145

Changes in Fish Community after Invasion and during Control of
Alien Fish Populations in Mizoro-ga-ike, Kyoto City
Kan ABEKURA1, Michio HORI2 and Yasuhiro TAKEMON3
1

Laboratory of Animal Ecology, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japane-mail: abekura@terra.zool.kyoto-u.ac.jp
2
Laboratory of Animal Ecology, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japane-mail: hori@terra.zool.kyoto-u.ac.jp
3
Laboratory of Global Hydrology, Water Resources Research Center, Disaster Prevention Research Institute,
Kyoto University, Kyoto 611-0011, Japane-mail: takemon@wrcs.dpri.kyoto-u.ac.jp

Abstract
Information was obtained on changes over a 30-year period in a fish community after invasion ofalien fish species in Mizoro-ga-ike, a natural pond of nine hectares in area located in Kyoto City, andthen an investigation done on changes over five years in populations of two alien fishes, largemouthbass (Micropterus salmoides) and bluegill sunfish (Lepomis macrochirus) while the local government
took measures to control their populations. The species richness of the native fish community of thepond decreased from 14 species in the 1970s to six species in the 2000s: i.e., more than half of thenative species became extinct during those 30 years. On the other hand, the percentage of alien
species increased from 7.7% in 1972 to 50.0% in 2002. A population control program using net fishingand spawning redd destruction was implemented by Kyoto City in 1998. This resulted in an effective
decrease in population estimates for M. salmoides from 84 in 1998 to several in 2002. Those for L.
macrochirus, on the other hand, showed only a gradual decrease from 9,500 in 1999 to 5,400 in 2002
after the start of population control. Although the population of 1+ cohorts (one year old) of the species
did not decrease distinctively after 1999, those of 2+ and 3+ cohorts decreased constantly. Population
estimates of Channa argus based on a mark and recapture method resulted in a decrease from 540 in1998 to 220 in 2002 in spite of no efforts to control on species. Causal relationships of alien fish invasion to changes in the fish community in Mizoro-ga-ike and the effects of the control measures on alienfish populations were examined.
Key words: alien fish, fish community, L. macrochirus, Mizoro-ga-ike, M. salmoides, population control

1. Introduction
Severe impacts of alien predatory fish on native
biotic communities have been reported in a lot of
freshwater ecosystems in various countries of theworld, including extinction of native populations andreduction in biodiversity (e.g., Myers, 1965;
McDowall, 1968a, b, 1984; Ogutu-Ohwayo, 1990).
The predatory fishes, largemouth bass (Micropterus
salmoides) and bluegill sunfish (Lepomis macrochirus),
which are native to North America, are two distinctive
representatives of such invasive alien species (Keast& Webb, 1966; Azuma, 1998). The piscivorous M.
salmoides has been introduced to 52 countries (Lever,
1996) and is listed as one of the world’s 100 worst
invasive alien species for its strong impacts on native
biotic communities (ISSG, 1999). The omnivorous L.
Global Environmental Research8(2)/2004:145-154
printed in Japan

macrochirus is also known for its strong impacts on
species compositions of native biotic communities
through predation on a wide range of prey species,
including eggs and brood or fry of fishes (Bain &Helfrich, 1983; Hall et al., 1970; Minckley, 1973,
Gilinsky, 1984; Hambright et al., 1986; Mittelbach,1988). Impacts of the two predatory fishes on preyspecies or on competitive species have been well
documented in the United States (Power et al., 1985;
Carpenter et al., 1985; Hambright et al., 1986; Werner& Hall, 1988; Carpenter & Kitchell, 1993; Hill &Lodge, 1994; Baca & Drenner, 1995).
In Japan, M. salmoides and L. macrochirus wereintroduced from the United States in 1925 and 1960,
respectively (National Federation of Inlandwater
Fisheries Cooperatives, 1991), and both have become
representative of invasive alien freshwater fishes©2004 AIRIES146

