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River Systems Vol. 21/4 (2015), p. 229–240
Published online June 2015
An inundated Salix stand provides spawning and nursery
habitat for native fish in a periodically flooded reservoir zone
Kazuhiro Azami1,*, Takaya Higuchi2, Chisato Konishi3, Hitomi Hashimoto4,
Tomonori Osugi5, Takashi Asaedea6 & Katsuki Nakai7
With 7 figures and 1 table
Abstract: The water level in the Miharu Dam reservoir (northeastern Japan) is drawn down to 8 m during the flooding season (4 months on average per year), exposing a section of emergent shoreline termed the “drawdown zone.”
Over many years of periodic water-level control, Salix species (willows) have established dominant stands in this
zone. Salix species are typical inundation-tolerant plant species found on reservoir shores, especially behind dams
subjected to water-level regulation. Thus, Salix stands are inundated or emergent for several consecutive months in
each year. We aimed to determine whether inundated Salix stands in the drawdown zone provide a native fish (silver
crucian carp, Carassius auratus langsdorfii) with spawning and nursery habitats. We tracked three tagged silver
crucian carp telemetrically in the front reservoir behind the Miharu Dam. The fish spent considerable periods of time
during the spawning season in inundated Salix stands. Over 3 months of field observations, we observed spawning
events, eggs, and juvenile carp in the inundated Salix, whereas areas without submerged vegetation did not support
spawning or offspring. Thus, this fish species uses flooded Salix stands as spawning grounds and juvenile habitat.
Keywords: Dam reservoir, limited water-level method, drawdown, Salix species, spawning habitat, nursery habitat,
silver crucian carp
Introduction
Artificial reservoir regulation, hydroelectric generation,
flood control, agricultural irrigation, and the provision of
industrial water supplies often causes rapid, drastic waterlevel changes. These changes have considerable ecological repercussions. In Lake Constance (Germany), the wa-
ter level may fluctuate by 1.5 m between March and June
(Probst et al. 2009). Other examples are as follows: 5.5 m
annually in Buttle Lake in western Canada (Northcote &
Atagi 1997); 11.6 m between September and October in
the Sau Reservoir, Spain (Benejam et al. 2007); 20 m annually in Gardiken, Sweden (Northcote & Atagi 1997);
and 30 m between April and May in Poyang Lake behind
Addresses of the authors:
1
OYO Corporation, 2-61-5 Toro-cho, Kita-ku, Saitama-city, Saitama 331-8688, Japan
OYO Corporation, 275 Aza Ishibatake Oaza Nishikata, Miharu-machi, Tamura-gun, Fukushima 963-7722, Japan
3OYO Corporation, Currently at Stanford University, Department of Geophysics, 397 Panama Mall Mitchell Building, Stanford,
CA 94305-2215
4OYO Corporation, 275 Aza Ishibatake Oaza Nishikata, Miharu-machi, Tamura-gun, Fukushima 963-7722, Japan
5Water resources Environment technology Center, 2-14-2 Kouji-machi, Chiyoda-ku, Tokyo 102-0083, Japan
6Saitama University, Department of Environmental Science, 255 Shimo-okubo, Sakura, Saitama 338-8570, Japan
7Lake Biwa Museum,1091 Oroshimo-cho , Kusatsu, Shiga 525-0001, Japan
*Corresponding author: [email protected]
2
DOI: 10.1127/rs/2015/0090
© 2015 The authors
E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany, www.schweizerbart.de
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Kazuhiro Azami et al.
the largest dam in the world, The Three Gorges, located
west of Shanghai, China (Zhang et al. 2012).
In Japan, the rainy typhoon season lasts from June to
October. The country has an Asian monsoon climate, which
formerly caused severe flooding and human disaster in
river basins. For this reason, large numbers of dams have
been constructed for flood control, and the “limited waterlevel method” has been implemented across the country.
The Ministry of Land, Infrastructure, Transport and Tourism and the Japan Water Agency manage approximately
100 dams, and 70 of these are regulated by the limited water-level method (Japan Dam Foundation 2001).
