Aquatic Invasions (2014) Volume 9, Issue 2: 221–226
doi: http://dx.doi.org/10.3391/ai.2014.9.2.11© 2014 The Author(s). Journal compilation © 2014 REABIC

Open AccessResearch Article

Replacement of nonnative rainbow trout by nonnative brown trout
in the Chitose River system, Hokkaido, northern Japan
Koh Hasegawa 1, *, Nobuo Ishiyama 2 and Hideyuki Kawai 2,†
1

Hokkaido National Fisheries Research Institute, Fisheries Research Agency, Nakanoshima, Toyohira, Sapporo, Hokkaido 062-0922, Japan
Department of Forest Science, Graduate School of Agriculture, Hokkaido University, Kita 9 Nishi 9, Kita-ku, Sapporo 060-8589, Japan†Present address: Department of Forest Management, Mitsubishi Materials Real Estate Corporation, Kita 22 Nishi 5, Kita-ku, Sapporo 0010022, Japan2

E-mail: hasekoh@affrc.go.jp (KH), night7mare@gmail.com (NI), yamameman55@yahoo.co.jp (HK)

*Corresponding author
Received: 1 March 2014 / Accepted: 26 April 2014 / Published online: 2 May 2014
Handling editor: Vadim Panov

Abstract
In this study, evidence for interspecific interaction was provided by comparing distribution patterns of nonnative rainbow trout
Onchorhynchus mykiss and brown trout Salmo trutta between the past and present in the Chitose River system, Hokkaido, northern Japan.
O. mykiss was first introduced in 1920 in the Chitose River system and has since successfully established a population. Subsequently,
another nonnative salmonid species, S. trutta have expanded the Chitose River system since the early 1980s. At present, S. trutta have
replaced O. mykiss in the majority of the Chitose River, although O. mykiss have persisted in areas above migration barriers that prevent S.
trutta expansion. In conclusion, the results of this study highlight the role of interspecific interactions between sympatric nonnative specieson the establishment and persistence of populations of nonnative species.
Key words: biological invasions, interspecific interaction, migration barrier, nonnative salmonids

