Southwestern Association of Naturalists
Feeding Ecology of Three Omnivorous Fishes in Lake Texoma (Oklahoma-Texas)
Author(s): Keith B. Gido
Source: The Southwestern Naturalist, Vol. 46, No. 1 (Mar., 2001), pp. 23-33
Published by: Southwestern Association of Naturalists
Stable URL: http://www.jstor.org/stable/3672370
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THE
SOUTHWESTERN
NATURALIST
46(1) :23-33
THE SOUTHWESTERN
NATURALIST
46(1):23-33
MARCH
MARCH 2001
2001
FEEDING ECOLOGY OF THREE OMNIVOROUS FISHES IN LAKE
TEXOMA (OKLAHOMA-TEXAS)
KEITH B. GIDO*
Universityof Oklahoma,Biological Station and Departmentof Zoology,Norman, OK 73019
*Correspondent:kgido@ou.edu
ABSTRACT-Feeding ecology of 3 omnivorous fishes in a large southern United States reservoir
was investigated to develop hypotheses on the potential functional roles of these species in this
ecosystem. I examined distribution, abundance, and diet of smallmouth buffalo (Ictiobus bubalus),
river carpsucker (Carpiodes carpio), and gizzard shad (Dorosoma cepedianum) relative to the availability of potential resources during summer 1997 and 1998. In July and August, abundance of
smallmouth buffalo was significantly greater at stations with depths of 10 m than at 1 m or 3 m
stations. There was no significant difference in abundance of gizzard shad or river carpsucker by
depth or month. Relative proportions of detritus and zooplankton in the diet varied among species. Smallmouth buffalo primarily ate copepods, whereas gizzard shad primarily ate detritus. The
diet of river carpsucker was intermediate in relative proportions of zooplankton and detritus to
that of smallmouth buffalo and gizzard shad. Of the 3 species examined, only gizzard shad showed
a significant decline in dietary crude protein, phosphorus, and organic content over the summer.
This corresponded to a decline in condition of gizzard shad. Overall, benthic invertebrates had a
heterogeneous distribution within the reservoir, and organic content of sediments was not different across sample stations. Relative importance to ecosystem functioning of these species, all of
which are highly abundant in southern reservoirs, likely depends on species-specific feeding ecology and environmental conditions.
RESUMEN-La ecologia de tres peces omnivoras en una presa del sur de Estados Unidos fue
investigada para desarrollar hip6tesis del papel funcional potencial en este ecosistema. Yo examine
la distribuci6n, abundancia, y dieta de Ictiobus bubalus, Carpiodes carpio, y Dorosoma cepedianum,
relativo a la abundancia potencial de los recursos durante los veranos de 1997 y 1998. En julio y
agosto la abundancia de Ictiobus bubalus fue significativamente mayor en los sitios con profundidades de 10 metros que en los sitios con im o 3 m. de profundidad. No hay diferencias significativas en la abundancia de D. cepedianumo C. carpiopor profundidad o por mes. Las proporciones
relativas de detritus o zooplancton en la dieta fue variada entre las especies, Ictiobus bubalus come
principalmente copepodes mientras que D. cepedianum come detritus. La dieta de C. carpio fue
intermedia en proporciones relativas de zooplancton y detritus a la de I. bubalusy D. cepedianum.
De las tres especies examinadas, solo D. cepedianumexhibi6 una reducci6n dietetica significativa
de proteina cruda, f6sforo, y contenido organico durante el verano. Esto corresponde a un declive
en la condici6n de D. cepedianum.En general, los invertebrados bent6nico tienen una distribuci6n
heterog6nea dentro la presa y el contenido organico del sedimento no fue diferente a traves de
los sitios de muestreo. La importancia relativa de estas especies en la funcionalidad del ecosistema,
todas altamente abundantes en el lado sur del reservorio, probablemente depende de la relaci6n
ecol6gica de la alimentacion especie-especies y las condiciones ambiental.
Fish assemblages in southern reservoirs typbenthic
ically are dominated
by large-bodied
omnivores (Mensinger, 1971; Robison and Buchanan, 1988). Such fishes can have important effects on ecosystem processes such as
of sedinutrient
cycling and bioturbation
ments (Lamarra, 1975; Brabrand et al., 1990;
Stein et al., 1995; Drenner et al., 1996; Flec-
ker, 1996). The degree they affect ecosystem
processes,
however, depends on their abunmodes
of feeding, and abiotic condidance,
tions (e.g., nutrient loading from the watershed and sedimentation
rates). To assess poomnivores in restential roles of large-bodied
ervoirs, a requisite first step is understanding
where and when they forage and how this re-
24
The SouthwesternNaturalist
lates to resource availability (e.g., Power,
1997).
