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BioInvasions Records (2014) Volume 3, Issue 3: 175–184
doi: http://dx.doi.org/10.3391/bir.2014.3.3.07
© 2014 The Author(s). Journal compilation © 2014 REABIC
Open Access
175
Research Article
On the natural history of an introduced population of guppies (Poecilia reticulata
Peters, 1859) in Germany
Jonas Jourdan1,2*, Friedrich Wilhelm Miesen3, Claudia Zimmer2, Kristina Gasch4, Fabian Herder3,
Elke Schleucher4, Martin Plath1,2,5 and David Bierbach6
1Biodiversity and Climate Research Centre (BiKF), Senckenberganlage 25, 60325 Frankfurt am Main, Germany
2Goethe University of Frankfurt, Department of Ecology and Evolution, University of Frankfurt, Max-von-Laue-Straße 13,
D-60438 Frankfurt am Main, Germany
3Zoologisches Forschungsmuseum Alexander Koenig, Sektion Ichthyologie, Adenauerallee 160, D-53113 Bonn, Germany
4J.W. Goethe-University of Frankfurt, Department of Integrative Parasitology and Animal Physiology, Max-von-Laue-Straße 13,
D-60438 Frankfurt am Main, Germany
5College of Animal Science and Technology, Northwest A&F University, Yangling, Shaanxi 712100, P.R. China
6Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Department of Biology and Ecology of Fishes, Müggelseedamm
310, D-12587 Berlin, Germany
*Corresponding author
E-mail: JonasJourdan@googlemail.com
Received: 26 March 2014 / Accepted: 22 July 2014 / Published online: 11 August 2014
Handling editor: Vadim Panov
Abstract
Artificially heated water bodies represent unusual habitats in temperate regions and form a refuge for exceptional fish communities. The
Gillbach, a tributary of the river Erft in Germany, receives thermally polluted cooling water from a power plant. Here, we present data on the
composition of the fish community in the Gillbach and found a high abundance of invasive species from all over the world, mostly
introduced by releases from home aquaria. We found a species composition that is dominated by invasive species containing the same
species as 15 years ago. We focused on guppies (Poecilia reticulata) and determined population size using the mark-recapture method.
Furthermore, we investigated the lower thermal tolerance limit (CTmin) to determine if Gillbach guppies have already adapted to colder
conditions compared to ornamental and Venezuelan wild type fish. We caught guppies of all sizes, and densities of 3.6 adult guppies per
square meter were comparable to densities found in their natural distribution area, pointing toward a self-sustaining population in the
Gillbach. The CTmin varied between populations and was significantly lower in ornamental and Gillbach guppies compared to guppies from
Venezuela. Despite differences in CTmin and their well-known potential to adapt to new environments, guppies originally stem from the
tropics, and a further spread will likely be restricted by low winter temperatures. Thus, P. reticulata in the Gillbach might not represent a
threat for local fauna in Central Europe, but provide a unique semi-natural experiment for various questions related to local adaptation of
invasive species, as well as ecological interactions with indigenous species.
Key words: Amatitlania nigrofasciata, Europe, exotic species, invasive, Rhine drainage, thermal pollution, thermal tolerances
Introduction
Alien species are among the major drivers of
species extinctions and, thus, loss of biodiversity
(Millennium Ecosystem Assessment 2005),
especially in freshwater ecosystems (Mack et al.
2000). In the European Union alone, 12,122 non-
native species have been reported so far (DAISIE
European Invasive Alien Species Gateway 2013).
However, not all of these are predicted to
reproduce and expand their current distribution
ranges (‘invasive alien species’, IAS; Williamson
and Fitter 1996; Sakai et al. 2001). Nevertheless,
even species that are not currently assumed to
successfully reproduce in their new environments,
or have a very localized occurrence, may
occasionally overcome reproductive constraints –
and thus reach IAS status – due to global warming,
niche shifts, or local adaptation to altered environ-
mental conditions (Whitney and Gabler 2008).
