Body traits in rainbow trout

Aquaculture, 80 (1989) 7-24 Elsevier Science Publishers B.V., Amsterdam

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Body Traits in Rainbow Trout I. Phenotypic Means and Standard Deviations and Sex Effects BJARNE

GJERDE

The Agricukural Research Council of Norway, Institute of Aquaculture Research, 1432 (Norway) (Accepted

k-NLH

12 August 1988)

ABSTRACT Gjerde, B., 1989. Body traits in rainbow trout. I. Phenotypic sex effects. Aquaculture, 80: 7-24.

means and standard

deviations

and

Data recorded at slaughter on 2-J-kg rainbow trout (Salmo gairdneri) from three year-classes were analysed. The following traits were recorded for all fish: ungutted and gutted body weight; body length; dressing percentage; condition factor; gonad weight and gonad index; belly thickness, scored and measured; abdominal fat score, viscera weight and viscera index; and meat colour score. The following traits were recorded for a sample of the fish from each year class: body circumference at the pelvic, dorsal and anal fins; area of the cross sections at the pelvic and dorsal fins; two belly thickness traits measured on each cross section; total height, height and width of each cross section; water, fat and protein percentages in the meat of the cross section at the dorsal fin; three body shape traits; and five cross section shape traits. The means and standard deviations for most traits were very similar across year-classes. The phenotypic variation was high for body weight, belly thickness, abdominal fat and percent water and fat in the meat. The variation was also substantial for meat colour score, condition factor and for most of the shape traits. Except for gonad weight, gonad index and viscera weight, sex effects accounted for a small proportion of the total variance, but were significant for most traits and should be taken into account in making a model. The few signiScant sex-by-year-class interaction effects were negligible. As high selection intensity can be practised in this species, the possibilities for genetic improvement of most of the examined traits are very good, on the assumption that the traits also show genetic variation.

INTRODUCTION

In Norway rainbow trout are raised in net cages in the sea to a body weight of 2-4 kg. Growth rate of fish raised at different farms and phenotypic variation for body weight and length recorded at slaughter were reported by Gunnes and Gjedrem (1981). Gjerde and Gjedrem (1984) reported on means and

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phenotypic variation for dressing percentage, meat colour score and belly thickness score and on sex effects on these traits as well as on body weight and length. Schmidt (1985) studied the height and width of the body anterior to the dorsal fin and height relative to the body length as a measure of body shape. Thus, basic parameters such as means and phenotypic variation and sex effect for traits other than body weight and length recorded at slaughter are very limited for rainbow trout. Means and phenotypic and genetic variation are of particular value in evaluating results from different experiments. The variation in a trait is an important parameter when planning a breeding scheme as it relates to the selection potential of the trait. In addition, if significant sex differences exist for an important trait, it may be economically feasible to produce a monosex stock for market. Moreover, significant sex effects must be corrected for in analyses of experimental data and in predicting breeding values of candidates for selection. Little attention has been paid to body quality traits of farmed salmonids. However, quality is difficult to define because it may vary from one market to another. For rainbow trout and Atlantic salmon in Norway, quality has been a question of (1) body size, larger fish demand a higher price per kg; (2) meat colour, salmon red colour is required; and (3) external appearance, silvery fish with no sign of secondary sex characters, abnormal body shape or other defects. Although rainbow trout belong to the “fat” class of fish, it is likely that an optimum body composition exists. However, this optimum may vary from one market to another as well as within a market, depending on how the final product is prepared. To be able to meet consumers’ preferences the phenotypic and genetic variation in body composition should be known, and traits of economic importance which show genetic variation should be included in the breeding objective. In Norway, feed constitutes 60-70% of the variable costs of raising rainbow trout from smolt to market size (Olsen, 1986). Because of difficulty in measuring food consumption in fish, feed efficiency cannot yet be included in a selection program. Nevertheless, feed efficiency may be improved indirectly be reducing excessive fat deposited in the meat and in the abdominal cavity. Rainbow trout are sold as ungutted or gutted whole carcasses, as cross section slices (steaks) or as smoked sides. The external body shape as well as the shape of parts of the carcass might therefore be of economic importance from a marketing point of view. Whether such traits show phenotypic and genetic variation should be known. This paper presents means and phenotypic variation for a number of body traits recorded on rainbow trout at slaughter. The effects of sex and sex-byyear-class interaction on the traits were also studied.