In the limited water-level procedure, the water level is
set below a normal high-water level during the monsoon
season from June to October to ensure capacity for flood
control; the water level is set higher (normal high-water
level) during the non-flooding season from October to
June, usually with dramatic water-level changes between
seasons. Water drawdown occurs between May and June,
and refilling takes place in October. The extent of drawdown or refill is administered year by year in the same
seasons. A predetermined water level is maintained during each season, and the levels are judged by the extent of
the shoreline slope area that is either immersed or emergent. This section of slope is called the “drawdown zone.”
Water-level regulation often has diverse and complex
impacts on reservoir ecosystems, and the effects change
considerably over time (Northcote & Atagi 1997). Although the effects of water-level regulation are clearly
species-dependent, negative effects are generally caused
by drawdown, with positive effects resulting from refilling. For fish, the negative effects of drawdown include
oxygen depletion in reduced water volume, stranding,
aerial exposure of spawning beds, and impairment of
spawning activities, as well as increased predation due to
loss of habitat (Greening & Doyon 1990, Ryan et al.
2013). These effects vary by the degree to which the depth
is lowered, timing, duration, and fish species (Wood &
Pfitzer 1960). Positive effects of refilling include increases
in spawning site availability, food, and protective cover
from predation, and reduction in wave action caused by
the hydrodynamic drag of submerged vegetation (Northcote & Atagi 1997).
Fox et al. (1977) reported that drawdown sometimes
promotes plant germination on aerially exposed sediments. Azami et al. (2013) reported that willows (Salix
species) in Japan can establish dominant stands in the
“drawdown zone” when the degree of slope and the timing of seed dispersal match the drawdown period. Since
Salix species are water-tolerant, they can survive inundation during the high-water period in winter. In many of the
Japanese reservoirs subjected to periodical water-level
regulation, such as the Miharu Dam, Sijyusida Dam,
Naruko Dam, and Gosyo Dam, Salix species have become
dominant in drawdown zones.
In Japan, Ministry of Land, Infrastructure, Transport
and Tourism conducts standardized fish survey in its approximately 100 dam reservoirs once in five years. According to the results, indigenous carp species usually compose
around 10% of the total fish populations in any dam reservoirs (Tanida et al. 2014). However, in recent years, exotic
species such as largemouth bass and bluegill have been increasing in number in Japan, and their effects on indigenous
fish populations have gained increasing attention.
The silver crucian carp (Carassius auratus langsdorfii) is a typical native fish species that occurs behind the
Miharu Dam (Azami et al. 2012, Kumazawa et al. 2012).
Angling for this fish is very popular and has contributed to
shaping the local culture in the vicinity of the dam. Consequently, management of silver crucian carp is an important issue for Miharu Dam administrators.
Silver crucian carp (Carassius auratus langsdorfii),
spawn around March to June, the peak is the early to middle of April. The carps lay eggs on water plants or on
clumps of drifting plant debris, such as tree branches and
fallen leaves (Nakamura 1969). In Miharu dam, Salix species have established dominant stands in drawdown zone
and the Salix are usually under water from March to May.
We hypothesized that silver crucian carps use flooded Salix
as their spawning substrate. Salix was thought to provide
certain fish species with some benefits. However, this has
yet to be critically documented. Observations on the utilization of inundated vegetation typical of reservoirs subjected to the limited water-level method provide valuable
data regarding the effects of dam management on the environment and the aquatic biota (native fish species in particular). In this study, we tracked silver crucian carps and
observed their spawning activities to determine the effects
of dam management on native fish, silver crucian carps.
Methods
Miharu Dam
The Miharu Dam was built on the Otakine River (37°N,
140°E), which flows into the Abukuma River and then on
to the Pacific Ocean off northern Honshu, Japan (Fig. 1).
