Rikunomizu（Limnology in Tokai Region of Japan）48 : 17 - 26（2011）

AuticlesEffects of catastrophic floods on daytime interstitial habitatof bagrid catfish Pseudobagrus ichikawai
Shiro SAGAWA1), Takashi TASHIRO2), Junjiro N. NEGISHI3) and Yuichi KAYABA1)

Abstract
We examined the daytime habitat used by highly endangered bagrid catﬁsh Pseudobagrus ichikawai before and after rarecatastrophic ﬂoods for the unaltered reach in a tributary ﬂowing into the Miya River of Mie Prefecture, Japan. Although themagnitude of the ﬂoods were large (>15-fold increase in water level), the ﬂoods had no effect on the population level of thecatﬁsh at both reach and channel-unit scales. The bagrid catﬁsh were consistently more abundant in the pool and shelteredinterstitial spaces formed by cobble and boulder substrates. Although the ﬂoods did not have recognizable effects on theoverall catﬁsh population at the reach or channel-unit scales, the catﬁsh’s spatial use at a microhabitat scale changed over theﬂoods. Only 20% of the patches where ﬁsh were observed before the ﬂoods contained individuals after the events. There wasan uncommonly deep (>5m) and large-volume pool in this small stream where individuals of various sizes occupied spaces
beneath single boulder and the number of individuals there increased by more than 3-fold over the events. These results
suggest that this unique microhabitat functioned as a critically important hydraulic refuge for the bagrid catﬁsh to minimize
the effects of the catastrophic ﬂooding event.
Key words: Bagridae, boulder, microhabitat, nocturnal, refugia(Received: 25 June 2010, Accepted: 8 February 2011)
relatively well studied for both water-column ﬁsh (e.g., salmonids)

Introduction

and demersal ﬁsh (e.g., cottids and gobiids) in terms of the effects

High flow events play important roles in the organization of

on populations that often require interstitial spaces between andlotic fauna (e.g. Resh et al. 1988). For fish communities, floods

underneath cobble-boulder substrates, particularly for concealment

may increase the abundance and species richness by temporarily(salmonids: Hartman, 1963; Meyer and Gregory, 2000, cottids:

connecting isolated pools and creating potential movement

Davey et al., 2005, cobitids: Bohlen, 2000, 2003). However, theopportunities previously impeded by barriers (Taylor, 1997; Taylor

actual habitat use of such space by demersal ﬁsh such as catﬁsh has

and Warren, 2001; Franssen et al. 2006). Conversely, ﬂoods could

not been rigorously studied by comparing before and after the highnegatively affect individuals by altering channel morphology,

ﬂow events in a relatively unaltered stream environment.

leading to death or displacement of individuals downstream

The bagrid catfish Pseudobagrus ichikawai is an endemic,(Harrell, 1978; Matthews, 1986; Fritz et al., 2002; Tew et al.

nationally endangered demersal freshwater ﬁsh, distributed in rivers

2002). Because severe ﬂoods may limit animal population levels

ﬂowing into the Ise and Mikawa bays in Japan (Nakamura, 1963;(Grossman et al., 1982), understanding the effects of catastrophic

Tokuhara and Hara, 2002; Miyamoto et al., 2004). The species

events with low recurrence intervals provides crucial management

has been designated as a“natural monument”in Japan since 1977

implications for lotic organisms. Effects of floods have been

and also listed in a threatened category under the IUCN Red List

1)

Aqua Restoration Research Center, Public Works Research Institute, Mubanchi, Kanyuuchi, Kawashima-kasada-cho, Kakamigahara, Gifu 501-6021, Japan
E-mail: sagawa77@pwri.go.jp
2)
Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
3)
Faculty of Environmental Earth Science, Hokkaido University, N10W5, Sapporo, Hokkaido, 060-0810, Japan17

Shiro SAGAWA et. al.
of Threatened Species (International Union for Conservation of

Their habitat requirements have also been examined (Shimizu andNature and Natural Resources, 2007). Mie Prefecture is one of

Shimizu, 1982; Shimizu, 1988; Watanabe, 1994a; Tashiro et al.,the species’primary distribution areas where their populations