While most species introductions are accidental
(Mack et al. 2000), several active introductions
J. Jourdan et al.
176
have also been documented. An example is the
introduction of Nile perch (Lates niloticus Linnaeus,
1758) into Lake Victoria, with its disastrous
consequences for the endemic fish fauna (Ogutu-
Ohwayo 1990; Seehausen et al. 1997; Goldschmidt
1998). Live-bearing fishes of the family Poeciliidae
have been widely introduced to tropical and sub-
tropical countries for malaria prophylaxis, i.e., to
help control vector (mosquito) populations
(Stockwell and Henkanaththegedara 2011). In
addition, some poeciliids like guppies (Poecilia
(Acanthophacelus) reticulata Peters, 1859), sword-
tails and platyfish (Xiphophorus spp.), and mollies
(Poecilia (Mollienesia) spp.) are among the most
popular ornamental fishes, and many introductions
may have been the result of occasional releases
from home aquaria (Padilla and Williams 2004;
Gozlan et al. 2010a; Stockwell and Henkanaththe-
gedara 2011; Strecker et al. 2011). The ability to
store sperm from multiple mates for several
months secures several consecutive broods and
allows a single gravid poeciliid female to found
an entire new population (Zane et al. 1999; Evans
and Magurran 2000).
Guppies are native to northern South America
between Venezuela and northern Brazil, and to
several nearby islands like Trinidad and Tobago
(Rosen and Bailey 1963; Magurran 2005). Studies
on natural populations of Trinidadian guppies
reported on geographical variation in morphological,
behavioral and life history characteristics,
primarily explained by variation in predator
regimes (Magurran et al. 1995; Magurran 2005).
Within few generations after the exposure to an
experimentally altered predator regime, guppy
populations responded with an earlier onset of
sexual maturity coupled with smaller offspring
size at birth (high predation), or delayed onset of
sexual maturity and increased offspring size at
birth (low predation; Reznick et al. 2008). This
ability to rapidly respond to altered selective
regimes highlights the invasive potential of
guppies (Magurran 2005; Deacon et al. 2011).
Guppies have broad environmental tolerances
and can withstand – at least for short periods of
time – marine salinity (Chervinski 1984), as well
as temperatures dropping to 12°C (Fujio et al.
1990) or rising to over 40°C (Chung 2001). This
renders a wide range of habitats suitable for
guppies and non-native guppy populations are
currently reported from at least 69 countries in
North and South America, Europe, Asia, Austral-
asia, and Africa (Deacon et al. 2011). However,
in contrast to Eastern mosquitofish (Gambusia
holbrooki Girard, 1859) that were actively released
in southern Europe during the 20th century for
mosquito prophylaxis, and are nowadays present
in virtually any southern European freshwater
system (Vidal et al. 2010; pers. obs. for Italian,
Spanish and southern French streams), guppies
are not widely established in Europe. Exceptions
are some isolated populations in a few southern
European rivers probably established in recent
years (Elvira and Almodovar 2001). Due to their
native distribution in the tropics, low winter
temperatures prevent self-sustaining populations
in large parts of Europe. Nonetheless, there are
occasional reports of small populations in Canada,
Russia and parts of northern Europe, but these
inhabit either geothermal springs or water bodies
with artificially increased water temperatures
due to thermal pollution arising through influx of
cooling water from power plants or surface
mining (Arnold 1990; Deacon et al. 2011). Such
permanently warm refuges, however, might serve
as source populations from where individuals
might start spreading into hitherto uninhabited
areas following adaptation to cooler conditions
(Klotz et al. 2013) or elevated temperatures as a
result of climate change (Rahel and Olden 2008;
Walther et al. 2009; Wiesner et al. 2010; Bellard
et al. 2013).
The present study reports on an artificially
heated ecosystem, the upper Gillbach, that is
verifiably inhabited by a guppy population since
the mid-1970s (Kempkes 2010) and receives cooling
water from a power plant. Although other non-
native fish are regularly found in the Gillbach
(Höfer and Staas 1998), we focused on the guppy
due their status as a model species in evolutionary
ecology (Magurran 2005; Evans et al. 2011), their
well-known success as an invasive species in
other parts of the world (Deacon et al. 2011) and
the present lack of information regarding their
invasiveness for central Europe (Nehring et al.