The Otakine River flows from Mount Otakine at an elevation of 1192 m ASL (above sea level); the river is 49 km
long and has a basin area of 386.5 km2. The Miharu Dam
has a catchment area of 226.4 km2 and an impoundment
area of 2.9 km2. The gross capacity of the reservoir is 4.28
× 107 m3.
Construction of a diversion canal for the Miharu Dam
began in March 1990, and the first impoundment was installed in October 1996. The surcharge water-level was
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Spawning and nursery habitat for native fish (Salix)
231
Fig. 1. Locations of the Miharu Dam and the study sites. The survey of drifting plant debris was conducted in the entire Miharu
dam reservoir except for the area of the Honsen front reservoir. The tracking of silver crucian carp was made in the HFR. The
observation of spawning activity was practiced in the HFR and TS.
reached in December 1997, and the dam began operation
in April 1998.
Between 2001 and 2010, the annual average air temperature was 11.4°C, the average temperature of the hottest month (August) was 23.2°C, and the average temperature of the coolest month (January) was –0.2°C. The annual average rainfall was 1194.4 mm, and the average
rainfall of the wettest month (July) was 195.3 mm. In the
driest month (February), precipitation was 27.2 mm.
The Miharu Dam operates under the limited waterlevel method protocol, which creates an 8-m water-level
difference between the non-flooding (8 months) and
flooding seasons (4 months). During the non-flooding
season, the water level is maintained below 326.0 m ASL
(normal high-water level); it is held below 318.0 m ASL
during the flooding season. In urgent flooding circum-
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S.W.L. (333 m ASL)
Drawdown
zone
N.W.L. (326 m ASL)
N.W.L in F.S. (318 m ASL)
Fig. 2. Profile diagram of the reservoir shore behind the Miharu Dam (ASL: above sea level, S.W.L: surcharge water
level, N.W.L: normal high-water level, F.S: flooding season).
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Kazuhiro Azami et al.
stances, a surcharge level (333 m ASL) is also set as an
upper limit during the relevant flooding season. The drawdown zone is subjected to exposure and inundation during
the same time periods each year. Over time, since periodic
water-level operations began, Salix species, especially S.
fragilis, have established dominant stands in the drawdown zone. The stands are inundated during the nonflooding season (Fig. 2).
Hebisawagawa front reservoir
For water-quality management, four front reservoirs have
been constructed where water flows into the Miharu Dam
main reservoir (MR) (Fig. 1). Among them, the Hebisawagawa front reservoir (HFR), which has an impoundment
area of 2.7 ha and a maximum depth of 15 m, was built in
1993. An “overflow bank” 5 m high (320.5 m ASL) and
5 m wide was built in the HFR dam body; it is located between the HFR and MR. During the non-flooding season,
HFR water is connected to the MR via the overflow bank.
The water level in the HFR is changed in correspondence
with changes in the MR. During the flooding season, the
MR water level was previously set at an elevation of
318 m ASL. However, the drawdown in the HFR is halted
at 320.5 m ASL by the overflow bank (320.5 m ASL).
Thus, a 2.5 m water-level difference exists between the
MR and HFR, making it impossible for fish to move between reservoirs during the flooding season (Fig. 3).
Distribution of Salix stands and drifting plant
debris
Silver crucian carp generally lay eggs on water plants
(Nakamura 1969). However, since few water plants grow
behind Miharu Dam, fish are believed to spawn on patches
of drifting plant debris, such as fallen leaves and branches
from Salix species. On April 15, 2011 (likely in midspawning season), we went along the entire shoreline of
the Miharu dam including front reservoirs except Honsen
front reservoir (total shoreline length: approx. 30 km) by
a boat and recorded locations and sizes of any observed
Salix stands and drifting plant debris.
a) Data analysis
With a GIS application, area of Salix stands, shores with
no vegetation and drifting plant debris located in each
area were determined.
Tracking silver crucian carp in the Hebisawagawa
front reservoir
Although fish are not able to move between the MR and
HFR in the flooding season, an exchange is possible during the non-flooding period because the water of the MR
flows over the overflow bank and the two water bodies
become linked (Fig. 3).