I examined feeding ecology of 3 omnivorous
fish species (gizzard shad, Dorosomacepedianum;
smallmouth buffalo, Ictiobus bubalus, and river
carpsucker, Carpiodescarpio) in the context of
the distribution and abundance of their food
resources in Lake Texoma (Oklahoma-Texas).
Omnivorous fishes, such as these, will switch to
detritus if higher nutritional quality prey declines (Ahlgren, 1990a; Lobon-Cervia and Rincon, 1994; Valladolid and Przybylski, 1996;
Yako et al., 1996) and will grow best when detritus is supplemented with invertebrate prey
(Mundahl and Wissing, 1987; Ahlgren, 1990b,
Bowen et al., 1995). Thus, abundance and condition of these fishes may depend on spatial
and temporal variation in benthic invertebrate
abundance. In flood control reservoirs, benthic invertebrates may vary with wave exposure
(Cooper, 1977), dissolved oxygen (Sublette,
1957; Cooper, 1980; Cooper and Knight,
matter
and particulate
1985),
organic
detritus
Moreover,
quality and
(Vaughn, 1982).
lentic
systems.
may
vary
throughout
quantity
For example, detritus from wind-exposed
shoreline habitats maintains fathead minnows
(Pimephalespromelas) better than detritus from
profundal habitats (Lemke and Bowen, 1998).
The goal of this study was to correlate distribution, abundance, and diet of 3 omnivorous fishes to spatial and temporal variation in
quality and quantity of resources in Lake Texoma. This information was then used to develop hypotheses regarding interspecific differences in potential effects of these fishes on material processing in reservoirs. Although studies previously have suggested gizzard shad can
play an important functional role in reservoir
ecosystems by processing detritus (Stein et al.,
1995; Vanni, 1996; Schaus et al., 1997), it is
possible that other benthic fishes such as smallmouth buffalo and river carpsucker perform
similar roles.
AND METHODS-Study Area-Lake TeMATERIALS
xoma is a 36,000 ha impoundment of the Washita
and Red rivers on the Oklahoma-Texas border. Reservoir releases and resulting fluctuations in water
level are primarily for hydropower and flood control. Near my study sites, Secchi depth transparency
typically ranges from 100 to 125 cm, but can decrease to 15 cm during turbid inflow episodes (Matthews, 1984). Study sites were located ca. 35 km up-
vol. 46, no. 1
lake from Denison Dam, within the Red River arm
of Lake Texoma near the University of Oklahoma
Biological Station (Fig. 1). Nine sampling stations
were established in 3 coves (3 stations per cove) located on the north shore of the reservoir. Within
each cove, stations were located at depths of 1 m, 3
m, and 10 m. One and 3 m stations were within a
given cove and the 10 m stations were located directly outside the cove in the main body of the reservoir. All sites were located off-shore by a minimum
of 20 m. Because depth profile, size, and wind exposure were similar among the 3 coves, each was
used as a replicate for statistical analyses.
Field Collectionsand DietaryAnalysis-Adult fishes of
the target species were collected monthly from June
to August 1997 and 1998 at each station using experimental gill nets (46 m by 1.8 m) with mesh sizes
ranging from 51 to 101 mm bar-measure mesh. Gill
nets were set with the lead line on the bottom during daylight for 2 to 4 h. At each station (n = 9) a
maximum of 2 individuals of each species (gizzard
shad, river carpsucker, and smallmouth buffalo) was
sacrificed for gut content analysis (i.e., maximum of
18 individuals of each species per sample date); remaining fish were counted and released. Intestines
from sacrificed fish were immediately removed and
transported on ice to a freezer.