2005), and the decreasing trends have been attributed primarily

have been historically recorded in eight watersheds. However, by

to degradations of interstitial habitats, which are suggested as

now, their distribution range is limited to only four watersheds

one of the most preferred microhabitats due, for example, to(Mizuno et al., 2007). For example, in a tributary of the Inabe

the channelization work (e.g., Watanabe, 1994a). On the otherRiver in the northern part of the prefecture, a catfish population

hand, it is less well known how bagrid catﬁsh respond to natural

decreased from several hundred to only a few individuals after adisturbances that may alter the interstitial habitats in a relatively

severe ﬂood in September, 1990, but the catﬁsh population has not

unaltered river environment. The overall aim of the present study

fully recovered from the damage caused by the flood (Board of

was to examine whether catastrophic floods with low recurrence

Education of Mie Prefecture and Tokai Research Association ofintervals (> 50-year events) act as a major factor in limiting theFreshwater Ecology, 1993; Board of Education of Mie Prefecture

population level of bagrid catﬁsh P. ichikawai in the Miya Riverand Mie Prefectural Science and Technology Promotion Center,watershed. Specific objectives were: (1) to assess the effects of

2003; Board of Education of Mie Prefecture 2006). As of today,

severe ﬂoods on abundance of bagrid catﬁsh inhabiting interstitial

most catﬁsh populations remaining in the prefecture are found inspace, (2) to examine habitat characteristics of bagrid catfish

the Miya River watershed, and therefore it is crucially important torelated to the persistence of the population over the ﬂoods, and (3)preserve their populations there and also to study them in relation

to elucidate the critical microhabitat factors for their conservation.

to severe ﬂood events.

Sagawa et al. Fig. 1

Various characteristics of bagrid catfish have been examined

Materials and methods

with regard to their spawning and growth (Watanabe, 1994a,
Study designb), morphology (Watanabe 1998a), diel activity (Watanabe,2008), genetic structure (Maeda et al., 1994; Okazaki et al., 1999;

The study reach was located in a fourth-order stream of the Miya

Watanabe et al., 2001; Watanabe and Nishida, 2003), distribution

River basin, Mie Prefecture, Japan (Fig. 1). The exact locations

areas (Okada and Kubota, 1957; Watanabe, 1998b; Watanabe andof the study tributary will remain undisclosed for conservation

Ito, 1999; Watanabe and Uyeno, 1999; Tokuhara and Hara, 2002;

purposes. The study reach was a 200-m long alluvial channel,

Miyamoto et al. 2004), and conservation (Watanabe, 1997).

which consisted of two 50-m channel units (glide and pool) and

125°

130°

135° 140°

145°

Mie Prefecture

45°

40°

Japan Sea

Miya River

Pacific
Ocean

35°

0

30°

Fig. 1 Location of the Miya River in Mie Prefecture, of which one tributary was studied.

18

20 40kmEffects of catastrophic ﬂoods on Pseudobagrus ichikawai

Glide

Pool

RiffleFlow direction
Fig. 2 View of the study reach consisting of a sequence of glide, pool, and rifﬂe.

one 100-m channel unit (rifﬂe), with a mean channel gradient ofTable 1. General characteristics of the study reach with three channel
units in the tributary of the Miya River.

approximately 0.3%, and a mean wetted width of 15-30 m (Fig.

Variables
Channel length (m)Wetted width (m)Dominant substrate
Dominant bank material
Bank vegetation

2, Table 1). The average depth of glide, pool, and riffle wereapproximately 60, 110, and 35 cm, respectively. For the pool,maximum depth was >5 m. Cobbles were the most abundantmaterials for both stream bed and riparian bank in the glide andriffle whereas boulders were the dominant bed material with

Glide
Pool
Rifﬂe
50
50
100
30
15
15
CobbleBoulder
CobbleCobbleRock
CobbleSparse grass under the riparian cover

bedrock forming riparian banks in the pool. The shore of the study
reach was covered with sparse grass under the riparian trees.
A ﬁeld survey was conducted from 27 September to 15 October,

Maximum 1-h and 24-h rainfall intensity from 28 to 29 September

2004 (Fig. 3). The timing of the survey was adjusted by checking

2004 in the upstream mountain area of the drainage reached

the typhoon track map on the weather chart. A pre-flood survey

119 mm (> 100-year event) and 732 mm (> 50-year event),

was conducted on 27 September a few hours before the onset

respectively. The daily mean and instantaneous peak water levelsof floods caused by the typhoon Meari, and a post-flood survey

reached 4.69 and 9.44 m, respectively, in our study tributary where

was conducted on 15 October after the water level and turbidity

water levels at pre- and post-ﬂood survey occasions were 0.62 andcompletely subsided to the level similar to the pre-ﬂood condition.

