SEPTEMBER1999
THESOUTHWESTERN
NATURALIST
44(3):287-295
EFFECTS OF RED SHINER (CYPRINELLA LUTRENSIS) ON RED RIVER
PUPFISH (CYPRINODON RUBROFLUVIATILIS )
KEITH B. GIDO, JACOB F. SCHAEFER,KIRSTENWORK, PHILIPW. LIENESCH,
EDIE MARSH-MATTHEWS, AND WILLIAM J. MATTHEWS
Departmentof Zoologyand Biological Station, Universityof Oklahoma,Norman, OK 73019
Present address of KW: EcosystemsRestorationDepartment, South Florida WaterManagement District,
3301 Gun Club Rd., P.O. Box 24680, WestPalm Beach, FL 33416-4680
Present address of PWL: Departmentof Biology, Universityof North Carolina Greensboro,P.O. Box 26174,
Greensboro,NC 27402-6174
ABSTRACT-A proposed desalinization project that would affect the entire upper Red River
basin, Oklahoma-Texas, would, if completed, raise the possibility of invasion by red shiner, Cyprinella lutrensis, into stream reaches from which it has been restricted by naturally high salinity.
Because introduced red shiners are known to impact native fish communities, we asked if red
shiner could have an adverse effect on Red River pupfish, Cyprinodonrubrofluviatilis,which typically
occurs in reaches of the Red River with high salinity. In an array of artificial streams we examined
effects of red shiner on survival, condition, and reproduction of Red River pupfish. In the presence
of red shiner, pupfish successfully produced larvae, but fewer juvenile pupfish survived to potentially recruit.
RESUMEN-Un proyecto propuesto de desalinizaci6n que afectaria a toda la cuenca del Red
River superior, Oklahoma-Texas, aumentaria, si es completado, la posibilidad de una invasi6n de
la sardinita roja, Cyprinella lutrensis, en riachuelos en donde no ha estado por una salinidad naturalmente alta. Preguntamos si la sardinita roja podria occasionar un efecto negativo sobre el
pez cachorito del Red River, Cyprinodonrubrofluviatilis, ya que la sardinita roja introducida suele
impactar las communidades naturales de peces. Cyprinodonrubrofluviatilistipicamente se encuentra
en las cuencas del Red River con alta salinidad. Examinamos los efectos de la sardinita roja en la
supervivencia, condici6n y reproduci6n del pez cachorito del Red River en una serie de riachuelos
artificiales. En presencia de la sardinita roja, el pez cachorito produjo larvas con 6xitopero menos
juveniles del pez cachorito sobrevivieron para su posible reproducci6n.
The parapatric distribution of species along
environmental gradients can reflect a trade-off
between the competitive ability of a species
and its ability to withstand environmental conditions (e.g., Connell, 1961). Abundance patterns of stream fishes are known to vary along
environmental gradients, and patterns of little
or no overlap between species have been interpreted as the outcome of competitive interactions (e.g., Taylor and Lienesch, 1996). However, it is often difficult to determine causal
factors and to separate relative effects of various biotic and abiotic factors without experimental studies. Human activities can inadvertently affect fish distributions by changing environmental conditions. For example, impoundment of rivers has dramatically affected
natural temperature and chemical gradients
(Baxter, 1977). In addition, changes in regional temperature regimes in response to global
warming may disrupt thermal gradients and
thus the distribution of fish species in streams
(Matthews and Zimmerman, 1990; Rahel et al.,
1996). The impetus for the present study is the
proposed desalinization of a naturally saline
river to improve
water quality
for municipal
and agricultural purposes, a process that could
alter distribution of fishes along a salinity gradient.
