Summerfelt, S. T., and R. C. Summerfelt. 1996. Aquaculture of walleye as a food fish. Pages 215- 

230 in R. C. Summerfelt, editor. Walleye culture manual. NCRAC Culture Series 101. North 

~ 

Central Regional Aquaculture Center Publications Office, lowa State University, Ames. ~ 

Aquaculture of Walleye as 

a Food Fish 

Steven T: Summerfelt, The Conservation Fund’s Freshwater Institute, PO. Box 1746, Shepherdstown, WV 

25443, and Robert C. Summerfelt, Department of Animal Ecologx lowa State Universitx Ames, lowa 

500 1 1-322 I 

Introduction 

Food-size walleye 

A food-size walleye is defined as the size that is 

acceptable for a given market (Figure l), but the smaller 

the acceptable size, the faster the turnover time in the 

culture system. The premises we used to define the 

minimum live weight of a food-size walleye are based 

on portion size and fillet dress-out percentage; a 40% 

dress out for a skin-on fillet would require a live weight 

of 1.25 lbs (567 g) to obtain two 4-oz slun-on fillets 

(Summerfelt et al. 1996). 

Culture systems for raising food-size walleye 

The technology selected to raise food-size walleye is 

critical for the success of commercial culture. The lack 

of a real commercial system means we must speculate 

about production strategies. Both extensive and 

intensive culture systems arc 

examined. 

Pond culture of phase I and 

phase I1 fingerlings has 

been a viable commercia1 enterprise for 

many years at many 

locations in the north 

central region, especially Minnesota. 

Presently, the entire 

crop of fingerlings is 

marketed to individuals, angler groups, lake 

associations, and Public 

agencies for stoclung 

(Kinnunen 1996). In a 1991 survey of fish producers in 

the 12 states of the north central region, 16.4% of the 

295 respondents reported that they were raising 

walleye; walleye represented 8% of total gross sales for 

the region, and ranked fourth of 61 cultured species 

(Hushak 1993). 

A scenario of pond culture of food-size walleye is 

given, and although some aspects are speculative 

because of research gaps of various aspects of the 

process, the facts are not encouraging for successful 

production of walleye to food-size in ponds. The culture 

practice for raising walleye to food size in ponds might 

proceed as follows: Phase I fingerlings (1.25-to 2.5-in 

[32-64 mm]), would be harvested from ponds, and 

habituated to formulated feed either in intensive culture 

or after restocking ponds at a density appropriate for 

growth to an advanced size. Phase I fingerlings 

must be harvested because fish density 

must be known before grow-out to 

phase I1 fingerlings 

can occur. Although 

we are not aware of 

any report describing 

habituation of walleye 

to formulated feed in 

ponds, they have been 

converted to feed in 

cages in ponds 

(Blazek 1996; Bushman 1996; Harder and 

Summerfelt 1996; 

Stevens 1996). Figure 1. A food-size walleye and a standard 10-in (254 mm) 

dinner plate for comparison. 

NCRAC Culture Series 101 - Walleye Culture Manual 21 5 
After stocking grow-out ponds at an appropriate 

density, fingerlings would be raised to the end of the 

growing season to a size known as “phase I1 fingerings”. The average size of phase I1 fingerling by the end 

of the first summer may be 5 to 8 in (127-203 mm). 

Stevens (1996) reported that fingerlings reached “about 

8 in (203 mm) by the fall of the first year in southern 

Iowa, but a 10-in (254 mm) average may be reached in 

the southern part of the north central region. Although 

phase I1 fingerlings must be overwintered, overwintering walleye fingerlings in Iowa has been accomplished with minimal mortality providing ponds are 

adequately aerated (Bushman 1996). Bushman (1996) 

fed caged walleye at the rate of 1% of their body weight 

per day from November through April, but the fish did 

not grow and their condition declined over-winter. 

The culture routine would continue in the second 

summer. Stevens (1996) said that most fish reached 12 

to 14 in (305-356 mm) by the end of the second 

summer. Based on the experience of Stevens (1996) and 

Bushman (1996), the short growing season in Iowa 

precludes walleye reaching food-size until the middle to 

latter part of the third summer. The culture interval 

needed to reach food-size would be even longer in 

northern Michigan, Minnesota, North Dakota, Wisconsin or Canada. 

Major impediments to the success of a pond culture 

strategy will be the long turnover time from fry to 

market size; potential winterlull; and although walleye 

may tolerate temperatures up to 90°F (32°C) for short 

periods (Collette et al. 1977), prolonged high summer 

temperatures may result in summerkills as well. 

On the other hand, intensive culture of walleye fingerlings is a well-established practice. Phase I pond-raised 

fingerlings are commercially available in abundant 

supply, and they can be habituated to formulated feed in 

intensive culture with survival rates as high as 90% 

(Nagel 1996). Also, if ponds are not available to 

produce phase I fingerlings, fry can be cultured 

intensively by feeding them brine shrimp or formulated 

feed (Colesante 1996; Moore 1996; Moode and Mathias 

1996; and Summerfelt 199613). Facilities for intensive 

culture of fry allow use of out-of-season spawning to 

obtain fry at least twice per year. Once feed-trained 

fingerlings (Le., fingerlings that are habituated to 

formulate feed) are produced, there are many commercial feeds suitable for grow-out to food size in intensive 

culture (Stettner et al. 1992; Bristow 1996), and open 

formula Qets have been developed for specific life 

stages of walleye (Barrows and Lellis 1996). 

Flow-through culture can be used to produce phase I1 

fingerlings and for grow-out to food size. It is the major 

cultural technology used for raising rainbow trout. 

Phase I walleye fingerlings are often habituated to 

formulated feed in raceways; many public agencies use 

this technology to produce phase I1 fingerlings. It 

should not be difficult to raise walleye to food size in 

raceways or circular tanks, although the tanks would 

have to be covered in order to reduce light intensity to 

levels suitable for light-sensitive eyes of walleye; 

walleye cannot be cultured in full sunlight. The major 

constraint to flow-through culture of walleye is the 

requirement for an abundant supply of water in the 

range 68-77°F (23.8-25°C). Growth rates of walleye at 

temperatures 40°F (155°C) are nearly zero. Thus, the 

major constraint to general use of flow-through culture 

is the availability of sufficient water sources with 

desirable water temperatures. In northern parts of the 

U.S. and Canada, ground water temperatures are 

typically 40°F (lO°C), and it is impractical to heat 

water for a single-pass system. 

Recycle systems are a third technology that can be used 

for walleye culture (Summerfelt 1993). The advantages 

of recycle culture systems for raising walleye to food 

size are the same as those for other species: controlled 

water temperature that creates a 12-month growing 

season; low water requirements relative to production 

capabilities; a small volume of concentrated waste; and 

the ability to locate a facility close to major markets. In 

recycle culture, fish are stocked at high densities, raised 

on pelleted feeds, and there is an intentional effort to 

minimize the use of new water to 15% or less of total 

system volume per day. The effluent from the culture 

tanks can undergo several treatment processes before it 

is returned to the culture tank: clarification to remove 

solids; nitrification (biofiltration) to convert ammonia to 

nitrate; reaeration or reoxygenation; and &sinfection by 

passing water through tubes with UV lamps or by 

ozone injection (Summerfelt 1996). 

Recycle systems can be used to culture walleye 

anywhere in North America because the volume of 

water used is typically 15% of the water requirements 

needed for a single-pass system (Summerfelt 1996). If 

water needs to be heated to obtain the desirable 

21 6 NCRAC Culture Series 101- Walleye Culture Manual 
temperature for growth, it must be reused if it is to be 

economic. Hushak's (1993) survey of fish producers in 

the North Central Region indicated that 89% used 

ponds, but a surprising 14% used some form of recycle 

system to produce a variety of fishes. There are many 

recycle culture facilities in the Midwest and elsewhere 

that raise food-size tilapia, and others that raise hybrid 

striped bass. Moode and Mathias (1996) have designed 

a recycle culture system for walleye for sites as far 

north as Winnipeg, Canada. Summerfelt (1 996) 

provides design criteria for large-scale recycle system 

for walleye culture. 

Objectives 

The objectives of this chapter are to report dissertation 

research by Summerfelt (1 993) in which walleye were 

raised in a recycle system for one year and to review 

selected topics related to intensive culture of walleye as 

a food fish. Data from the literature are used to analyze 

growth of walleye in relation to temperature and fish 

size. This information, as well as other factors that 

influence success of walleye culture in a recycle 

facility, is examined to determine the feasibility of 

culturing walleye to food size. 

Walleye cultured in a recycle culture system 

Background 

Summerfelt (1 993) raised walleye in 264-gal (1,000 L) 

circular tanks in a recycle system used for research on 

nitrification and clarification. Walleye were raised in the 

recycle system until they were 368 

days old. That study is used here as a 

usually ranged from 15 to 30 NTU's. However, during 

short and irregular intervals when the biofilter sloughed 

biofilm, turbidities reached 120 NTU's. Light intensity 

at the water level was about 10 lux. 

Fish 

Fingerlings were obtained July 17, 1990 from Welch 

Lake, a 57-acre (23 ha) production site of the Iowa 

Department of Natural Resources Spirit Lake Hatchery 

(Jorgensen 1996). Parent fish were from Spirit Lake and 

East and West Okoboji Lakes in northwest Iowa. Fry 

were stocked in Welch Lake when they were 3 days old. 

Fingerlings were stocked into the culture tanks when 

they were 55 days old; they averaged 1.8 in (46.4 mm) 

long and weighed 298.4/1b (1 -52 9). At the end of a 30- 

day training interval (84 days old) the fish averaged 

90.7/lb and 3.4 in (200kg and 87 mm). 

Feeding 

Fish were fed 16% of their body weight per day in the 

3-week interval they were habituated to formulated 

feed. Feeding rates and feed sizes were altered as the 

fish grew (Table 1). Feeding rates now used for habituation fingerlings are considerably less, and recommendations for that process are described by Bristow (1996), 

Flowers (1996), and Nagel (1996). 

Fish density 

Fingerlings were stocked at 35.2 fish/ft" (1 ,250/m3), a 

density of 0.38 lb/ft3 (6.25 kg/m3). Walleye were grown 

to a density of 4.5 lb/ft3 (72.1 kg/m3). A density of 6.0 

basis for estimating how long it 

would take to raise walleye to food 

Table 1. Feed type and size used in walleye growout 

experiment. 

size. 

Upon introduction of the 55-day-old 

pond-raised fingerlings to the 

intensive culture system and during 

the interval of training fingerlings to 

formulated feed, the temperature was 

held at 68°F (20°C) to prevent an 

outbreak of columnaris. During the 

rest of the culture period, the average 

temperature was 93.4"F. The maxmum temperature observed was 

80.6"F during the summer. Turbidity 

Fish weight Feeding rate Feed size Feed type' 

(9) (% BW/day) 

5 16.0 4 crumble Biotrainer 

11 8.0 2.4 (3/32-in) BioDry 

15 6.0 3 BioDry 

40 5.0 4 BioDry 

80 4.0 6 BioDry 

160 3.0 8 BioDry 

250 1.8 9.5 WG9015 

Biotrainer and BioDry feeds were from Bioproducts, Warrenton, Oregon: 

walleye grower diet (WG9015) was from Nelson & Sons, Murray, Utah. 

NCRAC Culture Series 101 - W a l l e y e C u l t u r e Manual 21 7 
lb/ft3 (96.2 kg/m3) was attempted for a short 

period, but it was discontinued because 

dissolved oxygen 25 mgL could not be 

maintained with only diffused air; pure 

oxygen was not used. 

Length- weight rela tionship 

Because the relationship between fish length 

and weight is curvilinear when expressed 

arithmetically, a log-log (base 10) transformation of length-weight data was used to 

obtain a straight line fit (Figure 2). 

The log-log equation was: loglO(W) = 2.929 

log,,(L) - 4.91, where log,,(W) is the log 

base 10 of the weight of the fish in grams and 

log,,(L) is the log base 10 of the length of the 

fish in millimeters. Using this equation, a 

16.3-in walleye (415 mm) would have a 

calculated weight of 1.25 lb (567 9). The 

calculated weights of walleye from 1 to 20 in 

(25-508 mm) are presented in Table 2. 

Y = (2.929 * loglo X) - 4.91; r2= 0.987 

I, I, I, 3.00 ~ 

0.00 ~ I' I' I' IN 

1.80 1.90 2.00 2.10 2.20 2.30 2.40 2.50 2.60 2.70 2.80 

loglo Length (mm) 

Figure 2. Logarthmetic relationship between length and weight 

of walleye grown in a recycle system (data from Summerfelt 

1993). Graph is of 2801 measurements of length and weight of 

walleye 55 to 532 days old. 

Length-weight data for fish 1 to 10 in (25-254 mm) 

total length from Piper et al. (1 982) that represented 

fish with a condition factor similar to walleye was used 

to calculate the following equation: log,,(W) = 3.033 

log,,(L) - 5.1 36. Data generated from this equation 

indicates that a 1.25-lb walleye would be 15.7 in (398.8 

mm) (Table 2). 

Robustness, or plumpness, can be expressed by the 

condition factor (k): 

k= 

w (g) x 100,000 

Urn) 

3 

Where w is the weight in grams and L the length in 

millimeters. The value 100,000 is used to produce a k 

value that will be near 1 .O. The mean k value for fish 

raised by Summerfelt (1993) was 0.895; the k value 

calculated for data from Piper et al. (1982) is 0.835; and 

the k value for the data calculated with the equation for 

Ws is 0.938. Mean k values reported for wild populations (sexes combined) range from 0.81 to 1.23 (Coby 

et al. 1979). Carlander (I 950) reported that k values 

> 1.02 were excellent, 0.89-0.97 average, and values 

<0.83 were poor. In wild populations, food availability 

seems to be the major factor affecting k values (Colby 

et al. 1979). 

