Table Of ContentBehavioural Responses of Two Native Australian Fish Species
(Melanotaenia duboulayi and Pseudomugil signifer) to
Introduced Poeciliids (Gambusia holbrooki and Xiphophorus
helleri) in Controlled Conditions
Kevin Warburton and Christine Madden
Department ofZoology and Entomology, University of Queensland, St. Lucia, Queensland 4072, Australia
Warburton, K. and Madden, C. (2003). Behavioural responses of two native Australian fish species
{Melanotaenia duboulayi and Pseudomugil signifer) to introduced Poeciliids {Gambusia holbrooki and
Xiphophorus helleri) in controlled conditions. Proceedings ofthe Linnean Society ofNewSouth Wales 124,
115-123.
Experimentaltreatmentstocomparebehaviouralresponsesincludednativefishspeciesonly,natives
plusoneexoticspeciesandnativesplusbothexoticspecies.Themosquitofish,Gambusiaholbrookifrequently
attacked both native species, but tended to nip Melanotaenia duboulayi (especially small individuals) and
chase Pseudomugil signifer The frequency ofattacks by G. holbrooki on M. duboulayi rose when all four
fish species were present. When food was added, aU four species showed a strong increase in aggression,
especially in the four-species treatment, where there were significant increases in the frequency ofattacks
bytheswordtailXiphophorushellerionM. duboulayandbyM. duboulayionG. holbrooki,andofconspecific
attacks by M. duboulayi. Increased attack frequency was associated with aggregation closerto the water's
surface,regardlessofthepresenceoffood. Theresults supportthehypothesisthatuittoducedpoeciliidscan
havedeleteriouscompetitiveeffectsonnativespecies. However,whilejuvenileM. duboulayiwereparticularly
vulnerable to the secondary effects offin-nipping, P. signifer appeared to be more susceptible to physical
displacement and reduced food capture success.
Manuscript received 16 April 2002, accepted for publication 21 December 2(X)2.
KEYWORDS:
rainbowfish, blue-eye, poeciliids, mosquitofish, aggression, displacement
INTRODUCTION on other fish through interference competition,
resource competition and predation (Lloyd 1990).
Prominent among the species of freshwater Gambusia holbrooki are aggressive towards other
fish that have been introduced to Australia are the species, and damage to fins and scales resulting fi-om
poeciliids Gambusia holbrooki (mosquitofish) and nipping by G. holbrooki (Lloyd 1987; Howe et al.
Xiphophorus helleri (swordtail). Gambusia holbrooki 1997J, coupled with secondary infection, can prove
is widespread and prolific throughout much of fatal. Such aggressive behaviour by Gambusia may
AustraUa (Arthington 1991). Xiphophorus helleri is cause physiological stress, and in topminnows
locally abundant in south-eastern Queensland {Poeciliopsis occidentalis) it leads to reduced rates of
(Arthington et al. 1983, McKay 1989, McDowall feeding, fecundity and survival (Schoenherr 1981).
1996), and there is a strong chance of further spread Gambusia holbrooki may also consume the larvae of
of this species in tropical and subtropical Australia Pseudomugilsignifer, melanotaemids(rainbowfishes)
&
(Arthington 1991). and other Australian native fish species (Aarn
Expanding ranges of introduced fishes and Ivantsoff 2001).
X
widespread declines of native populations have Theecologicalimpactsof helleriarepoorly
heightened interest in interactions between non- understood (Arthington 1991). Xiphophorus helleri
indigenous and resident species. Although the is primarily herbivorous (Arthington et al. 1983;
underlyingmechanismshavebeenpoorlydocumented, Arthington 1989) and its dietis likely to overlap more
it is likely that Gambusia species exert diverse effects with small omnivores (e.g. Melanotaenia duboulayi,
RESPONSE OF NATIVE AUSTRALIAN RSH TO INTRODUCED POECILIIDS
Arthington 1992) than insectivorous carnivores (e.g. net. Individuals of these four species were transferred
P. slgnifer). Xiphophorus helleri can occur at very to a laboratory at the University ofQueensland, where
highdensities in shallowAustraliancreeks (pers. obs.), they weremaintained in filtered, single-species aquaria
and it is possible that this may lead to enhanced (tankvolume0.05 m^;c. 50fishpertank) for 1-3 weeks
interspecific resource competition through the local prior to experimentation. During this time they were
depletion of invertebrate prey. Xiphophorus helleri fed on a commercial fish flake diet, which was the
and G. holbrooki often co-occur (Arthington et al. food used in the experiments. The tanks were filled
1983), and the pressure of high numbers ofthese two withagedtapwaterandmaintainedunderartificial light
species together could subject native species to (L:D 12:12) at a constant temperature of 23 "C
significant stress (McKay 1978, cited by Arthington
1991; Howe etal. 1997). Experiment 1. : Effects of species composition on
Several groups ofAustralian native fish tend behavioural interactions.