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MATERIAL AND METHODS

Data on rainbow trout (Salmo gairdneri) hatched in April 1978, 1979 and 1980 at the Institute of Aquaculture Research, Sunndalsera, were analysed. Each year-class had a different genetic origin, founded by brood fish from three Norwegian fish farms. A hierarchical mating system was used where sperm from each male fertilized eggs from 2 to 19 females. No sire or dam was used in more than one yearclass. The offspring were reared in freshwater for about 1 year. A random sample of about 50 individuals from each full-sib group was marked for family identification by fin-clipping and cold-branding and transferred to a net cage in the sea at Fla Fishfarm, Sunndalsera, where they were fed to excess on commercially produced dry pelleted food containing 50 mg/kg cantaxanthin as pigment source. The fish were slaughtered after 16 months in the sea. All fish in each yearclass were slaughtered within a period of 1 week. They were not fed for the last week before slaughter, and were slaughtered by anesthetizing with carbondioxide and cutting the gills. First slaughter dates were 3 September 1980,17 September 1981 and 10 September 1982 for year-classes 1978,1979 and 1980, respectively. Two data sets were obtained. Data set I

The following traits were recorded on 1749,1362 and 1355 fish in the 1978, 1979 and 1980 year-classes, respectively: ungutted body weight, to the nearest 50 g; body length, to the nearest cm; gonad weight, to the nearest g; abdominal fat, scored from 1 to 5, with 5 indicating the fattest; weight of viscera, to the nearest g (only abdominal fat and intestines were included); belly thickness, scored from 1 to 5, with 5 indicating the thickest belly; belly thickness, to the nearest mm, measured on the cut edge of the belly halfway between the pectoral and the pelvic fins; meat colour, scored from 1 to 5, with 1 indicating a pale and 5 the most red meat when judged by inspection of the abdominal cavity after removal of the viscera. In addition, the following traits were calculated: condition factor =ungutted body weight (g ) * lOO/ (body length (cm) )3; gutted body weight =ungutted body weight-weight of viscera, gonads and liver (kidneys were included in the gutted weight) ; dressing percentage = gutted body weight * lOO/ungutted body weight; gonad index = gonad weight * loo/gutted body weight; viscera index = weight of viscera * loo/gutted body weight. All scorings for a trait within a year-class were made by the same person.

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Data set II Additional traits were recorded on a sample of the fish included in data set I; the numbers were 132, 227, and 303 fish in the 1978, 1979 and 1980 yearclasses, respectively. The fish were a random sample from some randomly selected full- and half-sib groups as described in Gjerde and Schaeffer (1989). Body circumference was measured on ungutted fish at three positions; (1) posterior to the base of the pectoral fins, (2) anterior to the base of the dorsal fin, and (3) anterior to the base of the anal fin. The fish were then gutted and frozen. From each frozen fish a 1.52-cm cross section was taken at positions (1)and (2). On each section the traits shown in Fig. 1 were recorded. The areas of the sections (AP and AD) were measured using a planimeter on a tracing of each section. Average belly thickness (ABT) was calculated as (BT +BTPl +BTPB +BTDl +BTD2)/5, where BT is the belly thickness trait measured in data set I and the BTP’s and BTD’s are the belly thickness traits shown in Fig. 1. The following traits were calculated as measures of body shape (the ratios were multiplied by 100): BSl = CiD/BL, BS2 =THD/BL and BS3 = WD/BL, where CID is the body circumference at position (2), THD is the body height and WD is the body width as shown in Fig. 1, and BL is the body length.

T

Fig. 1. Total height (THX), height (HX), width (WX), area (AX) and belly thickness (BTXl and BTX2) recorded on a cross section slice taken just posterior to the base of the pectoral fins (X=P) and just anterior to the base of the dorsal fin (X=D).

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The following traits were calculated as measures of shape of each cross section (the ratios were multiplied by 100): SSl = WX/THX, SS2 = WX/THX, SS3=HX/WX, SS4=BTXl/WX, BTX2/WX and BT/WX, and SS5 = 0.5 * (IT* HX* WX* 0.5) /AX, where HX, WX, THX, BTXl and BTX2 are the traits shown in Fig. 1 and BT is the belly thickness trait in data set I. In SS4, WX was taken in mm. In calculating the numerator of SS5, the area dorsal to the WX line in Fig. 1 was assumed equal to half the area of an elipse with radii HX and WX/2. Each section from position (2) was skinned, deboned, freeze-dried and ground. The samples were kept frozen until the composition could be predicted. After thawing to room temperature, the water, fat and protein percentages of two independent freeze-dried meat samples from each section were determined by near-infrared-reflectance spectroscopy (Gjerde and Martens, 1987) and calculated on a raw meat basis. Statistical analyses A model including the effects of year-class, sex, sex-by-year-class interaction, sires nested within year-class and dams nested within sire and year-class was used. Sex differences were estimated for those traits in which the sex-byyear-class interaction was’non-significant. For those traits in which sex-byyear-class interaction effects were significant, sex differences were estimated within year-classes. The increase in error variance which resulted from omitting the sex-by-yearclass interaction effect or the sex effect was expressed as (ER-EF) * lOO%/ EF, where ER is the error variance when the model is reduced by omitting the effect and EF is the error variance from the full model. An F-test was used to test whether the effect was significantly different from zero. RESULTS