0.66 m, respectively. The swollen downstream caused inundation

The typhoon Meari was severe causing a catastrophic ﬂood withand damage to houses. In spite of the catastrophic magnitude ofan exceedingly low recurrence interval; it was the most severe

these ﬂoods, the longitudinal channel morphology (glide, pool, andtyphoon to hit Mie Prefecture in 2004 and resulted in by far the

rifﬂe structures) remained similar over the ﬂoods.

largest ﬂood since recording began in 1957 (Suwa et al., 2005).

19

7

Shiro SAGAWA et. al.
500

6.0
Precipitation
Water level

400

5.5
5.0
4.5

1

350

4.0

300

Pre-flood
survey

250
200

3.5

Post-flood
survey

3.0
2.5

2

2.0

150

1.5

100

1.0
0.5

0

0.0
2004.1.1
2004.1.10
2004.1.19
2004.1.28
2004.2.6
2004.2.15
2004.2.24
2004.3.4
2004.3.13
2004.3.22
2004.3.31
2004.4.9
2004.4.18
2004.4.27
2004.5.6
2004.5.15
2004.5.24
2004.6.2
2004.6.11
2004.6.20
2004.6.29
2004.7.8
2004.7.17
2004.7.26
2004.8.4
2004.8.13
2004.8.22
2004.8.31
2004.9.9
2004.9.18
2004.9.27
2004.10.6
2004.10.15
2004.10.24
2004.11.2
2004.11.11
2004.11.20
2004.11.29
2004.12.8
2004.12.17
2004.12.26

50

Ｗａｔｅｒ ｌｅｖｅｌ （ｍ）

Precipitation （mm）

450

FloodsFig. 3 Fluctuations of the precipitation and water level for the study tributary of the Miya River, which were measured at a gauging
station approximately 850m downstream of the study reach (Mie Preferctural Government).

Field survey

pre-ﬂood occasion, however, water level started rising just when

The daytime interstitial habitat use of bagrid catfish was

physical measurements were being taken forcing the termination

surveyed by four people making snorkeling observations over aof measurements. Thus, we only quantiﬁed physical environments

period of four hours (one hour each for the 50-m glide and pool,for the post-flood occasion following snorkeling observations.

and two hours for the 100-m riffle). Observations were made in

The quadrate set up for each observed site was divided into 16

the upstream direction, carefully examining shore vegetation,

subgrids (0.5 m × 0.5 m). At every corner of the subgrids (a total

beneath substrate materials (cobble, boulder), and crevices onof 25), current velocity (cm s-1), water depth (cm), substraterock (hereafter referred to as "shelter type"), all of which havecoarseness, and embeddedness were measured. Current velocity

been described as preferred habitats for bagrid catfish (Shimizu

was measured with an electromagnetic current meter (model VP-

and Shimizu, 1982; Shimizu, 1988; Watanabe, 1994a; Watanabe,

201, Kenek Co., Ltd., Tokyo, Japan) at 60% of the total water2008). Whenever necessary and possible, we checked concealed

depth. Dominant substrate size was recorded through visualinterstitial habitats by carefully moving substrate materials to avoid

examinations using 5 size codes: 1) sand (< 2mm), 2) gravel (2–disturbing the ﬁsh, and all the substrate materials were placed back16mm), 3) pebble (16–64mm), 4) cobble (64–256 mm), and 5)

immediately after each observation. Even when substrate materials

boulder (> 256mm). The longest diameter and the corresponding

Sagawa et al. Fig. 4

were moved, none of the fish reacted by swimming away, andthus our observation was considered to cause negligible impact

12

on their micro-distributions during the study period. When ﬁnding
the catﬁsh, we set up a 2 m × 2 m quadrate with the ﬁsh-locating

10
Number of fish

point as the center (hereafter referred to as "observed site"), and arecorded shelter type within the quadrate on a map. Furthermore,
we thoroughly searched for any other catﬁsh in the whole area ofthe observed sites, and if spotted, we visually estimated the bodysizes (total length) and classiﬁed into three different classes: ClassClass IIClass I