On February 26, the MR water level exceeded 320.5 m
ASL. To prevent fish with body lengths > 1 cm from moving out of the HFR and into the MR, a partitioning net was
deployed at the overflow bank.
On December 27, 2009, we caught three wild mature
silver crucian carp individuals (total body lengths: 290–
360 mm) in the HFR and internally attached a single
acoustic telemetry tag to each fish. The three fish were
labeled carps A–C. After keeping them in captivity for ~1
month, we released them in the HFR on January 22, 2010,
ensuring that the tags were securely fastened and that the
animals were in good condition. In the period January
21–22, 2010, we deployed acoustic receivers at three locations in the HFR: the dam body site, the center site, and
the Salix site (Fig. 4). Signals were received at the dam
body site and center site immediately following deployment. However, the Salix site did not receive signals until
March 22, 2010, when the receiver there became submerged as the water level rose. These fish were monitored
for 6 months.
The sample size we used is too small to compare fish
between the two dams. However, we chose to use this
small number because our purpose was to confirm the
presence of this species in the submerged area developed
from refilling, and not to investigate population trends of
silver crucian carp.
a) Acoustic telemetry system
• Acoustic tag: AMIRIX Systems Inc. V7-4L; weight:
1.8 g, diameter: 7 mm, length: 22. 5 mm. Transmission:
one pulse every 120 s, battery life: 246 days.
• Receiver: Submersible, multichannel receiver;
AMIRIX Systems Inc. VR2.
Fig. 3. Profile of the Hebisawagawa front reservoir (N.W.L:
normal high-water level, F.S: flooding season, ASL: above
sea level).
b) Receiver range test
On May 14, 2010, we checked the receiving range of each
of the receivers placed in HFR. We deployed a transmitter
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Spawning and nursery habitat for native fish (Salix)
233
(320.5 m ASL)
Fig. 4. Interception range of each acoustic receiver (each bounded by a white dotted line). The aerial image was captured
during the 2005 flooding season.
in multiple arbitrary locations in HFR and simultaneously
recorded the GPS coordinates of each location. Collected
data were plotted on a map showing the receiving range of
each receiver (Fig. 4).
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c) Data analysis
Collected data were sorted to examine the time the A to
C carp spent in the dam body site, center site and Salix
site.
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Kazuhiro Azami et al.
Spawning activity
To determine whether silver crucian carp engaged in
spawning activities in the inundated Salix stands, we collected field data at the HFR and at the Takinodaira site (TS)
(Fig. 1), which is part of the MR; data were collected at
intervals of 10 days from March 30 to June 17. At each site,
three 10 m transects were determined in each type of area,
one was inundated Salix area (Salix area) and the other was
area with no vegetation (no-vegetation area). We recorded
presence of drifting plant debris, the carps’ spawning activities, eggs, and juveniles along each transect.
Percentages occupied by drifting plant debris in Salix
and no-vegetation area were obtained with 3 scales, 0%,
10–50% and 50%<, sampling a 10 m x 2 m quadrat which
could represent amount of drifting debris in each area.
Observation was made in each transect for 30 minutes
and any signs of the carps at the surface, splashing, movement of tree branches above water, and rustling sounds
underwater were recorded as the carps’ spawning activities. Rustling was included because it generally accompanied the waving of tree branches above water or the
splashing of fish.
For 10 minutes, we visually explored juveniles of the
carp and captured them with dip nets within the range of
10 m x 1 m in each transect.
We searched eggs of the carps for 10 minutes in each
transect with a range of 10 m x 1 m and recorded data in
3 scales, no eggs, eggs attached 50%> of substrates’ surface and eggs attached 50%< of substrates’ surface.
a) Data analysis
Presence of spawning activities, eggs and juveniles, each
event counts as 1, between Salix area and no-vegetation
area were assessed by independent samples t-test with the
95% confidence intervals for the mean difference.