The Red River (Oklahoma-Texas) is characterized by high salt concentrations (>10
ppt) in the headwaters due to percolation of
ground water through marine salt beds
(Echelle et al., 1972). Salinity decreases downstream where freshwater tributaries enter the
river. Fish assemblages in the headwaters typi-
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The SouthwesternNaturalist
cally are depauperate (<10 species/collection), with Red River pupfish, Cyprinodonrubrofluviatilis, and plains killifish, Fundulus zebrinus,
being the dominant species (Echelle et al.,
1972). In lower reaches, red shiner, Cyprinella
lutrensis, western mosquitofish, Gambusia affinis, and numerous other native minnows are
abundant (Echelle et al., 1972; Taylor et al.,
1993). Although pupfish can tolerate low salinity (Echelle et al., 1972), they are thought to
be poor competitors and to exist in high abundance only in areas where high salt concentrations limit entry of other native species (>10
ppt; Echelle et al., 1972).
Increasing demand for water in Oklahoma
and Texas has led to projects that reduce salinity of the Red River. Two desalinization structures already exist in the upper Red River basin and seven others have been proposed by
the United States Army Corps of Engineers as
part of the Red River Chloride Control Project.
Currently, the Red River is still highly saline;
however, completion of additional desalinization plants could cause a basin-wide reduction
in salinity. Under this scenario, it is likely that
other species that are abundant downstream
under lower salt concentrations could invade
the upper reaches of the basin. In particular,
red shiner has been shown to rapidly invade
disturbed areas and has had negative effects on
native fish species (Karp and Tyus, 1990; Rinne, 1991; Douglas et al., 1995). Given the potential for increased interaction between red
shiner and Red River pupfish if desalinization
were to occur, we examined interactions between these species to assess direct effects of
the former upon the latter.
vol. 44 no. 3
Pool Depth
45cm
Elevated
Holding Tank
46cm
183cm
Riffle Depth
15cm
183cm
FIG. 1-Design
of each of eight experimental
streams located at the University of Oklahoma Biological Station.
1995). Experimental streams were filled with water
pumped from Lake Texoma (an impoundment of
the Red River, conductivity 1,200 jxmhos/cm, salinity
0.5 ppt). Streams were filled 2 months prior to addition of fish to allow colonization of algae and invertebrates. Streams were monitored weekly; screens
were cleaned of algae and debris, and any fish mortalities were noted and dead fish removed.
Each stream unit was treated as n = 1 in analyses,
with mean values within each unit entered as the
individual variate. The configuration of the stream
offered at least a semi-natural setting for small fishes,
with pools interspersed by riffles. In this setting, fishes had the potential to use a wide array of microhabitats reflecting differences in depth, current, and
shelter, and the potential to escape spatially from
competitors or predators.
Fish Collections-Pupfish for this experiment were
METHODS-Experimental Streams-Experimental
collected in the Red River near Eldorado, Oklahoma
streamswere located at the Universityof Oklahoma on 7 May 1997. At the time of collection, water temBiological Station and consisted of eight indepen- perature was 23 °C, conductivity was 18,000 pimhos/
dent riffle-pool units (Fig. 1). Each unit was filled cm, and salinity was 10 ppt. From 30 seine hauls (225
with natural sand, gravel, and cobble. Substratum m2), mean density of pupfish was 2.2 individuals/m2.
was sculptured to form concave pools and riffles Red shiners were captured from a nearby tributary
with sinuous flow to simulate a natural stream. In (Sandy Creek, salinity ca. 1 ppt) on 21 March 1997.
each unit, water was pumped from a downstream Each species was held in separate 183 by 50 cm cycollection box using a submersible 0.25 hp (horse lindrical holding tanks for at least 1 week prior to
power) pump into an elevated holding tank where placement in experimental streams. Experiments
water overflowed into an upstream collection box. ran for 65 days, from 14 May 1997 to 18 July 1997.