60-110mm

<50mm

8
6
4

Class III>120mm2

I = < 50 mm; Class II = 60-110 mm; and Class III = > 120 mm.
The size classes were deﬁned according to the frequency-histogram

0 0

based on 83 individuals of bagrid catﬁsh recorded from primarily
the Miya and Inabe rivers from late August to late September in

50

150

100
Total length (mm)Fig. 4 Frequency-histogram for the total length of bagrid catﬁsh (N = 83)
caught in the rivers of Mie Prefecture from late August to late September
in 2001-2002 (Board of Education of Mie Prefecture, Mie Prefectural
Science and Technology Promotion Center 2003). Fish were categorized
into one of three size classes: Class I (below 50mm TL), Class II (60-110
mm TL), and Class III (above 120 mm TL).

2001-2002 (Fig. 4, Board of Education of Mie Prefecture and Mie
Prefectural Science and Technology Promotion Center, 2003).
Our intention was to measure physical environments for eachobserved site at both pre- and post-ﬂood survey occasions. At the20

8Effects of catastrophic ﬂoods on Pseudobagrus ichikawai
Table 2. Fifteen environmental variables used for PCA and their calculation methods.
Variables
1.
2.
3.

Mean water depth
Mean current velocity
Mean substrate size

4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.

Water depth complexity
Current velocity complexity
Substrate size complexity
Percentage of substrate area (>pebble)
Percentage of substrate area (>cobble)
Percentage of substrate area (>boulder)
Percentage of over-lying substrate area (>pebble)
Percentage of over-lying substrate area (>cobble)
Percentage of over-lying substrate area (>boulder)
Number of substrates (>pebble)
Number of substrates (>cobble)
Number of substrates (>boulder)Calculation methods
The mean of values from 25 measurement points in the quadrate
The average value of two diameters (the longest diameter and the shorter one
perpendicular to the longest one) of the substrates larger than the size of pebble.
The coefﬁcient of variance (CV, the standard deviation divided by the mean) for
25 measurements in the quadrate
The percentage covered by materials larger than respective substrate sizes among
25 measurement points in the quadrate
The percentage covered by over-lying materials larger than respective substrate
sizes among 25 measurement points in the quadrate
The number of materials larger than respective substrate sizes within eachquadrate.

Table 3. Observed sites and observations per unit effort (OPUE) of
bagrid catﬁsh for each size class.

perpendicular shorter diameter were measured for the substrates
larger than the size of a pebble, and the substrate embeddedness

Channel-unit
type

were classified as "over-lying" or "embedded" according to thepresence or absence of interstitial spaces underneath substrateGlidematerials, respectively. The total water depth at the points with

Pool

over-lying substrate was corrected by adding median diameter (theaverage of two diameters) to the depth measured from the water
surface.
Data analysis
The effects of ﬂoods on bagrid catﬁsh were evaluated at threespatial scales: reach (200-m reach), channel unit (rifﬂe, pool, andRifﬂeglide), and observed site (2-m quadrate area). Chi-square tests
were used with a signiﬁcance level of 0.05 to examine whether theeffects of ﬂoods on population composition differed among three

Observedsite No.
G1
G2
P3
P4
P5
P6
P7
P8
P9
P10
P11
R12
R13
R14
R15

Pre-ﬂood
survey

1

I

II1
I1
1

1

I , II , III
I1
I1
I1

Total

size classes at the reach scale, and to examine whether the effects

Post -ﬂood
survey

II1
I0.5
I0.5
I0.5
10

10.5

1

I1
I , II3, III1
I1
6

I1
I1

I0.5
7 15.5

Shelter typecobblecobbleboulderboulderboulderboulder
cobblecobblecobbleboulderboulder
cobblecobblecobblecobbleFish size classes (mm TL): Class I< 50mm TL, Class II= 60-110mm TL, and Class III>
120mm (Board of Education of Mie Prefecture, Mie Prefectural Science and Technology
Promotion Center 2003).
The OPUE is indicated as superscripts for ﬁsh size class symbols.

