a method for determining stomach fullness for planktivourous fishes
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8/2/2019 A Method for Determining Stomach Fullness for Planktivourous Fishes
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A Method for Determining Stomach Fullness for
Planktivorous Fishes
QUINTON E. PHELPS*1
Department of Wildlife and Fisheries Sciences, South Dakota State University, Box 2140B,
Northern Plains Biostress Laboratory 138, Brookings, South Dakota 57007, USA
KIPP A. POWELL AND STEVEN R. CHIPPS
U.S. Geological Survey, South Dakota Cooperative Fish and Wildlife Research Unit, 2
Department of Wildlife and Fisheries Sciences, South Dakota State University, Box 2140B,
Northern Plains Biostress Laboratory 138, Brookings, South Dakota 57007, USA
DAVID W. WILLIS
Department of Wildlife and Fisheries Sciences, South Dakota State University, Box 2140B,
Northern Plains Biostress Laboratory 138, Brookings, South Dakota 57007, USA
Abstract. —Mean stomach fullness provides a useful index for
quantifying fish diets. However, estimating stomach fullness for
planktivorous fishes can be time-consuming and prone to error
because of small prey and unidentifiable remains. In this study
we developed a predictive equation for estimating the stomach
volume of yellow perch Perca flavescens as a function of total
length (TL). We then used an optical plankton counter (OPC) to
estimate the biovolume of invertebrate prey consumed. The OPC
quickly estimated the digital size and abundance of zooplankton
prey, which can then be converted to estimates of prey
biovolume. Stomach volume (V [mm3]) for yellow perch (113–
279 mm TL) was significantly related to body size ( L [mm]) andwas estimated as V ¼ 33 10À7 L2.96. Using the OPC, yellow
perch stomach contents (99% Daphnia pulex ) were converted to
prey biovolume (mm3) and then divided by stomach volume
(mm3) to estimate stomach fullness (%). This approach provided
reasonable estimates of stomach fullness ranging from 3% to
85% (mean ¼ 21%). Although the initial cost for the OPC
equipment is relatively high, this method provided substantial
time and labor savings compared with traditional approaches for
quantifying zooplankton abundance and biomass (e.g., micro-
scopic identification and enumeration and length–mass conver-
sions). Similarly, the OPC can be used to estimate the abundance
and biomass of freshwater zooplankton, thus reducing the time
and costs associated with traditional plankton analyses. The
approach is limited, however, in cases where very small prey
(,250 lm) are a dominant proportion of the sample because of
the potential errors involved in detecting and estimating the
biovolumes of small particles.
Accurate quantification of fish diets is an important
aspect of fisheries management. Traditionally, prey items
removed from fish stomachs are identified, counted, and
measured for mass or volume. While this approach
works well for larger prey such as fish or macroinver-
tebrates, estimating the abundance, biomass, or volume
of zooplankton prey can be time-consuming. Moreover,
estimates of zooplankton biomass in fish diets can be
compounded by error associated with subsampling and
conversions used to estimate zooplankton mass. An
optical plankton counter (OPC) developed for automated
counting and sizing of zooplankton has been widely used
in quantifying plankton biomass in marine environments
(Wieland et al. 1997). Recent application of OPC
technology to freshwater zooplankton indicated signif-
icant correlations between OPC and field-derived
estimates of freshwater zooplankton biomass (Sprules
et al. 1998). Compared with traditional methods for
calculating mass and biovolume for freshwater zoo-
plankton (Wetzel and Likens 1991), the OPC has the
potential to provide a new, rapid means for assessing
zooplankton biomass from fish diets.
A variety of approaches have been used to estimate
the stomach volume of fishes, including subjective
visual estimates (Hynes 1950), maximum observed
prey volume among different fish size categories
(Knight and Margraf 1982), and injection of water or
air into empty stomachs (Burley and Vigg 1989).
Estimates of stomach volume based on maximum
observed prey volume provide logical measures, but
require large sample sizes across a range of fish
lengths. Satiation of the stomach can require fewer fish,
but must be performed under artificial settings that may
affect the appetite and feeding level of fish.
The objective of this study was to assess the
usefulness of OPC technology for estimating zoo-
*Corresponding author: [email protected] Present address: Fisheries and Illinois Aquaculture Center
and Department of Zoology, Southern Illinois University, LifeScience II, Room 173, Mailcode 6511, Carbondale, Illinois62901, USA.
2 The South Dakota Cooperative Fish and Wildlife Research
Unit is jointly supported by the U.S. Geological Survey, SouthDakota State University, South Dakota Department of Game,Fish & Parks, and the Wildlife Management Institute.
Received February 13, 2006; accepted January 10, 2007Published online August 2, 2007
932
North American Journal of Fisheries Management 27:932–935, 2007Ó Copyright by the American Fisheries Society 2007DOI: 10.1577/M06-066.1
[Management Brief]
452-F
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plankton biovolume and to use these measures to
calculate stomach fullness for yellow perch Perca
flavescens. Stomach fullness is a useful diet measure
that is correlated with other diet indices such as total
prey mass and prey caloric contribution (Pope et al.