Upstream and downstream boxes were closed with To give an accurate count of adults, the experiment
0.5 cm plastic mesh to block movement of adult fish was terminated before juvenile fish were large
into collection boxes. Current velocity within each enough to be confused with fish initially stocked in
the streams.
unit was heterogeneous, ranging from 0 to 25 cm/
ExperimentalDesign-Interspecific interactions besec in riffles and 0 to 10 cm/sec in pools (Gelwick,
September
1999
Gido et al.-Effects
of red shiner on pupfish
tween pupfish and red shiner were examined using
an additive design experiment (Silvertown, 1987;
Goldberg and Scheiner, 1993). Three units were
stocked with 26 pupfish (PF treatment) and three
units were stocked with 26 pupfish plus 30 red shiner (PF+RS treatment) to give a pupfish density of
2.72 fish/m2 in each treatment. Treatments and individual fish were randomly assigned to experimental streams. To assay performance of pupfish between treatments we measured 1) larval abundance
in light traps, 2) juvenile abundance in light traps,
3) adult survivorship, 4) adult condition, 5) juvenile
survivorship at the end of the experiment, and 6)
diet of pupfish. We also examined diet of red shiner
to determine dietary overlap between the two species. If red shiner outcompeted pupfish for resources we predicted pupfish to have lower survivorship
and condition of adults, lower larval production,
and perhaps a shift in diet in the presence of red
shiner. If predation on larval and juvenile pupfish
was an important interaction, we predicted lower larval and juvenile survival in the presence of red shiner.
Larval and juvenile abundance in light traps were
used to estimate reproduction and recruitment of
pupfish. We assumed the number of individuals captured in light traps was directly related to abundance
of larval and juvenile pupfish, regardless of treatment. Larval fish (<5 mm SL) were assumed to have
recently hatched, within 1 to 2 days, and represented
reproductive output. Juveniles (5-25 mm SL) were
assumed to be those individuals that had survived
the first days of life and were able to feed and grow
in the experimental streams. Light traps were similar
to the quatrefoil trap described by Floyd et al.
(1984), with 3.2 mm slits. The center illumination
rod was lit by a 2.9 W bulb powered by a 6 V battery.
Approximately every week, three light traps were set
in each stream (one per pool) just after dusk for one
hour. Larval and juvenile fish from each trap were
preserved in 5% buffered formalin and returned to
the laboratory for measurement and enumeration.
On 22 June 1997 (day 31), baited minnow traps
were used to capture adult pupfish to examine condition. Of those captured, five individuals from each
stream were randomly selected for analysis. Pupfish
were measured, eviscerated, and dried at 600C for 24
h. Because dried body weight showed a linear relationship with length (r2 = 0.729, P < 0.0001), condition was calculated as dried weight/standard
length X 1,000. At the end of the experiment (day
65), adult and juvenile survivorship was determined
by collecting and counting individuals from streams.
A thorough effort was made to recover all adult and
juvenile fish while draining streams. Condition also
was calculated for adult pupfish collected at the end
of the experiment.
Stomach contents from adult pupfish and red
289
shiner removed at the end of the experiment were
examined under a stereomicroscope at 20 X magnification and food items were identified into major
resource categories. Percent volume and percent occurrence were calculated for all food items. Because
both measures gave similar results, only percent occurrence was used in analysis.
Intraspecific density effects on pupfish performance were examined at two densities. The three
PF units described previously (density of 2.72 fish/
m2) were used as a low density treatment and the
two remaining streams were stocked at a density of
5.43 fish/m2 and were used as a high density treatment (PF + PF treatment). Response variables for
these comparisons were 1) adult survivorship, 2)
adult condition, 3) per capita larval abundance in
light traps, 4) juvenile abundance in light traps, and
5) juvenile abundance at the end of the experiment.
All variables were measured as described previously
except larval abundance in light traps. To estimate
per capita reproduction, larval abundance in light
traps was adjusted to the number of female pupfish
in each stream. Expected number of female pupfish
on each sample date was based on predicted mortality rate (number of adults/day). This rate was determined by the difference in number of adults
found at the end of the experiment divided by number of days in the experiment. The assumption that
there was a constant mortality rate was supported by
an even distribution of pupfish mortalities recovered
throughout the study.