Details of observations on drifting plant debris are provided in Table 1. In total 124,171 m2 of drifting plant debris
was found in all of the MR and 64% (79,453 m2) was located on shores with immersed Salix stands. This debris
occurred in 40% of the area occupied by inundated Salix
stands; only 3% of the no-vegetation area contained debris.
Tracking silver crucian carp
The telemetry component of the study, which occurred
January–June, demonstrated that tagged carp remained
within the HFR immediately following their release (Fig.
6). At this time, the Salix site had not yet been flooded. As
part of dam management, the water level was gradually
raised by ~5 m from March 3 to April 12. Detection of the
fish in the reservoir on central tracking stations became
intermittent after mid-March. The water temperature during this period was ~5°C.
Transmitter pulses were first confirmed at the Salix
site on March 22 (water temperature was 7°C), the date
when the receiver at the Salix site was flooded by rising
water. From March 22 through early April, all carp, especially A and B, were frequently detected at the Salix site.
Carps A and C were not detected by any of the receivers
after April 10 (water temperature: 10°C). Carp B was detected until the end of May, after which the detection rate
dropped significantly.
Carp A spent 34.9% of its time in the Salix site from
March 22 to April 10, with a peak of 37.3% from 1 to 10
April. Carp B spent 15.3% of its time in the Salix site
from March 22 to April 10, with a peak of 37.5% from 1
to 10 April. Carp C spent 1.9% of its time in the Salix site
from March 22 to April 10, with a peak of 5.2% during 22
to 31 March.
Spawning activities of silver crucian carp
Results
Distributions of Salix stands and drifting plant
debris
Distributions of Salix stands and drifting plant debris are
plotted in Figure 5. The total 1987,387 m2 of Salix stand
were observed and area of Salix stands locates 11.76% of
in Miharu dam reservoir. Both Salix stands and drifting
debris were most common in the vicinities of reservoir
inlets.
On April 15, the water level in the MR was increased
and most of the Salix stands growing in shore areas were
flooded, although some Salix trees were sufficiently tall to
stand above the water. Drifting debris was often trapped
in the canopies of these tall individuals.
Field observations in the HFR and TS are reported in Figure 7. From March 30, the day the observations began, to
June 2, drifting plant debris was continually sighted both
in inundated Salix stands and shore zones with no inundated vegetation (no-vegetation sites). Nevertheless,
compared to Salix stands, drifting plant debris was rare in
the no-vegetation sites; there were only two sightings of
debris (April 18 and 27) in the HFR and one on April 11
in the TS in the no-vegetation zones.
During April–May, we took eggs assumed to be those
of carp species to a laboratory. After incubation, we identified them as silver crucian carp eggs.
Spawning behaviors and eggs were first observed on
April 11 in the Salix stands (in both the HFR and TS)
when water temperatures exceeded 10°C. We made similar observations consistently after this date. Sightings of
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Spawning and nursery habitat for native fish (Salix)
235
Fig. 5. Distribution of Salix stands and drifting plant debris in 2011.
eggs peaked on April 27, when the water temperature was
12°C. Spawning behaviors and eggs were recorded
through May 16.
Spawning behaviors of silver crucian carp were observed only in the Salix stands and most eggs were collected in that area; we collected eggs on only two occasions (April 18 and 27) in the no-vegetation sites of the
HFR. All eggs were observed in patches of drifting plant
debris.
Juveniles of silver crucian carp were first observed on
April 27 in the HFR Salix stands. After May 6, carp juveniles were found in both the Salix and no-vegetation sites.
Artificial drawdown began on May 12 and ended on May
29. Juveniles were consistently sighted during and after
drawdown. At that time, no exposed spawning beds or
stranded juveniles were detected on emergent surfaces.
Presence of spawning activity (spawning behaviors,
eggs and juveniles) between Salix area and no-vegetation
area were assessed by independent samples t-test with the
95% confidence intervals (CI) for the mean difference. It
t was indicated that that a number of spawning events in
the Salix area (1.15 ± 0.87) was significantly higher than
the no-vegetation area (0.4 ± 0.25) (t(20) = 3.1644, p =
0.0004) with a difference of 0.75 (95% CI, 0.24 to 0.44).
