Growth, carcasss composition, and taste of rainbow trout of different strains fed diets containing primarily plant or animal protein

Aquaculture, 70 (1988) 309-321 Elsevier Science Publishers B.V., Amsterdam

309

-

Printed

in The Netherlands

Growth, Carcasss Composition, and Taste of Rainbow Trout of Different Strains Fed Diets Contatining Primarily Plant or Animal Protein ROBERT R. SMITH’, RUMSEY’

HAROLD L. KINCAID’,

JOE M. REGENSTEIN3

and GARY L.

‘Tunison Laboratory of Fish Nutrition, U.S. Fish and Wildlife Service, Cortland, NY 13045 (U.S.A.) ‘National Fishery Research and Development Laboratory, R.D. # 4, Box 63, Wellsboro, PA 16901 (U.S.A.) 3Cornell C’niversity, Rice Hall, Department of Poultry and Avian Science, Ithaca, NY 14853 (U.S.A.) (Accepted

20 January

1988)

ABSTRACT Smith, RR., Kincaid, H.L., Regenstein, J.M. and Rumsey, G.L., 1988. Growth, carcass composition, and taste of rainbow trout of different strains fed diets containing primarily plant or animal protein. Aquaculture, 70: 309-321. Ten rainbow trout (Salmo gairdneri) strains were evaluated during early growth from 30 g to 250 g on two diets - one based on plant protein (soybean and cottonseed meal) and the other on animal protein (fish meal). Diets were formulated to be nutritionally isocaloric and isonitrogenous. Fish were fed identical starter diets until they weighed 30 g. Significant differences in growth rate were found attributable to fish strain. Differences associated with diet were nonsignificant. Percent dress-out data based on eviscerated weight, deboned weight, and fillet weight also showed significant differences in yield attributable to fish strain, but not to diet. Carcass composition varied among strains, but none of the differences could be attributed to diet. Organoleptic tests showed no differences in flesh acceptability associated with either fish strain or diet, and all trout tested were equally acceptable to human taste panels.

INTRODUCTION

Growth rates in trout populations reared under similar environmental conditions often vary widely for many reasons including: diet, feeding behavior, rearing density, oxygen and fish strain (Embody and Hayford, 1926; Brown, 1946; Pyle et al., 1961; Pyle, 1964; Gall and Gross, 1978a; Refstie and Steine, 1978; Kiincaid, 1981; Piper et al., 1982; Dwyer and Piper, 1984) Historically, trout have been raised on diets in which the primary source of protein was fish

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0 1988 Elsevier Science Publishers

B.V.

310

meal derived from any of several marine species. Because fish meal is costly and not always readily available, the use of soybean protein in fish feeds has increased in recent years (Smith, 1977). Questions have been raised about the effects of plant versus animal protein on production efficiency and human organoleptic qualities. The present study was initiated to determine the effect of fish strain and dietary protein source on growth rate, feed conversion efficiency, market yield, body composition, and organoleptic qualities on 10 distinct strains of rainbow trout (Salmo gczirdneri) fed diets formulated with either plant or animal protein. METHODS AND MATERIALS

Identification of strains Rainbow trout strains from divergent sources were introduced into the fish genetics program of the U.S. Fish and Wildlife Service in 1973-1979. The fish evaluated included five winter-spawning strains and five fall-spawning strains (Table 1) . Because of the difference in spawning time and space limitations, we studied the five winter-spawning strains in experiment 1 and the five fallspawning strains in experiment 2. Strains for the studies represented a broad cross-section of genetic backgrounds. In experiment 1, strains 14 (Sand Creek) and 20 (Desmet) were deTABLE 1 Identification of five winter-spawning diet studies

and five fall-spawning rainbow trout strains evaluated in

Strains

No. parents

No.’

Name

Experiment 1 14 20 22 23 24

winter-spawning Sand Creek Desmet Winthrop Ennis Donaldson

Experiment 2 12 15 17 25 32

fall-spawning strains Manchester Standard-Fall Growth-F3 Formalin Growth-F6

‘Strain names and identification

Date shipped

Male

Female

15 29 5 8 15

15 29 5 8 15

29 January 29 January 29 January 29 January 29 January

1982 1982 1982 1982 1982

12 12 11 16 17

12 12 11 16 17

21 21 21 21 21

1982 1982 1982 1982 1982

strains

numbers are taken from Kincaid (1981).