Introduction
Interspecific interactions between nonnative andnative species play a key role in the decline ofnative species populations and the collapse ofnative ecosystems (Parker et al. 1999) and also
the establishment and/or persistence of nonnative
species (Southward et al. 1998; Melbourne et al.
2007). Since multiple nonnative species co-occur
in many regions, interspecific interactions between
nonnative species may also influence nonnative
species (Ricciardi and MacIsaac 2011). For
example, the invasional meltdown hypothesis,
which proposes that nonnative species facilitate
successful establishment of other nonnative species,
is a commonly accepted theory among ecologists(Simberloff and Von Holle 1999). Moreover, species
replacement is also likely to occur between theinteracting nonnative species; this process has
not been well evaluated to date.
A number of salmonid species have been
introduced into many nonindigenous regions, andhave had devastating effects on native species(Fausch 1988; Townsend 1996). One of the mostserious problems of the devastating effects is species
replacement of native salmonids by nonnative
salmonids (Fausch 1988). Several kind of interspecific interactions have been proposed as potential
mechanisms of replacement such as competition,
predation, hybridization and introduction ofparasites and disease (Krueger and May 1991).
Moreover, multiple nonnative salmonids co-occur
in many regions (e.g. Houde et al. 2014; Kitano
et al. 2014). Species replacement is also likely tooccur between nonnative salmonid species, if
they interact with one another. That is to say,interspecific interactions are closely related to theestablishment and persistence of populations ofeach nonnative salmonid species.
In Hokkaido, northern Japan, nonnative rainbow
trout, Oncorhynchus mykiss (Walbaum, 1792),
and brown trout, Salmo trutta (Linnaeus, 1758),
co-occur in some regions (Takami and Aoyama
1999). Both O. mykiss and S. trutta are listed
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among 100 of the world’s worst invasive alien
species by IUCN, and effective management
strategies are urgently needed for the purpose of
aquatic ecosystem conservation (Lowe et al. 2000).
Previous studies have noted that both species havesuccessfully established populations in stableflow streams such as spring-fed streams (Fauschet al. 2001; Inoue et al. 2009; Kawai et al. 2013).
In these streams, O. mykiss and S. trutta arelikely to successfully establish by competitive
exclusion of white-spotted charr, Salvelinus
leucomaenis (Pallas, 1814), a common native
salmonid species (Morita et al. 2004; Hasegawa
and Maekawa 2009). Conversely, habitat and/or
food niche segregation between both nonnative
salmonid species and native masu salmon,
Oncorhynchus masou (Brevoort, 1856), anothercommon native salmonid species, may also
facilitate successful establishment of nonnative
salmonid species (Hasegawa and Maekawa 2006;
Inoue et al. 2009; Hasegawa et al. 2010; Hasegawa
et al. 2012a). Interestingly though, the nature ofinteractions between O. mykiss and S. trutta have
not yet been evaluated.
This study documents the species replacementof nonnative O. mykiss by nonnative S. trutta in
the Chitose River system, southwestern Hokkaido
between the 1990s and 2010s, then discusses
how interspecific interactions may influence
theirpopulation
establishment
and/orpersistence.
Methods
Study area
The Chitose River system is located insouthwestern Hokkaido, northern Japan (Figure1). The river has several unpassable dams forfishes (Figure 1). Underground flow isolates apart of Mamachi stream (around reaches 13–15)(Figure 1). Since the 1970s, the BOD (biochemical
oxygen demand) values have been almost lower
than 2.5 mg/L in the study area (Ishikawa et al.
2013). Thus, water quality has been well enoughfor inhabiting salmonid species. Nonnative O.
mykiss had been introduced into the system from
1927 to 1955 as a result of national policy(Bureau of Fishery, Ministry of Agriculture andForestry of Japan 1927; Yoshizumi 2005). And
also, a fish hatchery beside a main stem of the
Chitose River (nearby study reach “p” in Figure1a) had kept O. mykiss until 1985 and someindividuals might have escaped from the hatcheryto the river (Yoshimizu 2005; Nogawa pers.
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comm.). As a result, a self-sustaining population
was confirmed in the early 1980s (Urawa 1989).
Subsequently, nonnative S. trutta were first recorded
in 1984 (Urawa 1989) and are now widelydistributed in the system (Kawai et al. 2013).
Personal transportation for recreational fishing is
considered a major cause of S. trutta introduction(Takami and Aoyama 1999). However, the introduction of S. trutta has been legally prohibited
for a decade (Shimoda 2012).
Fish distribution
The distribution of fish in the 1990s wasdetermined from existing literature (reaches e–s:
Chitose City and Research Institute of Takugin
1997; reaches a–d: Takayama et al. 2002). In thesestudies, some study reaches were not recorded indetail. For example, locations were recorded at
the spatial level of “tributary” only (square symbols
in Figure 1a). Fish sampling was conductedusing minnow traps, dip nets, and electrofishingunit. The presence or absence of a species wasrecorded in all studies.
The distribution of fish in the 2010s was
surveyed in 44 reaches. Two pass electrofishing(Smith-Root Inc., Vancouver, WA, USA) was
conducted in the tributaries of Chitose River andthe inlets of Lake Shikotsu (length 50–150 m). In
the main stem of Chitose River, electrofishing
was conducted along the river bank for 25 to 45
min (reaches 38–44). Thirty-five reaches (reaches 1–
5, 8–12, 16–22, 27–44) were surveyed from theend of May to the beginning of June 2011
(datasets on S. trutta and study reach environments
were converted from Kawai et al. 2013). Sevenreaches (reaches 13–15, 23–26) were surveyed in
the summer of 2013. The number of individuals
captured for each species was recorded. All S.
trutta and O. mykiss individuals were anesthetizedusing ethyl 3-aminobenzoate methanesulfonicacid, and measured to the nearest 1 mm (forklength). Then, young-of-the-year and the older
individuals were distinguished. In this study, weregarded that S. trutta and O. mykiss smaller than
50 mm and 60mm were young-of-the-year withcertainty, respectively. Young-of-the-year individuals represent evidence of successful establishmentof a population (Kolar and Lodge 2001; Kawai etal. 2013)
In two reaches (reaches 6 and 7), a fish survey
was conducted by underwater observation in the
beginning of October 2013. An observer (KH)
entered the stream during daytime at the lower
end of each site and crawled slowly upstream inReplacement of rainbow trout by brown trout