Although gender differences in length-weight relationship or condition factor of walleye have not been found 

in natural populations (Colby et al. 1979), the k-value 

may be affected by genetic dfferences among populations, the size of fish used for the computation (larger 

fish tend of have larger k-values), and environmental 

factors, a standard weight (Ws) for each length can be 

calculated from an equation that is based on data for 

many walleye populations (Murphy et al. 1990). The 

equation for Ws is: loglO(Ws) = 3.180 x loglO(L) - 

5.453, where Ws is the standard weight of the fish in 

grams. The length of a walleye that weighed 1.25-lb 

(567 g) would be 15 in (480.8 mm). Data for computing 

Ws were from wild caught walleye which had a more 

robust body form than the cultured walleye reported by 

Summerfelt (1993). 

Ws may be used to compute relative weight (Wr), which 

is the ratio of the observed weight (Wo) that of the Ws 

for a fish of a given length: Wr = (WON,) x 100, Table 2 

provides data for Ws for walleye from 1 to 20 in (25.4- 

508 mm) from Murphy et al's (1990) equation, and 

relative weight from calculated weights from L- W 

equations generated from data from Summerfelt (1 993) 

and from Piper et al. (1982). 

21 8 NCRAC Culture Series 101- Walleye Culture Manual 
Table 2. Calculated weight and relative weight (Wr) of walleye from length-weight regression 

equations of Summerfelt (1993) and Piper et al. (1982), and Murphy et al’s (1990) data on 

standard weight (Ws). 

Summerfelt (1 993) Piper et al. (1982) Standard weight 

Length 

inches (mm) bs (9) Wr Ibs (9) Wr (Wd Ibs (9) 

1 (25.4) 

2 (50.8) 

3 (76.2) 

4 ( 101.6) 

5 (127.0) 

6 (152.4) 

7 (177.8) 

8 (203.2) 

9 (228.6) 

10 (254.0) 

11 (279.4) 

12 (304.8) 

13 (330.2) 

14 (355.6) 

15 (381 .O) 

16 (406.4) 

17 (431.8) 

18 (457.2) 

19 (482.6) 

20 (508.0) 

0.004 

(0.1 60) 

0.0027 

(1 .220) 

0.0088 

(4.002) 

0.0205 

(9.294) 

0.0394 

(1 7.867) 

0.0672 

(30.476) 

0.1 055 

(47.869) 

0.1 560 

(70.780) 

0.2203 

(99.939) 

0.3000 

(1 36.07) 

0.3966 

(1 79.89) 

0.51 17 

(232.103) 

0.6469 

(293.43) 

0.8037 

(364.56) 

0.9837 

(446.20) 

1.1 884 

(539.04) 

1.41 93 

(643.79) 

1.6780 

(761.1 2) 

1.9659 

(891.72) 

2.2846 

(1 036.28) 

155.0 

130.3 

117.7 

109.5 

103.5 

98.9 

95.1 

92.0 

89.3 

87.0 

84.9 

83.1 

81.4 

79.9 

78.6 

77.3 

76.1 

75.0 

74.0 

73.1 

0.0003 

(0.127 

0.0023 

(1.04) 

0.0078 

(3.56) 

0.01 88 

(8.51) 

0.0369 

(1 6.75) 

0.0642 

(29.1 1 ) 

0.1024 

(46.47) 

0.154 

(69.67) 

0.220 

(99.58) 

0.302 

(137.1) 

0.404 

(1 83.0) 

0.525 

(238.3) 

0.670 

(303.8) 

0.838 

(380.3) 

1.033 

(468.8) 

1.257 

(570.2) 

1.51 1 

(685.3) 

1.797 

(815.1) 

2.1 17 

(960.3) 

2.474 

(1 ,122.0) 

122.9 

111.0 

104.6 

100.2 

97.0 

94.4 

92.3 

90.5 

89.0 

87.6 

86.4 

85.3 

84.3 

83.4 

82.5 

81.8 

81 .O 

80.4 

79.7 

79.1 

0.0002 

0.0021 

(0.94) 

0.0075 

(3.40) 

0.01 87 

(8.49) 

0.0381 

(1 7.26) 

0.0680 

(30.82) 

0.1 109 

(50.32) 

0.1696 

(76.95) 

0.2467 

)(0.100) 

(1 11.91) 

0.3449 

(156.45) 

0.4670 

(21 1.84) 

0.61 59 

(279.36) 

0.7944 

(360.34) 

1.0055 

(456.1) 

1.2522 

(568.0) 

1.5375 

(697.39) 

1.864 

(845.67) 

2.2360 

(1,014.2) 

2.655 

(1,204.5) 

3.1 26 

(1,417.9) 

NCRAC Culture Series 101 - Walleye Culture Manual 21 9 
Growth rate 

Fish were 84 days old at the end of the interval when 

they were habituated to formulated feed. At that time 

they were 3.4 in (87 mm) long and weighed 90.7 fish/lb 

(5.0 9). After an additional 284 d (368-d old), they grew 

sex was not determined, gender differences are masked. 

In wild populations, males are typically younger and 

undergo sexual maturation at a smaller size than 

females (Scott and Crossman 1973). 

to an average of 342.5 mm 

and 342 g. We use data 

from this group of fish to 

examine two questions: 

1) How long must walleye 

be cultured to reach the 

minimum market size of 

1.25 lb (567 g)? 

2) Will growth rates show a 

sudden change as fish 

undergo sexual maturity? 

Walleye had not reached 

market size by 368 days, 

therefore, an estimate of 

their age at 1.25 Ib (567 g) 

requires extrapolating their 

growth hlstory (Figure 3). 

There was a good fit of fish 

weight-age data for both 

the largest 20% and the 

average fish; r2 values were 

0.99 and 0.98, respectively. 

An estimate of the days 

needed to reach 567 g 

would be <410 days (13.5 

months) for the largest 20% 

Largest 20% 

Mean size 

Y = -136.01 + 1.431 * X + 0.001 * X2; r2 = 0.993 

Y = -64.89 + -714 * X + ,001 * X2; r2 = 0.984 

550 

500 - 

450 - 

400 

Q 350 - 

300 

250 - M 

c, 

%I 200 - .i 6 150 

100 

50 

0 -. 

Mean 

size 

50 100 150 200 250 300 350 400 450 500 

Fish age (days) 

Figure 3. Growth curves for the average and the 

largest 20% of the walleye in an experimental 

recycle system raised at an average temperature of 

about 23°C. Extrapolation of the growth curves 

suggest that it will take 410 days for the fastest 20% 

of the population and 480 days for the average fish 

to reach food-size (1.25 Ib [567 91). 

Y = 1.875 - ,004 * X; R"2 = ,809 

111111 

1.80 

of the population and 480 5 . 

days (15.8 months) for the 5 1.40 

average fish. We used the 8 1.20 - 

largest 20% as an indcator 2 1.00 - 

of what might be achieved 9 0.80 - 3 

with domestication and 8 0.60 - 

selective breeding 0.40 - 

0.20 - (Kapuscinski 1996). 

Growth rates declined 

continuously with increase 

in age (Figure 4). The 

rectilinear change from 55 

to 368 days does not 

suggest a sudden change 

related to onset of sexual 

maturity. However, because 

\ t 

0.00 1 , , 1'1'1 I 

50 100 150 200 250 300 350 400 450 500 

Fish age (days) 

Figure 4. Growth rates (mm/day) of walleye raised in 

a recycle system from 84 to 368 days. Data for 

growth rates are plotted at the end of the growth 

interval for which the rate was determined (e.g., the 

first interval was from 55 to 84 days, the last from 

337 to 368 days). 

Review of walleye 

growth 

Growth rate determines 

how long it will take any 

fish to reach a market size. 

Data for reports on intensive culture of walleye are 

summarized in Table 3. 

These data can be used to 

examine the relationship 

between temperature and 

growth, optimum temperature for growth, and 

another opportunity to 

determine whet her growth 

rate decreases with the 

onset of sexual maturity. 

Growth rate, expressed in 

idd (mdd) or as UGR 

(unit growth rate), which is 

growth rate as a function of 

temperature (cm/d/"C) 

(Westers 1987), declines 

with increasing fish age 

(Figure 5). The regression 

coefficient (r2)indicates that 

size accounts for about 40- 

42% of variability in 

growth rate. 

The mean growth rate for 

the data in Table 3, which 

is a mean of means, was 

0.0240 idday (0.61 mm/ 

d)-0.72 idmonth- at a 

mean temperature of 

7 1.4"F (21.9OC). 

If walleye has a precisely 

defined optimum temperature for growth, a &splay of 

bivariate data on growth 

rate and temperature should 

220 NCRAC Culture Series 101- Walleye Culture Manual 
Table 3. Summary of research on growth rates (mm/d and unit growth rates [UGR]) of walleye 

fingerlings raised in intensive culture. UGR is mm/d per "C (Westers 1987). 

Temper at u re Length Growth rate Length of Reference 

"F Initial Final i n/d UGR grow-out 

("C) in (mm) in (mm) (m m/d) (days) 

71.6 1.2 4.2 0.026 113 Nagel (1 976) 

(1 975 data) 

69.8 4.9 6.6 0.025 70 Barrows et al. 

(21 1 (1 24) (1 68) (0.63) 0.030 (1 988) 

69.8 4.6 6.7 0.024 85 Malison et al. 

69.6 2.7 4.8 0.032 65 Kui pers 

69.6 2.7 5.1 0.036 65 

(20.9) (68.1 ) (1 29) (0.920) 0.044 

76.5 3.3 6.2 0.033 84 

(24.7) (84.5) (1 57.3) (0.848) 0.034 

62.6 5.8 6.1 0.0047 73 Siegwarth 

(1 7) (146.9) (1 56) (0.12) 0.0007 and Summerfelt 

(0.67 0.0031 (22) (31.5) (1 08) 

(21 1 (117) (1 70) (0.62) 0.030 (1 990) 

(20.9) (68.1 ) (123) (0.82) 0.039 (1 990) 

69.8 5.7 7.0 0.01 8 73 (1 990) 

(21 1 

(23) (313) (344) (0.26) 0.001 1 (1 991) 

(1 45.2) (1 78.3) (0.45) 0.0021 

73.4 12.3 13.5 0.01 0 119 Yage r 

73.5 10.2 12.9 0.021 

(23) (258) (329) (0.54) 0.0023 131 

77 6.9 8.5 0.022 72 Siegwarth and 

(25) (1 76.2) (21 5.6) (0.55) 0.0022 Summerfelt (1 992) 

67.6 6.3 7.9 0.023 70 Stettner et al. 

(1 9.8) (1 59.2) (200.4) (0.58) 0.0029 (1 992) 

67.3 8.8 10.2 0.01 3 98 

(1 9.6) (224) (259) (0.32) 0.001 6 

69.3 11.2 12.8 0.01 2 126 Siegwarth and 

(20.7) (285.3) (323.9) (0.31) 0.001 5 Summetfelt 1993 

75.2 3.4 13.6 0.036 286 Sum m e rfe I t 

(24) (87) (345) (0.90) 0.0037 (1 993) 

20.5 

20.5 

20.1 

24.9 

69.4 

(20.8) 

77.4 

(25.2) 

68.1 

68.1 

69.3 

69.3 

2.7 

(67.9) 

2.7 

(67.9) 

74.3 3.0 

(23.5) (76.2) 

90.6 

96.5 

84.1 

87.2 

3.4 

(85.7) 

3.4 

(90.6) 

0.804 

1.01 6 

.529 

,640 

0.025 

(0.636) 

0.032 

(0.81 0) 

.0039 28 Kui pers 

.0050 28 and 

.0026 28 Sum m e rfe I t 

.0026 28 (1 994) 

28 

28 

0.0031 

0.0032 

0.024 0.0025 104 Flickinger 

(0.58) (1 996) 

0.021 0.0023 104 

(0.53) 

0.01 7 0.001 9 222 

(0.45) 

NCRAC Culture Series 101 - Walleye Culture Manual 221 
show a domed (parabolic) curve with a peak at the 

optimum temperature. We plotted information from 

Table 2 and applied the equation for a parabola (second 

degree polynomial) to determine an optimum temperature (Figure 6). The regression coefficients were not 

statistically significant (p 2 0.05); therefore, inferences 

drawn from Figure 6 are only suggestive. The graph 

suggests no growth at culture temperature 40.O"F 

(15.6"C). The maximum growth rate, 0.00288 idday 

(0.7307 mdday) would be at a temperature of 74.1 OF 

(23.4"C); however, the dome of the parabola is quite 

flat, and the optimum temperature may be from 73.4- 

750°F (23.0 to 23.9"C). The graph (Figure 6) suggests 

no growth at a temperature 160.O"F (156°C). 

0.00 

Reported optimum temperature for growth of fingerling 

walleye ranged from 71.6"F (22°C) (Smith and Koenst 

1975) to 78.8"F (26°C) (Hokansen and Koenst 1986), 

but Hokansen and Koenst (1986) found that optimal 

temperature was higher at lower light intensity; 26°C 

was optimal at 5 lux. Cai and Summerfelt (1 992) 

estimated that the optimal temperature for metabolism 

of juvenile walleye was 25.3"C at 45 lux. Lack of 

agreement among these reports may be related to size 

of the experimental fish, genetics, nutrition, or cultural 

conhtions. Fish should not be handled for gradmg or 

transportation at temperatures >75"F (23.9"C). 

a 

I 1'1'1' I' 

Carrying Capacity 

The first requirement in the design of a system to 

produce food-size walleye is to determine the system 

carrying capacity. The carrying capacity is based upon 

available oxygen and the production of metabolites 

(ammonia), both of which are functions of the species 

of fish and life stage. Carrying capacity establishes the 

maximum quantity of feed that can be sustainably fed 

in a given system for a given set of conditions and 

avoids under- or over-utilization of the culture system. 

Once the biomass and feed loading have been determined, feeding and stocking/harvesting strategies can 

be developed to optimize growth and to maintain the 

culture facility at or near its maximum carrying 

capacity . 

The carrying capacity is the maximum weight of fish 

that can be sustained in a given volume of culture space 

and at a given flow rate before oxygen is depleted or 

production of fish metabolites deteriorates water quality 

to the point that fish are stressed and growth declines. 