to occur in much lower numbers in the presence of G. Experimental trials were conducted in May
holbrooki (Arthington et al. 1983; Lloyd 1990), but and September 1998 in a 90 litre tank (1.5 x 0.5 x
further work is necessary to disentangle the impacts 0.35m) maintained under the same conditions as the
of introduced species from the effects of habitat holding tanks. Grid markings on the tank allowed the
modification (Arthington 1991). To this end, there is vertical height in the water column of the fish to be
a clear need for behavioural analyses of interspecific estimated. The tank was screened by black plastic
interactions (Lloyd 1990), which are likely to vary sheeting to avoid external visual bias and an eye sUt
&
with the species involved (Arthington Lloyd 1989). was cut into the sheet to allow observations.
In particular, because the impact of such interactions In these trials, mixed species groups totalling
tends to be more severe in confined or still water 20fishwereusedandindividualswereusedonlyonce.
environments (Pen and Potter 1991) and when the The ratio ofmean catch rates from repeated sampling
nativefisharerelatively small, itisimportanttogather at the field site (7:3:6:4 for M. duboulayi, P. signifer,
comparative information on the effects ofdensity and G. holbrooki andX. hellerirespectively) was used as a
body size (Howe et al. 1997). guide when determining the relative numbers of fish
The present study investigated the in the four treatments (Table 1). The treatments were
behavioural responses of Melanotaenia duboulayi as follows: Treatment 1, native species; Treatment 2,
(Family Melanotaeniidae) and Pseudomugil signifer native species plus X. helleri; Treatment 3, native
(FamilyPseudomugiUdae)to Gambusiaholbrookiand species plus G. holbrooki; Treatment4, native species
Xiphophorus helleri (Family Poeciliidae). plus both exotic species.
Melanotaeniids and pseudomugiUds are speciose and Attempts were also made to equalise sex ratios
abundant in tropical inland waters in the Australia / and to ensure that the fish used in each trial were ofa
Papua New Guinea region (McDowall 1996). representative range of sizes. Fish biomass was not
However, although M. duboulayi and P. signifer are measured directly but on average the aggregate total
the commonest representatives of these groups in lengthofthefishused (all species combined) was55.6
south-eastern Queensland streams, they have been cm, 58.4cm, 50.0 cm and 53.4 cm forTreatments 1 to
driven to very low densities in disturbed habitats that 4, respectively. The order of trials was randomised
support large populations of introduced poeciliids with respect to treatment, and ten replicate trials were
(Arthington et al. 1983). To assess the relative performed for each treatment.
competitive potential of M. duboulayi and P. signifer In each trial, the fish were placed in the tank
in the presence of G. holbrooki and/or X. helleri, we andleftto settiefor 15 minutes. This wasfollowedby
tested the prediction that the frequency ofchasing, fin aperiod offocal sampling (Martin and Bateson 1990)
nipping and displacement ofthe native species would to record the behaviour ofindividual fish: a randomly
increaseinthepresenceoftheexotics-especiallywhen selected fish of each species was watched for two
food was available, when both exotic species were minutes, during which time its depth was recorded
present, and when the body size of the native every 20 seconds by reference to the markings on the
individuals was relatively small. tank. This was followed by a five-minute sampling
period in which the occurrence and direction of all
agonistic interactions (chases, nips) involving a
MATERIALS AND METHODS randomly chosen focal individual were recorded. A
singlefishflakewasthenplacedonthewater's surface
On several occasions in winter (May- in the central, intermediate depth section of the tank
September) 1998, fish were collected from a site in and the above procedure, involving both periods of
Moggill Creek, Brisbane, using bait traps and a seine focal sampling, was repeated. Pilot studies indicated
116 Proc. Linn. Soc. N.S.W., 124, 2003
WARBURTON AND MADDEN
K. C.
Table 1. Summary ofexperiments and treatments with mean (±SE) total length (excluding tail sword
ofX.helleri) and numbers ofindividuals.