2001). To calculate stomach fullness, information is
needed on stomach volume of the predator and total
biovolume of prey in the stomach. Stomach fullness is
then calculated as the ratio of total prey volume to
predator stomach volume, thereby providing a diet index that accounts for fish size (Knight and Margraf
1982).
Methods
Estimating stomach volume.—To measure the stom-
ach volumes of yellow perch, we used a modification
of the injection technique described by Burley and
Vigg (1989). Yellow perch (113–279 mm total length
[TL]; n ¼ 18) were collected through a hole in the ice
from East 81 Lake (Kingsbury County), South Dakota,
transported to the laboratory, identified to sex, andmeasured for TL (mm) before removal of stomach
contents. Stomachs were excised from each fish and
then contents were flushed with water from a 50-mL
syringe. After flushing, stomachs were clamped at the
pyloric valve (Bond 1979) and liquid epoxy was slowly
injected through the anterior end (i.e., posterior end of
esophagus) until the entire stomach was distended; the
esophagus was then clamped as close to the stomach as
possible. After 24 h of drying, stomach linings were
carefully scraped away and the epoxy plugs were
measured by water displacement to the nearest 0.5 mL.Stomach volumes were then related to fish TL using
log10
–log10
linear regression analysis.
Estimating zooplankton biovolume.—We collected
40 yellow perch (203–267 mm TL) by angling from
Waubay Lake in northeastern South Dakota in
February 2002. Yellow perch were collected through
a hole in the ice, transported to the laboratory, and
examined to determine gender and TL before removal
of stomach contents. However, only 55%
of theseyellow perch (n ¼ 22) contained food items; hence,
estimates of zooplankton biovolume were based on
these fish. Stomachs were excised at the junction of the
esophagus and pyloric valve as previously described,
and contents were removed by flushing the stomach
with distilled water. Stomach contents were then frozen
in distilled water until analysis (,2 weeks) to reduce
distortion and degradation of zooplankton prey. The
freezing process appeared to have little influence on
zooplankton degradation; most zooplankton were fully
intact after thawing from the ice before processing.
The stomach contents of individual fish were
processed using an OPC (Model 1 L) connected to a
laptop computer (Figure 1). The system was configured
as a recirculating system following the manufacturer’s
guidelines (MacKay 1996; Focal Technologies Corpo-
ration 1999). To facilitate flow through the OPC unit,
3–5 drops of a wetting agent (i.e., detergent) were
added to distilled water heated to 308C. Water in the
recirculating system was pumped from a 20-L holding
tank to a 1.3-L reservoir fitted over the OPC.
Zooplankton samples from fish diets were added to a
beaker and brought to 250 mL with distilled water.Diluted samples were then slowly (;2 min/sample)
added to the 1.3-L reservoir while the system was
circulating water and allowed to flow by gravity
through the OPC before reaching the main reservoir.
Samples were then collected in a bucket lined with a
63-lm mesh screen. Before processing any sample, the
OPC system was allowed to equilibrate to eliminate air
bubbles. The collecting bucket was emptied after each
sample and water in the recirculating system was
changed after five to eight samples to maintain a water
temperature of 30 6 28C. Following OPC processingall stomach contents were combined into a 500-mL
beaker and 10% of the sample was examined under a
dissecting microscope to quantify zooplankton com-
position.
To estimate the size distribution, the digital size of
the zooplankton passing through the OPC was
converted to an equivalent circular diameter (ECD)
that approximated the diameter of a circular disk
blocking the same amount of light as the organism
(Sprules et al. 1998; Focal Technologies Corporation
1999). The ECD values generated from the OPC werethen converted to volumes (V ) using the equation
V ¼P
63
ECD3
1:7689
FIGURE 1.—Diagram of the system using an optical
plankton counter (OPC) to estimate the biovolume of
zooplankton prey from the stomachs of yellow perch collected
at Waubay Lake, South Dakota.
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(Sprules et al. 1998). Total biovolume generated by the
OPC for each sample was divided by the estimated
stomach volume of the fish to provide a measure of
percentage stomach fullness.
Results and Discussion
The maximum stomach volume for yellow perch
was predicted from the equation
V ¼ 33 10À7 L2:96
;
where V is the empirical stomach volume (mm3) and L
is the empirical fish TL (mm) (log10
–log10
least-
squares regression; n ¼ 18; r 2 ¼ 0.96, P , 0.001;
Figure 2). Because bursting capacity was not measured,
our approach probably underestimated true maximum
stomach distension. Nonetheless, our equation predict-
ed stomach volumes (as a proportion of body size)
similar to those reported for other fish species (Knight
and Margraf 1982; Pope et al. 2001).
A composite sample of stomach contents revealed
that Daphnia pulex was the predominant prey item
(99%) in yellow perch collected during winter 2002.