Table 1. Surface area of two types of reservoir sites containing drifting plant debris and proportions of total reservoir area
containing debris.
Inundated Salix sites
2
No-vegetation sites
2
Total
Site Area
198,387 m
1,487,434 m
1,685,821 m2
Drifting debries
79,453 m2
44,718 m2
124,171 m2
Percentage (Drifting debries/Total amount)
64%
36%
100%
Percentage (Drifting debries/Site area)
40%
3%
–
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Kazuhiro Azami et al.
Fig. 6. Tracking results for three silver crucian carp individuals. The upper panel shows the relationship between water level
and water temperature and the lower panel describes tracking results. The light gray, black, and dark gray bullets represent
carps A–C in the dam body, center, and Salix sites, respectively. Carps A–C were released on January 22, 2010. The listening
station at the Salix site did not function until March 22, when the receiver was fully covered by water. Listening stations are
indicated in Fig. 1.
Discussion
Fish use of inundated Salix stands
Between January 22 and March 11, pulse recordings from
the tagged fish were logged continually. Thereafter, the
pulses became intermittent in the dam body and center
sites (Fig. 6). Analysis was hindered by a lack data for the
Salix site at the beginning of inundation. However, considering the receiving ranges of the receivers and the infrequency of pulses at these sites after March 11, the fish
had likely moved to inundated Salix stands. The water
level was raised from March 3 onward as a component of
dam management, and the Salix stands gradually flooded.
By March 11, the water-level had risen about 2 m and a
considerable proportion of the Salix stands was likely adequately immersed for carp utilization.
After the receiver at the Salix site began to function on
March 22, the three tagged fish were frequently detected
in the tree stands to the middle of April. The tagged carps
spent 2 to 38% of their time in the Salix area from March
22 to April 10. The peak of spawning in silver crucian
carp generally occurs from early to mid-April (Nakamura
1969). Although the tagged fish were not recorded exclusively in the inundated Salix stands, the amount of time
they spent there may have been adequate for spawning
activities since this species does not protect its eggs or
offspring. Although the tracking results could not represent population trends of crucian carp, we can definitively
report the presence of tagged carp among the flooded Salix species during the spawning period.
We observed spawning behaviors of silver crucian
carp only in the Salix stands in both the HFR and the TS,
which is a sector of the MR (Fig. 7). Eggs were consistently observed from April 11 to 27 in inundated Salix
stands both in the HFR and TS, whereas eggs were only
found on April 18 and 27 in no-vegetation sites of the
HFR. Thus, the silver crucian carp spawned exclusively
in the inundated Salix stands. This choice of sites may
have been promoted by the presence of large amounts of
drifting plant debris among the Salix. In total 9 observations were made for eggs and all of the eggs were found
within drifting plant debris.
Our survey demonstrated that 64% of drifting plant
debris observed in the MR was located on shores with immersed Salix (Table 1). This debris occurred in 40% of the
area occupied by inundated Salix; only 3% of the no-vegetation sites contained debris. When the water level is
raised, large amounts of plant debris on exposed Salix
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Spawning and nursery habitat for native fish (Salix)
eggs attached < 50% of substrates‘ surface,
237
eggs attached > 50% of substrates‘ surface
Fig. 7. Spawning activities of silver crucian carp. The upper panel shows seasonal changes in water level and water temperature. The lower panel shows the presence of drifting debris, breeding behaviors, eggs and juveniles in the Salix and no-vegetation site of two water bodies in spring/early summer 2011.
stands are likely carried to the surface and become trapped
among the Salix trees projecting through the surface.
These tree stands reduce water action through hydrodynamic drag, enabling the trapped debris to remain in situ
over periods sufficiently long for carp spawning.