Treatment
Species/size group Mean length SE Expt 1 2 3 4 5 6
(cm)
X. helleri
Mixed 3.39 0.25 1 6 4
Small 2.50 0.13 2 8 12
Large 4.50 0.11 2 8 12
M. duboulayi
Mixed 3.07 0.15 1 14 9 9 7
Small 2.61 0.11 2 12 11
Large 3.89 0.20 2 12 11
P. signifer 2.10 0.07 1 6 5 4 3
2.10 0.07 2 8 8
G. holbrooki 2.00 0.05 1 7 6
2.00 0.05 2 9 9
that a single food source was sufficient to elicit Treatment 4, small M. duboulayi, G. holbrooki;
behavioural responses in all fish, while promoting Treatment 5, Psignifer, large X. helleri; Treatment 6,
competition. Each trial therefore yielded data on P. signifer, smallX. helleri. Table 1 gives numbers of
aggressive encounters and the depth of the species fish used in the different treatments.
concerned, both in the absence and presence offood. There were seven replicate trials for each
Treatment differences in mean chasing frequency, treatment. The focal sampling and data analysis
mean nipping frequency and mean depth were procedures were as described for Experiment 1.
examinedusinganalysisofvarianceandmultiplerange Separate analyses were carried out to compare the M.
(Tukey) tests, separate analyses being carried out on duboulayi treatments (1-4) and the P. signifer
the "food absent" and "food present" data. Primary treatments (5-6).
data (numbers of nips and chases) were standardised
by dividing the number of attacks received by focal
fish per trial by the number of potential aggressors. RESULTS
Forinterspecific attacks the numberofaggressors was
n, where n = group size for the attacking species. For Experiment 1
conspecific attacks the number ofaggressors was n-1.
Data were log-transformed prior to analysis since Aggressive encounters
means and standard deviations were linearly related. There were several statistically significant
between-treatment differences in attack frequency
Experiment 2. Effects of fish size on interspecific (Fig.l; Table 2). Two main trends were apparent,
interactions. namely:
This experiment was designed to investigate (a) Attack levels were significantly higher with
the impact of relative body size on interspecific the combined treatment (Treatment 4) than
A
miexdiCtioTiS,._Melanotaenia duboulayi and X. helleri the other treatments. notable exception to
were each divided on the basis of body size into two this trend occurred in the case of chasing of
groups (large andsmall) (Table 1). BecauseP. signifer G. holbrooki by P. signifer, where the mean
and G. holbrooki arerelatively small species, only one frequency for Treatment 3 was marginally
sizeclass(adultfish,whichweresimilarinsizetosmall significantly higherthanthatforTreatment4
M. duboulayi andX. helleri) were used. (F,,,= 3.99;p = 0.061).
There were six mixed-species treatments, as (b) Significant treatment differences occurred in
follows: Treatment 1, large M. duboulayi, small X. thepresenceoffood. Theonlyexceptionhere
heller; Treatment 2, small M. duboulayi, large X. involved attacks by G. holbrooki on M.
helleri; Treatment3,largeM. duboulayi, G. holbrooki; duboulayi, where although the difference
Proc. Linn. Soc. N.S.W., 124, 2003 117
;
RESPONSE OF NATIVE AUSTRALIAN FISH TO INTRODUCED POECILIIDS
between treatment 0.2 Gh-Md (c-)
-i
means for nipping was
only marginally 0.15 -
significant in the
0.1
presence of food (F^
^^
= 4,12; ;7=0.057), 0.05 -
treatment means for
chasing varied
significantly in the
absence of food (Table
Gh-Md (n+)
2).
