IJERD – International Journal of Environmental and Rural Development (2018) 9-1

Review article
erd

Status and Issues in Ecological Engineering Techniques forConserving Fish in the Paddy Field Areas of JapanKAZUYA NISHIDA*
National Institute of Environmental Studies, Otsu, Japan
Email: nishida.kaz@gmail.com
Received 31 December 2017

Accepted 19 May 2018(*Corresponding Author)

Abstract Though land consolidation for paddy field areas has contributed to an increase infood supplies and an improvement in convenience, it has changed the physical structuresand deteriorated paddy field ecosystems throughout Japan. Eco-friendly land consolidationin paddy field areas has been conducted based on the implication of ecosystem deterioration.
Ecological engineering techniques, such as the placement of fish ladders between paddy
fields and irrigation canals, were developed and applied to actual paddy fields to conservefish. However, the techniques were not evaluated sufficiently. Therefore, I reviewed status
and issues of ecological engineering techniques for conserving fish, which are often
reported in Japan. This review showed the installation of fish ladders between paddy fields
and drainage canals, as well as the construction of artificial ponds and wetlands, havecontributed to providing fish with spawning and nursery areas in the paddy fields areas ofJapan. On the other hand, the effects of ecological engineering techniques in canals, such as
the construction of concrete block habitats, are not clear, although the techniques havealready been applied to actual paddy field areas. Studies evaluating the effects of thesetechniques, which were applied for canals especially, should be conducted, and researchersand engineers should elucidate the most suitable structures, locations, scales andmaintenance techniques for these constructions to best conserve fish in paddy field areas.
Keywords ecological engineering technique, paddy field ecosystem, fish, conservation,Japan

INTRODUCTION
Paddy fields have substituted for the back marshes of rivers, and play a role in the wetlands of low
and flat-land areas (Moriyama, 1997; Natsuhara, 2013). They are not only food production areas,
but also habitat, spawning, and nursery areas for aquatic organisms, such as fish, frogs, and aquatic
insects.
Land consolidation in paddy field areas has contributed to an increase in food supplies and an
improvement in convenience for farmers, and has exceeded 60 % of the paddy field areas in Japan.
On the other hand, land consolidation has changed the physical structures of paddy fields, anddeteriorated paddy field ecosystems in Japan (Lane and Fujioka, 1998; Hata, 2002a). For example,
fish species that use paddy fields as spawning areas such as Japanese killifish (Oryzias latipes andO. sakaizumii) and mud loach (Misgurnus anguillicaudatus), cannot migrate from canals to paddy
fields, because the creation of drainage canals create large drops between the paddy fields andthemselves. Consequently, many endemic and/or dominant species in paddy field areas such as
Japanese killifish and mud loach were designated as endangered species by the Ministry of theEnvironment and several prefectures of Japan.
Eco-friendly land consolidation in paddy field areas has been conducted based on theimplications of ecosystem deterioration. A section of the “Land Improvement Act” of Japan wasrevised in 2001, and “consideration of harmony with the environment” was added a principle of theland improvement project.
Ecological engineering has been researched with the aim of designing sustainable ecosystems
that integrate human society with its natural environment for the benefit of both humans andⒸ ISERD