Data Analysis-In all analyses the individual variate
was a single value for each stream unit, n = 3, 3, 2
for PF, PF + RS, and PF + PF treatments, respectively. A repeated measures ANOVA with time as the
repeated factor was used to test for differences between treatments in number of larval and juvenile
pupfish captured in light traps (SPSS, 1996). Comparisons between treatments for response variables
measured at the end of the experiment (adult survivorship, juvenile abundance, and adult condition)
were made using t-tests with a Bonferroni correction
for multiple comparisons (Rice, 1989). Additional ttests were used to test for differences in mean length
and sex ratio of adult pupfish at the end of the experiment. These tests were to used to evaluate potential differences that may have occurred during
the initial stocking and were not considered response variables. Differences in percent occurrence
of major food items in pupfish stomachs between PF
and PF + RS treatments and differences between
pupfish stomachs (all treatments combined) and red
shiner stomachs were tested using a G-test (Sokal
and Rohlf, 1995). Because of the small number of
replicates we a priori set our alpha level at 0.10 to
decrease the likelihood of Type II error.
RESULTS-Interspecific Interactions-No
signif-
290
vol. 44. no. 3
The Southwestern Naturalist
60
PF
PF + RS
50
40
30
larvae/hour
of
20
10
Number
0
3 June
10 June
17 June 22 June
10 july
FIG.2-Mean and standard error for number of larval pupfish captured in light traps for PF and PF +
RS treatmentsduring each sample period.
icant differences were found between PF and
PF + RS treatments in mean number of larval
pupfish captured in light traps (P = 0.228; Fig.
2). Regardless, the PF treatment had a greater
mean number of larval pupfish on each sampling occasion, suggesting sample size may
have been inadequate to detect any difference.
There was significantly higher juvenile abundance (P = 0.012) in the PF treatment, with
only two juveniles captured from light traps in
the PF + RS treatment across all sample dates
(Fig. 3). Because no juveniles were captured
on the first night of sampling in either treatment, this date was excluded from analysis.
We found no significant differences between
treatments for any of the variables measured at
the end of the experiment (P> 0.10; Table 1).
In addition, mean length of adult pupfish
(28.2 versus 28.4 mm) and sex ratio (0.85 versus 0.66-males:females) was similar between
treatments (P > 0.10). Mortality was high in
both treatments and ranged from 15.4 to
73.1%. Whereas there was no significant difference between treatments in number of juveniles recovered (P = 0.295), two of the three
PF units had notably higher juvenile abundance than the PF + RS treatment. This corresponded to the greater abundance of juve-
nile pupfish taken in light traps from PF units
than from PF + RS units.
Percent occurrence of major food items in
pupfish stomachs did not differ significantly
between PF and PF + RS treatments (G =
0.575, P > 0.90) indicating that presence of
red shiner did not cause a diet shift (Fig. 4).
Pupfish appeared to be opportunistic feeders;
the majority of their diet was filamentous algae. Pupfish did have a significantly different
diet than red shiner (G = 113.7, P < 0.001;
Fig. 4). In general, red shiner consumed a
greater proportion of amorphous detritus and
terrestrial insects (particularly ants) than pupfish.
IntraspecificDensity Effects-Larval abundance
in light traps was not significantly different (P
= 0.271) between PF and PF + PF treatments
(Fig. 5). However, on all sample dates mean
number of larvae/female pupfish was greater
in the PF treatment. Similarly, mean juvenile
abundance in light traps did not differ significantly between treatments (P = 0.212) but was
greater in the PF treatment on all but one sample date (Fig. 6). Although these results could
have been due to a weak density effect, our
experiment was not sensitive enough to detect
any difference. Condition of adult pupfish on
September
Gido et al.-Effects
1999
of red shiner on pupfish
291
12
PF
10
PF+RS
8
6
juveniles/hour
of
4
2
Number
0
3 June
10 June
17 June 22 June
10 july
FIG. 3-Mean and standard error for number of juvenile pupfish captured in light traps for PF and PF
+ RS treatments during each sample period.
80
70
60
pupfish (PF)
pupfish (PF+RS)
red shiner
50
40
occurence
30
20
Percent
10
0
Fil. algae
Veg. debris
Fish scales Diptera
Am. detritus
Amphipod
Terr. insect
FIG. 4-Percent occurrence of major food items taken by Red River pupfish with red shiner absent (PF)
and red shiner present (PF + RS). Diet of red shiner is given to assess interspecific overlap in diet. Fil. =
filamentous, Veg. = vegetative, Am. = amorphous, and Terr. = terrestrial.