For analysis of diet, gut contents were taken from
the esophagus (gizzard shad) or anterior quarter of
the intestine (smallmouth buffalo and river carpsucker) and preserved in 70% ethanol. Esophageal
contents of smallmouth buffalo and river carpsucker
were not used because food items were rarely found
in this region. Moreover, food items in the anterior
gut were intact and did not appear to have undergone much digestion. Gut contents from each fish
were stirred to create a homogeneous mixture of
food items; then a 1 ml subsample was taken from
this mixture for dietary analysis. A preliminary investigation showed that replicate samples of this homogenate were not necessary because of little difference in percent volume of major food items
(+5%) among replicates. The subsample was placed
in a Sedgwick-Rafter counting slide and analyzed at
40X magnification. Relative volumes of food items
for each sample were estimated by approximating
the area occupied by individual items in 20 fields of
view. Invertebrates were identified into major taxonomic groups as follows: phylum Rotifera; orders
Copepoda and Ostracoda; family Chironomidae;
and genera Daphnia and Bosmina. Because all species
had a relatively fine-grained diet, determination of
the nature of vegetative food items was difficult.
Therefore, I classified as detritus vegetative debris,
algae, and amorphous organic matter.
Contents of the anterior one-fourth of the intestine (for all species) also were examined for crude
protein, total phosphorus, and percent organic mat-
Gido-Feeding ecology of fishes in Lake Texoma
March 2001
25
Washita River
Enlargementof study area
\ I
'
I
N
I'f
I
I
N
Lake
Texoma
10 km
FIG. 1-Location of the 9 sample stations in the Red River arm of Lake Texoma. Samples were taken at
depths of 1, 3, and 10 m in each of 3 coves.
ter to assess nutritional value of the diet. A maximum of five individuals of each species was examined each month; only fish with intestines more than
75% full were considered in these analyses. Intestinal contents were oven-dried at 60?C for 24 h and
cooled in a desiccator. They were then ground with
a mortar and pestle to homogenize the contents. Total Kjedahl nitrogen and total phosphorus were determined from a 0.25 g subsample of the ground
contents that was digested in concentrated sulfuric
acid at 440?C for 5 min. Total nitrogen was determined by the Nessler method and total phosphorus
by the ascorbic acid method (American Public
Health Association [APHA], 1985). Total nitrogen
was converted to crude protein by a standard conversion factor (total nitrogen X 6.25). Organic content was estimated in the remainder of the sample
by ash-free dry weight (AFDW) based on the difference in weight of dried samples from those combusted in a muffle furnace at 550?C for 1 h. Although crude protein and percent organic matter
may poorly reflect detritus nutritional quality because of the importance of non-protein amino acids
(e.g., Bowen, 1980), these measures should show dif-
ferences in diet due to relative proportions of invertebrate prey.
Benthic Invertebrates and Detritus-Abundance of
benthic invertebrates and the organic fraction in
benthic sediments were estimated from core samples. Core samples were taken with a combination
of an acrylic tube (8 cm diameter) and an Eckman
dredge (15 by 15 cm). First, a sample of the benthic
substratum was brought to the surface with the
dredge and a subsample was then taken from within
the dredge with the corer before opening the bottom of the dredge. This allowed retrieval of core
samples from deep stations with only minimal disturbance of organisms and organic matter at the surface-water interface. Only the top 1 cm of each core
sample was retained for analysis.
Two samples were taken at each station for each
sample date (n = 18) and stored at 4?C. In the laboratory, a 5 g (wet-weight) subsample from each
core was dried at 60?C for 24 h, and organic content
was determined by combustion as previously described. The remaining sample was passed through
a 210 Jm sieve to retain organic debris and macroinvertebrates. Macroinvertebrates were identified and
26
The SouthwesternNaturalist
enumerated under a stereoscope at 20X magnification and placed into major taxonomic categories as
previously described; with the exception of cladocerans, that were combined into one group. Herein,
all invertebrates, including those associated with the
sediment-water interface (i.e., Copepoda and Ostracoda) are considered benthic invertebrates.
SedimentationRates-Because sedimentation of organic and inorganic materials can influence benthic
resources (e.g., invertebrate abundance and deposition of phytoplankton and detritus), cylindrical
sediment traps were placed on the bottom at each
station concurrent with the gill net sampling in 1997
to estimate settling rates and organic fraction of sediments. Traps were retrieved ca. 48 h after deployment. Each trap had a diameter of 9.9 cm and depth
of 30.5 cm (height:diameter ratio > 3:1; Blomqvist
and Kofoed, 1981) and was placed so the opening
was 40 cm above the substratum. Before removal,
each trap was capped in place (using SCUBA) and
then brought to the surface. Sediment samples were
allowed to settle for 1 h and then excess water was
decanted. Dry weight of each sample was determined after drying at 60?C for 24 h, and percent
organic content was determined by combustion (as
described previously).