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animals (Mitsch, 2012). Therefore, ecological engineering techniques in land consolidation projects
are required to strike a balance between the increase in production and improvement inmaintenance by farmers, and at the same time aid the conservation of aquatic organisms. The
ecological engineering techniques, such as fish ladders between paddy fields and drainage canals,
were adapted for paddy field areas where eco-friendly land consolidation was conducted. However,
the techniques have been rarely evaluated.
OBJECTIVE
I have reviewed studies of ecological engineering techniques in the paddy field areas of Japan to
discuss the status and issues with these techniques. In this paper, I have placed focus on thetechniques for conserving fish in paddy field areas, because the development of these techniques
has been adapted for actual paddy field areas in Japan frequently.
ISSUES OF PREVIOUS REVIEWS
Some researchers have reported studies on fish ecology and habitats in paddy field areas. Koizumi
et al. (2012) showed that studies on fish lifecycles and ecology in the paddy field area haveincreased since 2000; however, the development of methods for estimating and predicting the fish
populations and ecosystem of paddy field areas has been limited. Takemura et al. (2012) reviewed
the methods of evaluating habitat quality, and the connectivity between the habitats and spatial
arrangement of habitats for fish in paddy field areas, and discussed the issue of these methods. In
addition, Nakano (2017) reviewed that previous studies have examined habitat preference of fish
growth, spawning, wintering, migration, and dispersal of fish, in paddy field areas. These reviews
did not focus on ecological engineering techniques.
Several researchers have reviewed conservation measures and ecological engineering
techniques. Ogino and Ota (2007), Suzuki (2011), and Natuhara (2013) referred to the structure andinstallation method of fish ladders installed in paddy field areas of Japan; however, othertechniques were not reported sufficiently. Mizutani (2011) and Minagawa (2015), and Mori (2017)presented some conservation measures for fish in paddy field areas; however, the status and issueof these measures and techniques were not discussed sufficiently. In addition, a large number ofstudies have been conducted with respect to ecological engineering techniques in the paddy fieldareas of Japan; however, it is difficult for people except those who speak Japanese to recognize
these ecological engineering techniques, because many existing studies are published solely inJapanese.
Previous reviews in other countries have indicated the importance of paddy fields as habitatsthat are deteriorated by modern agricultural practices (Fernand, 1993; Herzon and Helenius, 2008;
Luo et al., 2014). In addition, Chen et al. (2015) reported preliminary installation of fish ladders in
the paddy field area of Taiwan. However, reviews and studies that focused on the ecologicalengineering techniques in the paddy fields have been rarely reported.
Sato (2014) conducted a questionnaire survey on the current status of ecosystem conservationin land consolidation projects to be carried out in the prefectures of Japan. He focused on
ecological engineering techniques for fishes and frogs, because these measures have been
conducted frequently based on their ecology and behavior, which has been revealed by previous
studies (e.g., Saitoh et al., 1988). The techniques for fish in land consolidation projects in Japan
shown by his questionnaire are summarized as follows (1-4 in Table 1). Although Sato’squestionnaire survey did not report this fact, artificial wetlands and ponds were also constructed inland consolidation projects (Sugihara and Mizutani, 2006; Moriyama et al., 2010; Takemura et al.,2010; Nishida et al., 2014). Therefore, I have focused on and reviewed the below ecologicalengineering techniques for fish in land consolidation projects in Japan (1-5 in Table 1).

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Table 1 Ecological engineering techniques for conserving fish in paddy field areas, whichwere often conducted in Japan
Conservation measures
1 Improvement of canal revetment
2 Improvement of canal bed

3 Improvement of canal form
4 Installation of migration pathway

5 Construction of calm waters

Ecological engineering techniquesa) Construction of masonry and wooden revetment works (Takahashi et al.,2009).
b) Construction of concrete block habitats.
a) Avoiding installation of concrete lining (Sato and Azuma, 2004).
b) Installation of habitat pool section (Hiramatsu et al., 2010; Minagawa etal., 2015).
c) Installation of spur dikes (Mukai et al., 2011) and blocks (Watabe et al.,2016).
a) Construction of meandering section (Nishida et al., 2011).
b) Construction of broaden section (Conservation Society-Japan, 1995;
Nishida et al., 2011).
a) Installation of fish ladders between paddy fields and drainage canals(Hata, 2002b; Suzuki et al., 2004; Tanaka, 2006; Sato et al., 2008;
Nakamura et al, 2009; 2012).
b) Installation of fish ladders in canals (Moriyama et al., 2008).
a) Construction of ponds (Sugihara and Mizutani, 2006; Takemura et al.,2010).
b) Construction of wetlands (Nishida et al., 2014, 2015).

(a) Masonry revetment work(b) Wooden revetment work

Fig. 1 Example of (a) masonry and (b) wooden revetment works in canals
Riparian plants grow between the wooden stakes