Means and variances Means and phenotypic standard deviations for most traits in data set I were very similar across year-classes (Table 1). However, mean dressing percentage was lowest in the 1979 year-class, and the condition factor was lowest in the 1978 year-class. Both absolute and relative gonad weight were smallest in the 1978 year-class followed by the 1980 and 1979 year-classes. Mean scores for abdominal fat, viscera weight and viscera index were all lowest in the 1980 year-class. The coefficients of variation were very high for gonad weight (78% averaged over the year-classes) and gonad index (70% ), high for body weight (22% ), belly thickness score (23% ), belly thickness, (20% ), abdominal fat score (22% ),

12 TABLE I Means and phenotypic

standard

Trait

deviations

for traits in data set I

1978 X

1979 SD

SD 0.82 0.72 5.0 2.1 0.16

SD

3.46 3.03 59.9 87.7 1.41

Gonad weight (g ) Gonad index

59.3 1.9

40.2 1.2

88.0 2.8

80.6 2.3

70.2 2.3

52.4 1.5

3.4 12.5 3.7 332.0 11.1

0.7 2.3 0.7 77.8 2.4

3.7 12.0 3.6 342.0 11.9

0.8 2.5 0.8 90.1 2.8

3.5 12.4 3.3 283.2 9.7

0.8 2.5 0.8 69.3 2.0

4.5

0.6

4.3

0.6

4.1

0.7

Meat colour (score l-5)

3.41 2.94 58.6 86.1 1.66

X

Ungutted body weight (kg) Gutted body weight (kg) Body length (cm) Dressing percentage Condition factor

Belly thickness (score l-5) Belly thickness (BT) (mm) Abdom. fat (score 1-5) Viscera weight (g) Viscera index

0.64 0.58 4.0 1.7 0.13

X

1980

3.36 2.97 58.5 88.0 1.68

0.72 0.65 4.3 1.8 0.17

viscera weight (25% ) and viscera index (22% ), medium for meat colour score (15% ) and rather low for condition factor (10% ) and body length (8% ) . Means and phenotypic standard deviations for traits in data set II are given in Table 2. Means and standard deviations of gutted body weight in data set II were very similar to those in data set I, showing that the sampling of the fish with regard to body weight was random. The mean value for BSl in the 1978 year-class was lower than in the two other year-classes. The standard deviations were higher for fat than for water percentages, and quite low for percent protein. Averaged over the year-classes, the area dorsal to the WX line was calculated to constitute 53% and 59% of the total area of the pelvic and dorsal sections, respectively (not given in the table). The coefficients of variation were high for the BTP’s (23% averaged over the year-classes), medium for the BTD’s (17% ) and area of the cross sections (17% ), and rather low for body circumference (CiX), total height (THX), height (HX) and width ( WX) (g-10% ), and for the body shape traits (6% 1. Standard deviations were quite similar for the same shape trait at the pelvic and dorsal sections. Therefore, only simple averages of the estimates within each of the five SS groups are given in Table 2. The coefficients of variation were high for SS4 (34% averaged over the year-classes) and low for SSl, SS2, SS3 and SS5 (6% ) . Frequency distributions of the scored traits are shown in Fig. 2. The distributions for meat colour were skewed, with only one observation in category 1.

13 TABLE 2 Means and phenotypic standard deviations overall effect of sex (males minus females) tained by omitting sex from the model Trait

within year-classes for traits in data set II, and the and the percent increase in error variance (%) ob-

1978 8

Gutted body weight (kg) Body circumference at pelvic fins (CiP) (cm) dorsal fin (CiD) (cm) anal fin (CiA) (cm) Section at pelvic fins area (AP) (cm’) total height (THP) (cm) height (HP) (cm) width (WP) (cm) belly thick. (BTPl) (mm) belly thick. (BTPB) (mm) Section at dorsal fin area (AD) (cm*) total height (THD) (cm) height (HD) (cm) width (WD) (cm) belly thick. (BTDl) (mm) belly thick. (BTDB) (mm)

1979 SD

3.09

0.53

x

1980 SD

2.85

0.73

R 3.02

Sex effect SD

d-9

%”