Meade (1988), Colt (1991), Colt and Orwicz (1991), 

1 Y = 0.834 - 0.002 * X; r2 = 0.423 

l.OO 1 : . 0.80 

0.60 

0.40 

0.20 1 

a 

0.0050 

0.0040 

0.0030 

0.0020 

0.0010 

e 

a 

0.0000 ~ 1'1' 1'1' I 

0 50 100 150 200 250 300 350 

Initial length (mm) 

Figure 5. Growth rates of walleye relative to length 

(data from Table 2). 

1.2 1 Y = -5.784 + 0.556 * X - 0.012 * X2; R2 = 0.177 

1.1 1 

0.9 1 I 

0.8 1 

0.7 

E 0.6 

S 0.5 v 

r 0 0.4 

0 

0 

.a .\ 

e 

0.0 O.l 1 i' III I 'IIIIII + 

15 16 17 18 19 20 21 22 23 24 25 26 

Temp ("C) 

Figure 6. Growth rate-temperature data from Table 2 

fitted with second degree polynomial equation. The 

optimal growth rate calculated from the equation is 

0.0288 in/d (0.731 mm/d) at 74.1"F (23.4"C). 

222 NCRAC Culture Series 101- Walleye Culture Manual 
Losordo and Westers (1994), and Soderberg (1995) 

have shown how to calculate the carrying capacity of an 

intensive culture system based upon oxygen consumption and ammonia production (i.e., metabolic functions). Calculation of carrying capacity, oxygen 

demand, and biofilter surface area requirements of a 

recycle system commonly use metabolic functions per 

unit of feed fed. Metabolic functions per unit of feed 

fed depend upon species, life stage, feed quality, and 

water temperature. In general, about 0.2-0.5 lb (200- 

500 g) of oxygen is consumed and 0.027-0.032 Ib 

(28.0-32 g) total ammonia nitrogen (NH,-N) is produced for every 1.0 lb (kg) of feed consumed (Summerfelt 1996). 

The sensitivity of a species to crowding is an important 

factor in determining the economic feasibility of its 

culture. A potential aquaculture species must be able to 

tolerate crowding because a commercial facility must 

maximize the use of space and raise fish at high density. 

Density, the mass of fish supported per unit culture 

volume, (e.g., lbs/ft3, lbdgal, kg/m3, gL) is a quantitative expression of the degree of crowding. Piper et al. 

(1982) describes a density index (D = WNL, where D 

= density index, W the carrying capacity [lbs/ft3], V the 

volume (ft'), and L, fish length [in]) to determine the 

carrying capacity (W = DI X V X L) of a hatchery 

raceway for any species of fish for which a density 

index has been determined. Held and Malison (1 996) 

reported a density index of cO. 15 lb/ft3/in of fish length 

for walleye, which is 30% of the 0.5 value used for 

rainbow trout (Piper et al. 1982). A density index of 

0.1 5 indicates a maximum density of 2.4 lb/ft3 for a 

1.25 lb (567g)) walleye at the time of harvest . Summerfelt (1993) raised 0.5-1.5 lb (227-681 g) walleye at a 

density of 3.72 lbs/ft3 (60 kg/m3) in a recycle culture 

system with ambient oxygen. That data suggests a 

density index between 0.21 to 0.32 lb/ft3/in. 

Loading, which is an expression of mass of fish 

supported per unit flow (e.g., Ib/gpm, kgkpm), is a 

useful measure of carrying capacity of an intensive 

culture system. Flickinger (1996) suggests that walleye 

growth with ambient oxygen condtions (at about 5,000 

ft[ 1,524 m] above sea level) was reduced (Le., loading 

was exceeding carrying capacity) at loading above 2 

Ibs/gpm (0.24 kg/Lpm), which is about 20% of the 

permissible carrying capacity of 10.4 lbs/gpm (1.25 kg/ 

Lpm) for a 10-in (254 mm) rainbow trout at 5,000 ft 

(1,524 m) at a temperature of 60°F (15.5"C). Current 

research at Iowa State University (1,000 ft above sea 

level [304.8 m]) in a recycle system indicates that a 

loading of 3.3 lb/gpm (0.4 kgkpm) is not excessive for 

10- to 14-in (254-356) walleye if available oxygen is 

sufficient. 

Feeding Strategies 

Improved feed utilization leads to improved water 

quality, fish growth and health, and better production 

economics. Feeding strategy, ration level, feed composition, amount of fines in the feed, and amount of 

uneaten feed all affect feed utilization (Westers 1992). 

Feeding fish to satiation with a high quality feed is 

particularly important in order to maximize growth. 

Uneaten feed is the leading contributor to poor feed 

utilization; uneaten feed can be as much as 30% or 

more of the total feed (Seymour and Bergheim 1991). 

Reducing wasted feed is particularly important in water 

re-use systems because uneaten feed places an additional load on solids removal and the biofiltration 

system. In recycle aquaculture, reducing wasted feed 

can effectively increase carrying capacity because 

loading is a function of amount fed. 

Because fish feed makes up the single largest operating 

expense (Blyth et al. 1993; Wade et al. 1996), reducing 

feed wastage will decrease operating expenses. Reducing the amount of uneaten feed will also reduce nutrient 

discharge and/or requirements for downstream treatment at aquaculture facilities, an important criterion if 

the water must be treated before release. 

Feed utilization and growth rate (Le., production) can 

be increased with feeding strategies that allow fish to 

control the amount of food that is available (Hankins et 

al. 1995; Thorpe and Cho 1995). Several methods for 

feeding fish to satiation were reviewed by Hanluns et al. 

(1995). The way fish are fed has a large impact on the 

amount of feed consumed and the amount of waste feed 

generated. Fish feeding has traditionally been done by 

hand, with motorized units, or with units which 

dispense feed upon demand by strilung a feedmg 

actuator. Each method has shortcomings. Feeding 

methods which are not based upon drectly observing 

fish feeding generally waste the most feed. Feeding 

mechanically requires the presumptive calculation of 

feeding rates by charts and/or growth rate estimation. 

Conservative (sub-optimal) estimates are generally 

made to reduce the amount of wasted feed. Hand 

feeding is labor intensive and increases operating costs, 

NCRAC Culture Series 101 - Walleye Culture Manual 223 
and it can fail when it is difficult to observe feeding, as 

with walleye. Demand feedmg fails when feeders are 

improperly set or when fish trigger the actuator but do 

not actually feed. Also, demand feeding does not 

always feed fish to satiation, which is required to 

maximize growth. 

Because walleye do not feed at the surface, but consume feed while it is sinking, it is difficult to observe 

feeding. One method of feeding walleye to satiation 

would be to watch the effluent stand-pipe or solids trap 

for uneaten feed pellets while hand feeding (Figure 7). 

If the feed is delivered slowly over the course of a halfhour, fish will feed to satiation, and feedmg can be 

terminated after waste feed is seen in the tank effluent. 

Another method consists of an automatic feeding 

control device that uses ultrasound to detect uneaten 

feed. Ultrasonic waste feed controllers are a new 

technology that can be used to feed fish to satiation 

with minimal wasted feed (Juell 1991; Juell et al. 1993; 

Summerfelt et al. 1995). The device uses an ultrasonic 

probe in the tank effluent pipe to detect uneaten feed 

and then turns off the feeder after a pre-set quantity of 

waste feed has been detected (Figure 7). Rainbow trout 

fed to satiation with the ultrasonic waste feed controller 

were 64% heavier and had the same feed conversion 

(1.15) as rainbow trout that were fed using a feeding 

schedule based upon growth rate estimates (Durant et 

al. 1995). 

Stocking and harvesting strategies 

Optimizing fish stocking and harvesting strategies can 

maximize production per unit system volume because 

they maintain the culture system at or near its biomass 

capacity, and they also can increase product value by 

providmg uniformly sized fish for the market (Summerfelt et al. 1993; Hanluns et al. 1995). There are three 

main methods for stocking and harvesting fish that can 

be used to accomplish these goals: batch culture (BC); 

concurrent batch stocking and harvesting (CBSH); and 

concurrent mixed-stoclung and graded harvesting 

(CMSGH) (Summerfelt et al. 1993; Hankins et al. 

1995). It is easier to understand these procedures if one 

visualizes a facility with many culture tanks, perhaps 12 

or more. BC is the practice of stocking all tanks in the 

facility at the same time with fish of the same age. 

Because growth rate of fish in separate tanks is about 

the same, fish in all tanks are harvested when the 

average fish reaches market size. This results in a large 

harvest in a relatively short interval. The sub-marketsize fish that are graded out during harvest may be 

placed in one or a few of tanks and retained 1-2 months 

longer to raise them to market size. Then the process is 

repeated with another group of fish. The advantage of 

BC is that stoclung and harvest are done infrequently, 

and feed types and sizes are similar for all tanks 

throughout the production cycle. In BC, the system’s 

overall production is equal to the maximum biomass 

obtained at harvest, and it can be predicted by applying 

growth models which acount for temperature, food, and 

size of fish (Ricker 1979; From and Rasmussen 1984; 

Hewett and Johnson 1987; Bjorndal 1988). However, in 

the course of a production cycle, average fish density is 

about half the carrying capacity, thus annual production 

from the facility is about half that which can be 

obtained using the other two strategies and it does not 

provide a steady supply of fish for the market (Summerfelt et al. 1993). 

CBSH uses the production capacity more efficiently 

because it uses one or several tanks of a multiple tank 

facility for fish of different ages. It is like the BC 

system except that each batch of fish is cultured in one 

or a few of the total number of tanks in the culture 

system. It requires the availability of small fish yeararound for restocking. CBSH procedure involves three 

steps: (1) one or several tanks within a facility that has 

many tanks are stocked with fish of similar size; (2) fish 

in each group of tanks are cultured independently of 

fish in the other tanks until the average size within a 

tank reaches market size; and (3) harvesting all fish in 

each tank when mean length reaches market size. 

Advantages of CBSH are continuous operation at or 

near the carrying capacity of the facility, and continuous 

stocking and harvest (Watten 1992). Disadvantages of 

CBSH are frequent stoclung and harvest, and addtional 

costs are involved for keeping several feed sizes on 

hand at all times, and increased labor costs to inventory 

and culture several size groups at the same time. If a 

supply of fingerlings can be obtained year round, the 

CBSH strategy would probably be most effective 

method used to produce walleye because it does not 

mix fish of dfferent sizes in the same tank. 

CMSGH procedure involves stocking and culture of 

fish of different sizes together in the same tank and 

continuous harvest of the market-sized fish by frequent 

gradmg. CMSGH would not be suitable for walleye 

because the larger cohorts of walleye would probably 

224 NCRAC Culture Series 101- Walleye Culture Manual 
Stand 

Pipe 

high-solids effluent 

(a) hand feed until waste feed is detected leaving the effluent stand pipe; 

n 

; high-solids cfflucnt 

(b) hand feed until waste feed is detected in the solids trap; 

I power 

nsducer assembly 

high-solids effluent 

(c) mechanically feed until waste feed is hydracoustically detected in the effluent. 

Figure 7. Feeding strategies for intensive culture of walleye. 

NCRAC Culture Series 101 - Walleye Culture Manual 225 
eat smaller, newly stocked fish. Frequent grading also 

places stress on the fish that can affect growth and 

incidence of disease. 

Prospects 

The chapters and case studies in the Walleye Culture 

Manual provide many reasons to develop food-size 

walleye culture at this time: 

Walleye is characterized as having white meat with 

very light delicate flavor; it is classified in the low fat 

and high protein category of food fishes. 

Walleye has widespread name recognition as a sport 

fish and has a favorable reputation as a food fish. 

A commercial market for food-size walleye already 

exists in the U.S., but these are wild fish imported 

from Canada as frozen 6- to 8-oz (170-227 g) skin-on 

fillets. 

. The market price for walleye is among the highest of 

freshwater fish; the $6.97/1b ($15.35/kg) price for 

skin-on fillets was 7.8% more than the price for 

cultured salmon fillets in the same markets (Summerfelt 1996a; 

walleye (Collette et al. 1977; Colby et al. 1979; Craig 

1987). 

The information in this culture manual demonstrates 

that a strong foundation of aquacultural technology 

for walleye exists-broodstock capture, methods to 

spawn walleye, out-of-season spawning, egg incubation, options for pond-culture of fingerlings, habituation of pond-raised fish to formulated feed, production scale systems for intensive culture of fry, feeds, 

limited development of domestic broodstock. 

. Fillet yield for processed walleye is similar to channel 

catfish but 10% higher than tilapia. 

. The standard 4 oz portion size allows use of 1.25 lb 

(568 g) fish as minimum harvest weight, a size which 

can be produced in <16 months in intensive culture 

(Figure 3). 

Walleye have been found to be tolerant of high density 

culture in recycle aquaculture systems. 

An abundance of fingerlings are commercially 

available for habituation to formulated feed, and they 

can be raised on a variety of commercially available 

feeds. 

. Substantial information exists on the biology of 

Although all of these propositions have been thoroughly 

documented, the reality is that commercial culture of 

walleye to food-size is nearly non-existent. The few 

commercial efforts to produce walleye to food-size have 

been unprofitable. Stevens (19961, an early pioneer in 

cage culture of walleye and entrepreneur in the commercial production of food-size walleye, raised walleye 

to 1.5 to 2.0 lb (0.68-0.91 kg) in cages. However, 

because of the relatively short growing season for fish 

in southern Iowa, the fish did not reach market size 

until the middle to latter part of the third year and the 

enterprise was not economically viable. Bushman 

(1996) concluded that because of the short growing 

season, cage culture in ponds in northeast Iowa would 

be unprofitable. In 1989, Aquaculture Inc., Rolla, 

Missouri, developed a relatively large-scale commercial 

facility, with circular tanks and a single-pass of ground 

water, for the explicit purpose of raising walleye to 

food-size (NCRAC 1990). Their efforts to culture 

walleye to food-size have terminated. Likewise, 

research on walleye culture to food size has also been 

limited, and few reports have been published (Summerfelt 1993; Flichnger 1996; Held and Malison 1996; 

Yager and Summerfelt 1996). 