IMPROVEMENT OF CANAL REVETMENT
Construction of Masonry Revetment Works
Stream and canal riparian structure influence the habitats of aquatic organisms (Pusey et al., 2003).
The growth of riparian plants influences the habitation of fish and aquatic organisms (Kawaguchi,
2003). Masonry and wooden revetment works (Fig. 1), and similar structures, were traditional
methods of canal improvement and repair. These structures have many spaces between materials,
and it is considered that these spaces and the plants that grow in them provide habitats for fish
(Katano, 1998). Takahashi et al. (2009) installed an experimental dummy of a wooden mattressdike, one of the traditional methods of revetment (shown in the middle right of Fig. 6), in adrainage canal to evaluate to the effects on fish habitation. They showed that this dike was
frequently used by fish, such as crucian carp (Carassius spp.), in paddy field areas. However, otherrevetment works in canals were not investigated for their effects on fish habitats.
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Concrete block habitats (Fig. 2) were often installed for canal revetments to enable fish to use canal
improvements as habitats in Japan (Sato, 2014; Japanese Society of Irrigation, Drainage and RuralEngineering, 2016). These blocks have cavities inside in the expectation that fish may use them as
habitats. However, no study has evaluated the exact effects of these concrete block habitats on fish
and other aquatic organisms in irrigation canals. In addition, there is no knowledge on which
species and growth stages of fish can use these concrete blocks as habitats.

Fig. 2 Example of a concrete block habitat installed in a drainage canal
IMPROVEMENT OF CANAL BED
Avoiding the Installation of Concrete Lining
Concrete lining was usually installed in canals during conventional canal improvements. However,the number of fish species and individuals was reported to drastically decrease because fish habitats
were often removed by the improvements (Kihira, 1983; Tsubokawa, 1985). Therefore, theinstallation of concrete lining was partially avoided as much as possible to allow eco-friendly canal
improvements. Sato and Azuma (2004) investigated the difference in fish habitation among
construction methods after canal improvements. Some canal sections improved by eco-friendly
construction methods such as concrete revetment works with concrete block habitats, and concrete
canal beds were not installed in these sections to enable fish to inhabit (Fig. 3).
(a) Straight section(b) Meandering section

Fig. 3 Example of (a) straight and (b) meandering sections of the drainage canal
These photographs were taken five years after canal improvement was undertaken

Numbers of fish species and individuals, and diversity indices in conventional concrete
lining canal sections were significantly smallest than those in other sections. On the other hand,these values in concrete revetment canal sections were not significantly different from those inearth canal sections. These observations indicated that avoiding the installation of concrete lining isⒸ ISERD

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the key to conserving fish in paddy field areas.
Installation of a Habitat Pool Section
Sato (2014) indicated that habitat pools (Fig. 4) were often installed during land consolidation
projects in Japan. Hiramatsu et al. (2010) and Minagawa et al. (2015) investigated fish assemblages
and physical environments in pools that were installed by canal improvement projects. These poolbeds were concrete and 0.30-0.50 m deeper than other canal sections. Their findings indicated thatthese pools were important for fish to winter in. On the other hand, Minagawa et al. (2015) also
reported that sand and silt had accumulated excessively, and the water depth was lower in somepools. Further studies are needed to elucidate how to wash away or remove these sand and silt
accumulations to maintain a water depth that is inhabitable by fish.

Fig. 4 Example of a habitat pool installed in a drainage canalThis pool bed was dug 0.3 m deeper than the normal canal section.
Sand and silt were accumulating, and the water depth had been reducing gradually

Installation of Spur Dikes and Blocks
Mukai et al. (2011) experimentally installed spur dikes with notches on the canal bed crossing thecanal to form sand bars, riffles, and pools for aquatic organisms. They investigated the temporalchange in the formation of sand bars and invertebrate assemblage before and after the installation.
The formation of sand bars was observed by investigation after one year of the installation of thedike. Riparian plants were formed on the sand bars six months after the sand bar formation. The
number of invertebrate individuals increased after the formation of sand bars and riparian plantzones, indicating these formations created invertebrate habitats. These methods might provide fish
habitats, as well as help to explain the findings of Nishida et al. (2011) described below. However,
the effect on fish habitation was not investigated.