2010). Our major aims were (1) to provide an
overview of the fish community found in the
Gillbach 15 years after the last survey (Höfer
and Staas 1998), (2) to estimate the population
size of guppies using mark-recapture analysis,
and (3) to evaluate whether guppies from the
Gillbach have already been adapted to colder
conditions and tolerate a lower critical minimum
temperature (CTmin) compared to guppies from
Venezuela and domesticated ornamental guppies.
Our study – even though largely descriptive – is
intended as a primer to future projects assessing
the status (and invasion potential) of guppies in
Central Europe.
On the natural history of an introduced population of guppies
177
Materials and methods
The Gillbach is a stream located west of Cologne
in North Rhine-Westphalia (Germany), and
meanders for approximately 28 km before it
drains into the river Erft, a Rhine tributary
(Figure 1). It receives thermally polluted water
from the coal-fired power plant “Niederaußem”
(50°59'46.82", 6°39'50.56", RWE Power Inc.).
The Gillbach is approximately three meters wide
and 30–80 cm deep. We measured abiotic water
parameters (pH, conductivity, dissolved oxygen
and temperature) with a Multi-Parameter Meter
(HQ40d Portable Meter, HACH, Loveland, USA)
approximately 100 m downstream of the influx
pipe at least once a month between August 2011
and April 2012.
To characterize the fish community in the upper
Gillbach, we performed electrofishing during 4
days in June 2013 along the first 400 meters
downstream of the influx pipe (section I; Figure
1), the next 400 m downstream (section II), and
for several hundred meters further downstream
using a portable electro fishing device (Hans
Grassl IG200-1).
First reports of introduced guppies date back
to the 1970s, when first individuals were initially
released to the Gillbach by hobbyist fish breeders
(Kempkes 2010). Although we assume this feral
population has been derived from a mixture of
various domesticated ornamental strains, guppies
nowadays caught in the Gillbach show an amazing
male color polymorphism typically found in
natural guppy populations (Figure 2; Haskins and
Haskins 1951; Endler 1983; Houde 1997; Brooks
2002). Electrofishing is not an ideal method for
catching guppies, mainly due to their small size.
To estimate the population size of guppies, we,
thus, used a standard mark-recapture approach
(Reznick et al. 2001). Guppies were collected in
June using a seine (2 mm mesh width) and dip
nets along sections I and II. Only adult fish were
considered in this approach. Females were included
if their body size exceeded 10 mm standard
length (SL), they appear to be gravid and they
had a clearly visible dark gravid spot above the
anal fin, while maturity in males was evaluated
by inspecting their gonopodium (the transformed
anal fin that develops into a copulatory organ at
maturation) and checking for color ornaments on
the body. Fish were transferred into well-aerated
coolers and subsequently anesthetized with clove
oil. Anesthetized fish were marked individually
with color polymer tags and transferred to a tank
with aerated water for recovery for at least half
an hour. No mortality was detected. Recapture took
place one week later at the same sites and with
comparable sampling effort. At both capture
events, body size of adult fish was measured to
the nearest millimetre using plastic rulers.
We initially captured 93 adult males and 145
adult females, all of which were marked and
released on the same day. During the second capture
event, a total of 131 males and 235 females were
captured, of which 7 males and 13 females were
recaptured individuals. For the estimation of
population size (with 95% CI), we used the R
package Rcapture (Baillargeon and Rivest 2007;
R_Core_Team 2013) assuming a closed population
(Mt model). For the Gillbach, this assumption
seems reasonable, at least over short periods of
time, as the sampling area starts at the influx
pipe of the power plant and is confined downstream
by a railroad tunnel (Figure 1).
For the measurement of lower thermal tolerance
limits (CTmin), guppies were collected from the
Gillbach, and carefully transported to the
laboratory of the University of Frankfurt/Main.
Fish were acclimated to the laboratory conditions
for three months before the measurements started.
We further included a color polymorphic stock
of guppies derived from various ornamental
strains and a stock of guppies descended from
animals imported from Venezuela by Aquarium
Dietzenbach. All fish were maintained in mixed-
sex stock tanks (80 to 180-l) at a constant
temperature of 28°C under a 12:12 h illumination
cycle. Tanks were equipped with natural gravel,
internal filters, as well as stones and artificial
plants for shelter. Fish were fed twice daily with
commercial flake food. For the investigation of
CTmin, we concentrated on females and followed
the protocol provided by Bierbach et al. (2010).