Ceriodaphnia spp. and adult copepods were also
present but represented less than 1% of total zooplank-
ton composition. Zooplankton biovolumes, estimated
from the OPC, ranged from 0.07 to 3.7 mm3. Dividing
zooplankton biovolume by stomach volume provided
reasonable estimates of stomach fullness that ranged
from 3 to 85% (mean¼ 21%; Table 1).
The OPC provided a rapid analysis of zooplankton
biovolumes; the average processing time was about 10
min/fish. Because the OPC does not distinguish
between zooplankton species, however, prey composi-
tion must be quantified by means of traditional
approaches. Moreover, because the minimum detection
limit of the OPC is about 250 lm, it does not detect most rotifers or small copepod nauplii (Sprules et al.
1998). If these taxa are abundant in the diet, then the
OPC would probably underestimate true prey biomass.
Similarly, small pieces (,250 lm) of zooplankton
remains may be underestimated (i.e., not counted) by
the OPC; although not evaluated here, this may
influence total prey volume of zooplankton removed
from fish stomachs. Often times, diet items removed
from fish stomachs are in various stages of degradation
or digestion. To reduce these problems, we suggest
using only prey items found in the stomachs of fishesand not in the intestines. We also recommend that
subsamples of fish diets be taken and examined under
the microscope, which will allow evaluation of species
composition as well as relative abundance of zoo-
plankton remains.
Although the shapes of freshwater zooplankton are
variable, Sprules et al. (1998) showed that the use of a
single geometric model (oblate spheroid) provided
accurate estimates of biovolume for mixed zooplankton
assemblages. Using mixed assemblages of freshwater
zooplankton, they demonstrated that size distribution
of zooplankton measured using the OPC were
remarkably similar to actual measurements determined
from the same samples with a microscope (Sprules et
al. 1998). Although comparisons between traditional
FIGURE 2.—Maximum stomach volume as a function of
total length for yellow perch (n ¼ 18) from East 81 Lake,South Dakota. See text for details.
TABLE 1.—Total length, food volume, maximum stomach
volume, and percent stomach fullness for yellow perch (n ¼
22) from Waubay Lake, South Dakota. Means (SEs) are
shown in the last row.
Length
(mm)
Food
volume (mm3)
Stomach
volume (mm3)
Stomach
fullness (%)
203 0.28 1.94 14.4
204 1.02 1.97 51.8
209 0.07 2.12 3.3215 0.07 2.30 3.0
215 0.24 2.30 10.4216 0.61 2.33 26.2
216 0.37 2.33 15.9
217 0.33 2.37 13.9221 0.63 2.50 25.2
228 0.73 2.74 26.6
231 1.76 2.85 61.8234 0.54 2.96 18.2
234 0.31 2.96 10.5
240 0.36 3.19 11.3241 0.84 3.23 26.0
245 0.82 3.39 24.2
247 0.32 3.47 9.2255 0.16 3.81 4.2
258 0.22 3.94 5.6265 0.68 4.27 15.9
265 0.21 4.27 4.9
267 3.70 4.36 84.9234 (4.3) 0.65 (0.17) 2.98 (0.16) 21.2 (4.3)
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approaches (e.g., microscopy) and OPC measurements
help validate OPC-based estimates, traditional
approaches can be influenced by sampling and
measurement error, thus affecting the accuracy of
biovolume estimates. For this reason true validation of OPC-based measurements would be difficult to obtain.
Nonetheless, when combined with information on
stomach volume, the OPC approach provided quick
and reasonable estimates of stomach fullness for
yellow perch feeding on cladoceran prey. Depending
on the questions being addressed, stomach fullness can
provide a useful measure for diet studies; Pope et al.
(2001) demonstrated that mean stomach fullness was
highly correlated with caloric-based indices that require
more time and information to calculate. Moreover, the
OPC can be used to estimate abundance and biomass of freshwater zooplankton from field samples, thereby
providing multiple uses that reduce the time and costs
associated with traditional plankton analyses.
References
Bond, C. E. 1979. Biology of fishes, 2nd edition. Saunders,
Orlando, Florida.
Burley, C. C., and S. Vigg. 1989. A method for direct
measurement of the maximum volume of fish stomachs
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Focal Technologies Corporation. 1999. Optical plankton
counter: users guide. Focal Technologies Corporation,
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Hynes, H. B. N. 1950. The food of freshwater sticklebacks(Gasterosteus aculeatus and Pygosteus pungitius) with a
review of methods used in studies of the food of fishes.
Journal of Animal Ecology 19:35–58.
Knight, R. L., and F. J. Margraf. 1982. Estimating stomach
fullness in fishes. North American Journal of Fisheries
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MacKay, I. 1996. Using the OPC 1L laboratory unit:
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largemouth bass. Environmental Biology of Fishes
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Wetzel, R. G., and G. E. Likens. 1991. Limnological analyses.
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