0.71

0.24***

3.9

32.1 37.3 25.9

2.0 2.4 1.7

33.9 40.0 26.5

3.6 4.3 2.9

35.1 40.9 26.9

3.4 4.0 2.7

1.6*** 1.5*** 0.7***

6.8 4.4 2.1

56.1 12.8 5.1 7.7 14.4 7.7

7.5 1.0 0.4 0.6 3.0 1.8

60.5 12.9 5.1 7.9 12.1 7.6

10.7 1.3 0.5 0.8 2.9 1.6

62.6 13.8 5.2 7.7 11.7 7.0

11.4 1.4 0.6 0.7 2.9 1.5

4.4*** 0.6*** 0.2*** 0.3*** 0.3 0.3*

5.1 6.4 3.7 4.3 0.1 1.0

83.2 15.4 7.4 8.4 22.2 16.2

11.2 1.1 0.6 0.7 2.8 2.3

83.2 15.1 7.3 8.3 19.6 15.8

15.5 1.6 0.8 0.9 3.5 2.6

85.4 15.2 7.5 8.4 18.6 14.8

16.2 1.5 0.8 0.8 3.7 3.0

4.5*** 0.5*** 0.3*** 0.1 0.4 0.8***

2.5 3.6 4.8 0.4 0.1 2.2

64.3 14.6 19.6

1.9 2.3 1.4

63.8 13.9 20.8

2.3 2.6 1.6

62.9 15.8 19.8

2.6 2.8 1.3

-0.7*** 0.7*** -0.1

2.2 2.2 -0.1

11.2 14.4

1.8 1.7

11.7 13.4

2.5 2.1

11.9 12.8

2.7 2.2

0.2 0.4*

-0.1 0.8

Body shape BSl BS2 BS3

59.5 24.7 13.4

2.7 1.3 0.9

68.6 26.1 14.4

4.0 1.5 0.8

70.0 26.1 14.4

4.3 1.6 1.0

0.6* 0.1 - 0.2**

0.6 0.0 1.6

Section shapeb SSl ss2 ss3 ss4 ss5

43.7 57.2 77.1 17.1 56.8

2.6 3.8 4.7 5.3 3.9

43.8 58.0 76.1 16.1 55.2

2.1 2.9 3.8 5.5 3.3

43.7 55.7 78.8 15.6 55.2

2.3 4.1 5.5 5.8 3.2

0.0 - 0.8** 1.3** 0.1 0.1

-0.2 1.6 2.4 -0.2 -0.1

water (% ) fat (%) protein (% ) Belly thick. (BT) ABT (mm)

(mm)

*p
Belly Thickness

hbdomlnol

1978

1979

Fat

1980

Fig. 2. Frequency distributions, within year-classes, for traits given subjective scores from 1 to 5.

15 0. 3

T

1978

D.2 +

F

Fig. 3. Frequency

distributions,

within year-classes,

of the gonad index for males and females.

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For belly thickness and abdominal fat the total number of observations in category 1 was six and seven, respectively. Frequency distributions for the gonad index of females and males in each year-class are shown in Fig. 3. Males had broader distributions. Sex and sex-by-year-class interaction-s In data set I the sex-by-year-class interaction effect was non-significant for ungutted and gutted body weight, body length, belly thickness (BT) and meat colour score. The overall sex effects for these traits are given in Table 3. In terms of the error variance, the sex effect was important for meat colour, less important for body weight and length, and unimportant for belly thickness (BT) . Males had meat which was less red and they were heavier (7.8% ) and longer (2.7% ) than the females. For the other traits in data set I the interaction effect was significant, but small in terms of change in error variance (Table 4). Sex effects within year-classes are given in Table 4. In terms of the error variance, sex effects were relatively large for weight of gonads and gonad index, particularly in the 1980 year-class, and for viscera index in the 1978 and 1979 year-classes. A less pronounced sex effect, but significant and consistent in sign across year-classes, was observed for abdominal fat score and viscera index. Dressing percentage was the only trait for which the sex difference was significant, but not consistent in sign over year-classes. Males had heavier gonads compared to females (59,102 and 95% for gonad weight and 48,86 and 78% for gonad index in year-classes 1978, 1979 and 1980, respectively), and had less abdominal fat (11,12 and 8% for score and 12,ll and 8% for viscera index ) . The sex-by-year-class interaction effect was non-significant for all traits in data set II, including gutted body weight. However, it was close to significant (P= 6.8% ) for protein percentage (not given in Table 2). For this trait the TABLE 3 The overall effect of sex (males minus females) for traits in data set I, and the percent increase in error variance (% ) obtained by omitting the sex effect from the model Trait

d-?

%

Ungutted body weight (kg) Gutted body weight (kg) Body length (cm) Belly thickness (BT) (mm) Meat colour (score l-5)

0.25*** 0.23*** 1.6*** 0.2*** -0.4***

2.8 2.8 2.8 0.2 1.2

17 TABLE 4 Percent increase in error variance (% ) obtained for traits in data set I by omitting the sex-byyear-class effect or the sex effect from the model, and the effect of sex (males minus females) Trait

SexXyear (%)

Sex (o/o)’ 1978 3.0*** O.B***

Males - females 1979 0.1 0.1

Dressing percentage Condition factor

1.5** 0.2***

Gonad weight (g) Gonad index

2.0*** 1.9***

15.1*** 18.1*** 13.4*** 16.9***

Belly thick. (score l-5) Abdom. fat (score l-5) Viscera weight (g ) Viscera index

0.3** 0.1*** 0.6*** 0.5***

2.7*** 5.8*** 2 8*** 11:6***

0.2 4.9*** 1.2*** 7.8**+

1980

1978

1979 -0.14 -0.01

1980

1.5*** 0.0

0.60 -0.03

33.6*** 31.6***

27.2 0.73

61.2 1.70

49.5 1.38

0.0 l.B*** -0.1 4.4***

0.23 -0.32 -24.1 -1.5

0.08 -0.33 - 18.0 - 1.4

0.04 -0.20 2.1 -0.8

-0.38 0.01

**p
increase in error variance by omitting the effect was only 0.6%, while it was less than 0.3% or negative for the other traits. For traits in data set II the overall sex effects are given in Table 2. In terms of change in error variance the effect was not important for any trait. The sex differences for the body size traits reflect the tendency for males to be bigger for all traits (from about 2 to 7% ). The sex differences for percent water and fat were highly significant but small, identical in size but with the sign reversed. For protein percentage the sex effect was non-significant. A significant and positive sex difference was observed for BSl, while it was negative for BS3, and non-significant for BS2. The significant sex differences for SS2 and SS3 were due to a significant sex difference for traits at the dorsal section, while the sex differences were not significant for any trait at the pelvic section. DISCUSSION