October October October October October

311

rived frorn natural populations within the previous 15 years and expressed average growth rates; while strains 22 (Winthrop), 23 (Ennis), and 24 (Donaldson) were established domestic brood stocks that had been maintained in hatcheries for at least 25 years and showed above average growth. All strains in experirnent 2 were derived from the Wytheville strain, but they had become differentiated over the past 15 years due to different selection regimes. Strain 12 (Manchester) was maintained as a random mating population taken from the Mane hester, Iowa, National Fish Hatchery (NFH) in 1965 and 1966. Strain 32 was developed by six generations of selection for accelerated growth rate. Strain 15 was produced by the hybridization of strain 12 and the Wytheville strain (Wytheville, Virginia NFH) in 1967 and 1968, and was maintained thereafter as a random population without further selection. Strain 17 was developed from strain 15 by selection for accelerated growth for three generations from 1967 to 1973 and maintained thereafter as a random breeding population. Strain 25 was also developed from strain 15 by using a selection program for formalin resistance for two generations from 1967 to 1972, followed by maintenance as a random breeding population after 1972. Growth At the .appropriate time, eggs from each of the test strains were transferred from the U.S. Fish and Wildlife Service genetics program at Leetown, West Virginia, to the Hagerman, Idaho, field station of the Tunison Laboratory of Fish Nutrition. The two experiments were carried out about 1 year apart, but the husba.ndry techniques used and the environmental conditions were similar. Eggs were water hardened and incubated in 10°C spring water until the eyed stage. They were then transferred to the Hagerman facility where they were incubated and hatched in spring water cooled to 10’ C. When fish had adsorbed their yolk-sacs and were ready to begin feeding, the temperature of the water was allowed to rise to 15’ C. At swim-up, about 2000 fish from each strain were transferred to separate 160-l fiberglass tanks and fed a commercial trout starter diet (Rangen Inc., Buhl, Idaho) for about 1 week until all fish were actively feeding. Then, six groups of 120 fish each were selected from each strain, weighed and placed in 160-l fiberglass tanks. At the beginning of the experiment all six groups of fish from each strain were fed a commercial starter diet (Rangen Inc.) until the slower growing groups reached an average weight of 30 g. The fish were then reweighed and three tanks were randomly assigned to either Diet AP or Diet PP which provided most of the protein from animal or plant sources, respectively (Table 2). During the course of the studies, the fish were weighed every 2 weeks until they reached an average weight of about 30 g, and every 4 weeks thereafter. Tanks were cleaned and checked for mortality daily. The flow rate through the tanks was about 10 l/min, and the spring water

312 TABLE 2 Ingredients

(g/100 g dry diet), calculated nutrients,

and cost of experimental

diets

Diet AP

PP Ingredients Fish meal, anchovy Soybean meal, fulifat Cottonseed meal Corn gluten meal Blood flour Yeast, brewers dry Wheat middlings Whey, dried Fish oil Soybean oil Molasses, sugar beet (70% DM) Vitamin mix’ Mineral mix’ Calculated nutrients Total protein (% ) Plant protein (% ) Animal protein (% ) Metabolizable energy (kcal/kg) Cost of ingredients ($U.S./kg) ‘Vitamin and mineral supplements

7.5 40.0 15.0 7.0 5.0 5.0 6.0 3.0

30.0 7.5 10.0 7.5 5.0 5.0 20.0

3.0 2.0

3.0 6.0 3.0 3.0 2.0

42.4 31.7 10.7

45.0 14.9 30.1

10.0

3529

0.37

3430 0.43

to equal National Research Council requirements

(NRC, 1981).

temperature was a constant 15’ C. The fish were fed all they would eat without excessive waste. To keep within safe loading and density limits, we successively reduced the population in each tank to 100 fish when the fish reached an average weight of about 2 g, to 50 fish at 30 g, and to 20 fish at 100 g. The reduction was by randomly assigning the fish in each tank into retain and cull groups. The group selected for continuation was then weighed. If the average weight was not within 10% of the average weight before reduction, individual fish were exchanged to bring the average weight to within 10% of the previous average weight. The daily growth rate was calculated at the end of the weighing period in which the average weight of three replications of each strain reached 200 g. The number of days required to reach 200 g was calculated using the daily growth rate and the average weight at the end of the period.