K. ABEKURA et al.

listed in Japan (Murakami & Washitani, 2002).
Intentional release of the two species resulted in the
rapid spread of their distributions throughout Japan
except in Hokkaido and Okinawa Prefectures (Azuma,1992). Reductions in stocks of native fish species
after their invasion were reported in various regions inJapan (Hamada, 2000; Takahashi et al., 2000; Minobe& Kuwamura, 2001). In addition, changes in benthos communities caused by their invasion have also
been reported in a number of irrigation ponds in Japan(Maezono & Miyashita, 2003). Such ecosystem degeneration caused by M. salmoides and L. macrochirus is now one of the most important issues for biodiversity conservation in a wide range of freshwaters inJapan. Most ponds and lakes, however, lack data on
past biological communities before the introduction ofalien fishes, and this hinders effective planning of restoration programs.
In Mizoro-ga-ike Pond in Kyoto City, the biological community has been designated as a nationaltreasure because of the unusual richness of rare species in the pond, and thus quantitative data on its floraand fauna in the past are available (Kyoto City Tourist
Bureau, 1981). In spite of considerable conservation
efforts by citizens and the local government, however,M. salmoides and L. macrochirus were released bysomeone into the pond some time since the late 1970s,
resulting in the extinction of eight native fish species
in total and drastic changes in the benthic fauna of thecommunity (Takemon, 2000; Takemon et al., 2002).
Kyoto City decided to start work on controlling M.
salmoides and L. macrochirus populations in Mizoroga-ike in 1998. The City consigned the work to ‘The
NGO for Mizoro-ga-ike Freshwater Animals andPlants,’ composed of volunteer members, including

academic scientists, students and citizens. Amongthem, in particular, retired seniors played an importantrole in fieldwork such as net fishing and fish measurement.
The present study aims to describe changes infauna and species composition in the fish communityof Mizoro-ga-ike during these thirty years based on areview of past surveys and on monitoring dataobtained in the course of the work to control the twoalien fish populations. We also present annual changes
in the total number of fish removed and estimates ofeach alien population for the five years in relation to
the fishing efforts and we evaluate the effects of thecontrol work on the alien fish populations.

2. Methods
2.1 Study site
Mizoro-ga-ike (35º03'N, 135º50'E, 75m a.s.l.) is asmall natural pond of nine hectares in area and one
kilometer in circumference, located in the north of
Kyoto City. Mizoro-ga-ike has no river flowing in orout but it has a floodgate and an irrigation gage (Fig.1).
The age of the pond as a wetland has been estimatedat more than 20,000 years based on a pollen analysisof sediment core samples (Nasu, 1981). This may be
a reason for the unusual richness of flora and fauna
species in the pond, where a set of relic species of thecold-temperate and subtropical zones coexist withinsuch a small pond. The former include Menyanthes
trifoliata, Menyanthaceae; Argyroneta aquatica, Argyronetidae; and a syrphid fly, Eurinomyia lineata, in thehigh moorland and floating islands in the middle ofthe pond; and the latter include Phragmites communis,
Zizania latifolia, Gramineae; Neohydrocoptus bivittis,

Fig. 1 Map of Mizoro-ga-ike. Arrows a-d and numerical numbers 1-30 represent the sites of‘Eri-ami’ fishing and ‘Mondori’ trap fishing, respectively. Net sites a and b were used in
1998, all four sites in 1999, and c and d in 2000, 2001 and 2002. ‘Mondori’ trap fishing
was carried out at the sites 1-30 in 2001 and 2002. The aerial photograph was taken in
June 2003.

Changes in Fish Community and Alien Fish Control

Phreatodytidae; and Hydrovatus bonvouloiri, Dytiscidae, in the littoral zone of the pond. Based on such
unique natural properties of the pond, the plant community of Mizoro-ga-ike was designated as a natural
treasure by the Japanese government in 1927 and thewhole biological community was designated likewisein 1988.
2.2 Fish collection for faunal survey and alien
population control
Kyoto City consigned the alien fish control work at
Mizoro-ga-ike to ‘The NGO for Mizoro-ga-ike Freshwater Animals and Plants’ in 1998. The NGO group
was established by one of the authors, Yasuhiro Takemon, who contacted citizens using online mailing lists.
A total of 40 volunteers have participated as members,
including academic scientists, students and citizens.
Among them, in particular, the first author, Kan
Abekura, and retired seniors took the initiative in activities such as net fishing and surveys to monitor the
biological community and water quality. Fish collection methods have been selected and improved
through discussion among the authors and citizens at a
meeting held beside the pond every month. We decided to remove the two alien populations of M.
salmoides and L. macrochirus first. There were otheralien species such as snakehead (Channa argus) andtopminnow (Gambusia affinis affinis) at the start of
the work in 1998. When snakehead was collected,
however, it was released alive after tagging for
population estimation, because we assumed thatsnakehead had less of an impact on the community
compared with the former two species. As to for the
topminnow, we have never found any effective
methods for their removal, and its control will be a
subject for future discussion. Consequently, we haveconducted fishing surveys and population control
work at Mizoro-ga-ike using the series of methods
listed in Table 1. All native fishes caught werereleased except for a small number of samples forresearch purposes.
Although gill nets, electro fishing and trawling areoften adopted for fish collection to control fish populations (Berg et al., 1997; Parker et al., 2001; Raborn
et al., 2002; Hamrin, 1999), they are unsuitable inMizoro-ga-ike because it has a lot of endangered species of aquatic plants, submerged macrophytes andbenthic animals, which could be damaged by the
trawling. Gill nets have been reported to be one of theTable 1147