Test fish were acclimated to 25°C prior to the
experiments in 60-l tanks for at least two weeks.
The test apparatus consisted of a 10-l test tank
connected to a circulating pump with an internal
cooling aggregate. An air-pump ensured saturated
oxygen concentrations throughout the tests. We
gently introduced a test fish and started to
decrease water temperature at a constant rate of
0.780±0.007°C min-1 once the fish was swimming
calmly. Down-regulation of the water temperature
was aided by the addition of ice cubes every two
minutes. We noted at which temperature the test
fish lost motion control as a proxy for the test
subjects’ absolute physiological tolerance. Directly
after the trials, fish were transferred into an
aerated 10-l tank in which temperatures were
gradually increased again to 25°C. All test fish
J. Jourdan et al.
178
Figure 1. Overview of the study area. Schematic view of the watercourses (blue) and electrofishing sites (I and II) in the Gillbach (North
Rhine-Westphalia, Germany).
Figure 2. Variation in body coloration in a sample of males caught from the Gillbach. Photographs by the authors.
regained motion control within few minutes, and
no mortality was associated with this experiment.
After completion of a trial, test fish were weighed
to the nearest 0.1 g using a Sartorius PT 600 scale
(accuracy ± 0.1%). In order to compare CTmin
among populations, we used a linear mixed
model with population as a fixed factor and log-
transformed body mass as a covariate. The
interaction term ‘population by body mass’ was
not significant (F2,22=2.77, P=0.085), but since
the Akaike’s Information Criterion (AIC) increased
by 21.6% in the simplified model, we used the
On the natural history of an introduced population of guppies
179
more complex model retaining the interaction
term. To identify significant differences between
populations, we used LSD tests for pairwise post
hoc comparisons of the estimated marginal
means derived from our model where body mass
was adjusted to -0.29 log(g). The analysis was
conducted using SPSS 22 (SPSS Inc. 2013).
Results
Fish community
Electrofishing revealed 11 different fish species,
seven of which were non-native (Table 1). The
most abundant native species were chub (Squalius
cephalus Linnaeus, 1758) and barbel (Barbus
barbus Linnaeus, 1758). Guppies were the most
abundant non-native species, but – given the
high capture success during the mark recapture
approach (see below) – were clearly under-
represented during electrofishing. Beside guppies,
six other non-native species were recorded
(Table 1), of which the Central American convict
cichlid (Amatitlania nigrofasciata Günther, 1867)
was the most abundant.
Sex ratio, population size and body size
distribution of guppies
The sex ratio, combined from both capturing
events, was female-biased (#males/#females =
0.58). We estimated a total population size of
4,305 (95% CI: 2,963–6,726) adult guppies,
translating into an approximated average density
Figure 3. Body size distribution of (a) male and (b) female
guppies collected in the Gillbach.
of 3.6 adults per square meter. Males showed a
very narrow body size range with a mean SL of
16.2 mm (95% CI: 16.0‒16.4 mm) while female
body size was much more variable, with a mean
SL of 19.7 mm (95% CI: 19.2–20.1 mm; Figure 3).
Abiotic habitat characteristics
As a consequence of a constant warm water
discharge from the power plant, abiotic water
parameters (temperature, pH, specific conductivity
and dissolved oxygen) remained stable during
the winter months (Table 2) and water temperatures
Table 1. Fish communities in the Gillbach inferred by electrofishing in June 2013.
Species Origin I II
Further down-
stream Total
Cichlidae
Amatitlania nigrofasciata (Günther, 1867) Central America 11 6 3 20
Oreochromis sp. Africa 6 0 1 7
Loricariidae
Ancistrus sp. South America 1 2 10 13
Cyprinidae
Barbus barbus (Linnaeus, 1758) native 2 1 37 40
Carassius auratus (Linnaeus, 1758) Ornamental fish 0 5 0 5
Chondrostoma nasus (Linnaeus, 1758) native 0 0 1 1
Cyprinus carpio (Linnaeus, 1758) Asia 0 0 2 2
Gobio gobio (Linnaeus, 1758) native 0 0 22 22
Pseudorasbora parva (Temminck & Schlegel, 1846) Asia 1 0 0 1
Squalius cephalus (Linnaeus, 1758) native 6 12 70 88
Poeciliidae
Poecilia reticulata (Peters, 1859) South America 10 20 11 32
Total 37 46 148 224
J. Jourdan et al.
180
Table 2. Fluctuation in temperature and water chemistry of the Gillbach in 2011 and 2012.