Mean values for most traits were very similar across year-classes. This is likely due to the fact that each year-class was reared at the same fish farm, slaughtered at about the same time each year and the data were recorded by the same person. As sires and dams were nested within year-classes, the year effect was confounded with the genetic level of the sires and dams. This confounding could be removed in the future by reusing some sires or dams in each year as reference points or controls. Use of relationships among sires and dams would also reduce the effect of confounding if some sires or dams were used in more than one year.

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Body size traits The coefficients of variation for body weight (19-24% ) and length (7-9% ) in this study tend to be lower than most previously reported values of 2535% for weight and 8-12% for length in 2-4-kg rainbow trout (Gunnes and Gjedrem, 1981; Gjerde and Gjedrem, 1984; Schmidt, 1985). In comparison, the coefficient of variation for growth rate is 8-12% in pigs (Standal, 1977)) meat-type chickens (Sorensen, 1986) and beef cattle (Wilton et al., 1973). Schmidt (1985) measured the body height and width anterior to the dorsal fin on ungutted rainbow trout weighing 2.8 kg on average. The standard deviation was 1.7 cm for height (mean 14.8 cm) and 0.8 cm for width (mean 6.5 cm). These values agree well with the variation in THD and WD in this study. Dressing percentage Mean dressing percentages were 5 to 6 percent units higher and the standard deviations 2 to 3 percent units lower than reported by Gjerde and Gjedrem ( 1984) for rainbow trout slaughtered in early December. The mean values were also significantly higher than mean values (77.4-84.6% ) reported by Asgard and Austreng (1985a,b, 1986) for rainbow trout slaughtered in late August or early October, but lower than mean values (91.2-93.8% ) reported by Storebakken and Austreng (1987) for rainbow trout slaughtered in June. Schmidt (1985) reported 84.4% as mean value with a standard deviation of 3.8% for rainbow trout slaughtered in August. In portion-sized rainbow trout mean values from 85.6 to 86.8% (Austreng et al., 1977) and from 86.7 to 91.1% (Morkramer et al., 1985) were reported. Gonads of maturing males and particularly of maturing females grow fast from September until the fish become sexually mature in February/March. The mean and phenotypic variation of dressing percentage are therefore expected to be largely influenced by date of slaughter. In the present data this was reflected by the lower mean value and higher standard deviation in the 1979 year-class. Condition factor In fish biology the condition factor is used to measure the variation from the expected weight for length of individual fish or groups of individuals, as an indication of differences in fatness, changes in nutritional status, suitability of environment, stage of sexual maturity, body shape, etc. (Le Cren, 1951). The condition factor is also frequently reported for farmed fish. In rainbow trout, 1.58, 1.68 and 1.70 were reported as mean values for three year-classes (Gunnes and Gjedrem, 1981)) and Schmidt ( 1985 ) reported 1.52 as mean value with a phenotypic standard deviation of 0.16, which agree well with the variation found in this study. Asgard and Austreng (1985a,b, 1986) reported mean values from 1.52 to 1.74, and with only minor differences between groups fed different diets in the same experiment. However, significant differences in mean

19

condition factor between groups of rainbow trout fed different diets (Refstie and Austreng, 1981; mean values ranging from 1.35 to 1.45) and between groups fed different ration levels (Storebakken and Austreng, 1987; mean values ranging from 1.25 to 1.64) have been reported. Significant differences between groups of portion-sized rainbow trout fed different diets were also reported (Edwards et al., 1977; Austreng and Refstie, 1979). As body composition was confounded with body size in the above experiments, the cause of the differences in condition factor could not be addressed. Significant changes in the condition factor have also been observed among sexually maturing rainbow trout (Tveranger, 1985) and Atlantic salmon (Aksnes et al., 1986). Although the value of the condition factor as a trait on farmed fish is not well understood, its use and interpretation becomes even more difficult as different formulas are used to calculate it (Le Cren, 1951) . The one used in this and most studies, including those referred to above, is based on comparison with an ideal fish whose body weight is proportional to the cube of its length. However, from the weight-length relationship which can be described by the formula W=aL” where W is body weight, L is body length and a and n are constants, it does not always follow that n= 3. In the present data n was estimated as 2.63 (SEE + 0.03), 2.96 ( 5 0.03) and 2.81 ( 2 0.04) in year-classes 1978,1979 and 1980, respectively. Nevertheless, the cube law was assumed to hold in this study. This implies that the condition factor will be affected by length itself as n was significantly different from 3 in the 1978 and 1980 yearclasses (Le Cren, 1951). Condition factor as calculated in this study showed phenotypic variation within as well as between year-classes. However, more research is needed to evaluate its value as an economic trait in farmed fish. Although it can be used to distinguish fish with abnormal body shape (McKay and Gjerde, 1986), its main value today lies in eliminating errors when editing records on weight and length. Abdominal fat Mean viscera index was about twice the mean value reported for Atlantic salmon at 5 to 6 months before spawning (Torrissen et al., 1984; Aksnes et al., 1986). This indicates a species difference in the amount of abdominal fat deposited, assuming a constant ratio of weight of intestine to body weight. Then a high (low) content of abdominal fat will cause a high (low) viscera index. In fish farming, the abdominal fat must be considered as a waste product. From a feed efficiency point of view, it may therefore be of importance to reduce the amount of fat deposited. However, the abdominal fat might have important biological functions during sexual maturation. Thus, in sexually maturing Atlantic salmon, the viscera index was found to decline from about 6% to 2.5% from May through November (Aksnes et al., 1986).