Figure 1. Distribution of Onchorhynchus mykiss and Salmo trutta in the Chitose River system in the (a) 1990s (circles and squares) and (b)2010s (triangles). Locations of major dams are marked by short black bars, and underground flow is represented by the dashed line. (a) The
study reaches for which the location is correctly identified are represented by circles, whereas those that could not be located accurately are
represented by squares. Slashed symbols: O. mykiss only; Black symbols: S. trutta only; Grey symbols: both O. mykiss and S. trutta; Whitesymbols: no O. mykiss or S. trutta were recorded.

a zigzag pattern (Thurow 1994). The number ofindividuals observed was recorded for each species.
Plots of each study reach were color-coded to
distinguish four patterns of fish distribution, i.e.
O. mykiss and S. trutta were sympatric, O.
mykiss or S. trutta were allopatric, and O. mykiss
and S. trutta were absence (Figure 1). Then, we
visually compared distribution patterns of O.
mykiss and S. trutta between 1990s and 2010s,
and evaluated the change of distribution patternsof the two species.

Results
Both O. mykiss and S. trutta were widely distributedin the Chitose River system in the 1990s (Figure1a). The two species were found sympatrically innine reaches (grey symbols in Figure 1a). In study
reach a–d, O. mykiss was dominant in abundance(Table 1). O. mykiss and S. trutta were foundindividually in five (slashed symbols) and three(black symbols) reaches, respectively (Figure 1a).
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Table 1. Number and proportion of Onchorhynchus mykiss and Salmo trutta in study reach a-d in 1990s. Data sets were originally shownin Takayama et al. (2002).
reach

a

b

c

d

O. mykiss
S. trutta

239 (63%)273 (62%)283 (99%)

30 (79%)142 (37%)

169 (38%)

3 (1%)

8 (21%)

Discussion

Figure 2. Body size composition of Onchorhynchus mykiss
captured in (a) Mamachi stream (reaches 13–15) and (b) Naibetsu
stream (reaches 23–25).

In the 2010s, S. trutta was still widelydistributed in the Chitose River system (Figure1b). Conversely, the distribution of O. mykiss
was mostly restricted to the area above themigration barriers (reaches 13–15, 23–25)(Figure 1b). Young-of-the-year O. mykiss were
captured in these areas (Figure 2). Although O.
mykiss were found together with S. trutta in two
reaches (grey symbols in Figure 1b), thepercentages of O. mykiss were only 2.9% (3individuals) (reach 5) and 1.3% (1 individual)(reach 26) (see supplementary material Table 1Sfor details). In Ninaru stream (reach 6) andOkotanpe stream (reach 7), only O. mykiss wereobserved by underwater observation.
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In the Chitose River system, nonnative O. mykiss
were widely distributed, and greatly exceeded S.
trutta in abundance until the 1990s (Urawa 1989;
Chitose City and Research Institute of Takugin
1997; Takayama et al. 2002). However, the current
study documented a dramatic decline of nonnative
O. mykiss from the 1990s to the 2010s, and thewidespread distribution of S. trutta in the 2010sin the river system. Thus, species replacement of
nonnative O. mykiss by nonnative S. trutta has
likely occurred. The end of cultivation in early
1980s (and also stocking) may cause the declineof O. mykiss (Yoshimizu 2005; Nogawa personalcommunication). Our results cannot rule out thepossibility that S. trutta successfully expanded
using niches occupied by O. mykiss. Additionally,
interspecific interactions between O. mykiss andS. trutta must be considered to understand themechanism of the species replacement.
Although piscivory by nonnative S. trutta
represents a potential mechanism to explain the
decline in O. mykiss distribution (Lowe et al.
2000), Hasegawa et al. (2012b) concluded thatfish were not primary prey of S. trutta in the
Chitose River system. Instead, Blanchet et al.
(2007) suggest that there is intense interspecificcompetition between O. mykiss and S. trutta due
to large niche overlap in an artificial channel. In
the case of the Chitose River system, therefore,
interspecific competition may have facilitatedspecies replacement of O. mykiss by S. trutta.
The early stages of their life history may playa key role in our understanding of the nature ofinterspecific interactions. Fausch (2007) notedthat recruitment of O. mykiss fry was hamperedby competition from S. trutta fry, which emerge
earlier than O. mykiss, and can therefore occupy
the preferred feeding habitat. However, there
appear to be conflicting outcomes from theirinteractions. For example, Scott and Irvine(2000) suggested that superimposition of spring
spawning O. mykiss on the redds of S. trutta(spawn in late autumn to winter) caused elimination