313

Processing and chemical analysis When the fish reached an average weight of about 250 g, the growth phase was terminated and all fish were individually weighed and measured. Ten average sizle fish were selected from each tank, fasted for 24 h, and then killed by asphyxia.tion. They were placed in buckets and mixed with crushed ice (ice&h ratio of :1:3) for transportation to the nearby Clear Springs Trout Company processing plant. In experiment 1, the fish were individually weighed, measured, and eviscerated by h.and at the processing plant; in experiment 2, the fish were eviscerated and weighed by laboratory personnel before being taken to the processing plant. Whole and eviscerated weights were obained for each fish. Fish from each tank were placed in a separate net bag and placed in a chill tank at 1.7” C for about 15 min. Individual identity was lost at this point. Upon removal from the chill tank, the fish were boned (i.e., the spine and ribs were removed but all fins and the head and tail remained on the fish), and boned weight for the fish in each tank (10 fish) was obtained. Next, fish were filleted (i.e., all fins, head and tail were removed) and the weight of filleted fish from each tank (10 fish) was obtained. The fillets were then ind.ividually packaged and frozen at - 40’ C in a spiral freezer. The processing procedure used in this study duplicated the “usual” commercial practice as nearly as possible. The processed fish were stored at - 20’ C until transfer to Cornell University. Analysis for fat, protein, and dry matter was done by AOAC standard methods (Williams, 1984) on whole fish and on the edible portion (fillet). Taste test The frozen and wrapped skin-on fillets were air-freighted to Ithaca, New York, where they were held at -20°C. About 0.5-2 h before the taste test, samples were thawed, arranged on baking trays that had been sprayed with a lecithin-based anti-sticking agent, and held at 1°C until they were cooked. Fillets representing each of the 10 set treatments (five strainsx two diets) were placed on a single tray and baked for 20 min at 310°F (154” C ). Because the size of the fillets used in these tests differed slightly, any slight texture changes due to the degree of cooking might unduly influence specific texture responses. Consequently, we measured only overall acceptability and flavor. Each fillet was used to prepare three individual tasting samples. Each of the persons conducting the test received all 10 samples from the same baking tray on two paper plates. Panelists were seated in the test kitchen at tables with dividers to separate individual panalists and ensure privacy. Water, salted soda crackers, and celery were provided for clearing the palate between samples. A g-point hedonic score

314

sheet (1, dislike extremely; 5, neither like nor dislike; and 9, like extremely) was used. Six panels of six people each (i.e., 36 panel members) were used for experiment 1 and four panels of six people each (24 panel members) for experiment 2. For each experiment, the test was conducted near noon, on four successive days. Panelists were students, faculty, and staff of Cornell University, recruited by the Poultry and Avian Sciences Department. Some persons participated in the panels for both experiments. The only prerequisite was that the panelist not dislike eating fish. Various ages and ethnic background were represented, and many participants had had previous experience on taste panels. In the jargon of taste-panel specialists, the people used would be classified as semi-trained, but not expert. They were aware that they were eating trout, but they did not know that the trout differed in strain or diet.

Statistical analysis A two-way analysis of variance was used for growth and yield data. Duncan’s Multiple Range Test was used to test individual comparison means (Duncan, 1971) where appropriate. The average score and standard deviation were determined for the organoleptic data. A ‘Y” test for the difference among means was made using paired data. RESULTS Growth

