Rikunomizu（Limnology in Tokai Region of Japan）74 : 29 - 34（2016）

ReportRediscovery of Japanese charr in the Denjogawa River and its tributary
in 2016 after a disturbance from the Ontake Landslide in 1984:
significance of a tributary as a refugium from disturbanceYukio ONODA1), 2), 3)
Abstract
Japanese charr (Salvelinus leucomaenis japonicus) was rediscovered in Denjogawa River and its tributary
in 2016 against reports of its extinction following huge sediment inflows caused by the Ontake Landslide
associated with the Western Nagano Prefecture earthquake in 1984. All of the 27 individuals of the charr
were observed in pools more than 30 cm deep. Most (25 of 27) of the fish were observed in the tributary,and relative density (represented as count per unit effort [CPUE]) was higher at upstream sites in thetributary. Moreover, the fish in the tributary had wider ranges of body size. In Denjogawa River, the species
was observed downstream of the confluence but not upstream although both sites had pools of depths
more than 30 cm. As relative density gradually decreased from upstream of the tributary to downstream ofthe confluence, the population in the tributary should be considered as the source population. Furthermore,
the population in the Denjogawa River should have been maintained by the individuals drifting from thetributary. Additionally, the wide ranges of the body size of fish in the tributary suggest that the populationin the tributary was maintained through reproduction. Because the small population in the tributary may
function as a source population for that in the Denjogawa River, it will be significant to secure migration
pathways between the mainstream and the tributary for continuity of the population in the Denjogawa
River system.
Key words: mountain stream, disturbance regime, refugia, Mt. Ontake, the Western Nagano Prefecture earthquake(Received: 20 November 2016; Accepted: 25 November 2016)
such monitoring is difficult because massive disturbances
sometimes cause extinction of fish populations and because
records of changes in fish population are scarce. Fortunately,
I had the rare opportunity of finding a population of theJapanese charr (Salvelinus leucomaenis japonicus), which
had been reported to be extinct in the Denjogawa Riversystem after the Ontake Landslide caused by the Western
Nagano Prefecture earthquake in 1984 (Takeda, 1985).
As the population was found about the confluence of theDenjogawa River and a tributary, this provides opportunities
to examine the significance of tributaries for resilience ofthe fish population in the mountain rivers. In this study,

Introduction
As mountain rivers tend to undergo massive disturbancessuch as from landslides due to high gradient, habitats offishes in the rivers tend to be unstable and fail to sustain theirpopulations (Geertsema et al., 2009). However, mountain
rivers have many tributaries (Horton, 1945; Allan, 1995),
which can function as refugia from such disturbances (Sedellet al., 1990; Koizumi et al., 2013). Accordingly, monitoringof the recovery processes of the fish population after
disturbances is significant for understanding the mechanismsthat sustain fish populations in these rivers. Nevertheless,1)

Mie University Research Center, 1577 Kurimamachiya-cho Tsu city, Mie 514-8507, Japan
Mount Ontake Volcanic Eruption Research Group of the Tokai Branch of the Japanese Society of Limnology (with Drs. KentaroNOZAKI, Takashi TASHIRO, and Tomomasa TANIGUCHI)
3)Kiso Biological Station, Kyoto University, 6388 Fukushima Kisogun Kisomachi, Nagano 397-001, Japan2)

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Yukio ONODA

the significance of the tributary for resilience of fish
population was evaluated by comparing fish density and size
distribution among sites around the confluence.

September 15, 2016, respectively (K. Nozaki, unpublisheddata). Each site was chosen to include one or more pools
with a depth greater than 20 cm, which is preferred by thecharr (Yagyu, 2009) and considered as potential habitatof the fish. All the sites were separated by placing weir(s),except between sites 2 and 5 (Fig. 1B).
Fish censuses were performed at all sites on September
15 and 16, 2016, by snorkeling for 15 or 30 min according
to the observed fish density. The fish species was identified
under water as Salvelinus leucomaenis japonicus based on
the patterns of colors and spots (Nakabo, 1993). Because
adults of the subspecies S. l. pluvius were absent, young
fish with white spots on the sides of body but not on theirback or vertex of the head were also identified as S. l.
japonicus, moreover (Naruse and Yoshiyasu, 1983). Thesize distribution was approximated by measuring the total
length of the fish to the nearest multiple of 5 cm by visual
estimation, corrected using a ruler under water. The numbers
observed were divided by the searching times to convert
counts into relative density as counts per unit effort (CPUE),according to the method described by Onoda and Kayaba(2016).