Table 3 summarises total
1.5
attack frequency, the frequency
of nips relative to chases, and w
1
o
total attack frequency in the n
presence of food relative to that < 0.5
in the absence of food. The
following trends were clear:
(a) On a standardised per
capita basis, M.
Xh-Md (c+)
duboulayi received
more attacks than the 0.4 n
other species. Most of
0.3-
these came from G.
holbrooki but 0.2
conspecific attacks by 0.1
M. duboulayi were also
relatively common. In
contrast, attacks on P. Natives only Natives +X.h. Natives + G.h. Natives + X.h.
signifer were less + G.h.
frequent and were
mainly conspecific in Treatment
nature.
(b) AttacksbyG. holbrooki
onM. duboulayimainly Figurela. Standardised(percapita)attackfrequenciespertrial. Bars
involved nipping. represent means and standard errors, c, chases; n, nips; - no food; +
Other interspecific and food added. Species names are abbreviated (see caption for Table 2)
conspecific attacks thefirst-named ofeach pairis the attackingspecies. Attacksby exotic
were dominated by and native species are shown in Fig. 1(a), above, and Fig. 1(b), facing
page, respectively.
chasing.
(c) The addition of food
increased attack
frequency. Food addition had a greater by G. holbrooki, M. duboulayi and X. helleri in order
relative impact on attacks by X. helleri than ofincreasing depth (Fig. 2). However, in the presence
on attacks by the other species, but absolute of food, P. signifer moved deeper while G. holbrooki
levels of aggression by X.helleri were very moved closest to the surface. The addition of food
low. Observations indicated that X. helleri had little impact on the preferred depth of M.
were territorial and defended localised areas duboulayi, but it affected X. helleri differently
of the tank. Attacks by M. duboulayi on dependingontreatment: whenX. helleriwasalonewith
conspecifics and on P.signifer also increased the native species, its mean depth increased, but when
markedly in relative terms. G. holbrooki was also present, the mean depth of X.
helleri decreased (Fig. 2).
Depth preferences ForP. signifer, G. holbrookiandM. duboulayi
The four species tended to swim at different there was a consistent effect of treatment, individuals
depths, with P. signifer nearest the surface, followed of all these species moving closer to the surface in
118 Proc. Linn. Soc. N.S.W., 124, 2003
—
—
WARBURTON AND MADDEN
K. C.
2.5 Experiment 2
Md-Md (c+)
2
Larger individuals of M.
1.5 - ii duboulayi were significantly less
susceptible to attack by G.
1 ^ ^ i 1
holbrooki. In thepresence offood,
0.5 G. holbrooki chased small M.
duboulayi approximately six times
moreoftenthanlargeM. duboulayi,
0.4 n i and G. holbrooki-small M.
Md-Md (n+) duboulayichasesweresignificantly
0.3 - more frequent than those for the
il
other three combinations of M.
0.2 -
duboulayiandexotic species (F^^^=
m
6.74; p = 0.0019). Similarly, there
.2 0.1 -
II were significant differences
5 - between treatments in terms ofthe
frequency ofnips on M. duboulayi
(F^24=34.75,p=0.0001),withsmall
M. duboulayi being nipped by G.
0.35 -,
Md-Gh (c+) holbrooki about eight times more
0.3 -
frequently than were large M.
0.25 -
duboulayi. AttacksbyX. hellerion
0.2 - ^H
P. signifer and M. duboulayi were
0.15 -
^ ^H^H infrequent and not significantly
0.1 -
affected by the relative size of X.
0.05 -
helleri.
- 1 When offered food and
mixed with G. holbrooki, large M.
0.7 -, Ps-Gh (c+) duboulayi spent significantly more
0.6 -
time in the deep section ofthe tank
0.5 - =
than did smaller individuals (F3
^
0.4 - 4.54, p = 0.012). There were no
0.3 - ^ significantdifferencesintankusage
0.2 - —— with respect to depth between the
0.1 - treatments with natives plus X.
"
- 1 1 ' 1 helleri.
Natives only Natives +X.h. Natives + G.h. Natives +X.li.+
G.h.