Data Analysis-Differences
in abundance among
depths, months, and years for each species of fish,
major benthic invertebrate taxa, percent organic
content in core samples, and sedimentation rates
were tested using a repeated measures ANOVA with
month as the repeated factor. Mean values from
paired samples were used for benthic invertebrates
and percent organic content from core samples.
Thus, for all variables there were 3 replicates (1 per
cove) at each depth. Due to this low replication, statistical power for comparisons was low and subtle
differences among treatment effects were probably
not detected. Log (x + 1) transformations of fish
and benthic invertebrate abundances were performed to maximize homogeneity of variances. Multiple comparisons among depths and months were
analyzed with Tukey HSD tests (SPSS, 1996).
Principal components analysis (PCA) was used to
characterize differences in diet among species and
across sample dates (Crow, 1978). Prior to analysis,
percent volume of major resource categories for
each species was arcsine square root transformed to
reduce deviation from normal distributions. Eigenvalues and loadings were calculated using PC-ORD
(McCune and Mefford, 1995) based on a correlation
matrix of variables. A one-way ANOVA was used to
detect differences among species in crude protein,
total phosphorus, N:P ratio, and organic content of
the diet. In addition, a general factorial ANOVA was
used to test for differences in these variables between years and among months for each species.
A Fulton-type condition index (weight/length3;
vol. 46, no. I
4
Gizzard shad
Im
-
3
_
T
3m
10m
21Ce
0
iU
0
U
*.
,
II
v
.-
II'I
Smallmouth buffalo
4-
cc 3-
-cr
0
2 -
.0
E
z
River carpsucker
2-
,
0
June
11n
~
July
1997
Aug
June
July
Aug
1998
FIG. 2-Mean abundance (individuals per hour
gill netting) of fish species taken from 3 coves in
Lake Texoma by station depth, month, and year. Vertical bars represent 1 SE.
Anderson and Gutreuter, 1983) was used to assess
condition of the three species throughout the summer. To increase sample size, lengths and weights of
fish taken from a separate gill net survey in the same
area of the reservoir (Gido, 1999) were used to calculate mean condition from June to August in 1996
and 1997. No analyses were performed in summer
1998 because of low sample sizes. Differences in condition among months were determined using a oneway ANOVA. Similarly, differences in sedimentation
rates and percent organic contents of sediments
among depths and months were examined using a
general factorial ANOVA. All post hoc comparisons
among depths and months were made by Tukey
HSD tests (SPSS, 1996). All ANOVAs were performed using SPSS (1996).
RESULTS-Fish Abundance and Diet-Differences in fish abundance
were found across
depths and years, but not among months (Fig.
of gizzard shad was sig2). Mean abundance
in
1997
than in 1998 (F1,9 =
nificantly higher
7.22, P = 0.025), but abundance did not vary
with station depth. There was a significant difference in mean abundance among depths for
smallmouth
buffalo (F2,9 = 7.07, P = 0.014),
but not between years. Although there was no
difference
in overall
abundance
among
months for smallmouth
buffalo,
they were
more evenly dispersed across depths in June
March 2001
Gido-Feeding
n
l
c.E.
.
,
r
.
3
*
All species
-e
C
DO n ,2 22 1 -
a
1 -A
G
s
,
sJRr,
-
June
July
August
River
carpsucker
J-
co
-3
O
Q-
-2
1
0
.
3
2-
1
2
3
.
Gizzard shad
a\
-3
-2
-1
0
1
2
3
3
2
/
Smallmouth
buffalo
\
.-2
-1 r-
C
- -21
'
-3
-3
-3
-2
Copepoda
Daphnia
-1
0
1
<
2
3
-3
-2
PC 1(21.3%)
-1
0
1
>
2
3
Detritus
FIG. 3-Differences
in diet of 3 omnivorous fish
species in Lake Texoma as revealed by the first 2
axes of a principal component analysis. Top left
graph represents differences in diet among species:
S = smallmouth buffalo, G = gizzard shad, R = river
carpsucker. Other graphs represent monthly changes in diet for each of the 3 species.
but primarily
occurred
at 10 m depths
27
of fishes in Lake Texoma
ecology
in July
and August. Because only 2 river carpsuckers
were taken in 1997, comparisons of mean
abundance among depths and months were
made only for 1998 and were not significant (P
> 0.10).