Methods
Japanese charr was searched at five sites located near theconfluence of the Denjogawa River and a tributary locatedon the southern side of Mt. Ontake in central Japan (Figs.
1 and 2A). The watershed areas of the Denjogawa Riverand the tributary were estimated at 8.52 and 1.95 km2,
respectively, by a spatial analyst tool (hydrological analysis)(ArcView 10.2; ESRI Japan) using fundamental geospatial
data from Geospatial Information Authority of Japan asdigital elevation model (DEM) data (T. Tashiro, unpublisheddata). Water quality were measured in the Denjogawa River(electrical conductivity: 41.40 mS m-1, water temperature:
14.1 °C, pH: 7.5, turbidity: 0.36 NTU, and chromaticity:1.54) and the tributary (ibid. 2.30 mS m-1, 12.8 °C, 6.8,0.17 NTU, and 2.62, respectively) at 11:00 and 12:45 h onResults
A total of 27 individuals belonging to one taxon (Japanesecharr, Salvelinus leucomaenis japonicus) were observed inthis study (Table 1, Fig. 2B, C, D). Relative density (CPUE)
ranged from 0 to 14 individuals per 30-min observation anddecreased gradually along the tributary from upstream toits confluence with the Denjogawa River. In the Denjogawa
River, some fish were present at the downstream site
(site 2) but not at the upstream site (site 1). The body size
distribution of the fish was wider at the sites in the tributary
than in the Denjogawa River (Table 1). Noticeably, among
all the sites, the body size distribution of the fish was widest(range: 10–30 cm) at the most upstream site of the tributary(site 3).

Discussion
Possibility of survival of the charr population after alarge disturbance
The fish populations observed in this study might havesurvived a large-scale disturbance (i.e., the Ontake Landslidein 1984). This possibility is supported by some collateralevidences. First, all the observed fish were identified asFig. 1. Location of the Denjogawa River and a tributary (A), andstudy sites around the confluence (B). Fig. 1B gives an enlarged
view of the inset in Fig. 1A. Area that had received debris fromthe Ontake Landslide in 1984 is redrawn in gray, according
to Mizuyama and Hara (1991). Each number (gray) near atransverse structure in Fig. 1B indicates its height (in meters)from the water surface of the site immediately downstream to it.

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30Figure 2

Significance of a tributary as a refugium for Japanese charr from disturbance

ABCDEFig. 2. Photographs of the study sites near the confluence (A), adult (B, C) and young (D) ofJapanese charr, and a dead small snake (Rhabdophis tigrinus) with some bite marks (E). The
Denjogawa River (left) and the tributary (right) meet near the mid-bottom of the photograph(A). The photograph (A) was taken by T. Tashiro and the other photographs (B-E) were taken
in pools of the tributary by the author.

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Yukio ONODA
Table 1. List of observed fish at the sites in the Denjogawa River and its tributary.
Site
no.*1

Observation
Date in2016

Time(min)Total bodylength(cm)SpeciesObserved fish
numberCPUE

Maximum
depth(cm)*30055

20

1

270

10

1

270

10

4490

15

3390

20

2290

25

4490

30

1190

10

33120

25

3312030

11120

10

1130

15

1170252270*2Denjogawa River1Sep. 16

152Sep. 15

15

Salvelinus leucomaenis

Tributary
345

Sep. 16

Sep. 15

Sep. 15303030

Salvelinus leucomaenis

Salvelinus leucomaenis

Salvelinus leucomaenis*1

Detailed information of the sites is presented in the text and in Figure 1. Table 1. Fish observed at the sites
in the Denjogawa River and its tributary.
*2Counts per unit effort*3
Maximum depth of the pool where fish were observed or searched.

Salvelinus leucomaenis japonicus, a species that wasdistributed in the Denjogawa River before the landslide in1984 (Takeda, 1985). Second, artificial release of the fishin the Denjogawa River is not known since the landslide
occurred (Y. Sawada, personal communication). Moreover,cultivation of the species is known to be much less preferredthan that of the subspecies S. l. pluvius (Tsuchida et al.,2008), suggesting that the observed fish could not have beenreleased. Third, according to aerial photographs taken after
the landslide, the tributary studied seems to be located on themargin of a range of the landslide (T. Tashiro, unpublisheddata, also see Fig. 1B). Given with the above information,
the fish population found in this study is likely to havesurvived the disturbance caused by the landslide, although
the possibility that the fish were artificially released should
be considered carefully.

in the tributary had a wider range of body sizes (10–30cm), which are equivalent to the body size of one- (bodylength: 13–16 cm), two- (16–22 cm), and three-year fish(17–25 cm), respectively (Miyadi et al., 1976). Of them,larger charrs, corresponding to two and three years of age,are known to reproduce (Miyadi et al., 1976). Yamamoto
et al. (1994) reported reproductive activity in male andfemale charr that had body lengths of more than 17 and 21cm, respectively, based on gonado somatic index (GSI).
Similarly, Kitano et al. (2005) found reproductive activity
in male and female charr that had fork lengths of morethan 20 and 22 cm, respectively. Additionally, the charr
intrinsically prefer tributaries to mainstreams as habitats forreproduction (Nakamura, 1999). Furthermore, the entire
watershed area of the tributary (1.95 km2) is larger than 1.01km2, which is needed to maintain charr populations (Endouet al., 2006) although the effects of fragmentation on thepopulation by weirs in the tributary should also be discussed
circumstantially in future studies. Therefore, the wide rangesof body size in the tributary suggest that the population has
been maintained through reproduction.