Replacement of rainbow trout by brown troutof the S. trutta population by damaging eggs ofS. trutta due to disturbance of spawning gravels.
In the Chitose River system, the former
hypothesis is likely to be more suitable for
explaining the observed results. However, there
is a clear need for further studies on the egg to
fry stages of both species, in order to confirm
this hypothesis.
The results of the survey in the 2010s fromreaches 13–15 (Mamachi stream) and 23–25(Naibetsu stream), where O. mykiss remained thesole inhabitants and their reproduction were
documented, suggest that O. mykiss populations
have been able to persist in areas where S. trutta
were unable to expand because of migration
barriers. Fausch et al. (2009) have noted that
increasing habitat connectivity can foster thewide-spread successful establishment of nonnative
species, a major threat to native species persistence.
Nonnative species that have negative effect onanother nonnative species may also suffer thesame risk associated with connectivity. These
results also support our hypothesis that interspecificinteraction likely caused the species replacementof O. mykiss by S. trutta.
O. mykiss also remained allopatrically in
Ninaru (reach 6) and Okotanpe stream (reach 7),
which are inlets of Lake Shikotsu. In thesestreams, S. trutta were also rare in 1990s in spiteof no migration barriers. Thus, the stream
morphology may be unsuitable for S. trutta rearingor spawning.
In conclusion, the results of this study suggestthat interspecific interactions have a significant
influence on population persistence and/or
establishment of nonnative species. In this study,
evidence for interspecific interaction and perhaps
exclusion was provided by comparing distribution
patterns of nonnative O. mykiss and S. trutta
between the past and present. To elucidate themechanism of the species replacement, further
studies focusing on interspecific interactions, forexample, experiments evaluating the effect ofinterspecific competition (Fausch 1998) are
required. Although water quality has been
sufficient for inhabiting salmonid species in the
study area (Ishikawa et al. 2013), other man-madechanges within stream environments such as lossof pool-riffle sequences due to channel alteration
may influence the outcome of interspecific interactions (Dunson and Travis 1991) and/or habitat
use of each species (Chen et al. 2014). It is likelythat these additional changes are also likely to
influence the distribution patterns of the two
nonnative salmonids in the Chitose River system.

Acknowledgements
We thank Futoshi Nakamura, Kazumasa Ohkuma, and Kazuo
Uchida for their support in planning and implementing this study.
We also appreciate Motohiro Kikuchi and Hideki Nogawa forproviding field information around the Chitose River, and two
anonymous reviewers for helpful comments on the manuscript.
This study was supported by Japan Society for the Promotion of
Science KAKENHI Grant Number 24780196 to KH.

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Supplementary material
The following supplementary material is available for this article.
Table 1S. Total number of fish captured (observed) for each fish species in each study reach.
This material is available as part of online article from:http://www.aquaticinvasions.net/2014/Supplements/AI_2014_Hasegawa_etal_Supplement.xls

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