of floods on habitat selection differed among three channel-unit
types at the channel-unit scale. In the latter analysis, we used theobservations per unit effort (OPUE) for each channel unit rather
than raw values of individual counts, which was the case for theformer analysis; OPUE was calculated as individual counts per 4

either of pre- or post-ﬂood occasion. Principal component analysis

persons per 1 hour. Because of the small sample sizes for observed(PCA) with 15 environmental variables (see Table 2) was used tosites where individuals were persistently found over the floods,

characterize habitat environments of “persistent” sites relative to

we could not apply adequate statistical tests for the data at the“less persistent” sites in multivariate dimensions. Environmental

observed site.

variables were standardized for PCA. Ward’s method of cluster
analysis was applied to the PCA scores for all observed sites toFurthermore, we examined physical habitat environments of theclassify the microhabitat type.

observed sites that we arbitrarily defined as either“persistent”
or“less persistent”based on how bagrid catﬁsh responded to theﬂoods: “persistent” sites were the sites where the bagrid catﬁshwere found at both pre- and post-ﬂood occasions, whereas“lesspersistent”sites were the sites where individuals were found at21

Shiro SAGAWA et. al.
was particularly high at both pre- and post-flood occasions, also

Results

exhibiting more than a 3-fold increase over the ﬂoods.

General patterns of observations
Comparisons of the environmental characteristics between the

The bagrid catﬁsh were found at a total of 15 observed sites (Table“persistent” and “less-persistent” sites

3). The total value of OPUE amounts to 26.0, which translates

The first two PC axes together accounted for 72% of total

into observations of 28 individuals; 12 and 16 individuals wereobserved at pre- and post-flood occasions, respectively. At bothvariance in the data (Table 4). The environmental variables thatthe pre- and post-flood occasions, we observed synchronous

had relatively high relationships with PC1 (factor loading > 0.80)

occurrence of the individuals of three size classes at P5 (Pool 5).

were mean depth, mean substrate size, percentage of substrate area

At the channel-unit scale, the number of observed sites and OPUE(>boulder), percentage of over-lying substrate area (>boulder)were the highest in the pool compared to the riffle and glide.

and substrate numbers (>boulder). Percentage of substrate

The shelter type used by catﬁsh was limited to interstitial spaces

area (>pebble) was the only variable that had relatively strong

underneath substrate materials such as cobbles and boulders.

influence on PC2 (factor loading > 0.80). The“persistent”
sites (P5 and P6) and“less-persistent”sites (G1, R15, P4, P9,

Changes of the numbers of observed sites and OPUE over the floods

and P10) were plotted in the 1st quadrant and 2nd-3rd quadrants,

The reach-scale analysis showed no significant relationshiprespectively. The cluster analysis produced two clusters with thebetween the occurrence of floods and the observed number of

one composed of the“persistent”sites and the other composed of2

individuals in each size class (χ =0.233, P=0.890). As a result

the“less-persistent”sites (Fig. 5). As the plots of the two habitatof the channel-unit-scale analyses, no significant relationshiptypes (“persistent”and“less-persistent”sites were separated

were found between the occurrence of floods and the number ofmostly along the PC1, environmental variables associated withobserved sites or OPUE for each channel-unit (the number of

the PC1 were considered important in characterizing the degree22observed site, χ =0.580, P=0.748; OPUE, χ =1.220, P=0.543). At

of habitat persistence over the floods. In fact, environmental

the observed-site scale, ﬁve sites were newly observed to containvariables that had high factor loadings showed exceedingly highﬁsh in the study reach after the ﬂoods, whereas individuals werevalues for“persistent”sites relative to“less-persistent”sites

not found at eight of the previously observed sites (80% for all 10(Table 5). Other examples include substrate characteristics suchobserved sites; Table 3). Individuals were persistently found overas substrate size and the proportional area covered by relatively

the flood only at two observed sites (P5 and P6). OPUE of P5

large substrate materials. The values of size (>40cm) and area

Sagawa et al. Fig. 5

Table 4. The results of principal component analysis using the environmental
variables measured for each observed site at post-ﬂood survey
occasion. The numbers in bold type denotes factor loadings greaterthan 0.80.
Variables
Depth
Velocity
Substrate size
Substrate area

Over-lying substrate area

Substrate numbers

Percent of variance(cumulative %)meancomplexity
mean
complexity
mean
complexity>pebble>cobble>boulder>pebble>cobble>boulder>pebble>cobble>boulder3