Principal components analysis revealed differences in diet among the three species,
though there was much overlap (Fig. 3). The
first 3 axes accounted for 48% of the variation
in diet among species (Table 1). Based on variable loadings, the first axis represented a contrast between individuals that consumed large
volumes of cyclopoid copepods (hereafter referred to as copepods) and Daphnia with those
that consumed detritus. This axis identified
both interspecific and seasonal differences in
diet of these species. The second axis represented the relative proportions of ostracods,
Bosmina, and terrestrial insects and primarily
identified intraspecific variation in the diet of
gizzard shad. Axis 3 (not shown) represented
a contrast between individuals that consumed
Daphnia and those that consumed zooplankton
ephippia and Bosmina and also showed weak
interspecific differences between smallmouth
buffalo and the other 2 species.
Averaged across months, copepods accounted for the greatest percent volume in the diet
of smallmouth buffalo (x = 50.0%), whereas
gizzard shad consumed primarily detritus (x =
80.2%, Table 2). The diet of river carpsucker
was intermediate between these species in percent volume of copepods (x = 32.3%) and detritus (x = 55.7%). In addition, all species
tended to have a greater relative volume of copepods in the diet in June (x = 40.5%) than
in July or August (x = 26.2%).
A one-way ANOVA revealed significant differences among species in dietary crude pro-
TABLE 1-Eigenvalues
and variable loadings for the first 3 principal component axes derived from an
analysis of relative volume of food items in the diet of 3 omnivorous fishes in Lake Texoma, 1997-1998.
Asterisk indicates items considered important for that axis.
Axis
Eigenvalue
Percent variance explained
Food item
Copepoda
Daphnia
Chironomidae
Ostracoda
Bosmina
Unknown zooplankton
Detritus
Rotifer
Ephippia
Terrestrial insect
PC1
PC2
PC3
2.132
22.2
1.382
13.7
1.214
12.1
-0.882*
-0.517*
-0.319
0.105
-0.206
0.161
0.916*
-0.297
-0.131
-0.222
0.153
0.065
0.004
-0.673*
-0.617*
-0.275
0.112
0.096
0.313
-0.565*
-0.165
0.454*
0.328
0.082
-0.466*
-0.315
0.216
0.211
-0.591*
0.326
G
28
vol. 46, no. I
The SouthwesternNaturalist
06
c
C~Co
M 11
ct
cli
0.3
C
~c
o 10
ao
z
a.
"5 11
Ct3
Cd
o dI
6
Co
0C
00
_
..
0.6 Q(1)
0.2-
I-0
0
C_
C'
C
0C
?r06
C
.,
d 0.3
d
o3j
cc
2
15
0.
12-
z
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9
00
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rd
56
cc
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t> I '
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N
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o
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ors
ccr
0
P.^ CO
t
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cc
m
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E
60*
40- "'
?"-,
C,.
m'o-e-
h0
06 cli
.- 20 O
Cd
(X
0
o
6
c,.
o
O
"6
/- . .^
tr c.
Gld
ltt
Jun
00 C
CC
CS --
rCl
101
0s CO
10
Clr
o00
0
:
6
6
11
c^
_
co" co^ c\r in"
"6
12
II
Cd
N
i;C
d;
r-< 0
^
C-(
Cf
d;
d
Cr1 C10
o-
o
^
r- r-(
d
0
^o~~~~o
d
d= d
^ t
o
o^
u
0C*(s
._
OI
_r
-0
Wl C
s
o
OGl
n
11
d d
d'
?
dj
?
?
73
o
C!UO~~~d.,
o
Cz
of
Aug
Jun
June
Aug
1998
Gur < f-
C,I
0_r -
1997
FIG. 4-Differences
in crude protein, total phosphorus, N:P ratio, and percent organic matter in the
diet of 3 omnivorous fish species in Lake Texoma.
Vertical bars represent 1 SE.
O
0
- 11
02
o
June
q
C*
^
_^r^SO
s_ . ^_/
?
II
.s
-S
0.0
18
0
in C3
-
.
o
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0.4
5a
Q.
. 12
C
"S
Smallmouth buffalo
Gizzard shad
River carpsucker
0o
0
d;
C
+1
.A
QI
o
(U
02
z
20
cnj
Co
10
o
*
*
30 -
U3
.
Q,H
5
tein, total phosphorus, N:P ratio, and percent
organic matter (P < 0.001; Fig. 4). Mean values
for all variables were greatest in the diet of
smallmouth buffalo, and least in gizzard shad.