Although there are needs for additional research, 

especially integrated research that takes fry to finished 

product on a production scale basis, many of the 

essential biological issues needed to develop walleye as 

a new aquaculture species have been addressed in this 

manual. 

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NCRAC Culture Series 101. North Central Regional 

Aquaculture Center Publications Office, Iowa State 

University, Ames. 

Summerfelt, R. C. 1996b. Intensive culture of walleye fry. 

Pages 161-185 in R. C. Summerfelt, editor. Walleye 

culture manual. NCRAC Culture Series 101. North 

Central Regional Aquaculture Center Publications 

Office, Iowa State University, Ames. 

Summerfelt, R. C., R. D. Clayton, T. K. Yager, S. T. 

Summerfelt, K. L. Kuipers. 1996. Live weight-dressed 

weight relationships of walleye and hybrid walleye. 

Pages 241-250 in R. C. Summerfelt, editor. Walleye 

culture manual. NCRAC Culture Series 101. North 

Central Regional Aquaculture Center Publications 

Office, Iowa State University, Ames. 

Summetfelt, S. T. 1993. Low-head roughing filters for 

enhancing recycle water treatment for aquaculture. 

Doctoral Dissertation. Iowa State University, Ames, 

Iowa. 

Summerfelt, S. T. 1996. Engineering design of a water 

reuse system. Pages 277-309 in R. C. Summerfelt, 

editor. Walleye culture manual. NCRAC Culture 

Series 101. North Central Regional Aquaculture 

Center Publications Office, Iowa State University, 

Ames. 

Summetfelt, S.T., J. A. Hankins, J. A., and S. R. 

Summerfelt, and J. M Heinen. 1993. Modeling 

continuous culture with periodic stocking and 

selective harvesting to measure the effect on 

productivity and biomass capacity of fish culture 

systems. Pages 581 -595 in J-K. Wang, editor, 

Techniques for modern aquaculture. American 

Society of Agricultural Engineers, Saint Joseph, 

Michigan. 

Summerfelt, S. T., K. H. Holland, J. A. Hankins, and M. 

D. Durant. 1995. A hydroacoustic waste feed 

controller for tank systems. Water Science and 

Technology 31 :123-I 29. 

Tave, D. 1986. Genetics for fish hatchery managers. AVI 

P u b I ish i n g C o m pan y, I nc . , West p o r t , Con n ect i c ut . 

Thorpe, J. E., and C. Y. Cho. 1995. Minimizing waste 

t h ro u g h b i o e n erg et ic a I I y and be h av i o ra I I y bas e d 

feeding strategies. Water Science and Technology 

31 129-40. 

Wade, E. M., S. T. Summerfelt, and J. A. Hankins. 1996. 

Economies of scale in recycle systems. In Success 

and failures in commercial recirculating aquaculture 

(Conference Proceedings). N RAES-98. Northeast 

Regional Agricultural Engineering Service. Ithaca, 

New York. 

Watten, B. J. 1992. Modeling the effects of sequential 

rearing on the potential production of controlled 

environment fish-culture systems. Aquacultural 

Engineering 11 :33-46. 

NCRAC Culture Series 101 - Walleye Culture Manual 229 
Westers, H. 1987. Feeding levels for fish fed formulated 

diets. Progressive Fish-Culturist 49:87-92. 

Yager, T. 1991. Effects of fish size and feeding frequency 

on metabolism of juvenile walleye. Master’s thesis, 

Iowa State University, Ames. 

Yager, T., and R. C. Summerfelt. 1993b. Effects of 

feeding frequency on metabolism of juvenile walleye. 

Aquacultural Engineering 13:257-282. 

Yager, T., and R. C. Summerfelt. 1996. Sensory evaluation of fillets from intensively cultured walleye. Pages 

237-240 in R. C. Summerfelt, editor. Walleye culture 

manual. NCRAC Culture Series 101. North Central 

Regional Aquaculture Center Publications Office, 

Iowa State University, Ames. 

230 NCRAC Culture Series 101- Walleye Culture Manual 
Held, J. A., and J. A. Malison. 1996. Culture of walleye to food size. Pages 231-232 in R. C. 

Summerfelt, editor. Walleye culture manual. NCRAC Culture Series 101. North Central Regional 

Aquaculture Center Publications Office, Iowa State University, Ames. 

Culture of Walleye to Food Size 

James A. Held and Jeffrey A. Malison, University o f Wisconsin-Madison Aquaculture Program, Department of 

Food Science, 123 Babcock Hall, Madison, Wisconsin 53706 

Introduction 

The success of culture systems for raising walleye 

fingerlings and advanced fingerlings coupled with the 

limited supply and high market value of walleye fillets, 

has spurred interest in developing methods to produce 

food-size walleye. Traditionally, the food market for 

walleye consists of fish that yield two 8-oz (230-g) 

fillets. Assuming a 50% yield, fish for the food market 

have to be 2 2 lb (0.9 kg). While considerable information is available on various walleye fingerling production techniques, information on the production of 

walleye to food size is scarce. To fully realize the 

potential of commercial production of walleye for the 

food market, significant research on postjuvenile culture is needed. 

Findings 

As part of a series of studes comparing various 

production characteristics and reproductive 

development of diploid and triploid purebred 

and hybrid walleye, we have generated some 

data on post-juvenile growth of walleye. In one 

study, we evaluated the growth rate of walleye 

raised under either constant temperature (70°F 

[21"C]) and photoperiod (16 h light: 8 h dark), 

and conditions that approximated the ambient 

temperature and light regimen found in southern 

Wisconsin (39-70°F [4-2l0C]; 8-16 h light). All 

fish used were the offspring of wild broodfish 

collected from southern Wisconsin lakes during 

the spring spawning season. Eggs were stripped, 

fertilized, and incubated in our laboratory. Newly 

hatched larvae were stocked into fertilized, 0.5-acre 

(0.2-ha), production ponds, and after about six weeks of 

pond culture, fingerlings were harvested and stocked 

into indoor 60-gal (223-L) flow-through fiberglass 

tanks to be trained to accept formulated feed and 

acclimated to intensive culture conditions. Growth 

studies were initiated after this process was completed, 

and they continued until the fish reached sexual 

maturity. Fish were cultured in 200-gal (757-L) flowthrough fiberglass tanks located in darkened rooms and 

provided with in-tank (underwater) lighting and 

airstone aeration. Water quality was maintained at a 

high level (e.g., dissolved oxygen not less than 6 ppm, 

and density indices (as calculated by Piper et al. [ 19821) 

at the end of the experiment were ~0.15 lb/ftVin of fish 

length (0.94 kg/m3/cm). After culture for almost 2 years 

at constant temperature of 70°F (21OC) the fish had not 

attained the 2 lb (0.9 kg) market size (Figure 1). 

However, because of our use of small tanks and low 

stocking densities, we consider these data to be limited 

in their application to large-scale commercial culture. 

Figure 1. Growth of walleye cultured under 

constant and ambient conditions in circular, 

flow-through, tanks. 

600 

500 - 

400 - 

Constant Temp (21 "C) 

300 

200 

100 

0 b- 1bO ' 260 ' 3iO 4iO 560 . 6b0 ' 71 

Days 

During the study, fish were fed to near satiation twice 

daily using appropriately-sized sinking trout pellets. 

The somewhat sluggish response to sinlung rations 

exhbited by larger walleye made it difficult to determine when the fish would stop feeding. Accordingly, 

the presence of excess food in the tanks was monitored 

several hours after feeding, and was used to adjust 

feeding rates. By the end of the experiment, all of the 

fish held at 70°F (21°C) consumed feed at a rate of 1- 

2% body weight/d. As one might expect, the fish 

experiencing winter conditions showed little feeding 

0 

NCRAC Culture Series 101 - Walleye Culture Manual 231 
Chapter 9 - Aquaculture of Walleye as a Food Fish 

response (they consumed ~0.25% body weight/d). We 

have recently tested the use of floating trout pellets to 

feed larger walleye, and while we do not yet have any 

performance data, acceptance of the floating diet was 

surprisingly good. The use of floating feed has the 

advantage of providmg a more visible feeding response 

and, therefore, gives a more imme&ate indication of the 

overall health and well-being of the fish. It also makes 

it easier to measure uneaten food, and thereby obtain 

more accurate feed conversions. 

When raising walleye > 8 in (20 cm) in tanks on 

formulated feed, we found that about 10% of a population over a year’s time stopped eating and slowly 

starved to death. Except for their emaciated condition, 

these fish appeared healthy. At present, we cannot 

explain the cause for this problem. Occasionally, 

however, we have found a preponderance of females 

(70-80%) in groups of large walleye raised in our 

laboratory, which suggests that the die off was mostly 

males. 

Female walleye reach a larger ultimate size than males, 

and are generally longer and heavier within a given year 

class. Becker (1983) indicated that in the wild this size 

hfference is especially apparent in walleye >2 years. 

We are currently undertaking a study to identify the size 

when sexual dimorphism begins under intensive culture 

conditions, and to determine if sexually related growth 

patterns will be important to commercial aquaculturists. 

We have recently developed a method to produce allfemale populations (Malison et al. 1994). To accomplish this, walleye fingerlings were fed a diet containing 

17 a-methyltestosterone which induced sex inversion in 

genotypic females. When these females mature they 

produced viable spermatozoa that were used to fertilize 

normal eggs, and the resultant offspring were 100% 

female. While this procedure does require the use of a 

hormone (a practice needing FDA approval if used 

commercially), it has the advantage that only a small 

number of broodfish and no fish destined for the table 

need to be treated with the hormone. 

In addition to our experience in raising adult walleye in 

flow-through indoor tanks, we have also raised adult 

walleye in ponds, either unconfined or confined in 

floating net pens. The walleye held in net-pens were 

exposed to rather high light levels for various reasons, 

includmg the shallow depth (4 ft [1.2 m]) of the netpens that were used. These fish exhbited stressful 

behavior and mortality quickly exceeded 50%. Whether 

this result was due to the high light conditions or other 

variables was not determined. Walleye in the open pond 

culture system were not fed formulated feed; instead the 

pond was stocked with live forage. Over an 8-month 

period (September-April) the survival (>85 %) and 

overall condition (as evidenced by normal reproductive 

development and robust body conformation) of these 

fish was quite good. 

Before a large commercial aquaculture industry based 

on the production of food-size walleye can develop, we 

need to gain a more thorough understandmg of factors 

such as loading and density limits, growth, feed 

conversion, and other performance parameters of large 

walleye produced under various conditions (e.g., 

raceways, open pond, net pens, etc.). 

References 

Becker, G.C. 1983. Fishes of Wisconsin. University of 

Wisconsin Press, Madison, Wisconsin. 

Malison, J.A., J.A. Held, and L.S. Procarione. 1994. The 

production of all-female populations of walleye 

(Sfizostedion vitreum) using partially sex-inverted 

broodstock. Abstract, 25th Annual Meeting of the 

World Aquaculture Society, 12-1 8 January 1994, New 

Orleans, Louisiana. 

Piper, R.C., I.B. McElwain, L.E. Orme, J.P. McCraren, 

L.G. Fowler, and J.R. Leonard. 1982. Fish hatchery 

management. U.S. Fish and Wildlife Service, 

Wash i ngton , D. C. 

232 NCRAC Culture Series 101- Walleye Culture Manual 
Flickinger, S. A. 1996. Production of food fish. Pages 233-235 in R. C. Summerfelt, editor. Walleye 

culture manual. NCRAC Culture Series 101. North Central Regional Aquaculture Center Publications Office, Iowa State University, Ames. 

Production of Food Fish 

Stephen A. Flickingep, Department of Fishery and Wildlife Biology; Colorado State University; Ft. Collins, CO 

80523 

Introduction 

The following is a report of my experience with raising 

walleye to advanced sizes from several research 

projects conducted from 1987 through 1989 at Colorado State University. Only small numbers of walleye 

were raised in tanks and cages, and culture conditions 

were altered several times to find conditions that would 

promote faster growth and improve food conversions. 

The value in reporting tlvs information is to provide 

data on a topic for which little information has been 

published. Commercial producers should not expand 

these results into business plans. Walleye used in our 

studies were derived from wild broodstock. 

Training 

Environmental conditions 

Pond-cultured walleye fingerlings, 3-in total length (8- 

cm), were stocked into a dark green-colored, 4-ft (1 2- 

m) diameter circular tank at a density of 25 fish/ft3 (51 6 

fish/m3). Water flow was about 3 gaVmin (11 L/min), 

and the temperature was a nearly constant 67°F (19OC). 

The tank was covered with a sheet of black plastic that 

had three slits for adding feed. Submerged lighting was 

provided by a 15-w bulb in a partially submerged 

hatching jar weighted with two bricks. 

Feeding 

Twice a day for the first five days, fish were fed thawed 

krill at a rate of 2% of body weight/d. Krill was slowly 

poured through the slits in the plastic cover. In about 

two days, the fingerlings fed ravenously on the krill and 

were fed to satiation. This is the only food for which I 

have seen walleye go into a feedmg frenzy. Five days 

into the training, walleyes were given only a little hll 

to excite them, and then they were given krill-based 

commercial fish feed initially at 3.5% of body weight/d, 

that had been specially screened for the largest particles 

that occurred during milling (this ration is normally 

sold in only small sizes as a starter feed). Each day, less 

krill was added until only krill-based fish feed was 

offered. Also, to compensate for growth during training, 

the amount of pelleted feed was gradually increased 

from 3.5% to 4% of initial body weight/d. In similar 

fashion, the fingerlings were converted from the krillbased feed to an early experimental BioMoist diet 

(Bioproducts, Inc., Warrenton, OR); 98% were trained 

to accept the BioMoist diet. We obtained about 90% 

survival with a second group of 1.5 in (3.5 cm) fingerlings that were trained in a similar manner. 

Tank culture to food size 

Environmen ta/ conditions 

Tank culture was done indoors in 4-ft (I .2-m) diameter 

circular tanks or in halves of plastic 55-gal (208-L) 

drums. To overcome the skittish behavior problem of 

walleye in tank culture, we covered some culture tanks. 