Factor loading
PC1
PC2
0.81
0.12
0.22
-0.45
-0.67
0.61
-0.76
0.56
0.95
0.04
0.48
-0.34
0.04
0.87
0.61
0.66
0.95
-0.13
0.54
0.68
0.79
0.41
0.96
-0.11
-0.07
0.79
0.45
0.64
0.87
-0.20
46.00

G1

“Persistent” sites

2

P6

R15

1

P5

0

PC 2

P9

-1

P10

-2
-3
-4

P4

“Less-persistent”
sites
-4

-3

-2

-1

0

1

2

3

4

PC 1
Fig. 5 The biplot of PC1 and PC2 using the 15 environmental variables
measured for each observed site at the post-ﬂood survey occasion.
The cluster analysis produced two clusters, each of which was
enclosed with elliptical shaped lines.

72.00

22

5

Effects of catastrophic ﬂoods on Pseudobagrus ichikawai
Table 5. Comparisons of environmental variables between “less-persistent” and “persistent” sites. The ranges of values
were shown for “less-persistent” sites.
“Less-persistent”
sites(N = 5)
Water depth (cm)Current velocity (cm/sec)
Substrate size (cm)
Substrate area (%)

Over-lying substrate area (%)

Substrate numbers (inds)

mean
complexity
mean
complexity
mean
complexity>pebble>cobble>boulder>pebble>cobble>boulder>pebble>cobble>boulder

25-88
16-56
1-12
58-136
8-34
51-109
44-80
39-60
0-33
33-52
28-40
0-22
8-18
7-10
0-6

“Persistentsites(N = 2)
<
inc.
inc.
>
<
inc.
inc.
<
<
inc.
<
<
inc.
inc.
inc.

P5
104
50
2
50
71
87
78
70
52
65
61
48
12
10
6

-

P6
147
21
2
57
43
81
72
64
40
52
44
28
17
14
8

inc.: the range of values measured for “persistent” sites were included in or overlapped with the range of values for “less-persistent” sites.

covered by boulders (>40%) at“persistent”habitats (P5 and P6)

entire tributary (Sagawa, unpublished)．We also observed nowere concurrently higher than those at“less-persistent”sites. In

sediment deposition in this pool after the events, which probably

particular, the value at P5 was exceedingly high with substrate size

resulted in no significant alteration of the channel morphologyof 71cm and percentage of boulder substrate of 52%, respectively

or water depth. These observations suggest that a large and deep

(Table 5).

pool, especially in a small stream with interstitial spaces formed bylarge substrate materials, is crucially important as a refugia for the
catﬁsh.

Discussion
Effects of catastrophic floods on bagrid catfish

Bagrid catfish abundance and spatial distribution

The effects of high flow events on fishes have been studied

Before and after the ﬂoods, bagrid catﬁsh were more abundantat relatively large spatial scales. For example, individuals werein the pool that included shelters provided by cobble and boulder

perished or displaced to downstream areas (Harrell, 1978;

substrates. Our ﬁnding is similar to that of Shimizu and Shimizu

Matthews, 1986; Fritz et al., 2002; Tew et al., 2002), and changes(1982) and Shimizu (1988), who reported that bagrid catfish

in abundance and species richness of assemblages occurred (Taylor,

inhabited the moderate pools with cobbles of various sizes during

1997; Taylor and Warren, 2001; Franssen et al., 2006). In our

the daytime. Furthermore, at finer scales (observed-site scales),study, however, reach and channel-unit scale analyses indicated that

we found no bagrid catfish in areas such as shore vegetation

there were no statistically signiﬁcant changes in total abundance atand crevices on rock that have been described as preferredboth scales before and after the events, suggesting that the effects

microhabitats (e.g. Watanabe, 1994a). The absence of individualsof the ﬂoods on catﬁsh were negligible. Although we are aware thatin previously reported types of habitats can be partly explained

our sample size was quite low and the interpretation of PCA resultsby the fact that habitats such as shore vegetation is naturally

must be carefully made, the results indicate that the study reach

uncommon in the study reach where alluvial characteristics keep

was provided with sufficient refuge functions for individuals toshore vegetation from colonizing (Fig. 2). Overall, all individuals

persist through the catastrophic events. Intriguingly, we observed

were found underneath substrate materials, providing further

bagrid catﬁsh of all three size categories to have inhabited together

evidence that bagrid catfish is highly dependent on interstitial

the same interstitial spot at P5 characterized by having single

habitats.