Inundation of terrestrial vegetation also increases production of food resources in the water, provides refuge,
and consequently enhances the growth and survival of juvenile fish (Applegate & Mullan 1967, Mullan & Applegate 1968, Aggus 1971, Keith 1975, Paller 1997, Sass et
al. 2006, de Mello et al. 2009, Schou et al. 2009).
Decomposition of underwater vegetation and leaching
from flooded soils often add nutrients to the water, promoting the growth of algae (Wetzel 1983). Flooded vegetation and the inundated substratum provide extra surface areas for periphyton and invertebrate attachment
(Werner & Hall 1988). Abundant macrofauna (mainly
chironomids) have been detected on inundated logs and
trees in the Missouri River reservoirs, USA (Claflin 1968).
Chironomids (midges) feed on periphyton attached to
these logs, and the adults search flooded trees for hatching
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sites. Inundation of vegetation supports a rich invertebrate
fauna and consequently provides young-of-the-year fish
with a food-rich habitat (Northcote & Atagi 1997). Summerfelt & Shirley (1978) examined terrestrial vegetation
immersed by water-level management in Lake Carl
Blackwell, OK, U.S.A. This vegetation reduces the effects of wave action, provides protective cover, and enhances the survival of early life stages.
Fisher & Zale (1991) reported a positive relationship
between the recruitment of young-of-year largemouth
bass and duration of inundation in flooded terrestrial vegetation stands during the spawning and nursery seasons.
In Lake Okeechobee, U.S.A., inundation of the vegetation
following a rapid expansion of the shoreline vascular
plant community in 2002 resulted in successful recruitment of young-of-year largemouth bass (Havens et al.
2005). Hoyer and Canfield (1996) found a correlation between percent volume infested with aquatic vegetation
and growth of age-1 largemouth bass.
Significant spawning activities observed in immersed
vegetation in the Miharu Dam indicates that submerged
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Kazuhiro Azami et al.
Salix provides favorable spawning and rearing environments for silver crucian carp during inundation periods.
Engel (1987) addressed the importance of channelizing through management: creating a room for large fish to
move between dense plant beds, which maximize the benefits of inundated vegetation to fish species. In the Miharu
Dam, during 10 years of periodic water-level control, Salix species have established dense vegetation, especially
in inlet areas with gentle slopes. Salix trees in the inlet
usually reach 3 to > 10 m in height, and the dense canopy
often prevents the understory from growing during exposed periods. Since the water level was raised to 8 m
after the flood season, the understory provides plenty
space for fish to move freely.
Havens et al. (2005) also reported that submersion of
highly diversified vegetation structures, including vascular plants, maximized the benefits to fish. In the Miharu
Dam, pre-impoundment environments such as marshes
with dense Phragmites (reed) species and pre-impoundment artificial structures such as remnants of roads or
building base structures are still present in some inlets,
which also adds complexity to immersed structures. The
dynamics of the submerged structures and the fairly large
area of flooded vegetation likely provide silver crucian
carp with suitable spawning and nursery environments.
Effects of water-level drawdown
Many studies have reported negative effects of artificial
water-level regulation. In Lake Biwa, Japan, artificial water-level drawdown from the end of May (to regulate
flooding) negatively impacts cyprinid fishes by destroying their preferred spawning ground in shallow areas
(Yamamoto et al. 2006). Greening & Doyon (1990) reported that drawdown has severe effects on fish fauna
through mortality caused by oxygen depletion in the reduced water volume, stranding, exposure of spawning
beds, and impairment of spawning activities. Gaboury &
Patalas (1984) reported that the year-class strength of
coregonid fishes is inversely related to amount of fall to
late spring drawdown.
The water level behind the Miharu Dam is drawn
down after May 15 for the duration of the flooding season
(June 11–October 10). Our field observations (Fig. 7)
showed a peak in carp spawning from approximately midApril to the end of the month; the largest numbers of eggs
were found on April 27, after which numbers greatly declined. Thus, spawning was largely completed before the
drawdown. In this study, we did not find any exposed
spawning beds or stranded juveniles on emergent surfaces. Schools of juveniles were consistently observed
before, during, and after drawdown. Although we do not
have comprehensive data on spawning, we believe that
the fish we studied are not as negatively impacted by
drawdown as populations in other water bodies.
