 h from the evening (17:00 h)
to the next morning (09:00 h) from 6 July until the
end of experiment. Each head injector in a net pen
released 5.0 L/min of bubbles.
RESULTS
Experiment 1: Daily fluctuation of DO vertical
profile in the fish farm
Fig. 3 Photographs of the head injector releasing
microscopic bubbles. (a) top view, and (b) side view
underwater.
Experiment 1: Daily fluctuation of DO vertical
profile of the water in the fish farm
Prior to the experiments that add DO into the
water in the net pens with the MBGS, the raft with
the OVPS was set at Stn 1 (Fig. 1). The vertical DO
profile was monitored every two hours, and velocity of the tidal current hourly from 18:00 h on June
29–18:00 h on 1 July 2004.
Experiment 2: Controlling DO levels in a net pen
during in the evening with the MBGS
A head injector was set at 4 m depth in each of the
neighboring net pens located at the north side and
south side of the raft with the OVPS and the MBGS
at Stn 1. Changes of the vertical profiles of DO were
The changes of the vertical profile of DO concentration of the water at Stn 1 for 48 h from 29 June−
1 July 2004 are shown in Figure 4a. Low DO levels
(<5.5 mg/L) were found at the net pen layer (0.5–
7.5 m) and the bottom layer (from the bottom to 2–
4 m above the sea floor), while DO levels of the
intermediate layer (8.5–12.5 m) were maintained
between 5.62 and 6.72 mg/L throughout the monitoring period.
In the net pen layer, DO decreased from the late
evening to the early morning of the next day, and
the lowest DO levels were recorded in almost the
whole layer until 6.5 m depth at 6:00 h on June 30
(4.84–5.51 mg/L) and in the layer between 2.5 and
4.5 m depth at 6:00 h on July 1 (4.43–5.33 mg/L). In
contrast, soon after daylight, DO rapidly recovered, and the highest DO levels (6.45–6.86 mg/L)
were found in the whole net pen layer at 18:00 h
on 30 June and 1 July. The decrease in DO levels
during the night may reflect the oxygen consumption by fish respiration, and the increase during
the daytime may be caused by the additional
DO supply from the photosynthetic activities of
phytoplankton.
In the bottom layer, low DO levels (3.34–
5.50 mg/L) were observed almost throughout the
monitoring period. Just below the net pens, organically enriched sediment forms from large
amounts of organic discharge. Therefore, the low
DO levels may be caused by aerobic decomposition of the organic matter deposited on the
seafloor.
FISHERIES SCIENCE
Controlling DO levels in net pen water
489
a
7.0
1
Depth (m)
5
7
9
5.5
5.0
5.0
6.5
6.0
6.0
5.5
5.5
6.0
5.0
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11
6.0
DO (mg/L)
6.5
3
4.5
13
15
5.0
4.0
5.5
3.5
18:00
20:00
22:00
00:00
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B-2
4.0
5.0
June 29
(2004)
b
The net pen layer
July 01
Intermediate layer
Bottom layer
7.0
6.5
6.0
DO (mg/L)
Fig. 4 Experiment 1. (a) Daily
fluctuation of dissolved oxygen
(DO) vertical profile of the water
in the fish farm. Numbers on the
figure indicate dissolved oxygen
of the water. (b) Mean DO levels
in the net pen layer (0.5–7.5 m),
intermediate layer (7.5–12.5 m)
and bottom layer (3–4 m above
the sea floor) at night (20:00–
06:00 h) and day (08:00–18:00 h)
from 29 June−1 July 2004. Error
bars = standard deviation.
June 30
5.5
5.0
4.5
4.0
3.5
3.0
Night
Figure 4b compares the differences in DO fluctuation patterns between night and day at the net
pen, intermediate and bottom layers. In the intermediate layer, the mean values of DO were maintained at almost saturated levels (6.04–6.17 mg/L)
throughout the monitoring period. In contrast, in
the bottom layer, the mean DO levels decreased
between 4.66 and 5.07 mg/L throughout the
monitoring period. In the net pen layer, the mean
DO was in the same levels with the intermediate
layer during the daytime, but decreased to 5.67–
5.76 mg/L during the night.
Figure 5 presents the changes in current velocity
at Stn 1 for 48 h from 29 June−1 July 2004. At the net
pen layer (4 m depth), the current velocities fluctuated between 0 and 4.50 cm/s (mean = 2.06 cm/s).