Date Daytime t [°C] pH
Specific conductivity
[µS/cm] DO [mg/L] DO saturation
[%]
Sept. 2011 11:30 23.2 8.10 1815 8.96 109.06
Oct. 2011 11:30 21.3 8.06 1877 8.46 99.16
Nov. 2011 12:00 19.4 8.34 1741 9.21 104.06
Dec. 2012 11:30 20.8 8.45 1778 9.14 98.13
Jan. 2012 16:00 19.0 8.28 1654 9.46 106.04
Feb. 2012 16:00 22.0 8.43 1915 8.73 103.72
Mar. 2012 14:30 23.1 8.46 1906 8.67 105.14
Apr. 2012 16:00 23.0 8.36 2026 8.30 100.50
never dropped below 19°C at the core area around
the water influx. However, additional measurements
approximately 2 km downstream in Rheidt
(+51°00'50.88", +6°41'03.3") revealed a decline
to 13.7°C in February 2012.
Thermal tolerances
Our linear mixed model detected a significant
difference in CTmin between populations
(F2,22=5.484, P=0.012). Venezuelan guppies had
a CTmin of 14.6 ± 0.6°C (mean ± SE), while it
was significantly lower in fish from the Gillbach
(12.4 ± 0.4°C; Post-hoc LSD test: P=0.007) and
the ornamental population (12.5 ± 0.2°C; Post-
hoc LSD test: P=0.004; Figure 4). The Gillbach
and ornamental populations did not differ
significantly in their thermal tolerance (Post-hoc
LSD test: P=0.842).
Discussion
Due to a constant warm water influx from the
coal power plant ‘Niederaußem’, the Gillbach
serves as a refuge for many tropical fish species
and seven out of eleven species found in our
survey are non-natives. Although most introduced
species fail to establish self-sustaining populations
(Williamson and Fitter 1996), high local densities
and the presence of juveniles are indicators of a
well-established population of guppies (P.
reticulata) in the Gillbach. This assumption is
further underpinned by consistent reports on guppies
that date back to the mid-1970s (Kempkes 2010)
and their wild-type-like morphology. Likewise,
convict cichlids (A. nigrofasciata) were reported
already 15 years ago (Höfer and Staas 1998) and
different age-classes have been found in the current
study. Beside guppies and convict cichlids, the
Figure 4. Lower critical minimum temperature (CTmin) of
guppies from the Gillbach, an ornamental strain and descendants
of wild-caught fish from Venezuela. Shown are estimated
marginal means (EMM) from a linear model with (log-
transformed) body mass as covariate. Different letters indicate
significant differences in post-hoc LSD tests.
survey from 1998 reported on other tropical species
like Lake Malawi cichlids (Pseudotropheus sp.),
as well as two additional undetermined represen-
tatives of the family Poeciliidae inhabiting the
Gillbach. Native fishes recorded in 1998 were
chub (Squalius cephalus), gudgeon (Gobio gobio
Linnaeus, 1758), barb (Barbus barbus) and
European eel (Anguilla anguilla Linnaeus, 1758).