Significant differences (PC 0.05) were found among the strains in growth rate expressed as days to increase average weight from 2 g to 200 g (Table 3 ) . Differences between the fee&gain ratios of fast-growing and slow-growing strains were also statistically significant showing that more feed was required per unit gain in the slow-growing strains. Differences in fee&gain ratios between fish in the two slow-growing strains and within the three fast-growing strains in experiment 1 were not statistically significant. Strains in experiment 2 consisted of three that grew at about the same rate as the fast-growing strains in experiment 1 and two strains that grew even faster. In experiment 1, there was a small but significant (PcO.05) difference in growth rate and feed conversion efficiency between the fish fed the two different diets. The AP diet was somewhat better than th’e PP diet in experiment 1, but there was no difference in either food conversion efficiency or growth rate of fish fed the two diets in experiment 2. In experiment 1, 106.6 g of the PP diet were required to produce the same amount of growth produced by 100 g of the AP diet. At the time these tests were run, the cost of ingredients per kg was $0.37 (U.S. ) for the PP diet and $0.43 for the AP diet. This equates to a feed

315 TABLE 3 Days required for rainbow trout to increase in average weight from 2 to 200 g and fee&gain ratios of 10 strains of rainbow trout fed diets containing mostly plant or animal protein (mean of three replications f SD). Means in each group with the same superscript are not significantly different (P
Days to reach 200 g ave. wt. Diet

Strain means

PP Experiment 1 215 f 14 237f 20 187k 22 205? 23 193k 24 207’ Diet mean Experiment 2 12 15 17 25 32 Diet mean

187f 192f 191+ 184k 18Ok 187”

Fee&gain ratios

AP

Diet

Strain means

PP

AP

18.23 5.51 1.00 5.29 3.79

209 k 13.50 237k 8.74 186k 0.58 193k 2.00 19Ok 3.46 203d

212” 237” 187b 199s 192b

1.90f0.25 2.05f0.13 1.25kO.03 1.43kO.12 1.32kO.10 1.59’

1.74 + 0.05 1.99+0.10 1.26 k 0.02 1.23 k 0.06 1.24 k 0.09 1.49d

1.82” 2.02” 1.26b 1.33s 1.28b

9.50 5.57 4.00 4.16 2.31

187k 1.53 192f 4.36 184f 7.02 181k 1.00 180f 1.53 185”

187” 192” 187” 183b 180b

1.22f0.13 1.19kO.08 1.16kO.05 1.09 k 0.05 0.98 + 0.03 1.13”

1.23 + 0.07 1.18*0.08 1.17f0.14 1.06f0.03 1.03Iko.05 1.13’

1.22” 1.19” 1.16’ 1.07s 1.10s

cost, per kg of fish weight gain, of $0.59 for the PP diet and $0.64 for the AP diet - an advantage of $0.05 or 8.5% in favor of the PP diet. At the fee&gain ratio of 1.13 for both diets in experiment 2 the cost advantage of the PP diet was $O.O’Y/kgor 13.5% lower. Mortality during the growth phase was small and was not correlated with strain or diet. Processing and chemical analysis Except, during the eviscerating procedure described in experiment 2, all eviscerating, deboning, and filleting was done by the same person in the two experiments. There were no significant differences
78.7 k 1.54 78.0 k 0.94 80.3 kO.91 81.0 k 1.69 79.0 k 1.85 79.4’

18.0 + 0.34 77.1+ 0.61 79.7 + 1.34 80.3 + 1.24 79.3 + 0.83 79.0”

Experiment 2 12 15 17 25 32 Diet mean

AP

85.7 k 3.24 85.7 i 1.30 84.7 k 0.46 84.7 + 1.57 85.0 k 0.45 85.2’

PP

85.9”

78.3” 77.8” 80.0b 80.7b 79.2”

84.7” 84.5” 85.1”

84.9”

69.0 k 0.17 69.0 + 0.35 70.7 2 1.30 72.32 1.89 70.3 k 1.00 70.3’

71.3 + 1.34 71.0 i 1.50 75.0 k 1.44 75.3 Ik2.09 75.3 + 1.70 73.6’

PP

70.7 k 1.43 70.3 + 1.17 70.3k1.11 72.0 k 0.78 70.0 k 2.58 70.7

72.0 k 1.98 70.0k3.12 74.3 f 0.54 75.0 k 2.88 75.0 f 0.89 73.3”

AP

70.5” 72.2” 70.2”

69.8” 69.7”

70.5” 74.7b 75.2b 75.2b

71.1”

56.7k0.71 56.0 f 0.49 58.3 k 0.79 60.3 f 1.91 57.3 * 0.87 57.7

55.7 + 0.76 56.3 k 1.60 58.3 f 1.16 57.3 k 1.41 58.0 + 0.39 57.1’