most effective fishing methods for removal oflarge-sized M. salmoides bass and L. macrochirus(National Federation of Inland Water Fisheries Cooperatives, 1991). Traditional gill-net fishing, however,causes an estimated 35%-70% mortality rate amongincidentally caught fish (Buchanan & Farrell, 2002)
and thus native fish apt to be killed by such fishing.
Electro fishing was also rejected for the same reason.
Thereby, we selected three methods, ‘cast net fishing,’
fixed net fishing using traditional Japanese ‘Eri-ami,’and ‘Mondori’ traps for our purpose (Table 1, Fig. 2).
We destroyed the spawning redd of the two alien
fishes during April to July. ‘Cast net fishing’ using a
net of 2 m in diameter with an 8 mm mesh-size wasconducted 1-4 times per month during May to
December every year from 1998-2002.
Two or four sets of ‘Eri-ami’ 20 m long and 1.5 m
deep, with 5 mm mesh size were set in the ponds todraw fish into a central net (tsubo-ami)(Fig. 1, Fig. 2).
The ‘Mondori trap’ is a basket type with two holes onboth sides, 60 by 90 cm and 60 cm in height, made ofnet with 1 cm mesh size (Fig. 2). Having entered the‘Mondori trap,’ fish can’t find the exit easily and will
stay for a while. To conserve water quality, no bait
was used. Trap fishing was performed 2-3 times per
week for the purpose of population estimates andcontrol of L. macrochirus in 2001 and 2002 (Table 1).
Age classes were judged by examining scales andthe total body length (TL) for M. salmoides and L.
macrochirus (National Federation of Inland Water
Fisheries Cooperatives, 1991). For M. salmoides, abody length of not less than 10 cm in TL was definedas the 1+ (one year old) stage or higher. Age classes
for L. macrochirus were identified using Cassie's
length-frequency example (Cassie, 1954) on samples
captured by ‘Eri-ami’ fishing in April, May and June.
2.3 Population estimates
M. salmoides and L. macrochirus were trapped for
population estimates using a marking and recapture
method in 1998 and 1999. Then they were removed
without marking or release in 2000, 2001 and 2002.
The populations were estimated in 2001 and 2002 byremoval methods. Petersen’s marking and recapture
method (Chapman, 1951) using fin cutting was
adopted for estimating 1+ or higher age populations of
L. macrochirus in 1999. Population estimates in 1999
were based on samples from four ‘Eri-ami’ fishingnets, a, b, c and d. The population in 1998 was

Fishing efforts invested for alien species control work in Mizoro-ga-ike for five years since 1998.
Eri-ami net fishing(No. of nets) (total man power days)

Casting net fishing(total man power days)

1998 March - July, September - November (2) (112) April - June

(3)1999 March - July

(4) (49) March - November(28)

Mondori trap fishing(No. of traps) (total man power days)

Removal of
spawning beds
April - July

2000 May- July, September - November(2) (39) April - December(29)

2001 May - July, September - November(2) (23) April - November(17)

May - November(10) (35)

April - July

April - July

2002 May - July, September - November(2) (30) April - October(11)

April - November(30) (52)April - July

148

K. ABEKURA et al.

a)b)Fig. 2 Fishing nets used for population control of largemouthbass and bluegill sunfish in Mizoro-ga-ike.
a) ‘Eri-ami’ fishing net and b) ‘Mondori’ trap.

estimated from the catch per unit effort (CPUE) basedon ‘Eri-ami’ fishing carried out at sites a and b in 1998
and 1999. A removal method (Delury, 1947, 1951) for
population estimation was used in August 2001 using
samples from ten ‘Mondori’ traps, 1-10, and samples
from 30 traps, 1-30, in June and July 2002 (Fig. 1). In
2000 the population was estimated using the CPUE
method, based on net casting carried out at the same
points in 2000 and 2001. Petersen’s marking and recapture method, attaching tags, was adopted to estimate 1+ or higher age populations of M. salmoides
bass and C. argus every year.