In our present study, we did not detect European
eel or Lake Malawi cichlids, and the guppy was
the only representative of poeciliid fishes. The
undetermined poeciliids found in 1998 were presu-
mably released by private pet fish keepers but,
unlike guppies, failed to establish. Nevertheless, in
addition to the previous report, we found a single
specimen of common nase (Chondrostoma nasus
Linnaeus, 1758), and two specimens of common
On the natural history of an introduced population of guppies
181
carp (Cyprinus carpio Linnaeus, 1758). Like in
1998, we also caught one specimen of the Asian
cyprinid Pseudorasbora parva Temminck &
Schlegel, 1846, which has been accidently
introduced in the 1960s with translocations of
cyprinids for aquaculture, and is nowadays widely
established in Europe (Kottelat and Freyhof 2007;
Gozlan et al. 2010b). Interestingly, our survey
found several adult individuals, breeding females
as well as juveniles, of the mouthbrooding African
cichlid Oreochromis sp., well known as ‘tilapia’
in aquaculture. This species was deliberately intro-
duced throughout the world to facilitate aquaculture
development, and is nowadays invasive in many
tropical countries, but has failed to establish in
Europe (Canonico et al. 2005; Garcia-Berthou et
al. 2005). In the Gillbach, the occurrence of
breeding adults and juveniles suggests a stable
population; founder individuals most likely
stemmed from a recently closed aquaculture
facility that used the power plant’s cooling
water. Beside the variety of invasive fish
species, also the invertebrate fauna of the
Gillbach is affected by exotic species. Recently,
Klotz et al. (2013) reported on two invasive species
of freshwater shrimps (Neocaridina davidi
Bouvier, 1904 and Macrobrachium dayanum
Henderson, 1893) from Asia in this stream.
Our density estimates of 3.6 adult guppies per
m2 are similar to those reported on Trinidadian
populations. Reznick and Endler (1982) found
slightly lower densities (approximately 2 guppies
per m2) in high predation sites (“Crenicichla-
sites”) and higher densities (app. 9 guppies per
m2) in low predation sites (“Rivulus-sites”),
while a subsequent study reported densities of
approximately 4 individuals per m2 at both high
and low predation sites (Reznick et al. 2001).
Similar to the Gillbach population, adult sex
ratios in Trinidadian guppy populations are often
female-biased, since males likely face higher
predation rates (Magurran 2005; Arendt et al.
2014).
Our data on adult body size distributions
reflect the findings from previous studies (Endler
1995). Like in many other poeciliids (Hughes
1985; Plath et al. 2003), male Trinidadian guppies
are smaller (SL: 13–19 mm) than females (18–24
mm; Magurran 2005) and wild-type guppies are
usually smaller than fish from domesticated
strains (males: 21.5–27.5 mm, females: 23.8–35.5
mm; Zimmer et al. 2014). Predation has been
identified as a major selective force for body size
evolution, whereby high predation rates select
for smaller body size in Trinidadian guppies. For
example, Reznick and Endler (1982) reported on
body lengths of 14.88 ± 0.10 mm (SL ± SE) for
males caught at ‘high predation’ sites with
abundant cichlid predators and 16.42 ± 0.14 mm
for ‘low predation’ sites without predatory cichlids
present (Reznick and Endler 1982).
Exposure to predation has a strong effect on
virtually all aspects of a population’s biology, as
selection from predation is a powerful driver of
behavioral, morphological and life-history trait
evolution (Endler 1995). Predation also influences
male ornamentation (Endler 1980), and guppy
males from ‘high predation’ sites are often less
conspicuous and bright (Rodd and Reznick 1997;
Magurran 2005). Even though body coloration
was not quantified in the present study, we found
color polymorphic male phenotypes qualitatively
resembling those found in natural guppy popu-
lations. Investigations of Gillbach guppies in the
1970s reported males carrying traits typical for
ornamental fish, like elongated fins and single-
color morphs (Kempkes 2010), and further
releases of ornamental breeds over the last years
cannot be ruled out. However, these forms seem
to have disappeared, leaving polymorphic traits
similar to native guppies from Trinidad (Endler
1983) or feral guppies from Japan (Karino and
Haijima 2001) with a high variation of dorsal
and caudal fin lengths and color spot patterns. As
a logical extension of this interpretation, we
argue that the Gillbach population likely faces
predation pressures comparable to natural
Trinidadian and South American ‘high predation’
populations (Reznick and Endler 1982; Figure 2) –
most likely by piscivorous species like the native
S. cephalus and B. barbus, as well as the invasive
A. nigrofasciata. Furthermore, bird predation
(e.g. by Alcedo atthis Linnaeus, 1758 or Ardea
cinerea Linnaeus, 1758) is expected to occur. An
alternative explanation is that the ornamental guppy
strains are not hardy enough (i.e. insufficient
thermal tolerance, handicapped due to elongated
fins), so that only wild-type strains survived.