PP

Diet

Strain means

Diet

Diet

Strain means

Filleted

Boned

Eviscerated

Experiment 1 14 86.0 + 1.20 20 84.0 + 1.28 22 84.1 kO.95 23 84.3 + 0.90 24 85.3 k 2.62 Diet mean 84.9’

Strain

57.0 k 2.43 58.0 k 1.10 57.3 k 1.13 59.7 + 0.28 57.0 k 2.85 57.8’

57.3 k 0.76 55.7 F 2.77 58.7 k 1.00 57.3 k 1.26 58.7 k 1.31 57.5”

AP

56.8” 57.0a 57.8” 60.0b 57.2a

56.5” 56.0a 58.5b 57.3b 58.4b

Strain means

Eviscerated, boned, and filleted yield (% of whole body weight) of 10 strains of rainbow trout fed diets containing mostly plant or animal protein (mean of three replications k SD). Means in each group with the same superscript are not significantly different (P~0.05)

TABLE 4

w g

Fillet

Fillet

Fillet

20 -

22 -

23 -

Fillet Fillet

32 - Fillet Mean-Fillet

17 -Fillet 25 - Fillet

12 15 -

32 - Carcass Mean - Carcass

Carcass Carcass

17 25 -

2

Carcass Carcass

12 15 -

Experiment

24 - Fillet Mean - Fillet

Fillet

14 -

Carcass

Carcass Carcass

23 24 -

Mean -

Carcass Carcass Carcass

1

14 20 22 -

Experiment

Strain

0.68 0.60 1.78 1.61

AP

74.5

78.5 + 1.03 76.1’

75.1+ 1.90 76.3k1.11

75.8 * 1.39 75.1 kO.11

5.59 1.78 2.38 1.69

72.8 + 5.44

k k f f

51.6’

50.7”

74.8 70.3 76.4 78.4

55.1* 1.04 51.8+ 1.02 50.4 + 1.54

54.5 f 1.86 52.3 f 0.93 50.5 f 1.90

48.4 + 4.12 47.6 + 1.2

50.0 k 2.07 50.7 + 1.45

69.2b

69.1b

68.4 k 0.80

68.0 i 1.88 66.1 k 1.77 70.0 * 0.54

72.2 k 0.82

69.8 I? 0.87

51.5 * 1.73 54.6 + 1.20 52.0

49.Ok2.63 55.1 t 1.52

49.8kO.68

68.4 k 1.03

68.2 i 2.01

70.3 + 1.30

70.3 I+4.31

53.3 * 0.27 51.9

50.6 + 49.5 k 52.9 k 53.32

PP

78.5b

75.0ab 72.7” 76.4

74.3”

50.5”

54.8b 52.1b

49.2” 49.4”

69.2”

67.3”

68.1”

71.3”

70.1”

16.7 rt 0.73 15.3 + 3.56 16.8+ 1.61 15.4 + 0.17 13.9 * 0.86 15.6’

14.9 k 3.41 14.3 + 1.06 17.1c

39.8 t 0.89 42.2k2.16 41.0’

42.7 + 2.99 42.3 t 1.50 38.1 k 2.10

27.1 to.66 26.5b

19.8-f 1.33 27.7 i 0.91 28.8 f 1.53

29.2 + 0.63

38.0 + 2.02 40.1+ 1.81 37.1+ 1.76 39.2b

41.3 k 1.16 39.7 + 2.27

AP

18.8+ 5.56 17.OL4.63 22.5 + 1.61

42.0’

40.8? 1.10 42.1 k 0.43

43.2+ 1.19 40.0* 1.91

43.7k4.31

29.8 k 2.01 27.3 k 0.70 28.2b

31.7kO.45 26.2 k 1.20

25.9 * 1.37

39.0b

39.1k2.55 38.1 k 1.59 37.6 k 1.89

54.0b 52.4b 54.0b

38.9 k 0.35 41.1_+0.82

50.2” 49.3”

PP

Diet

Diet means

Fat

Protein Strain

are not significantly

19.7” 15.2” 14.2”

17.8” 16.2”