These slow current velocities restricted the water
exchanges in the net pens, and emphasized the
effect of the fish respiration on DO levels. At the
intermediate layer (10 m depth) and the bottom
layer (1 m above the sea floor), the velocities
fluctuated between 1.40 and 15.6 cm/s (mean =
6.41 cm/s) and 1.30 and 24.8 cm/s (mean =
8.80 cm/s), respectively. Therefore, the water
Day
Night
Day
exchange rates of these two layers were 3.1 and 4.2
times, respectively, larger than that of the net pen
layer. Nevertheless, the DO levels of the bottom
layer decreased to 3.34–5.50 mg/L throughout the
monitoring period. These results indicate the large
benthic oxygen consumption on the sea floor with
the organically enriched sediment just below the
net pens.
Experiment 2: Controlling DO levels in a net pen
during the evening with the MBGS
According to Figure 4, the DO levels in the net pen
layer decreased from late evening to early morning. An experiment was conducted to control the
DO levels in the net pens, using the MBGS from
16:00 h on 9 June 2004. Figure 6 compares the vertical profiles of DO at Stn 1 on 8 June with those on
9 June 2004 after bubbles were added to the net
pen water with the MBGS.
On 8 June the vertical DO concentration profiles
at 16:00 h exhibited a sharp decrease from the
saturated conditions (8.00 mg/L) at the surface to
FISHERIES SCIENCE
Current velocity
(cm/s)
30
25
20
15
10
5
0
30
25
20
15
10
5
0
S Srithongouthai et al.
18:00
20:00
22:00
00:00
02:00
04:00
06:00
08:00
10:00
12:00
14:00
16:00
18:00
20:00
22:00
00:00
02:00
04:00
06:00
08:00
10:00
12:00
14:00
16:00
18:00
Current velocity
(cm/s)
30
25
20
15
10
5
0
Current velocity
(cm/s)
490
June 29
(2004)
June 30
DO (mg/L)
Depth (m)
6.0
6.5
7.0
7.5
DO (mg/L)
8.0
6.0
6.5
7.0
7.5
DO (mg/L)
8.0
6.0
6.5
7.0
7.5
DO (mg/L)
6.0
8.0
0
2
4
0
2
4
0
2
4
0
2
4
6
8
10
12
14
6
8
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8
10
12
14
6
8
10
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14
16
18
20
June 8, 2004 (16:00)
16
18
20
June 8, 2004 (18:00)
DO (mg/L)
6.0
Depth (m)
July 1
6.5
7.0
7.5
16
18
20
June 8, 2004 (20:00)
DO (mg/L)
8.0
6.0
6.5
7.0
7.5
16
18
20
6.0
6.5
7.0
7.5
0
2
4
0
2
4
0
2
4
6
8
10
12
14
6
8
10
12
14
6
8
10
12
14
6
8
10
12
14
June 9, 2004 (16:00)
16
18
20
June 9, 2004 (18:00)
16
18
20
June 9, 2004 (20:00)
6.82 mg/L at 4.5 m, and gradual increase of DO up
to 9.5 m depth (7.24 mg/L). The lowest DO concentration at 4.5 m in the net pen layer indicated that
the fish were most concentrated at this depth
in the evening. This lowest DO further gradually
7.0
7.5
8.0
DO (mg/L)
6.0
8.0
0
2
4
16
18
20
6.5
June 8, 2004 (22:00)
DO (mg/L)
8.0
Fig. 5 Experiment 1. Changes in
current velocity at (a) the net pen
layer at 4 m, (b) intermediate
layer at 10 m, and (c) bottom
layer at 1 m above the sea floor
from 29 June−1 July 2004.
16
18
20
6.5
7.0
7.5
8.0
June 9, 2004 (22:00)
Fig. 6 Experiment 2. Comparison of the changes in dissolved
oxygen (DO) vertical profile
before (16:00–22:00 h, 8 June
2004) and after (16:00–22:00 h, 9
June 2004) microscopic bubble
injection. A head injector of the
microscopic bubble generating
system (MBGS) was set at 4 m
depth of two net pens each
(dotted line) and released air
bubbles at 0.5 L/min flow rate on
9 June 2004.
decreased to 6.72, 6.35 and 6.32 mg/L at 18:00,
20:00 and 22:00 h, respectively.