Fig. 5 Example of concrete blocks on the concrete lined canal bed
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Watabe et al. (2016) also installed concrete blocks, such as the one seen in Fig. 5, on theconcrete lining of a canal bed, and investigated the fish assemblage and canal environment before
and after the installation. They arranged the long sides of the rectangular concrete blocks
perpendicular to the water direction in the canal. Their observation showed that the blocks createdcalm water and sand sediment zones in the canal. However, the change in the number of fish
individuals before and after the installation was small. There is no sufficient evidence that these
concrete blocks create suitable fish habitats.
IMPROVEMENT OF THE CANAL FORM
Construction of Meandering Sections
Nishida et al. (2011) investigated the temporal change in fish assemblages and physicalenvironments of drainage canals before and after canal improvements by eco-friendly construction
methods. In particular, they focused on the differences in temporal changes between meanderingand straight sections where revetments were installed (Fig. 3). Sand bars did not form, riparianplants did not grow, only mud loach were dominant, and the number of species and diversity
indices had declined in the straight sections five years after the canal improvement. On the other
hand, sand bars formed, riparian plants grew, and a number of fish species and diversity indices did
not change before and after canal improvements in the meandering sections. These differences
between straight and meandering sections may be due to the presence or absence of calm waters
made by sand bars and riparian plants, which create natural hiding spots for fish. Further studies areneeded to elucidate how canal form and structure can form sand bars and riparian plant zones.
Construction of Broadened Sections
Ecosystem Conservation Society-Japan (1995) reported that the number of fish species increased ina broadened section of a canal after canal improvements (Fig. 6). The construction of broadened
sections probably provided calm water areas in the canal. However, which factor increased thenumber of fish species was not clear. In addition, studies that elucidate the effect of construction ofbroadened canal sections on fish assemblage are limited.

Fig. 6 Example of a broadened section in a canal
The previous canal width before improvement was between the left bankand stakes located in the center of this photographSome meandering sections that Nishida et al. (2011) investigated were also broadened by theeco-friendly canal improvements. Large sand bars formed and riparian plants thrived in thesebroadened sections. This increased plant growth perhaps increased the management efforts offarmers and prevented drainage water flow. Research into construction methods that strike abalance between improving the convenient management of canals and conservation of aquaticorganisms is required.
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INSTALLATION OF A MIGRATION PATHWAY
Fish Ladder between Paddy Fields and Drainage Canals
Endemic and/or dominant fish in paddy field areas such as mud loach and Japanese killifish
migrate from canals and spawn in paddy fields (e.g., Saitoh et al., 1988). However, these fish
species often cannot migrate to paddy fields after a land consolidation project, because drainage
canals are dug to improve the draining ability of paddy fields (Hata, 2002b). This digging creates alarge drop between the paddy field and drainage canal.
Japanese researchers have tried to develop fish ladders to enable these fish species to migratefrom drainage canals to paddy fields (Hata, 2002b; Suzuki et al., 2001; Tanaka, 2006; Sato et al.,2008). These fish ladders were classified into two types, and were installed in paddy field areas asfollows.
1) Paddy field outlet ladder (Fig. 7a): This ladder type was installed at each outlet of paddy fields
in order to allow fish migration from drainage canals. Fish ladders of variable structures weredeveloped, and they were installed in every region of Japan by farmers and/or local governments.
2) Up-dam ladder (Fig. 7b): This fish ladder type was installed at drainage canals and dams with ahigher water level at the upper section of drainage canal from this ladder. This improvement
enables fish to migrate from downstream of the ladder to the paddy field because the water level
between the drainage canal and the paddy fields is lower. This fish ladder type has been utilized inpaddy field areas around Lake Biwa.
(a) Paddy field outlet ladder(b) Up-dam ladderFig. 7 Example of the two type of fish ladders in use(a) paddy field outlet ladder and (b) up-dam ladder between paddy fields
and the drainage canal