39.lb 40.3ab 42.2”

43.2” 42.8”

27.2”

29.3”

27.0”

27.6” 25.88

38.6” 39.1” 37.4”

40.4”

40.1”

means

24.1 kO.79 25.2’

25.4 k 1.44 26.6kO.66 24.3 ? 0.76

25.4 k 2.23

32.2 k 0.70 32.0 Z!Y 0.20 32.1b

32.3 f 0.87 31.3kO.45

32.9 * 3.09

28.gb

30.6 i 0.87 28.2 k 1.03 28.4 k 1.11

28.7 k 0.77

28.6 k 0.97

30.6

30.5 + 0.35

31.5kO.40 30.9 _+0.52 30.3kO.15 30.0 * 0.30

PP

Diet

Dry matter

(PC 0.05)

Strain

different

25.9+0.09 25.0k0.36 23.9 k 0.14 24.9”

25.2 kO.18 24.6 k 0.28

33.0 * 0.99 32.1b

31.6 t 0.20

32.6 + 1.06 31.2kO.75

32.1+ 1.51

28.4 + 0.69 28.3b

27.8 i 0.65 29.2 kO.91

27.3 k 0.46

28.8 k 0.83

30.5 + 0.79 31.1’

32.6 + 1.80 31.4k1.02 30.4 k 0.90 30.7kO.23

AP

24.0b

25.3 25.0” 26.3” 24.7b

32.5”

31.3” 31.9”

32.5” 32.5”

28.4”

28.7”

29.2”

28.7” 28.0”

30.4b 30.5b

30.4b

32.1” 31.2”

Strain means

of 10 strains of rainbow trout fed diets with mostly plant or animal protein

(fillet)

(mean of three replications I! SD). Means in each group with the same superscript

5

Protein and fat (as % of dry matter) and dry matter of carcass and edible portions

TABLE

318

The boned and filleted yields were statistically higher in the fast-growing strains (22, 23, and 24) than in the other two strains in experiment 1. There were no differences among the five fast-growing strains in experiment 2 in terms of boned and filleted yield, except for strain 25, which showed the highest yield of all strains tested. It is generally believed by nutritionists that there is a reciprocal relation between moisture and either fat or protein, or both, in all animals (Morrison, 1951; Maynard and Loosli, 1956; Love, 1970), and that phenomenon was observed in these studies. In experiment 1, since dry matter was significantly higher in the slow-growing strains (Table 5) than in the fast-growing strains, the slow-growing strains might be expected to have more fat. The whole body fat content was indeed found to be numerically higher (though not significantly) for the slow-growing strains. Higher fat content was also indicated by the significantly lower protein content of the slow-growing strains (Table 5 ). This finding suggests that the slow-growing strains may have grown slower because they directed higher proportions of dietary energy into fat deposition and less to protein synthesis. Although the cultural methods used and diets fed in both experiments were designed to be as nearly alike as possible, the fish in experiment 2 did store more fat in the viscera and less in the muscle than those in experiment 1 (Table 5 ) . This conclusion is supported by their lower eviscerated yields (Table 4), higher water and lower fat in the fillets, and higher protein content of the fillet (Table 5). Whether these differences were due to the genetic composition of the fish or to the cultural methods used cannot be determined from the present data, since the two experiments were not run concurrently and the diets were mixed from different lots of ingredients. It is unlikely, however, that such subtle differences in diet could account for the variation observed.

Taste test No significant differences, either for strain or diet, were indicated by flavor or overall acceptability scores (Table 6). Mean scores indicated that differences in flavor and overall acceptability were not detected by most panelists. The distribution of flavor and overall acceptability preferences for people testing the different strains fed either an AP or PP diet can be seen in Table 7. It should be pointed out that most panelists either did not choose to, or could not, designate a preference. A summary of the data for both experiments indicated that an overwhelming majority found no difference in flavor or overall acceptability of 10 strains of trout fed either the AP or PP diet. (Flavor preference: 229 neutral, 83 PP, 80 AP. Overall acceptability: 252 neutral, 73 PP, 76 AP.)