On 9 June, the vertical DO concentration profile
at 16:00 h showed a similar pattern to 8 June. The
surface DO was 7.79 mg/L and the lowest level was
FISHERIES SCIENCE
approached the study area. Figure 7 shows the vertical profile of daily mean DO 8 days before and
after the start of the MBGS operation. Eight days
before the start of the MBGS operation, DO concentrations in the net pen layer fluctuated between
5.17 and 6.24 mg/L. DO >6.00 mg/L was restricted
in the surface layer up to 1.5 m depth. Eight days
after the start of the MBGS operation, the DO of the
net pen layer up to 4.5 m depth increased markedly. DO >6.00 mg/L (maximum 6.84 mg/L) was
recorded in almost the entire net pen layer.
Figure 8 illustrates the daily fluctuations of the
vertical profile of mean DO, salinity and temperature at Stn 1 from 9 June−12 October 2004. As
shown in Figures 6 and 7, the net pen layer was
Experiment 3: Long-term monitoring of the
water quality in the fish farm with the MBGS
6.0
6.0
1
3
5
7
5.5
5.5
6.0
6.0
5.5
5.5
1
2
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9
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4.0
7
6
5
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7
Jul
5
6
6
5
5
5
Aug
32.5 32.0
31.5 31.0
32.5
6
8 7
4
6
32.5
32.5
29.5
31.5
33.0
30.5
32.0
31.5
33.0
33.0
32.0
32.5
33.0
Jul
Jun 2004
28 29
27
23
Jun 2004
Aug
24
30
29
26
25
28
27
28
25
27
Jul
Aug
Sep
33.5
33.0
32.5
32.0
31.5
31.0
30.5
30.0
29.5
29.0
31
30
29
28
27
26
25
24
23
22
21
Oct
Sep
29
10
9
8
7
6
5
4
3
Oct
Sep
33.0
1
3
5
7 22
9
11
13
15
B-2
3.5
10 11 12 13 14
July
b
Depth (m)
7
4.5
Salinity (psu)
Depth (m)
1
3
5
7
9
11
13
15
B-2
1
3
5
7
9
11
13
15
B-2
4.5
5.0
5.0
5.0
5.0
June 2004
Depth (m)
5.5
5.0
28 29 30
c
6.0
9
11
Jun 2004
Fig. 8 Experiment
3.
Daily
changes of mean vertical profiles.
(a) dissolved oxygen (DO) (b)
salinity, and (c) water temperature at Stn 1 from 9 June−12 October 2004. A head injector was set
at 5 m depth in four net pens, and
released air bubbles at 5.0 L/min
flow rate from 6 July 2004. Numbers on the figures indicate dissolved oxygen, salinity and water
temperature respectively.
6.5
5.5
5.5
6.0
6.0
13
15
B-2
a
7.0
6.5
6.5
6.0
6.0
Temperature (ºC)
Fig. 7 Experiment 3. Comparison of the daily changes of the
mean dissolved oxygen (DO) vertical profile at Stn 1 before and
after the supply of bubbles with
the microscopic bubble generating system (MBGS). A head injector was set at 5 m depth in four
net pens, and released air bubbles at 5.0 L/min flow rate from 6
July 2004.
Depth (m)
The MBGS operated at Stn 1 for 16 h/day from 6
July−12 October 2004, except when typhoons
DO (mg/L)
6.67 mg/L in the net pen layer at 5.5 m. However,
the lowest DO at 4.5 m depth in the net pen layer
gradually increased to 6.85, 6.85 and 7.30 mg/L at
18:00, 20:00 and 22:00 h, respectively. The depression of DO at 4.5–5.5 m depth was not found
at 22:00 h. Thus, the operation of the MBGS could
recover the DO levels in the net pen layer
effectively.
491
DO (mg/L)
Controlling DO levels in net pen water
Oct
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effectively controlled by supply of bubbles with the
MBGS. However, the long-term monitoring of the
DO vertical profile revealed that the bottom layer
with DO less than 5.0 mg/L occasionally extended
to the net pen layer during the summer (the periods 11–30 July and 9–18 August 2004). In July, a
halocline developed in the surface layer up to 3 m
depth due to the rainy season. From July to August,
a thermocline also developed in the surface layer
up to 7 m depth. Since the water column was well
stratified by the halocline and the thermocline, the
extension of the low DO concentration from the
bottom to the net pen layer may be caused by
the acceleration of DO consumption of the bottom
water.