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Shape traits Mean values for the body shape traits were lowest in the 1978 year-class. As sires and dams were nested within year-classes, this might reflect a genetic difference in body shape between year-classes. Schmidt (1985) reported a mean of 26 and a standard deviation of 1.5 for the body shape trait, BS2, which agree well with the values for the trait in this study. Although the coefficients of variation were low for all shape traits except SS4, the standard deviations were not small for BSl and the SS traits. The area of the cross sections ventral to the WX line constituted more than 40% of the total area. The thickness and composition of the belly are therefore likely of importance for the overall value of the carcass. However, consumers’ preferences or acceptance limits relating to different shape traits need to be investigated.

Body composition Average values for fat content in the meat (13.9-15.8% ) were in the upper range or higher than estimates previously reported. However, no other estimates have been reported for body composition of rainbow trout of body size comparable to that in this study. Asgard and Austreng (1985a, 1986) reported mean fat contents of 11.0 and 14.1% in the meat of trout weighing around 2 kg, while they found mean values from 4.3 to 7.3% in two other experiments with trout weighing 1.5 and 2.4 kg on average (Asgard and Austreng, 1985b, 1986). Linder et al. (1983) reported 7.1 and 8.2% fat in the meat of two strains of rainbow trout weighing 1.5 kg on average, and Tveranger (1985) reported about lo-12% fat in the meat of rainbow trout weighing on average 1.4 kg. Storebakken and Austreng (1987) reported mean carcass fat content ranging from 6.7 to 10.2% in six groups of rainbow trout fed different ration levels and weighing from 0.5 to 1.0 kg at slaughter. Morkramer et al. (1985) reported from 4.0 to 7.9% as average fat content in the meat in four experiments with portion-sized rainbow trout. Fat content is reported higher in the anterior than in the posterior end of the body of rainbow trout (Gjerde, 1987) and Atlantic salmon (Saito, 1969). This stresses the importance of the location of sampling on the carcass. Whether the change in composition in the longitudinal direction varies between individuals or groups of individuals should be investigated. This is of vital importance to whether a standardized position on the carcass is adequate or whether the whole fillet needs to be ground and analysed. In this study and in Asgard and Austreng (1985a,b, 1986) the reported composition was in the cross section below the dorsal fin. Morkramer et al. (1985) used a cross section from the anterior end of the body. Average values for protein percentage (19.6-20.8% ) were in goodagreement with previous reports (Linder et al., 1983; Morkramer et al., 1985; Asgard and Austreng, 1985a, 1986; Storebakken and Austreng, 1987 ). Higher values (22.2-

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24.1% ) , however, were reported by Asgard and Austreng ( 1985b, 1986) when the fat contents in the meat were low. Few reports have appeared on the individual variation in body composition of salmonids. Linder et al. (1983) reported standard deviations of 0.85 and 1.05% for percent fat and 0.29 and 0.44% for percent protein based on eight fish per strain. Morkramer et al. (1985) reported standard deviations varying from 1.3 to 2.8% for percent fat and from 1.1 to 1.7% for percent protein. From the data of Asgard and Austreng (1985a,b, 1986) average standard deviations of 3.5% for percent fat and 1.6% for percent protein were calculated. These results and those in the present study show that the phenotypic variation is higher for percent fat than for percent protein. Fat content in meat of rainbow trout appears to vary a lot both between and within groups of fish, while the protein content appears to be quite stable, although some variation might be expected for this component also. The variation in fat content may vary with date of slaughter, type of food, body size as well as genetic factors and others. The relative importance of these factors on body composition in rainbow trout should be investigated. This is of importance in order to meet consumers’ preferences. Meat colour The salmon-red colour of the meat of salmonids is of vital importance to acceptance by consumers and makes a major contribution to the elite image of the fish. Meat colour score shows substantial phenotypic variation in rainbow trout, as shown in this and other studies (Gjerde and Gjedrem, 1984; Asgard and Austreng, 1985a,b, 1986). As colour quality preferences vary, sorting of the fish prior to distribution to various markets is an advantage. However, the human eye (brain) has a poor ability to memorize colours (Hunter, 1975) and perceived colour is highly dependent upon viewing conditions such as source of illumination and colour of surroundings. Perceived colour might also be influenced by the amount of fat in the meat and by the body mucosa when the meat colour is judged by inspection of the abdominal cavity, as in the present study. Therefore, both from a marketing and breeding point of view, there is a need for instrumental techniques giving consistent qualitative and quantitative information on product colour. Recently a portable instrument (Minolta Chroma Meter) has been successfully used to measure meat colour (Skrede and Storebakken, 1986). Sex effects Males were significantly heavier and longer than females, a result which agrees with several studies on rainbow trout (see Gjerde, 1986 for a review). The sex differences observed for the other body measurement traits were likely a reflection of this general difference in body size between males and females. However, males lighter than females have also been reported (Gjerde and