319 TABLE 6 Human taste-panel flavor and overall acceptability scores for 10 strains of rainbow trout fed diets containing mostly plant or animal protein (mean f SD). Scores based on hedonic scale of l-9 (1 =dislike extremely; 9 = like extremely) Strain

Flavor scores

Acceptability scores

PP

AP

Strain means

PP

Experiment 1 14 20 22 23 24 Diet mean

6.512 1.58 6.11 !z 1.53 6.73 5 1.53 6.31 f 1.59 6.81 f 1.66 6.50

6.65 & 1.51 6.33 k 1.59 5.94 * 1.68 6.41 t 1.55 6.23 + 1.68 6.31

6.59 6.22 6.24 6.37 6.52

6.37 + 6.04 + 6.63 + 6.40? 6.94 + 6.48

1.58 1.64 1.52 1.52 1.37

6.69 * 6.26 k 6.13+ 6.37 f 6.31 f 6.35

1.41 1.42 1.59 1.73 1.68

6.53 6.15 6.38 6.39 6.63

Experiment 2 12 15 17 25 32 Diet mean

6.59 + 1.20 6.22 & 1.61 5.91+ 1.74 6.15 k 1.52 6.37 & 1.64 6.25

6.44 + 6.15 * 6.33? 6.79 + 6.51+ 6.44

6.52 6.19 6.12 6.42 6.44

6.65? 6.33 + 6.06 * 6.25 k 6.39 + 8.34

1.27 1.40 1.49 1.73 1.50

6.53 f 6.32 + 6.34 + 6.76* 6.52 + 6.49

1.10 1.56 1.62 1.46 1.49

6.59 6.33 6.20 6.51 6.46

1.22 1.65 1.84 2.50 1.54

AP

Strain means

TABLE 7 Human taste-panel flavor and acceptability preference for 10 strains of rainbow trout fed diets containing mostly plant or animal protein. Individual preference of paired samples Strain

Experiment 1 14 20 22 23 24 Total Experiment 2 12 15 17 25 32 Total Experimenti land2

Flavor preference PP

AP

6 7 10 9 11 43

7 6 3 8 16 40

6 11 6 8 9 40 83

Acceptability preference PP

AP

Neutral

22 22 22 18 8 92

4 7 9 8 12 40

6 6 4 6 6 28

25 22 22 17 17 103

4 9 12 8 7 40

36 24 24 26 27 137

4 9 6 6 8 33

5 8 12 13 10 48

37 29 28 27 28 149

80

229

73

76

252

Neutral

320 DISCUSSION

These studies reconfirm the published studies and conclusions of Gall and Gross (1978b) and Klupp (1979) that the fish strains differ significantly in growth rate and feed conversion efficiency during the early life stages of growth. The overall performance of the faster growing rainbow trout strains in our studies, when considered throughout the period from first feeding to 250 g body weight, was nearly double that of the slower growing strains. Also, although little difference was found in the dressing percentages of the different strains, tissue fat was higher and protein lower in the slow-growing strains than in the fast-growing one. These findings agree with the published results of Ayles and Baker (1983) and Linder et al. (1983) who found significant differences in carcass fat and protein in rainbow trout raised in Canada and Finland, respectively. This observation points out the opportunity for improving “product quality” through selective breeding for faster growth rate, higher protein deposition, more efficient feed conversion, and improved ability to utilize plant proteins. It also suggests that past selection for accelerated growth and adaption to hatchery environments may have modified the fishes biological mechanisms by which energy is stored and mobilized. Most important, fish in these studies were raised on diets high in plant protein without significantly affecting growth rate, feed conversion efficiency, or flesh taste acceptability compared to fish fed predominantly animal protein diets. Indeed, when production cost (i.e., relative cost to produce 1 kg of fish flesh) on the two diets was considered, cost of the plant protein diet was 8.5 13.5% lower than the animal protein diet. The choice of plant protein, animal protein, or in some combination in the production situation will be dictated by economic considerations. Since feed cost makes up such a high proportion of total production cost, the use of lower cost plant protein diets is a viable approach for reducing production cost. ACKNOWLEDGEMENTS

We thank Dave Erickson of Clear Springs Trout Company and Donna Gerwig of Cornell’s Institute of Food Science for their help in processing the fish and arranging the taste tests.

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