Excluding 1997 data because of low sample
size, gut contents of river carpsucker had intermediate levels of all variables except N:P ratio. Low values of crude protein, phosphorus,
and percent organic matter in the diet of gizzard shad were likely due to large volumes of
sand rather than low protein content of detritus. No significant differences between years or
among months for crude protein, total phosphorus, or percent organic matter were observed in the diet of smallmouth buffalo. However, there was a significant year by month interaction for N:P ratio (F2,14 = 5.406, P =
0.018). This interaction occurred because of a
decline in N:P into summer 1997 and an increase in N:P into summer 1998. Because of
the low abundance of river carpsucker in 1997,
only differences among months for 1998 were
tested. No significant differences were found
March 2001
Gido-Feeding
ecology
of fishes in Lake Texoma
1.00
29
4000
0.95 -
1m
3m
Ostracoda
Gizzard shad (n = 288)
3000 '
I
2000
a
1000 ?
E
0.90 -
u,
Chironomidae
10
.5cp 6000 ur
0.85 -
0
0
x
^-
0.80
m 0.200
Lm
I
4000
i
2000'
li
6.0
Smallmouth buffalo (n = 142)
E
z
Copepoda
8000
6000
a
0.18 -
|
4000 ?
2000 ?|
0
^
o_&L
l
July
Aug
June
m
1997
u
0.1 -
,
0.12 -
c
"0
o 0.36 -
z .
. .
7
. . . .
.
. . I
River carpsucker (n = 36)
July
Aug
1998
FIG.6-Differences in abundance of 3 major taxa
of benthic invertebrates in core samples taken from
3 coves in Lake Texoma by station depth, month,
and year. Vertical bars represent 1 SE.
a
0.32 -
0.28--
- _ June
a
a
T
0.24 -V//
0.20
June
July
August
FIG.5-Mean condition of 3 omnivorous fish species in Lake Texoma across summer months for 1996
and 1997. Vertical bars represent 1 SE. Different letters above error bars represents significant difference between means.
for any of the variables. Dietary nutrient content of gizzard shad showed a significant effect
of month for all measures of diet quality (P <
0.025). These differences were attributed to a
general decline in all variables from June to
August.
Mean condition of gizzard shad in 1996 and
1997 was significantly lower during July and
August than in June (P < 0.05); however, no
significant differences were observed for smallmouth buffalo or river carpsucker (Fig. 5).
Benthic Invertebrates and Detritus-Chironomidae (41.2%), Copepoda (29.8%), and Ostracoda (22.9%) accounted numerically for 94%
of the total individuals from core samples. All
taxa showed heterogeneous
distributions
across station depths, months, or years (Fig. 6).
Mean abundance of ostracods was not different (P > 0.05) among months, but varied significantly among depths (F2,10 = 7.509, P =
0.010) and years (F1,10= 8.408, P = 0.016). Ostracods were typically more abundant at 1 m
and 3 m stations, and overall were more abundant in 1997 than in 1998. Mean chironomid
abundance varied differently across months
and years as shown by the significant interaction between these variables (F2,20= 20.254, P
< 0.001). Their abundance was greater inJune
than in July or August and greater overall in
1997. Mean chironomid abundance in June
was also higher at 1 m than at the 3 m or 10
m stations. Mean copepod abundance also varied significantly across months and depths
(month by depth interaction, F4,20= 9.030, P
= 0.002). Copepods were most abundant at
the 10 m stations, although this difference was
only significant for July and August.
Organic Matter in CoreSamples and Sedimentation Rates-No significant differences in organic content of core samples were detected
among months, depths, or years (P > 0.05). In
general, mean percent organic matter in core
samples was low (3.87%) and varied considerably among paired samples (CV = 115%). Ad-
The SouthwesternNaturalist
30
700
vol. 46, no. 1
parently, this species either compensates for
the low nutritional quality of its food by increasing
consumption rates or assimilation efE 500;
ficiency (Grimm, 1988), or by supplementing
its diet with small amounts of invertebrates
I 400- when available (Yako et al., 1996).
300i
I
T
Food items consumed by these species were
similar
to those reported previously (Walburg
E 200and Nelson, 1966; Tafanelli et al., 1971; Sum(I
Q)U)
::
100'':
.