Maintenance of the local population of the charr in thetributary through reproduction
Even if the observed population of charr consisted ofreleased individuals, it is noteworthy that the population
has maintained itself through reproduction. The populationⒸ2016

The Tokai Branch of the Japanese Society of Limnology

32Significance of a tributary as a refugium for Japanese charr from disturbanceSignificance of local population in the tributary as asource population
This study suggests that local population in the tributaryfunctions as a source population in the Denjogawa Riverbecause relative density decreased along the tributary andthe confluence (Table 1). A higher density in the tributary
might be caused by the migration for reproduction fromthe mainstream, because this study was performed in theearly reproductive season of the charr (late September toearly November, Miyadi et al., 1976). However, the fish areunlikely to have migrated upstream in the tributary because
high weirs existed between the study sites (Fig. 1B) andthe charr could migrate only downstream in the tributary.
As the upper site in the Denjogawa River had no charr, thepossibility that the charr population in downstream site of
the Denjogawa River being caused by a drift from upstream
is quite unlikely. Therefore, the charr population in themainstream of the Denjogawa River is likely maintained by
the charrs provided by the tributary.

the resilience of the metapopulation. Changes in the charr
population of the Denjogawa River system, including its
tributaries, should be surveyed continuously and extensively
in further studies to comprehensively understand the effectsof tributaries as refugia and the recovery processes fromhuge disturbances such as landslides.

Acknowledgments
The author thanks T. TASHIRO for providing data, cooperationin the field survey, and valuable suggestions, Y. SAWADAof the Education Board of Otaki Village for valuable
information, Kiso District Forest Office, Forest Agencyof Japan for permission to use a forest road for the accessto the Denjogawa River, M. YAMADA of Kiso BiologicalStation, Kyoto University for her hospitality, and Editage(www.editage.jp) for English language editing. This study
was partly supported by grants from the Water Resources
Environment Center to K. NOZAKI (2015-04).

Implications for conservation of sustainable populationof the charr
It is important to secure the Denjogawa River and its
tributaries to allow re-colonization and conservation ofa sustainable charr population in the Denjogawa Riversystem. Although the studied tributary is expected tohave a watershed area larger than that necessary for thesustainability of the charr population, the usable area forthe population may have decreased by isolation due totransverse constructions such as weirs (Endou et al., 2006).
A decrease in the usable area should mean a decrease in the
carrying capacity. Indeed, a dead small snake (Rhabdophistigrinus) with bite marks, assumed to have been received
from a charr (Miyadi et al., 1976), was found in a pool of
the tributary (Fig. 1E), likely reflecting severe shortage ofprey resources in the small tributary. As the re-colonizationby the charr population increases usable area and thecarrying capacity at the watershed scale, it should contributeto conservation of the sustainable population. Additionally,
the re-colonization will act to disperse the risks encounteredduring the next disturbance because the tributary, whichfunctioned as a refugium in this event, might actually
receive greater disturbances, such as landslides, in thenext event. Moreover, the re-colonization can resolve theisolation, which causes decrease in genetic diversity (Moritaet al. 2009). Therefore, securement of the pathways formigration and re-colonization will be significant to improve

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要

1984年 に 発 生 し た 御 嶽 崩 れ（ 山 体 崩 壊 ）に よ る 撹 乱 後の伝上川とその支流における2016年のヤマトイワナの再発見：
撹乱からの避難場所としての支流の重要性
小野田幸生
王滝川水系の伝上川では，1984年の長野県西部地震に伴う御嶽崩れ（山体崩壊）による土砂流入によって，ヤマトイワナの個体群が絶滅したと報告されていた。しかしながら，2016年9月末に潜水目視調査によって，イワナ属魚類が27個体発見された。確認された地点は水深が30 cm 以上の落ち込み型の淵であった。確認個体の多くは，伝上川の左岸に流入する支流で確認され，上流ほど単位努力量当たりの確認個体数が多かった。伝上川本川では水深が30 cm 以上の淵があったにも関わらず，支流の合流点の下流側では2個体が確認され，上流側では確認されなかった。多くの調査地点の間に治山ダムがあることを合わせて考慮すると，調査対象の支流がソース個体群であり，流下した個体群が伝上川で確認されたと考えられる。さらに，伝上川本川と比べて，支流の個体群の体サイズは幅広いサイズで構成されたことから，支流の個体群は再生産を通じて維持されてきたと考えられる。支流などを単位とした局所個体群は小規模であるため，他の個体群との交流が確保されるような移動を保障することが，山地河川に生息するイワナの保全に重要であると考えられる。
キーワード：山地河川，撹乱体制，避難場所，御嶽山，長野
県西部地震

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