With a minimum water temperature of 19.2°C
in January 2012, and a total annual temperature
range of only 4.2°C, the Gillbach provides suitable
temperature conditions for guppies along its first
few kilometers. The constant outflow of cooling
water provides even more stable conditions than
in some natural Trinidadian habitats, where water
temperatures can be highly variable (up to 7°C
per day, Reeve et al. 2014). Nevertheless, water
temperatures dropped to 13.7°C within the first 2
km in February 2012 and even though Gillbach
guppies (and ornamental breeds) tolerated a lower
J. Jourdan et al.
182
critical minimum temperature (CTmin) than fish
from a wild-type Venezuelan stock, it is unlikely
that guppies will survive outside the core area
close to the warm water influx during winter.
While increasing air temperatures are expected
for the Rhine basin as an effect of climate
change (up to 2.3°C in the lowland area during
winter, projected to the year 2050; Middelkoop et al.
2001), water temperatures sometimes decrease to
less than +1°C in the river Rhine (e.g. February
2012; measuring station Mainz-Wiesbaden, HLUG
2013). Even if climate change alters pathways of
invasive species and modify ecological impacts
(Rahel and Olden 2008; Walther et al. 2009), the
overwinter survival of guppies should not be
possible in German river areas without warm
water influx. Poeciliids exhibit a great potential
to adapt to new environments (Meffe and Snelson
1989; Stockwell and Henkanaththegedara 2011),
and the CTmin of the Gillbach population recorded
here is congruent with another study reporting on
some domesticated guppy strains that tolerate
temperatures of 12°C for at least 24 h (Fujio et
al. 1990). However, guppies stem from the tropics
and a further spread in Central Europe will
probably be restricted by low winter temperatures.
Similarly, the survival of the other non-native
tropical fishes found in the Gillbach can be assumed
to fully rely on the power plant’s cooling water
discharge.
In summary, the Gillbach is characterized by
an unusual species composition, dominated by
invasive species that established stable populations
in the artificially heated creek. The guppy popu-
lation consists of more than 4000 individuals in
the core area around the water influx. The source
of the guppies is suspected to be ornamental
animals; nevertheless, we could not find evidence
for adaptation to lower temperatures, as no
difference in CTmin between ornamental- and
Gillbach guppies was detected. The establishment
of introduced species is influenced by many
factors. Understanding the complex interactions
between the invading species and the recipient
environment is a fundamental challenge to
ecologists and conservation managers (Mooney
and Cleland 2001; Hayes and Barry 2008).
Beside the limited invasive potential, the
Gillbach population of introduced guppies with
its assumed ‘spread and diminish’ characteristic
may provide a fruitful semi-natural experiment
for questions related to local adaptation of
invasive populations and ecological interactions
with indigenous ones. Even though the risk of a
further spread of guppies in Central Europe may
seem unlikely, we recommend the continuous
monitoring of this system with a special focus on
changes in the invasive status of the species
inhabiting the Gillbach, since the thermal gradient
is connected directly to native ecosystems and
may serve as a source habitat for species invasions.
Acknowledgements
The present study was prepared at the Biodiversity and Climate
Research Centre (BiK-F), Frankfurt a.M., and financially
supported by the research funding program “LOEWE –Landes-
Offensive zur Entwicklung Wissenschaftlich-ökonomischer
Exzellenz” of the Hessian Ministry of Higher Education,
Research, and the Arts as well as by the Leibniz Competition
(SAW-2013-IGB-2). We thank all participants of the student class
Experimental Ecology for help with the mark-recapture analysis.
The authors would like to thank U. Rose (Erftverband and
Erftfischereigenossenschaft) for supporting this study and for
providing permits to conduct surveys at the Gillbach, as well as F.
Wegmann (Untere Fischereibehörde Rhein-Erft-Kreis) for the
electrofishing permit. The thermal tolerance data was collected as
part of the university class in Frankfurt ‘Kompaktveranstaltung
Tierversuchspraktikum’ (F 69/Anz. 29 (§10)). We further thank
three anonymous reviewers for their valuable comments that
helped to improve the manuscript substantially.
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