'
merfelt et al., 1972; Pierce et al., 1981). However, the percent volume of detritus in their
0
June
diets was typically less. Other investigators have
July
Aug
reported mean volumes of detritus of 68% in
FIG.7-Sedimentation
rates taken from 3 coves on
river
carpsucker (Summerfelt et al., 1972) and
Lake Texoma across 3 depths in 1997. Vertical bars
than 65% in smallmouth buffalo (Tagreater
1
SE.
represent
fanelli et al., 1971) in Oklahoma reservoirs.
Corresponding values for the present study
were 57% and 36%, respectively. Because the
ditionally, there were no differences (P < 0.05)
in percent organic matter in sediment samples other studies were carried out over the entire
among months or depths, suggesting a relative- year, it is possible that these differences are
ly homogeneous distribution of organic matter due to the greater proportion of zooplankton
in sediments. Sedimentation rates did vary sig- in the diet during summer. In any case, previnificantly among depths (F2,13= 8.389, P = ous studies agree with the findings of this one
that both species are facultative detritivores
0.005) and months in 1997 (F2,13 = 7.837, P =
0.006; Fig. 7). Mean sedimentation rate was and consume detritus when availability of ingreatest in July, and, in all months, was signif- vertebrates is low (i.e., July and August).
Differences in feeding strategies among fish
icantly greater at the 3 m and 10 m stations
than at 1 m stations. Thus, even though organ- species may influence their relative imporic fraction of sediments appears homogeneous
tance in reservoir ecosystems. In this study, gizacross habitats, the rates of deposition dif- zard shad ingested large amounts of detritus
fered.
and inorganic sediments, whereas smallmouth
in
buffalo
and river carpsucker filtered inverteDIscussION-Interspecific Differences Feeding
Ecology-In Lake Texoma, diets of gizzard shad, brates and detritus without ingesting much inriver carpsucker, and smallmouth buffalo dif- organic matter. Thus, gizzard shad would prefered in invertebrate species composition, nu- sumably process more materials and have a
trients, and organic content. These differences
greater effect on fragmentation and decomwere partially due to the different feeding
position of detritus, microorganisms, and algae
in sediments in Lake Texoma. The effect of
strategies of these fishes. Based on morphology
and diet, gizzard shad can be classified as a foraging by river carpsucker and smallmouth
scooper (Gerking, 1994), that scoops mud buffalo on fragmentation and decomposition
from the bottom and roughly sorts and dis- of detritus is more likely through initial procards unwanted matter through the gill rakers. cessing of detritus and sediments rather than
Smallmouth buffalo and river carpsucker are direct consumption and passage of materials
suction feeders and more selectively filter de- through their alimentary canal.
tritus and invertebrates through their gill rakSpatial and TemporalVariationin Diet and Food
ers. Percent organic matter in gut contents of Resources-Reservoir ecosystems can present a
all fishes was higher than in core samples, in- spatially and temporally heterogenous environdicating selective consumption of food items ment for fishes. External inputs from the wafrom the substrate. However, in comparison to tershed, wave action, and fluctuation in water
the two catostomids, the percent organic con- level can influence deposition and oxidation of
tent in the diet of gizzard shad was low sediments and detritus. In addition, mid-sum(<20%), owing to consumption of large quan- mer succession of zooplankton (Threlkeld,
tities of inorganic sand along with detritus. Ap- 1986) and benthic invertebrate (Sublette,
-
600 -
r
3m
March 2001
Gido-Feeding
ecology of fishes in Lake Texoma
1957) assemblages creates seasonally available
food resources for many reservoir fishes. In
this study, I found that declines in invertebrate
abundance, particularly copepods, at the 1 m
and 3 m stations from June to August corresponded to declines in relative volume of invertebrates in the diet of the 3 fish species. For
gizzard shad, organic content and crude protein declined from June to August and was followed by a decline in condition. Thus, at least
for gizzard shad, food quality and quantity declines into the summer, and they shift to a diet
with lower nutritional value (i.e., detritus).
Numerous studies have shown that omnivorous fishes will switch to a lower quality diet
when invertebrate abundance declines (Brabrand, 1985; Mundahl and Wissing, 1988; Ahlgren, 1990a; Lobon-Cervia and Rincon, 1994;
Ahlgren, 1996). Mundahl and Wissing (1987)
found a similar pattern in an Ohio reservoir,
where gizzard shad switched from a mixed zooplankton and detritus to a primarily detritus
diet, resulting in lower growth and condition
during summer. Ahlgren (1990b) also reported
that juvenile white sucker (Catostomus commersoni) fed only detritus lost weight, whereas
those fed invertebrates gained weight. Bowen
et al. (1995) suggested that, because of limited
availability of invertebrates, many fish consume
detritus to supplement their diet; however, nutritional quality of this resource is not adequate to sustain growth and reproduction.