3. Results
3.1 Changes in the fish community
The fish fauna of Mizoro-ga-ike changed drastically, during 20 years since the 1970s (Table 2). In
1972 twelve native species were listed, with snakehead, C. argus, as the only introduced species. A continental alien species, Rhodeus ocellatus ocellatus,
invaded before 1977 and as a result, related native
species, R. ocellatus kurumeus, disappeared during thesame period. The M. salmoides bass and L.
macrochirus invaded in the latter half of the 1970s andG. affinis affinis invaded before 1985. During the several years between 1977 and 1985, a total of six native

species and even the established intruder, R. ocellatus
ocellatus, disappeared. Consequently, in the 2000s,
only six native fish species in total were surviving.
The ratio of introduced species of fish fauna in
Mizoro-ga-ike showed a constant increase from 7.7%
to 54.5% during those thirty years. Among the introduced species, however, three species, Carassius
carassius buergeri, Cichlidae sp. and Glyptoperichthes gibbiceps, were captured only once and were presumably temporal dwellers without successful colonization.
The relative abundance of alien species, Rhodeus
ocellatus ocellatus, in the fish community was only
23.9% in 1979, with the rest, 76.1%, composed of fivenative species (Carassius sp. 25.2%, Pseudorasbora
parva 17.7%, Oryzias latipes 12.3%, Rhinogobius sp.
12.3%, Cyprinus carpio 8.6%) in 1979 (Fig 3). Contrastingly, the relative abundance of introduced species had increased to 98% (L. macrochirus 97%, M.
salmoides 1%) in 2000. In 2002, five years after thestart of population control of the two alien species,
their relative abundance had decreased to 87% (L.
macrochirus 84%, M. salmoides 3%) and that ofnative species composition increased to 13% (Pseudorasbora parva 8%, Carassius sp 5%).
3.2 Effects of population control of alien fishes
Annual changes in population estimates of L.
macrochirus after the start of population control are
shown in Table 3. The estimated population decreased
in 1998 just after starting population control, from9,545 in 1998 to 5,744 in 2000. ‘Mondori’ traps wereused in addition to ‘Eri-ami’ and net casting from2001 and removed three times as many fish as in 2000,
but the populations have shown no significant
decrease since 2000. In 2002, the mean catch number
by all fishing methods decreased significantly from30.8 in 2001, to 15.9 in 2002 by ‘Eri-ami’ fishing,from 3.8 to 0.9 by net casting, from 13.0 to 4.2 by‘Mondori’ trapping and the CPUE of all fishingdecreased. Annual changes in population estimates foreach age class of L. macrochirus are shown in Table 4.
Fish of the 1+ class decreased to 3,984 in 2000 from
6,093 in 1998 but increased to 4,205 in 2002. The
composition of the 1+ class increased to 78.0% in
2002 from 63.7% in 1998. Fish of the 2+ and 3+
classes or higher decreased from 2,726 and 736 in
1999, to 1,014 and 168 in 2000, respectively.
Population estimates of M. salmoides decreased
from 84±23 in 1998 to 33±24 in 2000 (Table 5).
Quantitative estimates were not possible in 2001 and2002 because no M. salmoides were captured in spiteof fishing efforts. The capture efficiency (ratios ofcatches to population estimates) of the M. salmoides
bass was 2-3 times higher than that of L. macrochirusby ‘Eri-ami’ fishing. As for C. argus, population
estimates decreased significantly from 498±109 in1999 and 224±126 in 2002, though they were notsubject to population control (Table 6).

Changes in Fish Community and Alien Fish Control

149

Table 2 Changes in the fish fauna of Mizoro-ga-ike after introduction of alien species.
○：Native species， ●：Introduced domestic species， ▲：Introduced exotic species.
*：Temporal dwellers without successful colonization
FamilySpecies