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Gjedrem, 1984), probably an effect of stage of sexual maturation, as the fish were slaughtered in December. Secondary sexual characters that reduce the market value of maturing fish were not developed at the time of slaughter. Therefore, in all female stocks, the economic value of reduced growth rate as compared to males should be considered. Females also possessed a more desirable meat colour. This sex effect might, however, be confounded with the sex effect for fat percentage as a low negative phenotypic correlation was observed between these traits (Gjerde and Schaeffer, 1989). Also non-significant sex effects for meat colour score (Gjerde and Gjedrem, 1984) and carotenoid content (Torrissen et al., 1984; Torrissen and Naevdal, 1984) have been reported. The sex differences for dressing percentage were all very small. This agrees with Schmidt (1985) who reported a non-significant sex effect on this trait. However, Gjerde and Gjedrem (1984) reported 3.6-7.2 units higher dressing percentages in males. This indicates that observed sex differences might be largely dependent on stage of sexual maturation. In this study the fish were slaughtered early in their sexual maturation. This might explain the unimportant sex effect on the carcass composition traits, condition factor and shape traits, and the negligible sex-by-year-class interaction observed for most traits. As numbers of observations per subgroup are rather large in fish data, the rather unimportant sex effect observed for most traits is likely to have only a minor influence on estimates of genetic parameters and on predicted breeding values. However, sex is easy to record at slaughter and should be included in the model. Sex effects and maturation Gjerde (1984, 1986) reported that maturing Atlantic salmon and rainbow trout were heavier than immature fish 4 months before they were expected to spawn. Tveranger (1985) found that maturing rainbow trout males grew significantly faster than immature fish in the period October-January when maturing fish were expected to spawn in February/March. Aksnes et al. (1986) studied the change in body weight, condition factor, viscera index, body composition and meat colour score of maturing males and females and immature Atlantic salmon in the period May-December. The differences between the three groups of fish were largely influenced by date of recording. These results show that the maturation process has a significant effect on many traits. Hence, the sex effect might be different within maturing and immature fish. Among immature fish the gonad index remains fairly constant throughout the spawning season, while it increases sharply among maturing fish during the same period (Godfrey, 1961). Thus the gonad index can be used to distinguish between maturing and immature fish. However, this method does not permit positive identification of maturity in the case of “immature” fish re-

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corded very early in the spawning season that might have begun to mature at a later date had they not been slaughtered. Frequency distribution of the gonad index indicated that to distinguish accurately between immature and sexually maturing fish slaughter should take place later than in this study. This is supported by Kato (1975) who found it was difficult to distinguish the maturing females and impossible to distinguish the maturing males from the immatures by the gonad index in May; i.e., 5 to 6 months before the spawning season. ACKNOWLEDGEMENTS

The author wishes to thank Fla Fishfarm for allowing the data to be recorded there, and Johan Settem for skillful assistance during the data collection.