Water was heated to 72-77"F (22-25"C), dependmg on 

how well a water heater kept up with needed flows. No 

diseases were encountered at the higher temperatures. 

Tanks were cleaned as needed, and walleye were always 

weighed in 0.5% salt (NaCl) solution, a practice that 

may reduce stress and prevent hsease. 

Feeding 

Belt feeders were used on the tanks and the cages. 

Condensation on the belts caused some problem with 

mold growing on the fines that adhered to the belt. Food 

was placed on the belts either in perpendcular strips so 

fish would be fed in 3-h intervals, or in a smooth layer 

to provide nearly continuous feeding. Differences in 

growth between feeding methods was not significant. In 

trials 1 and 2, a feeding rate of 3% of body weight was 

used. Most of the time, the larger fish in trial 3 were fed 

at 1.5% of body weight per day but when food conversion was poor, as little as 0.8% of body weight was fed 

in an attempt to reduce waste. 

NCRAC Culture Series 101 - Walleye Culture Manual 233 
Chapter 9 - Aquaculture of Walleye as a Food Fish 

Resu Its 

In three trials, ranging from 104 to 222 days, 

walleye growth ranged from 0.45-0.5 mdd, 

with food conversions ranging from 2.5 to 3.7 

(Table I). Differences in growth rates of walleye 

raised in covered and open tanks were too 

variable to prove any beneficial effect from the 

cover. Similarly, no significant growth benefit 

could be attributed to external lighting, internal 

lighting, no lighting, or photoperiods of various 

lengths. Submerged lighting, however, reduced 

the shttishness of the fish and made it easier for 

fish to be observed. As might be expected, when 

excess feed accumulated in the tanks the water 

quality deteriorated. 

For the most part, mortality was low, usually an 

occasional fish seems to stop feedmg and 

becomes emaciated. However, on a couple 

occasions, 20% mortality occurred after we 

attempted to grade the walleye. We used 

commercial grader boxes, but walleye just 

Table 1. Growth and food conversions of foodtrained walleye reared for 104 to 222 days in 

indoor tanks. 

Trial 1 Trial 2 Trial 3 

104days 104days 222days 

Initial 

nu m be r/l b 

nu m be r/kg 

nu m be r/l b 

nu m be r/kg 

i n c h es/d ay 

mi Ilirneterdday 

mean 

range 

Final 

Growth 

Food conversion 

20 

45 

6 

14 

0.024 

0.58 

2.7 

1.5-3.6 

30 

70 

10 

22 

0.021 

0.53 

2.5 

1.5-3.2 

3 

6.8 

1.3 

2.8 

0.01 7 

0.45 

3.7 

0.9-9.2 

stayed in the box. The grader box had to be raised and 

shaken to grade the walleye, and this created considerable mortality. 

Cage culture 

Methods 

In the summer, walleye were cultured in ponds in l-yd3 

(0.8-m3) cages with 1/2-in (13-cm) bar measure mesh. 

Cages were covered with a solid plywood lid. Water 

clarity, based on Secchi disk measurements, generally 

exceeded the depth of the cage, which resulted in rather 

high light intensity in the cage. Water temperature in the 

ponds ranged from 60-85°F (16-30°C), but it was 

typically around 75°F (24°C). 

We experimented with feeding only at night for 46 d, 

but there was no obvious difference in growth rates of 

fish fed in the day compared with night-only feeding. 

Raccoons attracted to the feeders were a nuisance at 

times. Fish were fed at 3% of body weight until the last 

two weeks when they were fed at 2.5%. 

Results 

In the first cage trial (Table 2), walleye grew and 

converted food similarly to walleye in tanks (Table 1). 

Final density in the cage was 0.42 lb/ft3 (6.6 kg/m3). 

Table 2. Growth and food conversions of 

food-trained walleye reared 80 to 123 days 

in cages. 

Trial 1 Trial 2 

80 days 123 days 

Initial 

nu m be r/l b 

nu m b e r/kg 

nu m be r/l b 

nu m b e r/kg 

inchedday 

mi I I i meterdday 

Food conversion 

mean 

range 

Final 

Growth 

5.5 

12 

3.1 

6.6 

0.01 9 

0.63 

3.1 

1.3-9.9 

5.0 

11 

1.6 

3.5 

0,034 

0.85 

3.0 

1.5-4.8 

Density was low because few fish were available for 

this trial. Cage trial 2 was more successful, walleye 

growth was higher than in the first cage trial or in the 

two tank trials (Table 2). Final density in this trial was 

2.15 lb/ft3 (34.5 kg/m3). 

234 NCRAC Culture Series 101- Walleye Culture Manual 
Aquaculture of Walleye as a Food Fish - Chapter 9 

Discussion 

In tank culture of walleye, water flows were adjusted to 

provide loadings (weight of fish per unit water flow per 

minute) of 3 lb/gal/min (360 g/L/min), or less. Subsequently, however, our experiments indicated that 

walleye growth slows above a loadmg of 2 lb/gal/min 

(240 g/L/min). Perhaps the growth rates were affected 

because of a hgh loading. High loading implies low 

oxygen, high ammonia, or both. I have observed that 

growth of largemouth bass slows at 3 lb/gal/min (360 g/ 

L/min) and ceases at 5 lb/gal/min (600 g/L/min), but 

rainbow trout can be successfully raised at loadmgs 

well above 5 lb/gal/min (600 g/L/min). If further 

research supports the necessity for loading of 2 lb/gaV 

min (240 g/L/min), or less, to grow walleye at a 

reasonable rate, it will require more than 1,000 gal/min 

(3,785 Wmin) water flow to produce 1 ton (2,000 lbs, 

909 kg) of walleye in a flowing water system. 

Growth rates in tanks were disappointing, however, 

growth rates in cages were about 1 in/mo (25 mm/mo), 

which probably is acceptable in commercial production. 

Based on these observation, the underlying reason for 

poor growth of walleye to commercial size in our tank 

culture experiments seems to be the effect of stressful 

rearing condtions, especially after walleye reach 8 in 

(20 cm). In tanks, walleye were easily startled from 

activity in the room. Perhaps there is a need for 

revisions in the culture environment, but domestication 

of walleye stocks to obtain a fish that is more amenable 

to confinement seems to be a logical approach to this 

problem. 

Food conversions reported here are a little rnisleadng, 

because the diets had high moisture content. Moisture 

content was 14% in an experimental form of BioDry 

feed, 20% in BioDry Grower, and 26% in an experimental form of BioMoist feed. Correction to a standard 

moisture content for pelleted feed would decrease food 

conversions somewhat, but they still would be higher 

than expected for commercially reared, domesticated 

fishes. 

Walleye reared at Colorado State University were 

barely minimum market size of 2-oz (57-g) fillets, and 

larger markets exist for 4-, 6-, and 8-oz fillets (personnel communication, Morey Fish Company, Golden 

Valley, MN). For our largest fish, walleye approaching 

14 in (35 cm) and weighing 13.3 oz (377 g) had a handfilleting dressing percentage of 42% (range 39-46%). 

Similar-sized wild walleye dressed out at 45% (range 

43-46%). In a taste test, participants could not differentiate cultured walleye from wild walleye on the basis of 

taste, but they dd correctly identify cultured walleye on 

the basis of texture. 

NCRAC Culture Series 101 -Walleye Culture Manual 235 
Chapter 9 - Aquaculture of Walleye as a Food Fish 

236 NCRAC Culture Series 101- Walleye Culture Manual 
Yager, T. K., and R. C. Summerfelt. 1996. Sensory evaluation of fillets from intensively cultured 

walleye . Pages 237-240 in R. C. Summerfelt, editor. Walleye culture manual. NCRAC Culture 

Series 101. North Central Regional Aquaculture Center Publications Office, Iowa State University, Ames. 

Sensory Evaluation of Fillets from 

Intensively Cultured Walleye 

Timothy K Yager’ and Robert C. Summerfelt, Department of Animal Ecologx Iowa State Universitl: Ames, 

Iowa 50011-3221 

Introduction 

“The walleye is considered to be one of the best eating 

(sic) of all freshwater fishes “ (Carmichael et al. 1991), 

a sentiment echoed in many cook books: “But by any 

name the walleye is one of the most delicious of freshwater fishes. Its snow-white flesh is both delicately and 

distinctively flavored, and it invariable comes as a 

delightful surprise to a diner who tastes it for the first 

time” (Cameron and Jones 1983). 

Scientific proof for the well-regarded reputation of 

walleye as a food fish is not needed. However, intensive 

culture of walleye to a food-size fish is a new endeavor, 

therefore it is important to know whether walleye 

cultured in a recycle system will acquire an undesirable 

off-flavor and aroma that would be quickly recognized 

by the consumer who may compare the taste to walleye 

harvested from pristine Canadian lakes, the major 

source of food-size walleye sold in the U.S. (Summerfelt 1996). Taste and odor problems affecting the 

palatability of pond cultured channel catfish have been 

known for many years (Love11 1974). Raising walleye 

to food size on a commercial basis in a recycle culture 

system also raises concerns regarding potential offflavor problems. Recycle aquaculture is especially 

attractive in Canada and states with short growing 

seasons. A report on walleye culture in recycle systems 

is presented in ths manual (Summerfelt and Summerfelt 1996). 

To avoid marketing fish with off-flavors, most contemporary commercial recycle aquaculture facilities raising 

tilapia or hybrid striped bass place fish that have 

reached market size in a purge tank to depurate for up 

to 14 days to remove the off-flavors. Commercial 

producers of tilapia and hybrid striped bass typically 

use 2 to 5 days depuration time, but the literature 

review by Persson (1 984) indcates a range from 5 to 18 

days may be needed to purge a muddy odor from 

channel catfish, but it depends on temperature and 

intensity of the off flavor problem. The review by 

Stickney (1993) inhcates that channel catfish with a 

geosmin problem can depurated within a few days. 

The objective of this research was to assess the organoleptic qualities-those characteristics (aroma, flavor, 

and texture) that are evaluated by one’s senses-of 

cultured walleye. Walleye raised in an intensive, recycle 

system (Recycle), and in an intensive, single pass, flowthrough system (Flow-through) were compared with 

walleye purchased from a local store (Store). 

Methods 

Taste panel 

The taste panel members were students, faculty and 

staff of Iowa State University (ISU) that responded to 

an advertisement that was distributed to a few department offices on campus. The only prerequisite for 

acceptance of panelists was that they not dslike eating 

fish. Panelists were told that they were eating walleye, 

but they were not told anything else about the three 

groups of walleye they evaluated. Three trials were 

conducted on July 11, 13, and 14, 1990. Each taste 

panel consisted of 12 to 14 indwiduals untrained in 

sensory analysis. 

Sensory evaluation 

Panelists were instructed to follow a specific protocol 

for the sensory evaluation. They were given forms to 

record their assessment of taste, aroma, firmness, 

flakmess, and an overall rating (not an average) for each 

fish portion. The instructions told them to use their 

sense of smell to evaluate aroma; to use their fork to 

evaluate flakiness and firmness; and to taste a sample to 

evaluate flavor. Their responses were entered on a form 

Present address: US. Army Corps of Engineers Center, 190 5th Street East, St. Paul, MN 55101 

NCRAC Culture Series 101 - Walleye Culture Manual 237 
Chapter 9 - Aquaculture of Walleye as a Food Fish 

where they had to choose an option to which 

we assigned a 5-point scale (they were not 

given the point scale, only the options): 5=like 

extremely, 4=like, 3=neutral, 2=dislike, and 

1 =dislike extremely. 

The portions were 1-in (25 mm) squares, about 

1/2-in (12.5 mm) thck of slunless pieces of 

flesh from the dorsal musculature. Fish portions 

were baked at 325°F (163°C) for 6 minutes on 

aluminum broiling pans. Fish portions were not 

battered, or seasoned (i.e., unsalted). After 

preparation, samples were placed in small 

color-coded cups. Samples were presented to 

panelists in the cups on a paper plate. Panelists 

were seated in a test kitchen at tables with 

dividers to separate individual panelists and to 

ensure privacy. Water and salted soda crackers 

were provided and panelists were asked to use 

the cracker and water to cleanse their palates 

between samples. 

Table 1. Chemical composition of randomly selected 

samples of fish portions used in the sensory 

evaluation. Samples were small pieces of skinless fillets removed from walleye raised in a 

recycle or flow-through culture systems compared with samples of walleye purchased frozen 

from a supermarket (store). 

Recycle Flow- Store p-value for 

through ANOVA’ 

Protein (%) 20.6a 20.4a 21.la 0.71 2 

Fat (%) 0.10a 0.38a 1.62b 0.015 

Moisture (%) 78.3a 78.2a 76.3a 0.230 

NaCl (%) 0.14a 0.20a 0.94b 0.003 

TBA (PPm) 0.44a 0.18a 7.3613 <0.001 

There were 13 panelists for trial 1, 12 for trial 2, and 14 

for trial 3. Daily means for each parameter were used 

for statistical evaluation (i.e., there were only 3 replicates of each parameter). Scores were statistically 

analyzed using analysis of variance (ANOVA) procedures to determine significant differences among 

groups; posthoc tests of differences between each of the 

three treatments were done with Fisher’s PLSD. 

Biochemical analysis 

Samples of fish portions used in the sensory evaluation 

were analyzed for percent protein, fat and moisture and 

for concentration of NaCl and 2-thiobarbituric acid 

(TBA). Concentrations of NaCl were determined by the 

Quantab method (AOAC 1975). Fat content was 

determined by ether extraction and moisture was 

measured during this process (AOAC 1990). Protein 

was determined by difference between the sum of fat 

and moisture and 100%. TBA is a measure of the level 

of oxidative rancihty (Tarladgis et al. 1960; Tarladgis et 

al. 1964; Koniecko 1979). In this procedure, TBA 

combines with malonaldehyde, a compound produced 

by the oxidative breakdown of unsaturated fatty acids, 

to produce a red color, the intensity of the color as 

measured with a spectrophotometer is proportional to 

the level of rancidty of the sample. 

p-values 50.05 are statistically significant; values in row with same letter are 

not significantly different at 0.05 level. 