292
The Southwestern Naturalist
vol. 44, no. 3
3
PF
PF+PF
2
larvae/female/hour
1
pupfish
of
0
Number
3 June
10 June
17 June 22 June
10 july
FIG. 5-Mean and standard error for number of larval pupfish captured in light traps for PF and PF +
PF treatments during each sample period.
12
PF
10
PF+PF
8
6
juveniles/hour
of
4
2
Number
0
3 June
10 June
17 June 22 June
10 july
FIG. 6-Mean and standard error for number of juvenile pupfish captured in light traps for PF and PF
+ PF treatments during each sample period.
September
Gido et al.-Effects
1999
of red shiner on pupfish
293
TABLE1-Response variables measured for Red River pupfish. Values for each experimental unit and
means are given for each treatment.
Treatment
Variable
Adult survivorship
Juvenile abundance
Condition (day 31)
Condition (day 65)
PF
15.40
95.00
5.22
4.34
50.00
76.00
4.68
5.01
x
73.10
22.00
5.12
7.21
42.60
64.30
5.01
5.52
day 31 was significantly lower in the double
density
treatment
(t = 5.18, P = 0.014).
No
significant differences were found between
treatments in response variables examined at
the end of the experiment (P> 0.10; Table 1).
Mean length of adult pupfish (28.2 versus 28.1
mm) and sex ratio (0.85 versus 1.15-males:females) did not differ between treatments.
DIscussIoN-Interspecific
Interactions-The
greatly reduced abundance of juvenile pupfish
captured in light traps in the PF + RS treatment suggests high mortality from larval to juvenile stages. This effect could have resulted
from direct predation, predator avoidance, or
reduced survival of juvenile pupfish because of
limiting resources. Though food may have
been limiting for juveniles, there was minimal
dietary overlap between red shiner and pupfish adults. Pupfish typically consumed filamentous algae and red shiner consumed amorphous debris and terrestrial insects. The food
items these fish consumed in artificial streams
probably reflect natural food habits because
similar items have been reported for pupfish
(Echelle et al., 1972) and red shiner (Hale,
1962; Cross and Collins, 1995). Because of low
dietary overlap between species, competition
for food was probably not very strong. In contrast, there was some evidence to suggest that
predation by red shiner limited the abundance
of juvenile pupfish in the PF + RS treatment.
Foremost was the observance of red shiner
chasing, and on one instance consuming, a juvenile pupfish (ca. 10 mm SL). This is consistent with previous reports of red shiner preying on juveniles of other species (Ruppert et
al., 1993). In addition, there were trends for
fewer larval pupfish in light traps and fewer
juvenile pupfish at the end of the experiment
in the PF + RS treatment. These trends, how-
PF + RS
53.80
42.00
4.13
4.57
50.00
52.00
4.84
5.69
x
30.80
4.00
3.86
4.74
44.90
32.70
4.28
5.00
PF + PF
65.40
68.00
2.81
4.43
55.70
15.00
3.56
4.67
x
60.60
41.50
3.19
4.55
ever, were not significant, suggesting larval and
juvenile pupfish were able to survive in this
treatment. Finally, reduced abundance of juvenile pupfish in the presence of red shiner
may have been apparent because pupfish
avoided open water to escape predation, and
were not captured in light traps. Thus, it may
be that juvenile pupfish abundance was similar
for both treatments but their behavior differed
between treatments.
Predation of larval and juvenile fishes by introduced fishes has been implicated in the decline of several native fish species (Meffe, 1985;
Scoppettone,
1993). In addition, predator
avoidance has been shown to affect growth and
survivorship of juvenile fishes by forcing them
to use habitats with lower resource quality
(e.g., Persson and Greenberg, 1990). The importance of predation, however, depends on
other factors such as spatial overlap between
species and environmental heterogeneity. In
our experimental streams, pupfish tended to
use benthic areas but red shiner used both
benthic and mid-water column habitats. Additionally, occurrence of debris and algae in
stomachs of both species suggests they both
spent time foraging in benthic areas. Because
of the broad habitat use by red shiner, there
likely would be enough spatial overlap between
species that larval and juvenile pupfish would
be susceptible to predation by invading species.