DISCUSSION
The dissolved oxygen level of the water is a vital
condition for aerobic respiration in aquatic organisms. In this study, measurement of DO concentration in the water column with the OVPS close to the
net pen (Stn 1) revealed that the DO content of the
water of the net pen layer decreased markedly
during the night, even if the DO in the water was
saturated during the day (Fig. 4). In the day,
photosynthesis by phytoplankton produces and
supplies oxygen into the surface layer of the water,
while the phytoplankton activities stop then consume oxygen for respiration in dark conditions.
Because current velocity was slow (mean =
2.06 cm/s) (Fig. 5), water exchange in the fish farm
was minimal. Therefore, DO consumption in the
net pen water would naturally be accelerated by
the respiration of the reared fish and the phytoplankton at night.
In Experiment 1, DO concentrations decreased
to levels below 5.5 mg/L in the whole net pen layer
(4.84 mg/L in the lowest level) at night in an
uncontrolled net pen without adding bubbles in
early summer. Harada21 described that yellowtail
requires more than 5.7 mg/L of DO for normal
growth. Murata13 reported that yellowtail and red
sea bream began to stop feeding when DO levels
decreased to less than 4.8 mg/L and 3.3 mg/L,
respectively. Hirata and Kadowaki22 reported that
more than 20% reduction of DO concentration of
water (approximately 5.0–5.5 mg/L in water conditions in early summer) brought significant increase
in mortality of yellowtail. It is very likely that such
decrease in DO levels in the net pen could give
physiological stress to the growth and/or mortality
of the reared fish, even if this phenomenon
occurred only at night. As shown in Figures 6 and 7,
the operation of MBGS from the evening to the
morning of the next day succeeded in efficiently
S Srithongouthai et al.
preventing such decrease of DO concentration and
in creating DO-saturated conditions in the net pen
layer during the night.
In the long term experiment (Experiment 3), the
MBGS operated at 5.0 L/min air-flow rate at 5 m
depth in the net pen to control DO levels in water
throughout summer. Long-term monitoring of
the daily mean DO vertical profile with the OVPS
revealed that the mean DO concentration of the
water above the head injectors during this experiment (6 July−12 October 2004) was kept at 5.5–
6.6 mg/L, but low DO levels of 4.5–5.0 mg/L occasionally extended from the bottom to the net pen
layer during the neap tide (Fig. 8). The vertical profile of the water quality indicated that DO reduction was caused by the acceleration of oxygen
consumption on the sea floor, the deeper water
during July and August, and by the vertical mixing
of water with low DO levels in the vertical circulation period in September
Decrease in DO in the bottom water just below
fish farms under the stratified water conditions has
been documented during the warm seasons by
many authors.3,10,22–26 Degradation of the organic
matter deposited just below the fish farm is accelerated during the warm seasons, and causes the
depletion of the DO concentration of the bottom
water.5,9,27–30 In this study, the low DO levels of the
bottom water were observed to extend to the net
pen layer occasionally during the neap tide,
although bubbles were continuously supplied by
the MBGS to the net pen water. Therefore, to maintain the DO content of the net pen water at levels
suitable for fish farming throughout the warm seasons, DO supply needs to be increased to the net
pen water and the bottom water, and the decomposition of the organic matter deposited on the sea
floor needs to be accelerated during the cold
seasons. Further research is being undertaken to
investigate improvement of environmental conditions for sustainable development of coastal fish
farming. A head injector to add microscopic bubbles into the water is being developed, and a new
technique will treat organically enriched sediment
on the sea floor just below a fish farm with a small
deposit-feeding polychaete (Capitella sp. I) from
autumn to winter, when DO levels of the water are
suitable for the increase of the polychaete.31 The
effects of the treatment of the organically enriched
sediment to the maintenance of DO levels in the
net pen water will be reported elsewhere.
ACKNOWLEDGMENTS
This work was supported by Research and Development Program for New Bio-industry Initiatives,
Controlling DO levels in net pen water
FISHERIES SCIENCE
Japan. The authors thank the staff of Keiten Co.,
Ltd. for their collaboration during the sampling.
16.
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