DISCUSSION
Treatment
Figure lb.
The results supported the
prediction that the addition offood
X
Treatment4(i.e.,whenbothG. holbrookiand helleri and the number of exotic species present would
werepresent;Fig. 2). However,between-trialvariation positively influence the frequency of attacks on the
was high and these trends were only marginally native species. In most cases, the highest attack rates
significant (F^^^ = 4.51, p = 0.048 for M. duboulayi; wererecorded when all four species werepresent, and
F^^g= 4.11, p = 0.058 for P. signifer; comparisons of most significanttreatment differences occurred inthe
Treatments 1 and 4 in the absence offood). presence of food. Food has been shown to increase
rates of aggression in other fish species (Syarifuddin
Feeding success & Kramer 1996). It is noteworthy that, although G.
Individual M. duboulayi tended to take food holbrooki was responsible for most interspecific
before individuals of the other species (Table 4). attacks, chasing and nipping of M. duboulayi by X.
Overall, X. helleri had a lower per capita feeding helleri also increased to relatively high levels in the
success than the other species. four-species treatment. The species enhancement
Proc. Linn. Soc. N.S.W., 124, 2003 119
RESPONSE OF NATIVE AUSTRALIAN HSH TO INTRODUCED FOECILIIDS
Table 2. Main between-treatment difTerences in attack frequency. Species names have been abbreviated
(seekey); thefirst-named ofeach pairistheattackingspecies. Underlined treatmentsare notsigniflcantly
different. Keytoabbreviations: Gh=Gambusiaholbrooki; Md=Melanotaeniaduboulayi; Ps=Pseudomugil
signifer; Xh =Xiphophorus helleri
Interaction Type of Food / no F df P Treatment
attack food differences
Gh- Md chases No food 7.46 1,18 0.014 T4>T3
Gh- Md nips Food 4.12 1,18 0.057 T4>T3
Xh- Md chases Food 5.10 1,18 0.037 T4>T2
Md -Md chases Food 11.57 3,36 0.0001 T4>T3>UL^T2
Md -Md nips Food 2.90 3,36 0.048 T4>T2>T3>T1
Md -Gh chases Food 5.26 1,18 0.034 T4>T3
Ps- Gh chases Food 3.99 1,18 0.061 T3>T4
effect was associated with aggregation involving appeared to promote elevated levels of activity and
vertical movement toward the surface. When all four more aggressive interactions. The broader diversity
species were present in low numbers there was a ofspecies-specific behaviours and salient stimuli may
general tendency (for all species except X. helleri in also have encouraged heightened levels ofactivity. In
the absence offood) to movecloserto the surface. By a study of conspecific and interspecific interactions
increasing the local density of fish, such aggregation betweenbrooktrout,Salvelinusfontinalis, andrainbow
trout, Salmo gairdneri (=Oncorhynchus mykiss),
Newman (1956) postulated that the presence of
Table3. Frequencyofattacks(smnmarytable). 'Total
food increased feeding activity, which in turn
attacks' refers to nips plus chases. For each species
increased aggressive activity as the focus of
combination,theratioofnipstochasesandtheratioof
attacks was displaced from food to fellow fish of
totalattacksduringallfoodtrialstototalattacksduringall bothspecies. Henotedthatfeedingfishdisplayed
non-food trials are also shown. (->-) indicates an some movements that are associated with
increaseinthepresenceoffood, butwherea ratiocan-
aggression, such as body undulations, swift
notbecalculatedduetoazero"nofood"value. Species
darting and biting, and suggested that such
nameshave beenabbreviated (seecaptionforTable2). movements constituted sign stimuli eliciting
attacksfromotherfish. Theincreasedexcitement
Interaction Total Nips Food: associated with the four-species treatment
:
attacks/ chases no food (Treatment 4) could not be explained in terms of
trial thetotal numberoffish,whichremainedconstant
across treatments. Treatment 4 had the lowest
—
Gh Md 1.57 2.73 6.45 numbersofindividualsofeachofthefourspecies
—
Gh Ps 0.60 0.86 4.41 studied. Working with gouramis (Trichogaster
—
Xh Md 0.26 0.82 24.50 trichopterus), Syarifuddin & Kramer (1996)
Xh-*Ps 0.08 1.00 (+) found that fish were more aggressive in smaller
groups and attributed this to greater costs of
Md *Md
1.06 0.24 9.34 contest competition with increasing group size.