Results and discussion 

Biochemica/ composition 

Proximate analysis of samples of skinless fillets used in 

the sensory evaluation indicated that there were similar 

levels of protein and moisture in all samples (Table 1). 

The frozen samples from the store had significantly (p 5 

0.05) more fat, salt (NaCl), and higher levels of TBA 

than the samples from the two groups of cultured 

walleye. Differences in biochemical composition of the 

two groups of cultured walleye were not statistically 

significant for any of the parameters. 

Yurkowski (1989) reported that wild caught walleye 

from central Canada had a fat content of 1.4%, which is 

similar to the fat content of our store-bought fish, but 

substantially higher than either group of cultured 

walleye. Although we did not know how long the storebought fillets were in storage before we purchased 

them, the measurements of TBA suggest some oxidative rancidty of fatty acids had taken place. If the 

organoleptic characteristics of walleye degrade due to 

rancidity of its fat, then it is useful to note that the 

cultured fish had a lower fat content than wild caught 

fish. Given the problem with breakdown of fats, it 

seems prudent to avoid raising fish with high fat 

content, especially if the fish is to be processed and 

238 NCRAC Culture Series 101- Walleye Culture Manual 
Aquaculture of Walleye as a Food Fish - Chapter 9 

frozen. Geosmin, not rancid fat, is the primary chemical 

responsible for off-flavor in freshly slaughtered, pondraised channel catfish (Stickney 1993). 

Sensory evaluation 

Differences in organoleptic scores among the three 

groups of fish (store, flow-through, and recycle) were 

small and not statistically significant (Table 2). This 

suggests that the consumer can find the taste and aroma 

of freshly processed cultured walleye, including 

walleye raised in a recycle system, equal to a frozen 

walleye. However, comments made by a few panelists 

after the test offer other insights into the findmgs. They 

said that one of the samples (the store bought fish) 

“seemed to be seasoned’ and that the others (the two 

groups of cultured walleye) were “somewhat bland”. 

The chemical analysis indcate that the store-bought 

samples had a significantly higher salt (4.7 to 6.7 times 

greater) content than either group of cultured walleye, 

suggesting that the frozen samples had been treated 

with salt sometime before freezing. Thus, the sensory 

evaluation might have been biased in favor of the storebought fish because of its salt content. Paradoxically, it 

was our personal opinion that the store-bought fish had 

an off-flavor and undesirable aroma, which we now 

assume to be a degree of rancidity that is reflected by 

high levels of TBA. We think that the salty flavor of the 

store-bought fish enhanced the sensory evaluation of 

that product sufficiently to overcome a problem with 

rancid fat. 

A taste test with similar implications to the present one, 

53% of panelists that tasted fresh, farmed chinook 

salmon and previously frozen wild-caught chinook 

preferred the fresh farmed fish (Koch 1991). The 

importance of that test and the present evaluation of 

wild versus cultured walleye is that a fresh product is 

preferred or at least equal to that of a frozen product of 

wild-caught fish. 

In deference to purveyors of frozen walleye, we remind 

the reader that the frozen walleye used in this study 

were not a representative sample of all frozen walleye 

present on the U.S. market or even that of the two 

supermarkets in Ames, Iowa. The store-bought fish 

were very small samples of what was in stock at the 

time of this study. What we perceive to be rancidity 

problem in the store-bought fish cannot be attributed to 

any specific member of the dstribution chain involved 

from the capture of the fish to the retailer. On the other 

hand, the tank cultured fish were not depurated before 

slaughter, they represent fish taken directly from the 

culture tanks. 

References 

AOAC (Association of Official Analytical Chemists). 

1975. Official methods of analysis of the Association 

of Official Analytical Chemists, 12th edition. AOAC, 

Arlington, Virginia. 

Table 2. Mean sensory evaluation on a scale of 1-5, where 1 was “dislike 

extremely” and 5 “like extremely”) of walleye intensively cultured in 

flow-through and recycle systems walleye purchased frozen from a 

local supermarket. Differences among the treatment groups were not 

statistically significant (p 2 0.05). 

Scores for sensory evaluation 

Treatment group Taste Aroma Firmness Flakiness Overall’ 

Flow-through system 3.8 3.7 3.8 3.9 3.7 

Recycle system 3.6 3.8 4.1 4.0 3.8 

Store-bought fish 3.4 3.5 3.9 3.9 3.4 

This is an independent, composite assessment, not the average of the other scores. 

NCRAC Culture Series 101 - Walleye Culture Manual 239 
Chapter 9 - Aquaculture of Walleye as a Food Fish 

AOAC (Association of Official Analytical Chemists). 

1990. Official methods of analysis of the Association 

of Official Analytical Chemists, 15th edition. AOAC, 

Arlington, Virginia. 

Cameron, A., and J. Jones. 1983. The L. L. Bean, game 

and fish cookbook. Random House, NewYork. 

Carmichael, G., M. Ring, and J. McCraren. 1991. Sea 

fare cookbook, volume 1. American Fisheries 

Society, Bethesda, Maryland. 

Koch, A. 1991. Fresh farmed beats frozen wild at B.C. 

taste test. Northern Aquaculture 7(4):33. 

Koniecko, E. S. 1979. Handbook for meat chemists. 

Avery Publishing Group Inc., Newark, New Jersey. 

LoveII, T. 1974. Environment-related off-flavors in 

intensively cultured fish. Pages 259-262 in R. 

Krueuzer, editor. Fishery products. Fishing News 

(Books) Ltd., Farnham, Surrey, England. 

Persson, P. 1984. Uptake and release of environmentally occurring odorous compounds by fish: A review. 

Water Resources 18:1263-1271, 

Stickney, R. R. 1993. Channel catfish. Pages 33-79 in R. 

R. Stickney, editor. Culture of nonsalmonid freshwater fishes, 2nd edition. CRC Press, Boca Raton, 

Florida. 

Summerfelt, R. C. 1996. Introduction to the Walleye 

Culture Manual. Pages 1-10 in R. C. Summerfelt, 

editor. Walleye culture manual. NCRAC Culture 

Series 101. North Central Regional Aquaculture 

Center Publications Office, Iowa State University, 

Ames. 

Summerfelt, S. T., and R. C. Summerfelt. 1996. Aquaculture of walleye as a food fish. Pages 215-230 in R. 

C. Summerfelt, editor. Walleye culture manual. 

NCRAC Culture Series 101. North Central Regional 

Aquaculture Center Publications Office, Iowa State 

University, Ames. 

Tarladgis, B. G., A. M. Pearson, and L. R. Dugan. 1964. 

A distillation method for the quantitative determination of malonaldehyde in rancid foods. Journal of the 

American Oil Chemists’s Society. 37:44-49. 

Tarladgis, B. G., B. M. Watts, and M. T. Younathan. 1960. 

Chemistry of the 2-thiobarbituric acid test for determination of oxidative rancidity in foods. II. Formation of 

the TBA-malonaldehyde complex without acid-heat 

treatment. Journal of the Science of Food and 

Agriculture 15:602-607. 

Yurkowski, M. 1989. Lipid content and fatty acid 

composition of muscle from some freshwater and 

marine fish from central and Artic Canada. Pages 

547-557 in R. K. Chandra, editor. Health effects of 

fish and fish oils. ARTS Biomedical Publishers and 

Distributors, St. J oh ns, N ewfo u nd I and . 

240 NCRAC Culture Series 101- Walleye Culture Manual 
Summerfelt, R. C., R. D. Clayton, T. K. Yager, S. T. Summerfelt, and K. L. Kuipers. 1996. Live 

weight-dressed weight relationships of walleye and hybrid walleye. Pages 241 -250 in R. C. 

Summerfelt, editor. Walleye culture manual. NCRAC Culture Series 101. North Central Regional 

Aquaculture Center Publications Office, lowa State University, Ames. 

Live Weight-Dressed Weight Relationships 

of Walleye and Hybrid Walleye 

Robert C. Summerfelt, Richard D. Clayton, Timothy K. Yager,’ Steven T: Summerfelt,* and Kent L. 

Kui~ers,~Department of Animal Ecology; lowa State Universitx Ames, lowa 5001 1-3221 

Introduction Objective 

Dressed yield is the percentage of the live weight 

obtained for a specific processed product. The live 

weight-dressed weight 

we evaluated dressing percentage (dressed yield) and 

carcass characteristics of three year-classes (fish 

hatched in the same 

relationships for 

different forms of a 

food-fis h product 

year are members of 

the same year-class) 

of walleye and two 

Table 1. Hypothetical live weight of fish required to 

yield two 4- or 6-02 (1 14-170 g) fillets.’ 

strongly influences year-classes of 

which form of the Dressed yield Live weight of fish hybrid (interspecies 

dressed product will be % of live weight 4-02 (1 14 g) fillets 6-02 (170 g) fillets cross between 

marketed, as well as female walleye and 

male sauger) 

walleye grown 

under in ten sive 

objective of this 

study was to obta 

information on the dressed yield of walleye, and to 

determine whether yield is a function of fish size and 

gender. Data were obtained on yield for slun-on and 

40 1.25 Ibs (568 g) 1.88 Ibs (851 g) 

45 1.67 Ibs (756 g) 

50 1 .OO Ibs (454 g) 1.50 Ibs (681 g) 

the overall economic 

feasibility for commercia1 production. 

Obviously, processed culture. The 

yield affects the live 

weight of the fish 

needed to yield fillets of a commercial size (Table 1). 

Also, if dressing percentage increases substantively 

with fish size, it may be practical to relate fish size at 

1 .I 1 Ibs (504 g) 

’No assumptions made relative to effects of fish size on dressing percentage. 

n 

harvest to the size at which maximum dressed yield is 

obtained. 

skinless fillets; the relative weight of the head, scales, 

and shn; and the relationship between body weight and 

fillet yield. 

Information on dressing percentage is essential for cost/ 

benefit or break-even analysis for walleye food-fish 

production. A highly processed form of the fish 

(slunned fillets) results in more waste, higher processing costs per unit weight, and a more expensive final 

product. For example, for rainbow trout, yield from a 

0.55 lb (250 g) fish was 77.7 to 86.0% after evisceration 

(gills and viscera removed, but head, tail, and all fins 

retained), 69.0 to 75.3% for boned fish (spine and ribs 

removed but all fins and the head and tail remained on 

the fish), and 55.7-60.3% for filleted fish (shn and 

scales remained, but all fins, head and tail were removed) (Smith et al. 1988). 

Market forms 

The most common forms of dressed fish are (Dunn 

1974; Plutt 1986): 

(1) Whole: No processing. 

(2) Whole dressed (drawn): Fish that have been 

eviscerated and the gills have been removed; the 

scales, head, tail, and all other fins remain. 

(3) Pan dressed or dressed (Figure 1): Eviscerated and 

head, tail, scales, and fins removed. The pectoral and 

pelvic fins are removed with the head, or separately, 

but the dorsal and anal fins are removed by incision 

Present addresses: ’US. Army Corps of Engineers Center, 190 5th Street East, St. Paul, MN 551 01. 2The Conservation Fund’s 

Freshwater Institute, PO Box 1746, Shepherdstown, WV 25443. %oute 1 , Box 121 , Fullerton, NE 68638. 

NCRAC Culture Series 101 - Walleye Culture Manual 241 
Chapter 9 - Aquaculture of Walleye as a Food Fish 

1 ,; 

Figure 1. Dressed walleye: eviscerated and with head, tail, and fins removed. 

The most common processed 

product for walleye is a scaled, 

skin-on fillet (Figure 2). For 

comparison, trout are sold whole 

dressed, dressed, boned, or 

filleted, but whatever form, 

because of their small scales, 

trout and salmon are commonly 

marketed with scales on, 

however, walleye have much 

larger scales that must be 

removed for marketing as 

dressed or filleted forms. 

Channel catfish are marketed as 

whole dressed fish, regular 

fillets (skinless), shank fillets, 

fillets strips, nuggets, and steaks 

(Foster and Waldrop 1972). 

Figure 2. Skinless walleye fillet is about 40% of live weight. 

on each side of the fins to the depth of the vertebral 

column. 

(4) Steaked: Body cross-sectioned into pieces 5/8- to 1- 

in (15.9-25 mm) thick. Steaked fish contain a crosssection of the back-bone. Cultured walleye will 

probably not be steaked because it requires a large 

(> 4 pounds) dressed fish. 

(5) Filleted: Pieces of fish flesh that are cut lengthwise 

away from the backbone after scaling a whole 

dressed fish. Most retail sales are of skin-on fillets, 

but some consumers prefer skinless fillets because of 

the rubbery nature of the skin and taste problems that 

develop in frozen products from oxidative rancidty 

in the fatty layer underlying the shn. 

Source of fish 

Fish used in this study were 

raised in intensive culture on 

formulated feed in the aquaculture facilities at Iowa State 

University (ISU). When fish 

were processed, they were 227 

to 783 days old (i-e., 7.5-26 

months). The condition factor 

for each group is given for the 

date they were processed. The 

conhtion factor, k = (weight [g] 

x 100,000) + L (mm)’, is a 

measure of the robustness or 

degree of well-being of a fish. 

Piper et al. (1982) gives a value for walleye in English 

units, called C, which equivalent to a k value of 0.835. 

Three year-classes (1987, 1988, and 1994) of walleye 

were studied. All year-classes were first pond-raised 

then converted (habituated) to formulated feed. The 

1987 year-class was from Spirit Lake, Iowa (Spirit Lake 

walleye, SLW). They were pond-raised for about 55 

days, then habituated in a 28-day interval to formulated 

feed in intensive culture (Kuipers and Summerfelt 

1994). Thereafter, some of the 1987 year-class of SLW 

were used in feeding experiments by Stettner et al. 

(1992) in 120-L tanks; then raised in 277-L tanks until 

the final processing (467 days posthatch). 