The effects of the fish ladders have been investigated by some researchers. Suzuki et al.
(2004) investigated the effects of fish ladders between drainage canals and paddy fields. They
indicated that mud loach and crucian carp migrated from drainage canals to paddy fields throughfish ladders, and spawned in the paddy fields. After hatching, their larvae probably grew in the
paddy fields and then migrated to the drainage canals. As a result, increases in fish wet weights in
paddy fields after land improvement projects were equal to or larger than those before them.
Nakamura et al. (2009, 2012) reported that the drainage of water from paddy fields throughfish ladders was greater than that from the outlets of paddy fields in which the fish ladder was notinstalled. This finding indicated that irrigation water is required in the paddy fields that fish ladders
are installed. Therefore, new water management practices are crucial to enable fish to migrate
through the fish ladder and spawn in the paddy fields.
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Installation of Fish Ladders in Canals
Fish ladders were also installed at drop structures and outlets of the canals (Fig. 8). Pool and canaltypes of fish ladder that were developed by river engineering researchers were installed at the drop
structures. Moriyama et al. (2006) reported new fish species were detected in the irrigation canals
after the installation of fish ladders on outlets, suggesting that these fish ladders enabled fish to
migrate from lower water areas. On the other hand, the fish ladders may also allow alien fish
species to invade the irrigation canals.
(a) Fish ladder at a drop structure in a canal (b) Fish ladder at an outlet of a canalFig. 8 Examples of fish ladders installed (a) at a drop structure and (b) outlet of a canal
CONSTRUCTION OF CALM WATERS
Construction of Ponds and Wetlands
Sugihara and Mizutani (2006) investigated fish habitation in an artificial pond (Fig. 9), and themigration between an artificial pond and the adjacent water areas, in paddy field areas. A large
number of age-0 pale chub (Opsariichthys platypus) as well as crucian carp and Japanese killifish
were captured in the artificial ponds in winter, indicating that these fish species wintered there. In
addition, for fish that spawn in paddy fields, such as the field gudgeon (Gnathopogon elongatuselongatus), adults ascended from connected drainage streams to the pond, and oppositely, juveniles
and sub-adults descended from the pond to the drainage streams. These findings indicated thatthese fish species probably used the pond as a spawning and nursery areas, as well as paddy fields.
(a) Artificial pond(b) Artificial wetland

Fig. 9 Examples of an (a) artificial pond and (b) artificial wetland constructedin a paddy field area

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Takemura et al. (2010) reported that ninespine stickleback (Pungititus sp. 2) made nests onaquatic plants in artificial ponds. On the other hand, they also suggested that artificial ponds will
become unsuitable for nesting by ninespine stickleback if aquatic plant mowing is abandoned andplant cover is increased throughout the pond, because ninespine sticklebacks often made nests on
the edge of each plant zone, but did not make nests in the center of the plant zone. On the other
hand, intense mowing may remove all nest substrate for ninespine sticklebacks. In addition, in
these shallow and calm waters, sand and silt accumulates and aquatic plants grow easily. Therefore,
maintenance of these water areas may be crucial to conserve fish habitats. Further studies areneeded to elucidate how to suitably maintain these water areas.
Nishida et al. (2014) investigated fish habitation in an artificial wetland (Fig. 9), and migration
between an artificial wetland and connected stream, and the migration of the mature mud loach andeight-barbell loach (Lefua echigonia) was detected between late winter and spring. Juveniles andsub-adults of these two loach species migrated from the wetland between summer and winter.
These results confirmed that wetlands act as spawning and nursery habitats for these two species ofloach. On the other hand, fish species other than the mud loach and eight-barbell loach, frogs, andaquatic insects that were found in the adjacent paddy fields, were rarely detected (Nishida et al.,2015). The unique characteristics of artificial wetlands that are frequently used by many aquaticorganisms are not known yet.
CONCLUSION
This review showed that the installation of the fish ladders, which were developed by Japanese
researchers and engineers, between canals and paddy fields provides fish, such as mud loach andcrucian carp, with spawning and nursery areas . Similarly, artificial ponds and wetlands constructedin a paddy fields area were sufficiently used by fish as spawning and nursery areas especially. On
the other hand, the effects of ecological engineering techniques on canals where fish spent most of
their life cycles are not clear, although the techniques have already been applied to actual paddy
field areas. Studies evaluating the effects of these techniques in canals should be conducted, andresearchers and engineers should elucidate the most suitable structures, locations, scales, andmaintenance techniques for these constructions to best conserve fish in paddy field areas.
Middle- or long-term monitoring is required to evaluate these techniques, because ecosystems
and physical environments often fluctuate, and previous studies have indicated it can take a longtime until they stabilize after disturbance. However, this monitoring is rarely conducted in Japan;
the development of monitoring methods and mechanisms are needed.
In Japan, many studies evaluating the effects of these techniques are published in Japanese,
because the local language is useful to easily show the effects to the Japanese and local
governments, as well as domestic engineers. On the other hand, studies published in English areneeded to provide knowledge of these ecological engineering techniques to people globally. Thisknowledge may be useful in Southeast Asia especially, because paddy fields in this area could be
improved to increase future agricultural production and convenience.
ACKNOWLEDGEMENTS
I greatly appreciate the help of Dr. Akiko Minagawa in providing references for this review.
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