72

Cyprinidae

Zacco temmincki○

Zacco platypus

○

77

79

85

98

99

2000

01

2

○

○

○

○

○

○

○

Aphyocypris rasborella

○

○

○

Gnathopogon elongatus elongatus

○

○

○

Pseudorasbora parva

○

○

○

Cyprinus carpio

○

○

○

●

Gnathopogon elongatus caerulescens

Carassius auratus cuvieri●

Carassius auratus langsdorfii
Carassius carassius buergeri

○

○

○

○

○

○

○

○

●

●

●

●

●

●

○

○

○

○

○

○

●*

●*

●*

Carassius auratus●*●*

Pseudogobio esocinus▲

Rhodeus ocellatus ocellatus○

Rhodeus ocellatus kurumeus○

Acheilognathus tabira tabira
Cobitididae▲○

Misgurnus anguillicaudatus○○

Loricaria

Glyptoperichthes gibbiceps

Siluridae

Silurus asotus○○

Oryzias latipes
Gambusia affinis affinis

○

○

○

Channa argus
Micropterus salmoides salmoides

▲

▲

▲

Oryziatidae
Poeciliidae
Channidae
Centrarchidae
Cichlidae
Gobiidae

○

○

○

○

○

○

▲*

▲

Lepomis macrochirus
Cichlidae sp.

○

○

○

▲

▲

▲

▲

▲

▲

▲

▲

▲

▲

▲

▲

▲

▲

▲

▲

▲

▲

▲

▲

▲

▲

▲

▲

▲*

Odontobutis obscura○○

Rhinogobius sp.
Number of native species

○
12

○
11

○
9

○
5

○
6

○
6

○
6

○
5

○
5

Number of introduced species

1

4

4

4

7

6

8

6

5

Percentage of introduced species
Reference

7.7% 26.7% 30.8% 44.4% 53.8% 50.0% 57.1% 54.5% 50.0%
1)1) Hosoya (2001),

2) Nagata and Hosomichi (1981) ,

1)

2)

1)

3)

3)

3) Present study

100

Compositions(%)
Compsitions(%)

3)

Micropterus salmoides
salmoides
Lepomis macrochirus

80

Rhinogobius sp.
Cyprinus carpio

60

Oryzias latipes
Rhodeus ocellatus
ocellatus

40

Carassius sp.
Pseudorasbora parva

20
0
1979
(n=66)

Fig. 3

1998(n=79)

1999(n=136)2000(n=263)

2001(n=314)2002(n=119)Changes in species composition of the fish community in Mizoro-ga-ike from 1979 to
2002, based on fish with ages of more than 1+ captured in May to July by net casting.
Data for 1979 from Nagata and Hosomichi (1981).

3)

3)

150

K. ABEKURA et al.
Table 3 Annual changes in the total catch and the population estimates of bluegill sunfish (Lepomis
macrochirus). Petersen’s marking and recapture method (Chapman, 1951) was adopted in 1999. A
removal method (Delury, 1947,1951) was used in 2001 and 2002. The population in 1998 was estimatedfrom CPUE based on ‘Eri-ami’ fishing carried out at a and b in 1998 and 1999.The population in 2000
was estimated from CPUE based on net casting between 2000 and 2001. The mean number of capturesof the 1+ or higher age classes is based on data in May to July. That of the 0+ age class in September to
October.
Year
Total days of Eri-ami fishing
Number of Eri-ami nets per day

1998
112
2

1999
49
4

2000
39
2

2001
23
2

2002
30
2

Total days of net casting
Number of net casting per day

3
10

28
5 - 10

29
14

17
14

11
14

Total days of mondori trap fishing
Number of net casting per day

―
―

―
―

―
―

35
10

52
30

1+ and higher
Estimated population
Max 95％ confidence limit
Minimum 95％ confidence limit
Marking
Recapture
Total catch by Eri-ami fishing
Mean number of catch/net/week
Total catch by net casting
Mean number of catch/net
Total catch by Mondori trap fishing
Mean number of catch/trap
Total removal
0+
Total catch by Eri-ami fishing
Mean number of catch/net/week
Total catch by net casting
Mean number of catch/net
Total catch by Mondori trap fishing
Mean number of catch/trap
Removal

9,545
―
―
―
―
1,214
21.7
77
2.4
―
―
771

7,477
9,713
5,241
254
33
1,900
20.4
758
3.5
―
―
2,404

5,744
―
―
―
―
666
32.1
786
3.7
―
―
1,452

5,775
7,069
4,481
―
―
574
30.8
715
3.8
3,601
13.0
4,796

5,387
6,586
4,188
―
―
325
15.9
180
0.9
4,353
4.2
4,858

3,759
207
19
―
―
―
3,778

0*
―
494
2.8
―
―
494

12,679
756
222
2.4
―
―
12,901

4,707
315
97
1.1
119
2.8
4,923

2,792
248
76
0.8
1,329
2.8
4,197

Remaval of spawning beds

―

0

0

2

1

Total capture
5,069
3,152
14,353
9,813
Total removal
4,549
2,898
14,353
9,719* Eri-ami wasn't used in July to December in 1999 and no fish of the 0+ age class were caught.