substrate material (median particle diameters: 71 cm) remaining
Interstitial microhabitats as a flood refuge for bagrid catfish

in the same small area over the events. Furthermore, the maximum

The microhabitats, which persistently supported the catﬁsh overdepth of this pool was >5 m and its volume was the greatest in the23

Shiro SAGAWA et. al.
the floods, were limited to only two sites (20%) among all theof sediment pulse and movements of relatively large substrateobserved sites. In particular at P5, the abundance increased bymaterials over the events were not substantial. In contrast, themore than 3-fold over the events whereas most of the other sites

main-channel of the Miya River experienced tremendous changes

where ﬁsh were previously observed no longer held ﬁsh individuals

in channel morphology such as burial of pools and scouring ofafter the floods within the reach. It is difficult to fully attributerifﬂes in places largely because of sediment movements originating

this pattern to certain mechanisms; however, one possibility isfrom landslides and debris ﬂow this particular year (e.g. Hayashi

that certain microhabitats (i.e.“less-persistent”sites) were more

et al. 2004), which caused habitat loss and decreased bagrid catﬁshprone to floods relative to those with more persistent physicalindividuals by as much as 50 percent in the main-channel of thecharacteristics (i.e.“persistent”sites). Although our sample size

river (Kitamura 2008). Furthermore, similar habitat degradation

was quite low and the PCA conducted may not be powerful enough,after the same event diminished the adult bagrid catfish in thethese two types of observed sites seem to be distinguished by their

upstream reaches of the same tributary (Mizuno et al. 2007).

physical characteristics; the“persistent”sites were characterized

These results indicate that the presence of the uncommonly large

by greater areas covered by relatively large-sized substratepool functioned as a critically important microhabitat to prevent thematerials and greater water depth compared to the“less-persistent”

catfish’s local population from declining against the catastrophic

sites (Fig. 5 and Table 5). Although we have little quantitative

ﬂoods. If streams are to be restored for the conservation of bagrid

information as to the responses of riverbed materials to the ﬂoods,

catfish, creation of deep pools with substrate materials of sizes

the substrates of“persistent”sites P5 and P6 (median particle

that are larger than critical diameters for movement during severe

diameters: the former 71 cm, and the latter 43 cm) remained in thefloods might be effective. Furthermore, large substrate materials

same position over the events whereas there were apparent signs of

could be placed in spatial arrangements with which the plugging

bed movement in the rifﬂe and glide (cleansing of particle surfacesof interstitial habitats by sedimentation would be minimized.

and tumbling of certain particles). This agrees with the view that

Lastly, proper management of land use activities at the watershed

the riverbed materials in the rifﬂe and glide (8-34 cm), where all

scale is advisable to prevent excessive sedimentation in remaining

the“less-persistent”sites were found, moved during the event,distribution areas of bagrid catﬁsh.

taking into consideration that the critical diameter for sediment
movement was 50 cm in the study tributary (Tashiro et al., 2005).

Acknowledgements

Furthermore, the increase in the catﬁsh abundance at P5 over theﬂoods may imply that individuals might have aggregated at P5 to

The study was conducted as part of the project "Programs foravoid the impacts of events. The importance of habitat availability

Rehabilitation of Natural Habitats and Maintenance of Viable

within interstitial spaces underneath boulders has been reported

Populations of the Natural Monument Bagrid Catfish" by thefor the bagrid catﬁsh (Shimizu and Shimizu, 1982; Shimizu, 1988;

Board of Education of Mie Prefecture. We thank M. Saiki and K.

Watanabe, 1994a; Watanabe, 2008). However, these interstitial

Hasegawa for help in the field. We are grateful to Seiichi Mori,

cobbles and boulders habitats had not been studied with regard

Yasushi Harada, Katsutoshi Watanabe, Kazuyuki Muraoka, Futoshi

to their function during ﬂoods. Our ﬁndings suggest that a stable

Nakamura, and the handling editor for this journal, Yoshinori

riverbed with boulder clusters is important in providing hydraulic

Taniguchi for valuable comments throughout the study. We also

refugia for catﬁsh of various ages.

thank Brian Cullen (Nagoya Institute of Technology) who kindly
provided us with proofreading of the English text which greatly

Management implications for the conservation of bagrid catfish

improved correctness and clarity throughout the manuscript.

Persistence of catfish abundance over the events seemingly
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