Whereas the nutrient and organic contents
of the diet, along with condition of gizzard
shad, declined from June to August, there was
no significant decline in these parameters for
smallmouth buffalo or river carpsucker. Even
though the relative volume of invertebrates in
the diet of these species appears to decrease
into the summer, they still may be able to maintain condition by supplementing their diet
with detritus (Bowen et al., 1995). Moreover,
smallmouth buffalo likely selected deeper habitats during July and August because of the
greater abundance of copepods in these habitats.
Differences in nutritional quality of detritus
in sediments among habitats may also influence distribution and abundance of detritivorous fishes. Lemke and Bowen (1998) showed
that nutritional quality of detritus in sediments
was greater in areas that were exposed to turbulence from waves than in profundal zones
31
sheltered from waves. Bowen (1984) also
showed that the condition of male tilapia (Sarotherodon)was lowest in habitats with low quality foods. Although I did not rigorously examine nutritional quality of detritus in sediments
(e.g., examine amino acids), I did show percent organic matter in core samples and freshly deposited sediments did not vary across
depths. This homogeneous distribution of organic matter in upper layers of sediments appears to be due to deposition of sediments of
similar organic composition.
Thus, even
though spatial differences in benthic invertebrates occurred, organic matter in sediments
was relatively constant across habitats. Homogeneous distribution of organic matter in sediments may correspond to broad spatial distribution of gizzard shad, and perhaps river carpsucker, that have higher proportions of detritus in their diet. More detailed examination of
detritus quality (e.g., amino acids and hydrolysis-resistant organic matter) would be necessary to determine if detritus quality affected
distribution of these species.
Because benthic fishes must process sediments to attain nutritionally important food
material (Minckley et al., 1970; Mundahl,
1991), rate of sediment deposition presumably
can influence their effect on benthic communities. For example, Mundahl (1991) showed
that gizzard shad processed <4% of the sediments deposited in an Ohio reservoir. He concluded that, because of high rates of deposition of sediments, foraging by gizzard shad
should have little effect on benthic communities. Typical of many reservoirs (Neel, 1966),
sedimentation rates in Lake Texoma were
high. At the scale of the whole reservoir, Lake
Texoma has lost >11% of its initial storage capacity due to sediment deposition between
1942 and 1985, with the greatest areas of deposition in the riverine portions of the reservoir (H. Hartwell, United States Army Corps of
Engineers, pers. comm.). Sediment trap data
from this study further suggests sediment deposition also increases with depth at the scale
of individual coves. Therefore, these spatial differences in sedimentation will likely influence
the impact these species have on benthic invertebrate assemblages.
Conclusion-The success of omnivorous species such as gizzard shad, smallmouth buffalo,
and river carpsucker in many southern reser-
32
The SouthwesternNaturalist
voirs is likely due to their ability to switch to
lower quality food items when invertebrate
prey are in low abundance
(e.g., Cherry and
Guthrie, 1975) and the ability of adults to
evade predation because of their large body
size (Stein et al., 1995). Interspecific differences in distribution, abundance, and foraging behavior of these species along with variable abiotic conditions
their relative
likely influence
importance in reservoir ecosystems. This study
suggests gizzard shad should have the largest
per capita effect on ecosystem processes given:
1) low organic content in their diet and presumably high rates of sediment processing; and
and processing of detritus that
2) consumption
would otherwise be locked in sediments.
Assistance in the field and laboratory was provided
by R. Durtche, S. Gido, W. Luttershmitt, W. Matthews, J. Schaefer, and W. Wolfinbarger. I also thank
R. Durtche, E. Marsh-Matthews, B. Narin, and R.
Smiley for thoughtful discussions. Earlier versions of
this manuscript greatly benefitted from comments
by L. Canter, M. Kaspari,J. Schaefer, W. Shelton, W.
Matthews, and C. Vaughn. Partial funding for this
project was provided from the Graduate Student
Senate of the University of Oklahoma. This research
was in partial fulfilment of a Ph.D. at the University
of Oklahoma.
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Submitted 7July 1999. Accepted29 December1999.
AssociateEditor was David R. Edds.