242 NCRAC Culture Series 101- Walleye Culture Manual 
Aquaculture of Walleye as a Food Fish - Chapter 9 

Samples of the 1987 year-class were processed at 151, 

160, 170,357, and 467 days posthatch to determine 

whether fillet percent changed with live weight. The 

fish were 0.7 lbs (307 g) at 467 days posthatch 

(Table 2). 

Three groups of the 1988 year-class were used. One 

was another year-class of SLW converted (habituated) 

to formulated feed over a 28 days by Kuipers and 

Summerfelt (1 994); thereafter they were transferred to 

1,000-L tanks a recycle system for the next 701 days in 

a study by Peterson (1992). They were processed on 

June 17, 1990 when they were 783 days posthatch. A 

few were held until July 11-14, when they were 

processed for use in a sensory evaluation by Yager and 

Summerfelt (1996). 

The other two groups of the 1988 year-class were Rock 

Lake (Wisconsin) walleye (RLW), and hybrid walleye 

(RLH). The hybrids were produced by crossing sauger 

males, obtained near Bellevue, Iowa on the upper 

Mississippi River, with female RLW. RLW were halfsibs to RLH. Fish were initially raised in ponds to 40- 

60 mm at the Lake Mills State Fish Hatchery, Lake 

Mills, Wisconsin, then habituated to formulated feed in 

a study by Malison et al. (1990). RLW and RLH were 

transported to the ISU aquaculture facility October 17, 

1988 when they were about 5.7-5.9 in (145-150 mm). 

Table 2. Fish age and size and dress out percentages for the 1987 

year-class of Spirit Lake walleye (SLW). 

Sex (number in sample) 

Males (54) Females (69) p-va I u el 

Fish age and size 

Fish age (day) 

Mean 

Range 

Length 

Mean: inches (mm) 

Range: inches 

(mm) 

Weight: Ibs (9) 

Mean 

Range 

Condition factor (k) 

Processing characteristics2 

Head length (% total length) 

Head weight 

Scales 

Skin 

Fillets, skin-on 

Fillet, skinless 

227 

151 -467 

9.8 (248.4) 

7.9-1 3.6 

(200-345) 

0.30 (134.9) 

46-470 

0.81 

24.4 

15.0 

4.0 

9.0 

43.2 

34.2 

239 

151 -467 

9.9 (252.1) 

4.6-1 5.3 

(1 17-388) 

0.33 (151 .l) 

49-470 

0.92 

24.7 

16.0 

2.7 

8.1 

41 .O 

32.9 

0.52 

0.65 

0.28 

0.26 

0.30 

0.07 

0.39 

0.39 

0.04 

0.23 

'The p-value for unpaired t-test of difference between males and females; p-values 2 0.05 are 

considered statistically significant. 

*Except for head length, the parameters are expressed as percent of total live weight. 

They were raised in experiments by Siegwarth and 

Summerfelt (1990,1992, and 

1994) in 120-L tanks to 783 

days (25.7 months), the small 

tanks seemed to reduce their 

growth potential (Siegwarth 

and Summerfelt 1994). Six 

months prior to the end of 

that study, about 200 each of 

the RLW and RLH were 

transferred to 1,000 L tanks 

for growout until they were 

processed to obtained 

measurements of dressed 

weights, which was June 17, 

1990 when they were 783 

days posthatch. At that time, 

mean length of the three 

cohorts of the 1988 yearclass was 15.5 in (393 mm) 

for the SLW, 13.8 in (352) for 

the RLW, and 13.8 in for the 

RLH walleye (Table 3). A 

few of the RLW were raised 

another 24 days until July 11- 

14 when they were processed 

for use in a sensory evaluation (Yager and Summerfelt 

1996). 

The 1994 year-class included 

walleye and three interspecific hybrids which are 

designated by their place of 

origin: Spirit Lake hybrids 

NCRAC Culture Series 101 - Walleye Culture Manual 243 
Chapter 9 - Aquaculture of Walleye as a Food Fish 

(SLH), Mississippi River hybrids (MRH), and Rock 

Lake hybrids (RLH). Hybrids were produced by 

crossing female walleye with sauger collected from the 

Mississippi River near Genoa, Wisconsin. All groups of 

Table 3. Fish size and dress out percentage for groups in the 1988 

year class at about 783 days posthatch.’ 

Fish size 

Length (mm) 

Spirit Lake walleye (SLW) 

Rock Lake walleye (RLW) 

Rock Lake hybrid (RLH} 

Spirit Lake walleye 

Rock Lake walleye 

Rock Lake hybrid 

Spirit Lake walleye 

Rock Lake walleye 

Rock Lake hybrid 

Weight (9) 

Condition factor (k} 

Processing cha ra c fe ris tics3 

Head length (mm) 

Spirit Lake walleye 

Rock Lake walleye 

Rock Lake hybrid 

Spirit Lake walleye 

Rock Lake walleye 

Rock Lake hybrid 

Spirit Lake walleye 

Rock Lake walleye 

Rock Lake hybrid 

Spirit Lake walleye 

Rock Lake walleye 

Rock Lake hybrid 

Spirit Lake walleye 

Rock Lake walleye 

Rock Lake hybrid 

Head weight (%) 

Skin (YO) 

Fillet, skin-on (Yo) 

Fillet, skinless (Yo) 

Sex (number in sample) 

Males Females 

387.0 (5) 

338.0 (6) 

329.6 (5) 

578.4 (5) 

380.3 (6) 

324.6 (5) 

1 .oo (5) 

0.91 (5) 

0.97 (6) 

24.4 (3) 

24.1 (5) 

25.4 (5) 

9.8 (2) 

13.3 (5) 

14.5 (5) 

8.4 (2) 

8.3 (5) 

6.9 (5) 

44.5 (2) 

40.5 (5) 

39.6 (5) 

36.1 (5) 

32.2 (6) 

32.7 (5) 

396.5 (26) 

360.7 (9) 

648.1 (26) 

441.3 (21) 

431.9 (9) 

1.05 (26) 

0.98 (21) 

355.4 (21) 

0.91 (9) 

25.0 (21) 

24.9 (IO) 

25.0 (9) 

12.9 (19) 

13.9 (9) 

15.5 (1 0) 

5.9 (19) 

5.3 (IO) 

7.8 (9) 

39.3 (19) 

36.6 (IO) 

38.7 (9) 

33.4 (26) 

31.3 (21) 

30.9 (9) 

p-val ue* 

0.38 

0.1 5 

<0.01 

0.04 

0.1 9 

0.03 

0.47 

0.88 

0.95 

0.50 

0.20 

0.59 

0.02 

0.1 2 

0.54 

0.83 

0.21 

0.22 

0.06 

0.1 0 

0.61 

0.1 0 

0.56 

0.21 

’The large number of data entries for this table did not allow presentation in both English and metric 

2The p-value for unpaired t-test of difference between males and females; p-values 5 0.05 are 

3Except for head length, the parameters are expressed as percent of total live weight. 

units. 

considered statistically significant. 

this cohort were raised from 

hatching under intensive 

culture on formulated feed 

in the ISU aquaculture 

facilities until they were 

processed September 21, 

1995, when they were 516 

days old. Mean length was 

12.1 in (308 mm), and the 

mean weight was 0.6 lb 

(270 g)- 

Resu I ts 

Gender differences in size 

Female walleye and female 

hybrid walleye were longer, 

heavier, and they had a 

larger k value than males in 

all year-classes (Tables 2, 3 

and 4). However, differences in k values between 

males and females were not 

statistically significant 

within any year-class. 

Gender differences in 

length and weight of SLW 

of the 1987 year-class were 

slight and not statistically 

significant, perhaps because 

the fish were relatively 

young (227-239 days old) 

(Table 2). However, gender 

dferences in size between 

SLW of the 1988 year-class 

were not statistically 

signficiant and neither were 

differences of RLW. Gender 

differences in length and 

weight of the 1988 RLH 

were signficiant, females 

were considerably larger 

than males (Table 3). 

Gender differences in 

length and weight of fish 

576 days old were statisti244 NCRAC Culture Series 101- Walleye Culture Manual 
Aquaculture of Walleye as a Food Fish - Chapter 9 

cally significant; however, female RLH and MRH were 

larger than male hybrids. 

Head size 

Head length (percent of total length) was slightly larger 

for female than male walleye, but slightly larger for 

male than female hybrid walleye; however, dfferences 

between the sexes were not statistically significant 

(Tables 2 and 3). Head weight (% total weight) was also 

larger in females than males. The differences in head 

weight between males and females was statistically 

significant in the 1988 year-class of SLW but not in the 

1998 year-class of RLW or RLH (Tables 2 and 3). 

Scales and skin 

The scales of walleye, 

which has large, conspicuous, ctenoid scales, represented 2.7-4.0% of live 

weight (Table 2). For the 

1987 cohort of SLW at a 

mean age of 228 days 

posthatch, and a mean 

weight of 0.3 lbs, the shn 

(without scales) represented 

8.4% of live weight, a rather 

substantial amount (Table 

3). For the two 1988 cohorts 

of walleye (SLW and 

RLW), skin was 8.4-8.3% 

of live weight for males and 

5.3-5.9% for females, but 

quite the opposite for RLH, 

which was 6.9% for males 

and 7.8% for females. 

Fillets 

Obviously, slun-on fillets 

have a higher processing 

yield than skinless fillets, 

but the magnitude of the 

dfference is the important 

factor (Tables 2 and 3). For 

SLW, the dfference 

between skin-on and 

skinless fillets (sexes 

combined) was 8.4% in the 

1987 year-class and 5.9% in 

the 1988 year-class. The 

1987 SLW had a mean 

weight of 0.31 lbs (139 g) when processed (Table 2) 

compared with 1.39 lbs (629.3 g) for the 1988 yearclass of SLW that were 7 16 days old when processed 

(Table 3). The expected increase in processing percentage did not occur with an increase in fish weight. In 

both 1987 and 1988 year-classes of SLW, processing 

percentage for slun-on fillets was greater for males than 

females; the difference was statistically significant for 

the 1987 year-class and nearly so (p = 0.06) for the 

1988 year-class. Processing yield for skin-on fillets was 

substantially greater for male than female RLW and 

RLH walleye of the 1988 year-class, even though 

female fish were larger than male fish for the same 

Table 4. Fish size and dress out percentage for groups in the 1994 

year-class at about 516 days.' 

Fish size 

Length (mm) 

Spirit Lake hybrids (SLH) 

Rock Lake hybrids (RLH) 

Mississippi R. hybrid (MRH) 

Rock Lake walleye (RLH) 

Spirit Lake walleye hybrids 

Rock Lake walleye hybrids 

Mississippi River hybrids 

Rock Lake walleye 

Spirit Lake walleye hybrids 

Rock Lake walleye hybrids 

Mississippi River hybrids 

Rock Lake walleye 

Weight (9) 

Co n d i t i o n factor (k) 

Processing characteristics3 

Fillet, skinless 

Spirit Lake walleye hybrids 

Rock Lake walleye hybrids 

Mississippi River hybrids 

Rock Lake walleye 

Sex (number in samale) 

Males Females p-val ue2 

299.1 (11) 318.0 (14) 

294.7 (26) 316.1 (40) 

293.2 (15) 313.5 (24) 

307.6 (5) 313.8 (7) 

259.8 (1 1) 326.6 (14) 

226.8 (26) 294.9 (40) 

216.2 (15) 280.0 (24) 

269.8 (5) 273.1 (7) 

0.96 (11) 0.99 (14) 

0.90 (26) 0.92 (40) 

0.86 (15) 0.92 (24) 

0.87 (5) 0.88 (7) 

35.7 (1 1) 33.5 (14) 

35.3 (26) 34.8 (40) 

34.9 (15) 34.1 (24) 

36.2 (5) 35.3 (7) 

0.1 0 

co.01 

0.01 

0.75 

0.04 

co.01 

co.01 

0.96 

0.64 

0.46 

0.35 

0.74 

0.1 0 

0.45 

0.36 

0.64 

'The large number of data entries for this table did not allow presentation in both English and metric 

*The p-value for unpaired t-test of difference between males and females; p-values 20.05 are 

3Except for head length, the parameters are expressed as percent of total live weight. 

units. 

considered statistical I y sign if i can t . 

NCRAC Culture Series 101 - Walleye Culture Manual 245 
Chapter 9 - Aquaculture of Walleye as a Food Fish 

50 ~ 

49 ~ 

48 - 

47 - 

46 

45 ~ 

Males 

Y = 42.858 + 0.004 * X; R2 = 0.02 

0 

0. 

a 

a 0 

-1 

%401 Y I ' I I I 1'1 I 

9 50 100 150 200 250 300 350 400 450 9- 

Females 

Y = 38.973 + 0.012 * X; R2 = 0.069 cd 

0 a 

/ 

a 

0 

a. 

0 

30 I I I I 1'1' I I I' 

0 50 100 150 200 250 300 350 400 450 500 

Total weight (g) 

Figure 3. Relationship between skin-on fillet weight 

as percentage of total weight and total weight of 

walleye from the 1987 year class. The regression 

coefficients for both male and female walleye were 

not significant and the coefficient of determination 

indicates that weight accounts for only 2 to 6.9% of 

the variability in relative fillet weight. 

groups, but the differences were not statistically 

significant 

For sexes combined, the mean processing percentage 

for skinless fillets was 33.2% for the 1987 year-class, 

31.5-33.9 for the 1988 year-class, and 34.4-35.7 among 

the four year-class cohorts of the 1994 year-class, 

which is remarkabaly close agreement among a 

dwersity of years, fish ages, and size. Differences in 

processing percentages for skinless fillets between male 

and female walleye and hybrid walleye were not 

statistically significant for any year-class, but in every 

(eight groups of three year-classes) comparison, the 

dressed yield was higher for males than females. 

