A Method for Determining Stomach Fullness for Planktivourous Fishes

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



DAVID W. WILLIS



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



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.

MANAGEMENT BRIEF 933



(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)

934 PHELPS ET AL.



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

or digestive tracts. Journal of Fish Biology 34:707–714.

Focal Technologies Corporation. 1999. Optical plankton

counter: users guide. Focal Technologies Corporation,

Blacksburg, Virginia.

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

Management 2:413–414.

MacKay, I. 1996. Using the OPC 1L laboratory unit:

application note no. 4. Focal Technologies Corporation,

Dartmouth, Nova Scotia.

Pope, K. L., M. L. Brown, W. G. Duffy, and P. H. Michaletz.

2001. A caloric-based evaluation of diet indices for

largemouth bass. Environmental Biology of Fishes

61:329–339.Sprules, W. G., A. W. Herman, and J. D. Stockwell. 1998.

Calibration of an optical plankton counter for use in

freshwater. Limnology and Oceanography 43:726–733.

Wetzel, R. G., and G. E. Likens. 1991. Limnological analyses.

Springer-Verlag, NewYork.

Wieland, K., D. Peterson, and D. Schnack. 1997. Estimates of

zooplankton abundance and size distribution with the

optical plankton counter (OPC). Archive of Fishery

Marine Research 45:271–280.

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A Method for Determining Stomach Fullness for Planktivourous Fishes