Md *Ps 0.08 0.23 9.67 In the present study it is possible that fish were
Ps- Md 0.14 0.46 (+) responding more to the numbers of individuals
Ps- Ps 0.79 0.16 3.59 belonging to each species than to the size ofthe
group as a whole, but testing this hypothesis
Md — Gh 0.21 0.52 (+) would require more work.
—
Md Xh 0.05 1.00 1.33 The four species exhibitedconsiderable
—
Ps Gh 0.49 0.56 3.41 variation in the extent and type of aggression
—
Ps Xh 0.04 (+) (+) displayed. Although M. duboulayi were often
attacked by G. holbrooki, they concentrated
120 Proc. Linn. Soc. N.S.W., 124, 2003
WARBURTON AND MADDEN
K. C.
mainly on conspecific rather than
interspecific exchanges. In
Experiment 1, attacks by G.
holbrooki on M. duboulayi were
over 2.5 times more frequent than
attacks by G. holbrooki on P.
signifer. Further, G. holbrooki
tended to nip M. duboulayi but
chase P. signifer. However, these
differences were not due simply to
the difference inbody sizebetween
the two native species, since in
Experiment2 G. holbrookiattacked
small M. duboulayi more readily
than large M. duboulayi. Juvenile
M. duboulayithereforeappeartobe
particularly susceptible to nipping.
The depths occupied by
b) Food present the four species examined were a
function of (a) species- and size-
specific differences in mean depth
preference, (b) an increased
tendency to aggregate near the
surface as species diversity
increased, and (c) species-specific
changes in depth in response to the
addition offood. In the absence of
food, P. signiferoccurred closestto
the surface. However, when food
was present, G. holbrooki moved
Natives only Natives +X-h. Natives + G.h. Natives +Xh. closest to the surface while P.
+ G.h. signiferretreatedtowardthe deeper
regions of the tank. These results
Treatment
suggestthatP. signiferisarelatively
Figure 2. Mean (+ SE) swimming depths (in cm) by fish species, poor competitor for food at the
M
treatmentandtheabsenceorpresenceoffood.. duboulayi,oblique surface, as does the fact that for P.
signifer (unlike M. duboulayi, X.
bars; P. signifer, blacli bars; X. helleri, crossed oblique bars; G.
helleri and G. holbrooki) feeding
holbrooki, crossed vertical/horizontal bars. Exotic species names are
success was lowest when all four
abbreviated (see caption for Table 2).
species were present.
Table 4. Per capita feeding success of the four species. In this table the percentage of trials when any
member ofa given species took the offered food beforemembers ofany otherspecies has been divided by
the number of individuals of the focal species. Species names have been abbreviated (see caption for
Table 2).
Treatment
Species Natives only Natives + X.h. Natives + G.h. Natives + X.h. +G.h.
Md
6.6 8.9 6.4 8.7
Ps 1.2 4.0 4.5 1.0
Xh
- - 2.8
Gh
- - 3.4 4.2
Proc. Linn. Soc. N.S.W., 124, 2003 121
RESPONSE OF NATIVE AUSTRALIAN FISH TO INTRODUCED POECILUDS
In Experiment 2, larger M. duboulayi were
ACKNOWLEDGMENTS
less attracted to the shallow part ofthe tank than were
smallerM. duboulayi. In the wild, adult M. duboulayi
tend to occur at greater depths than juveniles and are We are very grateful to Louise Weaver for
less inclined to feed at the surface (Hattori and collecting data for Experiment 2, to Culum Brown for his
Warburton, in press). These observations may reflect helpinthedesignandpreparationofthisstudy,andtoAngela
the general tendency for small fish species, and small Arthington and anonymous reviewers for commenting on
individualswithinspecies, tooccurclosertothesurface the manuscript.
(Helfman et al. 1997).
Therearesubstantialoverlapsamongthefour
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