Processing percentage and fish size relationship 

One of the arguments for harvesting larger fish is the 

assumption that the processing percentage increases 

with size. We conducted a regression analysis of fillet 

yield-fish weight relationship for all year-classes, and 

stocks within year-classes, and for the six age groups 

within the 1987 year-class. Only three of the coefficients of determination (r2) were greater than 0.10 (i.e., 

body weight did not account for more than 10% of the 

variability in yield). Of all the regression coefficients 

(i.e., slope of the regression), only one, the combined 

data for the 1987 cohort, was statistically significant; 

however, even in that case, the r2 for that relationship 

was only 0.066, which means that for the best fitting 

regression, weight would account for 6.6% of the 

variability of processing percentage. Albeit the regressions was not statistically significant, dressed yield of 

shn-on fillets showed a slight increase with fish size 

(Figure 3). 

We also examined head weight and processing yield of 

fillets in relationship to fish age (Table 5). The analysis 

for five age-groups (1 51 -467 days old) of the 1987 

year-class inQcated significant differences in head 

weight, and yield of shnless fillets by fish size groups, 

but there was not a trend for the older (i.e., larger) fish 

to have higher processing percentages. For example, the 

highest processing percent for skin-on fillets was 44.5% 

at 15 1 days, but that value did not differ significantly 

from values of 40.7 and 42.5 at 170 and 357 days, 

respectively. Likewise, a comparison of the processing 

yield of the 1987, 1988, and 1990 year-classes, which 

were different ages when they were processed (467 

maximum for 1987,783 for 1988, and 516 days 

posthatch for 1990 year-class), did not indicate a trend 

for processing percentages to increase with age (i.e., 

weight). 

Processing percentages and stock differences 

An analysis of processing percentages for head weight 

and both skin-on and skinless fillets for 1988 and 1990 

year-classes indicated some statistically significant 

differences among stocks in the 1988 year-class. 

In the 1988 year-class, the relative head weight of SLW 

(12.65%) was less than that of RLW or RLH, and the 

skinless fillets of SLW were larger than of either RLW 

or RLH. However, dfferences among the mean 

processing percentage for slunless fillets of the four 

groups of the 1990 year-class were not statistically 

246 NCRAC Culture Series 101- Walleye Culture Manual 
Aquaculture of Walleye as a Food Fish - Chapter 9 

significant. These analyses do 

not indxate higher processed 

yields for hybrid walleye 

compared with walleye. 

Discussion 

Defining a food-size walleye 

References defining a foodsize walleye were not found 

but traditionally, in Canada, 

the actual size of a fish 

serving (referring to all fish, 

not just walleye) was 6 to 8 

ounce (1 80 to 225 g) (Iredale 

and York 1985). Presumably, 

that was for scaled, but skinon fillets. A large midwest fish 

company indicated a market 

for 4-, 6-, and 8-oz fillets 

(Flickinger 1996) 

Cookbooks provide another 

perspective on portion sizes 

(Table 6). They generally 

indicate 0.25 lbs, or 4 oz (1 14) 

for fillets and slightly larger 

portions (0.31 lb, 142 g) for 

steaks. 

Assuming that a 4-6 oz (1 14- 

227 g) shn-on fillet is the 

market size product, the 

Table 5. Comparison (analysis of variance, ANOVA)) of processing percentages of walleye and hybrid walleye: p-values 20.05 

are statistically significant. Means with a letter in common in 

the same column are not statistically different (p 2 0.05, 

Fisher’s PLSD test). 

Age (days posthatch) Head weight Fillet: skin-on Fil1et:skinless 

and year-class (% total wt) (% total wt) (% total wt) 

1987 (151 -467 days old) 

151 

1 60 

1 70 

357 

467 

p-value for F-test 

1988 (783 days old) 

Spirit Lake walleye 

Rock Lake walleye 

Rock Lake hybrid walleye 

p-value for F-test 

1990 (51 6 days old) 

Spirit Lake walleye hybrids 

Rock Lake walleye hybrids 

Mississippi River hybrids 

Rock Lake walleye 

p-value for F-test 

14.8a 

1 6.0a1b 

18.0b 

15.0a 

0.005 

~ 

12.65a 

14.77b 

14.13b 

0.01 0 

44.5a 

39.3b 

40.7a,b 

42.4a1b 

0.062 

39.83a 

37.93a 

39.05a 

0.31 3 

36.1 a 

31.2b 

31.5b 

35.3a 

41.2~ 

0.001 

33.94a 

31.54b 

31.52b 

0.006 

34.49a 

34.96a 

35.71 a 

35.72a 

0.472 

minimum live weight that is needed to for a 4-oz (1 14 

g) or 6-oz (1 70 g) skin-on fillet would range from 1 .O to 

1.88 lb (454-851 g), depending on processing percentages. 

Processing percentages of marketable product for 

walleye have been based on assumptions or approximations. It is probable that the Freshwater Fish Marketing 

Corporation (FFMC) of Canada (see Summerfelt 1996 

for an description of this quasi-government entity), 

which processes 6.7 to 10.9 million lbs (5.0 million kg) 

of walleye annually, has a substantial body of infomation on processing percentages of food-size, wildcaught walleye, but it was not unavailable. Stettner et 

al. (1993) assumed a 40% processing percentage for a 

1.87 lb (849 g) fish would provide two 6-oz (170 g) 

skin-on fillets. Held and Malison (1996) assumed a 

50% yield (the form, shnless or skin-on was not 

specified) for a 2-lb (900 g) food-size fish. Stevens 

(1 996) reported dressout yield of 45-50% for skin-on 

fillets. Flickinger (1 996) reported 42% (range 39-46%) 

processing percentage for cultured walleye weighing 

0.83 lb (377 g), and 45% (range of 39-46%) for wild 

caught walleye of similar size. Assuming that 

Flickinger (1996) used a scaled, but skin-on fillet 

processed in a similar manner to our procedures, the 

42% yield for his cultured walleye is similar to those in 

this study. We obtained means for slun-on fillets from a 

high of 44.5% for males from SLW in the 1988 yearclass to a low of 36.6% for female KW. 

Using an average yield of 40%, the minimum, food-size 

walleye would be 1.25 lbs (504 g) live weight to obtain 

two, 4-oz (1 14 g) slun-on fillets. Channel catfish are 

typically harvested at 1.25 lbs (568 g) (Foster and 

Waldrop 1972). A 1.88 lbs (853 g) walleye would be 

NCRAC Culture Series 101 - Walleye Culture Manual 247 
Chapter 9 - Aquaculture of Walleye as a Food Fish 

needed to obtain two, 6-oz (170 

g) fillets with a 40% processed 

percentage. Faster growing fish 

have a smaller head to body 

size than a slower growing fish. 

Perhaps, with genetic selection, 

optimal environmental con&- 

tions for fast growth, and 

skillful processing procedures, 

processed percentage for 

walleye can reach 45%, which 

would reduce the requirement 

for live weight of the cultured 

fish to 1.1 1 lbs (504 g) to 

obtain two 4-oz (1 14 g) fillets, 

or 1.67 lb (756 g) to obtain two 

6 oz (170 g) fillets. For now, a 

value of 40% seems to be the 

most valid estimate or processing percentage for skin-on 

fillets. 

Obviously, processing percentage is a function of the slull 

and/or time taken to fillet the 

Table 6. General recommendations (buying guides) for portion 

sizes for different forms of fish. 

Form Serving size' Reference 

Ibs (9) 

Whole 0.75 (340) 

Whole dressed (drawn) 0.5 (227) 

0.5-0.75 (227-340) 

Pan dressed (dressed) 0.5 (227) 

Steaked 

Fillets 

0.33 (151) 

0.31 (142) 

0.25-0.50 (1 14-227) 

0.33 (151) 

0.25-0.50 (1 14-227) 

0.25-0.50 (1 14-227)) 

0.22-0.25 (99-1 14) 

Dunn (1 974) 

Dunn (1 974) 

Plutt (1986) 

Dunn (1 974) 

Dunn (1 974) 

Plutt (1986) 

Hachfeld and Eykyn (1992) 

Dunn (1 974) 

Plutt (1986) 

Darling et al. (1989) 

Hachfeld and Eykyn (1992) 

'The information may have been given for 2, 4, or 6 servings, but for comparative purposes the 

values are expressed for a single serving. 

fish. In the present study, we endeavored to process fish 

with reasonable speed to approximate the speed used in 

a commercial facility using hand processing. There are 

tradeoffs relative to time and processed percentage. An 

experienced production worker may obtain a smaller 

processed yield in an effort to obtain a hlgher total 

volume of fish processed per unit time, especially if the 

worker is paid by the pound of fish processed. By 

comparison, we may have been slower than a production line worker but our processed yield might have 

been higher because of our emphasis on maximizing 

processing percentages of the dressed product. 

We think it is appropriate to define a food-sized walleye 

as whatever size can be marketed for human food. A 

flexible definition of the market size allows for the 

tra&tional size, but also for a smaller size that can be 

brought to market faster. A faster turnover time can be 

obtained if walleye are harvested at the inflection point 

of the growth curve (Le., at the size [age] when growth 

rates decline) and sold at a smaller than traditional size, 

perhaps not much larger than yellow perch. Yellow 

perch are usually marketed at 7.5 to 8 inches (190-203 

g), or 5 fish per pound live weight (3.2 oz, 90.8 g) per 

fish. Perch of this size yield two 0.67 oz (19.0 g) fillets 

with a 42% fillet yield. In January 1996, the price for 

yellow perch was $2.50/lb ($5.50/kg) live weight, 

$8.00-9.00 ($27.50-$19.80/kg) per pound for a skin-on 

fillet wholesale, and $1 1.00-12.50/pound ($24.70- 

$27.50/kg) retail (Chns Stan-, Bayport Aquaculture, 

personal communication). Most of that product is sold 

in restaurants, not retail. Restaurants use about 6 oz 

(170 g) of fish per serving, similar to the traditional 6 

oz size for walleye fillets. 

In summary, fillet sizes range from 0.67 oz for yellow 

perch to 4.4 oz for channel catfish, with a generic 

buying guide of 4 oz (1 14g) for fish fillets of 5.3 oz 

(1 50 g) per serving (Dunn 1974). This range in size is 

smaller than the traditional 6-8 oz size for walleye 

fillets. However, for a frame of reference, we recommend using 1.25 lb (567g) as the minimum size for 

cultured walleye. A 40% dressout yield for slun-on 

fillets would produce two 4 oz fillets. 

Processed yield and fish size relationship 

The dressing percentage of both sexes of channel 

catfish was positively correlated with body weight for 

13-month old fish but not for 22-month-old catfish 

(Dunham et al. 1985). We &d not find an relationship 

between fish size or age and processing percentages for 

248 NCRAC Culture Series 101- Walleye Culture Manual 
Aauaculture of Walleve as a Food Fish - Chanter 9 

the size range of fish in the study. Regression analysis 

consistently showed a slight positive slope (i.e., 

dressout percentage increases with fish weight), but the 

coefficient of determination (r2) did not indicate more 

than 10% (rz = 0.10) of the variation in fillet percentage 

was related to fish weight. Although statistically 

significant differences in the fish age-percentage 

dressout occurred, there were no overall trends to the 

data that suggested processed percentages increased 

with age or size. This is an important finding because it 

does not support a assumptions that processed percentage will increase by raising fish to larger size than the 

minimum size needed for to achieve the smallest 

acceptable fillet. 

Yield of walleye and hybrid walleye 

We did not find a consistent difference between 

processed percentages for walleye and hybrid walleye; 

i.e., processing yield would not be improved by 

culturing hybrid walleye. SLW had smaller relative 

head weight and larger processed yield of skinless 

fillets than RLW or RLH. This difference and growth 

comparisons we have made between SLW and other 

stocks in the Midwest provide evidence to recommend 

the SLW as a source of broodstock for domestication. 

Dressed yield for rainbow trout showed significant 

differences attributable to the strain of the trout but not 

between two diet types (Smith et al. 1988). 

Skin-on and skinless fillets 

The reduction in processed percentage from skin-on to 

slun-off fillets was as high as 8.4% for males from one 

cohort. The loss from removal of the skin would seem 

to require skin-on fillets as a matter of practice; 

however, the price of skinless walleye fillet can be as 

much as 32% greater than skin-on fillets based on 

observations I have recorded in regional fish markets. If 

consumers are willing to pay more for slunless fillets, 

then the loss in production from removal of the skin 

may be acceptable providing higher processing costs 

for labor or equipment do not exceed the differential in 

prices of the two types of fillets. 

Comparative dressout yield 

The head of walleye is a smaller percentage of body 

weight (9.8-15.5%) than the 21.6-23.6% reported for 

channel catfish (Dunham et al. 1985). Head weight as a 

percentage of body weight of female walleye was 

usually larger than that of males. In channel catfish, the 

relative head weight of 13 month old males is larger 

than same aged females, but 22 month old female 

catfish had a larger head than 22 month old male catfish 

(Dunham et al. 1985). 

It is reasonable to assume a dressout yield for skin-on 

walleye fillet is 40%. This seems to be a reasonably 

good value compared with several other species. Plante 

(1996) reported that “typical” fillets yields for common 

food-fish are 32% for tilapia, 40-45% for channel 

catfish, and 60% for salmon. Chettleburgh (1 991) gives 

32% as processing yield of skinless fillets for tilapia. 

However, fillet values for channel catfish are quite 

variable. McGilberry et al. (1989) reported that regular 

fillets” are about 43.5% of the live weight, which would 

be two 4.4 oz (125 g) fillets, but Wolters et al. (1 991) 

reported 28.8% dress-out percentage for fillets of 

female diploid channel catfish and 27.9% for fillets 

from male catfish. They processed catfish by removing 

fillets from whole fish, then they slunned the fillet on a 

mechanical slunner. Heidnger and Kayes (1 993) 

reported that dressout yield for yellow perch is 37-40%, 

with exceptional value of 42%, which is nearly identical 

to values for walleye. 

Acknowledgment 

we thank Robert Witmer for preparation of the two 

illustrations of walleye. This study was sponsored by 

North Central Regional Aquaculture Center, the Iowa 

State University Agriculture and Home Economics 

Experiment Station (project 2824 and 2982), and the 

Iowa Department of Economic Development. 

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250 NCRAC Culture Series 101- Walleye Culture Manual