9,055
9,055

Table 4 Annual changes in population estimates for each age lass of bluegill sunfish (Lepomis macrochirus) in
Mizoro-ga-ike. Estimation methods in 1998-2002 are as mentioned in Table 2 (a). Age classes wereidentified from the frequency distribution of body length of fish captured by ‘Eri-ami’ fishing in April,
May and June ('98 n=506, '99 n=401, '00 n=163, '01 n=231,'02=99). Relations of body length to age
were judged from scale examination of subsamples in 2000 and 2001 ('00 n=11, '01 n=86, '02 n=60).
Year
Age group
Number of 1+ and higher(Max 95％ confidence limit)(Minimum 95％ confidence limit)

1998

1999

2000

2001

2002

9,545

7,477

5,744

5,775

5,387

-

9,713

-

7,069

6,586

-

5,241

-

4,481

4,188

1+ (%)

6093(63.7)5145(68.8)3984(69.3)

4198(72.6)4205(78.0)

2+ (%)

2726(28.5)

1940(25.9)1334(23.2)

1192(20.6)1014(18.8)

736(7.7)

392(5.2)

426(7.4)

385(6.6)

168(3.1)

771

2,404

1,452

4,796

4,858

3,778

494

12,901

4,923

4,197

3+ or higher (%)
Removal of 1+ and higher
Removal of 0+Changes in Fish Community and Alien Fish Control
Table 5

151

Annual changes in the total catch and population estimates of largemouth bass (Micropterus
salmoides salmoides). Petersen's marking and recapture method (Chapman, 1951) was adopted in
1998,1999 and 2000. The population in 2001 could not be estimated because there was no capture by
Eri-ami fishing. See Table 2 for the total annual effort of each fishing method. Mean number of
captures of the 1+ and higher age classes was based on data from May to July.

Year
Total days of Eri-ami fishing
Total days of net casting
Total days of mondori trap fishing

1998
112
3
―

1999
49
28
―

2000
39
29
―

2001
23
17
35

2002
30
11
52

1+ and more
Estimated population
Max 95％ confidence limit
Minimum 95％ confidence limit
Marking
Recapture
Total catch by Eri-ami fishing
Mean number of catch/net/week
Total catch by net casting
Mean number of catch/net
Total catch by Mondori trap fishing
Mean number of catch/trap
Total removal
0+
Total catch by Eri-ami fishing
Mean number of catch/net/week
Total catch by net casting
Mean number of catch/net
Total catch by Mondori trap fishing
Mean number of catch/trap
Removal

84
109
58
24
12
45
0.7
1
0.03
―
―
12

48
75
22
12
4
19
0.3
21
0.20
―
―
24

34
57
10
7
2
12
0.5
2
0.03
―
―
7

―
―
―
0
0
0
0.0
3
0.04
0
0
3

―
―
―
0
0
1
0.1
6
0.04
4
0.004
11

1,290
54
7
0.22
―
―
1,297

591
16.4
14
0.35
―
―
605

616
26.9
59
0.22
―
―
675

285
15.8
33
0.33
19
0.1
337

36
1.8
39
0.14
37
0.03
112

Removal of spawning beds―

several

2

5

4

1,343
1,309

645
629

689
682

340
340

123
123

Total capture
Total removal

Table 6

Annual changes in the total catch and the population estimates of snakehead (Channa argus).
Petersen's marking and recapture method (Chapman, 1951) was adopted every year. See Table 2 forthe annual total effort of each fishing method.