Environmental heterogeneity and presence
of refugia may reduce effects of predation on
survival and recruitment of juveniles. For example, Meffe (1985) showed that two poeciliids (one introduced and one native) were
able to coexist at a structurally complex site;
whereas, the introduced species displaced the
native species at less complex sites. Echelle
(1970) reported that Red River pupfish burrow
294
The Southwestern Naturalist
into the sand to avoid avian predators. In our
experimental streams, a large number of juvenile pupfish were able to recruit into the population regardless of the small number of juveniles caught in light traps. Juveniles tended
to be concentrated near the substratum and
potentially could have sought refugia in algal
mats and interstitial spaces of gravel and cobble. Because red shiners are relatively small
fish, there is only a short length of time when
newly hatched pupfish are small enough to be
consumed. Based on gape measurements of
red shiner and juvenile pupfish growth rates
(from light trapping data), we estimated that
in 14-19 days at approximately 25°C, a juvenile
pupfish would grow to a size (>14 mm SL) at
which it could no longer be consumed by an
adult red shiner. The ability of pupfish juveniles to survive this period when they are susceptible to red shiner predation may greatly
influence their ability to coexist.
Intraspecific Density Effects-In
order to detect
competition, experimental densities should be
high enough to allow resources to be limited
but also encompass densities that are realistic
in nature (Goldberg, 1996). Overall, our experimental densities were shown to be high
enough and that resources were limiting. Condition of adult pupfish was significantly reduced in the double pupfish density treatment
on day 31, but not at the end of the experiment. Because of high mortality in these
streams, pupfish densities in the PF + PF treatment at the end of the experiment were not
significantly different than in the PF treatment
(P > 0.10). Thus, we did not expect as strong
a density effect at the end of the experiment,
after densities had declined.
Based on estimated density of pupfish during fish collections, our experimental density
should be representative of moderate to high
densities in nature. In fact, because of the
highly variable flows in the upper Red River
(United States Geological Survey, in litt.), it is
likely that, during low flow periods or in isolated pool habitats, pupfish densities greatly
exceed those we tested.
General Considerations--Echelle
et al. (1972)
suggested that the poor competitive ability of
Red River pupfish, rather than salinity, limited
their distribution in lower reaches of the Red
River. Our experiments further suggest that
pupfish larvae and juveniles are susceptible to
vol. 44, no. 3
predation by red shiner in these reaches. Reduction of salinity in headwaters of the Red
River would likely allow invasion of potential
competitors and predators of pupfish. Environmental heterogeneity (i.e., availability of refuge habitat) may reduce the effect of predation by allowing juvenile pupfish to reach a size
that exceeds the gape of red shiner.
Removal of natural environmental gradients
can affect distribution of fishes by changing
the chemical and physical environment as well
as influencing biotic interactions. In a study of
sheepshead
minnow,
Cyprinodon
variegatus,
Dunson et al. (1998) showed a negative relationship between specific conductance and
growth and reproductive output. If environmental conditions are changed, it is likely that
a combination of biotic and abiotic factors will
adversely affect some species while benefitting
others. To help predict the outcome of such
changes requires knowledge of the physiological response of a species to the change in environment and its response to corresponding
changes in the biota. In the Red River basin,
encroachment of red shiner into typical Red
River pupfish reaches may cause direct harm
to pupfish.
Conversations with L. Cofer stimulated this research. G. Welborn and C. Vaughn provided advice
on experimental design. Earlierversionsof this manuscript benefitted from comments by A. Echelle, G.
Garrett, and an anonymous reviewer. We thank the
Oklahoma Department of Wildlife Conservation for
scientific collecting permits required for this research.
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Submitted2 April 1998. Accepted2 September1998.
AssociateEditor was David Edds.