The periodic drawdown of the MR may well provide
benefits for silver crucian carp by promoting establishment of inundation-tolerant Salix species on the emergent
shoreline, which function as extensive spawning and
nursery habitats in the subsequent high-water period.
Nevertheless, confirmation requires the assessment of
yearly population changes and observations on physiologically viable spawning periods. To evaluate the overall
effects of artificial water-level regulation, research is also
needed on drawdown effects on other indigenous fish species.
Establishment of shore vegetation
In the Miharu Dam, Azami et al. (2013) investigated the
distribution of Salix species in the drawdown zone and
how it relates to days of inundation. They reported that
timing of drawdown, presence of favorable physical conditions, and time span of exposure and inundation are the
main factors promoting vegetation on the shore of the reservoir.
Since the life spans of Salix seeds are much shorter
compared to other plant species’ (Higashi 1979), it is critical that exposed shore is appeared during the seed-dispersal period. In the Miharu Dam, the drawdown period
from May to June corresponds to the seed-dispersal period of Salix (May to June). Due to current or wind, dispersed seeds tend to be carried along shorelines and seeds
are deposited on exposed sediment as water levels decrease. Thus, the temporal coordination of the two events
plays a key role in spreading seeds along the exposed
shoreline.
Azami et al. (2013) indicated that shore slope of < 20°
and low wind-induced wave actions are favorable to Salix
establishment. Yanai and Kikuzawa (1991) also reported
that Salix seeds need light and moist soil to germinate and
develop.
Salix species can tolerate at least 238 consecutive days
of inundation, 2–10-fold longer than can other common
Japanese plant species, such as oak (Quarcus serrata; 14–
97 days) and Japanese chestnut (Castanea crenata; 14–
37 days) (Azami et al. 2013). If dam administrators want a
dominant Salix community on their reservoir shoreline,
they must ensure at least 100 days of submersion. A longer
exposed period during the summer would promote plant
growth, including further growth and expansion of Salix in
following years. In the Miharu Dam, the exposed period
lasts ~4 months, from June to October, and the inundation
period lasts 8 months, from October to June.
The following points dam administrator should take
into account before making plans for establishing vegeta-
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Spawning and nursery habitat for native fish (Salix)
tion along reservoir shores. 1) Presence of inundationtolerant plants along shoreline; otherwise, dam managers
need to consider planting of inundation-tolerant plant species. 2) Drawdown before or during the seed-dispersal
periods of the target plant species and maintenance of the
drawn water level for a certain period during the growing
season; it is crucial that seeds of the target species deposit
on the exposed shore and adequate time period for the
plant establishment increases the likelihood of survival in
the following submerged period and consequently to the
recruitment success in the following period.
To benefit certain fish species, it is crucial to keep the
water level high and to avoid drawdown during spawning
periods. The effects of drawdown operated from May to
June might not be so severe for silver crucian carp since
their peak spawning period (early to middle of April) is
prior to the drawdown. However, the drawdown would
cause devastating effects on the species whose peak
spawning period coincides with drawdown period through
exposure of spawning beds and stranding of juveniles.
For establishing shore vegetation, it is essential to recognize the spawning periods of fish species found in a reservoir and carefully assess the potential effects of drawdown and refill, considering ecological and geographical
features, water level management regimes, and other restrictions.
Acknowledgments
This study was conducted with the support of the Water
Resources Environment, Watershed Ecology Research
Group. We thank the Miharu Dam Management Office for
providing us with weather and dam-related data. We also
thank Dr. Yamamoto Toshiya from Toyota Yahagi River
Institute for providing us with information on carp species. These studies were mainly conducted by Namiko
Kageyama Morikazu Nishida, Mariko Takemoto and Wataru Koketsu (OYO Corporation).
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Received 17 January 2014
Modified version received 08 February 2015
Accepted 26 March 2015
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