Year
Total days of Eri-ami fishing
Total days of net casting
Total days of Mondori trap fishing1+ or more
Estimated population
Max 95％ confidence limit
Minimum 95％ confidence limit
Marking
Recapture
Total catch by Eri-ami fishing
Total catch by net casting
Total catch by Mondori trap fishing
Total removal
0+
Total catch by Eri-ami fishing
Total catch by net casting
Total catch by Mondori trap fishing
Removal
Total capture
Total removal

1998
112
3
―

1999
49
28
―

2000
39
29
―

2001
23
17
35

2002
30
11
52

539
751
327
82
15
104
0
―
8

498
607
389
123
39
161
0
―
2

366
535
196
47
10
83
0
―
5

235
376
93
22
3
33
0
15
7

224
350
98
14
1
17
0
3
3

1
0
―
1

2
0
―
0

309
0
―
0

0
0
―
0

0
0

105
9

184
2

392
5

48
7

28
11

8

152

K. ABEKURA et al.

4. Discussion
4.1 Impacts of alien fishes on the fish community
in Mizoro-ga-ike
Out of twelve native Japanese fishes inhabiting
Mizoro-ga-ike in the 1970s, a total of six disappeared,
and in their place, introduced alien species, particularly M. salmoides and L. macrochirus, increased inabundance in the pond during these twenty years. It is
fairly difficult in general to prove that such missing
species are really extinct. The past records on fish
fauna in the period between 1972 and 1985, inparticular, were based only on sampling by net casting(Hosoya, 2001; Nagata & Hosomichi, 1981), and thus
the absence of species (Table 2) does not necessarilymean extinction of the populations. The faunal list of
the present study after 1998, however, was based onmuch greater efforts at fish collection (Table 1), thus it
seems highly probable that these fish became extinct
during the twenty years. Although faunal changes infreshwater ecosystems may be attributable to various
environmental changes, including water pollution,embankment, vegetation removal, etc., inter-specificinteractions within the community are plausible in thecase of Mizoro-ga-ike, since the pond has been reserved as a national treasure and development of its
catchment area was avoided during the twenty years.
In addition, the present situation of the fish species
composition (Fig. 3) being overwhelmingly occupiedby the two alien species of M. salmoides and L.
macrochirus indicates that these two invaders had serious impacts on the native community in the pond.
Strictly speaking, however, the impact of each alien
species is difficult to judge separately because they
invaded the pond within a comparatively short period
in the 1970s-1980s, except for C. argus, which established a population in the 1960s. We will discuss,hereafter, possible reasons for the changes in eachnative fish population in relation to the timing of invasion of the alien species.
When we investigate in detail the fish faunalchanges shown in Table 2, those of R. ocellatus
kurumeus and O. latipes may be explained as follows.
Since R. ocellatus kurumeus disappeared in 1977
before the invasion of L. macrochirus and M.
salmoides, the related continental species, R. ocellatus
ocellatus, could have threatened its congenic native
species with gene degradation by hybridization(Kawamura et al., 2001). In the case of O. latipes, its
disappearance in 1985 coincided with the invasion of
G. affinis affinis. Since G. affinis affinis is an egg
predator (Komak & Crossland, 2000) and has asimilar habitat to Oryzias latipes (Sahara & Kouchi,1980), the invasion of G. affinis affinis may be aprinciple cause of O. latipes' extinction.
As for C. argus, another predatory alien fish, its
impacts on the biological community are not clear
enough. Since C. argus had been introduced toMizoro-ga-ike in the 1960s, it coexisted at least withthe twelve native species recorded during 1972-1979.
In 1979, the percentage of R. ocellatus ocellatus
was 23.9% and five native species were coexistedwith it (Fig 3). In contrast, L. macrochirus accounted
for more than 94% of the fish in Mizoro-ga-ike in
1998-2001. Only three native species, Pseudorasbora
parva, Rhinogobius sp. and Carassius sp., constituted
a mere 0% -5% of the fish fauna in these years, except
2002.
As for the other cyprinids and Rhinogobius sp.,
their extinction or reduction in abundance may be attributed to the invasion of M. salmoides and L.
macrochirus. Since the two alien predators were introduced in the same period in the latter 1970s, it is difficult to determine their individual impacts on Mizoroga-ike.
4.2 Effects of control on the L. macrochirus
population
Population estimates of L. macrochirus decreased
from 9,545 in 1998 to 5,744 in 2000 but didn’t decrease appreciably from 2000 to 2002 (Table 3). The
populations in 1998 and 1999 were estimated basedon ‘Eri-ami’ fishing samples, and those in 2000, 2001
and 2002 were based on ‘Mondori’ trap fishing. Since
the difference in fishing methods might influence
population estimates, we compared the CPUEs of net
casting among these years, and this resulted in somewhat of an increase during the first four years. Thus,
the decrease in population from 1998 to 2000 may
have been less significant than the estimates in Table 3.
Population estimates in 2001 were based on ten‘Mondori’ traps, and in 2002, on 30 traps. Since thepopulation in 2002 was estimated based on samples
from a wider area than of 2001, the population in 2002
may have been overestimated in comparison withthose in 2000 and 2001. In conclusion, the population of L. macrochirus decreased rapidly in the firsttwo years, but it did not decrease appreciably, at leastin the following three years, in spite of increased control efforts.
Reasons for the slow decrease in the population
may be related to changes in its population structure.
The population estimates of the 2+ and 3+ age classes
or higher decreased distinctly from 2000 to 2002. Thismeans that the population of these age classes can becontrolled by a combination of ‘Eri-ami,’ net casting,and ‘Mondori’ trap fishing. Since population estimatesof the 3+ or higher age class didn’t decrease significantly before 2001, the addition of the ‘Mondori’ trap
method may be effective for population control ofolder age classes. In comparison with the effective
population decreases of these age classes, the 1+
population could not be controlled easily because ofrecruitment from the 0+ population. This phenomenon
seemed to derive from an increase in the survival ratioof the 0+ population. Such an increase in the survival
ratio can be explained by the following two hypotheses. Firstly, there is a strongly negative correlation

Changes in Fish Community and Alien Fish Control

between the biomass of M. salmoides and that of L.
macrochirus (Guy & Willis, 1990; Novinger, 1990),
and thus, the removal of M. salmoides may result in
an increase in the population of L. macrochirus.
Secondly, the removal of older age classes may cause
an increase in the survival ratio of younger age classes.
The latter hypothesis is ascribed to intraspecificcompetition or predation among the age classes. Sucha possibility may be clarified by further investigationof the annual changes in the population structure of L.
macrochirus in Mizoro-ga-ike.
4.3 Effects of control on the M. salmoides
population
The population estimate of M. salmoides in 1998
was only 84, which was much fewer than that of L.
macrochirus, and the estimates decreased to 33 in
2000. The recruitment of the 0+ age class of M.
salmoides was as much as 1,000 or more in order(Table 5). This means that the survival ratio of the0+ age class continued to be low even after the decrease in population of older age classes. A comparatively higher capture ratio of M. salmoides than L.
macrochirus by ‘Eri-ami’ fishing (Tables 3 & 5) may
be a reason for the effective decrease in the populationof M. salmoides. If the present efforts for population
control of M. salmoides are continued, it may be possible to achieve extinction of the population in thispond.
4.4 Effects of population control on other fishes
The population estimates of C. argus have been
decreasing from the year to year (Table 6). We imagined previously that removal of the two predators L.
macrochirus and M. salmoides would result in an increase in the population of C. argus. The result, however, was strikingly the reverse. Reasons for this phenomenon are unknown at present.
According to the decrease in M. salmoides and L.
macrochirus populations, Pseudorasbora parva andCarassius spp increased in abundance to 5% and 8%,
respectively, of the fish community in Mizoro-ga-ike(Fig.3), and this may be an effect of alien fish control.
Although the control measures were started in 1998,
no recoveries of native fishes were detected in the first
three years from 1999 to 2001. There are two possibilities which may explain such very slow recoveriesof the native fish species. Firstly, the impacts of M.
salmoides and L. macrochirus may be strong enougheven at present levels of population density, and secondly, habitat conditions such as food resources andspawning resources may be unsuitable at present. In
order to prove these hypotheses, changes in physicalenvironmental conditions and prey communities during population control of the alien fishes will need to
be investigated. Mechanisms of their impacts on thecommunity are also an important subject to study: e.g.,competition for food as well as direct predation onfish, eggs and/or fry have been reported as mecha-

153

nisms (National Federation of Inlandwater Fisheries
Cooperatives, 1991).

Acknowledgements
The present study was based on data obtained in
the course of alien species control work by Kyoto City
for conservation and restoration of the biological community in Mizoro-ga-ike. We would like to express
many thanks to members of the NGO for Mizoroga-ike Freshwater Animals and Plants, particularly to
Messrs. Akio Ito, Toshiharu Tasue and Ken-ichi Narita
for their field work on alien fish control, to Mr. Yohei
Morikawa for his cooperation in 1998 and 1999, andto Mr. Koji Ban and his students for their help withbody size measurement of specimens. The study was
financially supported in part by the Environmental
Technology Development Fund from the Ministry of
the Environment and by the River Environment Fund(REF) in charge of the Foundation of River andWatershed Environmental Management (FOREM),
Japan.
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(Received 16 December 2004, Accepted 22 December 2004)