REFERENCES Aksnes, A., Gjerde, B. and Roald, S.O., 1986. Biological, chemical and organoleptic changes during maturation of farmed Atlantic salmon, Salmo salar. Aquaculture, 53: 7-20. Asgard, T. and Austreng, E., 1985a. Casein silage as feed for salmonids. Aquaculture, 48: 233-252. Asgard, T. and Austreng, E., 1985b. Dogfish offal, ensiled or frozen, as feed for salmonids. Aquaculture, 49: 289-305. Asgard, T. and Austreng, E., 1986. Blood, ensiled or frozen, as feed for salmonids. Aquaculture, 55: 263-284. Austreng, E. and Refstie, T., 1979. Effect of varying dietary protein level in different families of rainbow trout. Aquaculture, 18: 145-156. Austreng, E., Risa, S., Edwards, D.J. and Hvidsten, H., 1977. Carbohydrate in rainbow trout diets. II. Influence of carbohydrate levels on chemical composition and feed utilization of fish from different families. Aquaculture, 11: 39-50. Edwards, D.J., Austreng, E., Risa, S. and Gjedrem, T., 1977. Carbohydrate in rainbow trout diets. I. Growth of fish of different families fed diets containing different proportions of carbohydrate. Aquaculture, 11: 31-38. Gjerde, B., 1984. Response to individual selection for age at sexual maturity in Atlantic salmon. Aquaculture, 38: 229-240. Gjerde, B., 1986. Growth and reproduction in fish and shellfish. Aquaculture, 57: 37-55. Gjerde, B., 1987. Predicting carcass composition of rainbow trout by computerized tomography. J. Anim. Breed. Genet., 104: 121-136. Gjerde, B. and Gjedrem, T., 1984. Estimates of phenotypic and genetic parameters for carcass traits in Atlantic salmon and rainbow trout. Aquaculture, 36: 97-110. Gjerde, B. and Martens, H., 1987. Predicting carcass composition in rainbow trout by near-infrared-reflectance spectroscopy. J. Anim. Breed. Genet., 104: 137-148. Gjerde, B. and Schaeffer, L.R., 1989. Body traits in rainbow trout. II. Estimates of he&abilities and of phenotypic and genetic correlations. Aquaculture, 80: 25-44. Godfrey, H., 1961. Method used to distinguish between immature and maturing sockeye and chum salmon taken by Canadian exploratory fishing vessels in the Gulf of Alaska. Bull. Int. North Pac. Fish. Comm., 5: 17-25. Gunnes, K. and Gjedrem, T., 1981. A genetic analysis of body weight and length in rainbow trout reared in seawater for 18 months. Aquaculture, 24: 161-174. Hunter, R.S., 1975. The Measurements of Appearance, Wiley, New York, NY, 348 pp.

24 Kato, T., 1975. The relation between the growth and reproductive characters of rainbow trout, Salmo gairdneri. Bull. Freshwater Fish. Res. Lab., Tokyo, 25: 83-115. Le Cren, E.D., 1951. The length-weight relationship and seasonal cycle in gonad weight and condition in the perch (Perca fluuiatilis). J. Anim. Ecol., 16: 188-204. Linder, D., Sumari, O., Nyholm, K. and Sirkkomaa, S., 1983. Genetic and phenotypic variation in production traits in rainbow trout strains and strain crosses in Finland. Aquaculture, 33: 129-134. McKay, L.R. and Gjerde, B., 1986. Genetic variation for a spinal deformity in Atlantic salmon, Salmo salar. Aquaculture, 52: 263-272. Morkramer, S., Horstgen-Schwark, G. and Langholtz, H.J., 1985. Comparison of different European rainbow trout populations under intensive production conditions. Aquaculture, 44: 303320. Olsen, B., 1986. Okonomiske forhold ved fiskeoppdrett. In: T. Gjedrem (Editor), Fiskeoppdrett med framtid. A/S Landbruksforlaget, Oslo, Norway, 328 pp. Refstie, T. and Austreng, E., 1981. Carbohydrate in rainbow trout diets. III. Growth and chemical composition of fish from different families fed four levels of carbohydrate in the diet. Aquaculture, 25: 35-49. Saito, A., 1969.1969. Colour in raw and cooked Atlantic salmon (Salmo salar). J. Fish. Res. Board Can., 26: 2234-2236. Schmidt, M., 1985. Produktionsleistung von Forellen unterschiedlicher Gewichtsklassen under besonderer Berticksichtigung des Schlachtkorperwertes. Dissertation, Universitit Gottingen, 59 PP. Skrede, G. and Storebakken, T., 1986. Instrumental colour analysis of farmed and wild Atlantic salmon when raw, baked and smoked. Aquaculture, 53: 279-286. Sorensen, P., 1986. Study of the effects of selection for growth in meat type chickens. Thesis, Doctor Agronomiae, Royal Agricultural and Veterinarian University, Copenhagen, 314 pp. (in Danish with English summary and subtitles). Standal, N., 1977. Studies on breeding and selection schemes in pigs. V. Phenotypic and genetic parameters estimated from on-the-farm test data. Acta Agric. Stand., 27: 13-31. Storebakken, T. and Austreng, E., 1987. Ration level for salmonids. II. Growth, feed intake, protein digestibility, body composition, and feed conversion in rainbow trout weighting 0.5-1.0 kg. Aquaculture, 60: 207-221. Torrissen, O.J. and Naevdal, G., 1984. Pigmentation of salmonids - genetical variation in carotenoid deposition in rainbow trout. Aquaculture, 38: 59-66. Torrissen, O.J., Hansen, T., Torrissen, K.R. and Naevdal, G., 1984. Causes of variation in carcass traits of Atlantic salmon (Salmo salar). ICES, C.M. F:25,14 pp. Tveranger, B., 1985. Variation in growth rate, liver weight and body composition at first sexual maturity in rainbow trout. Aquaculture, 49: 89-99. Wilton, J.W., Burgess, T.D. and Batra, T.R., 1973. Ultrasonic measurements of beef bulls in performance-testing programs. Can. J. Anim.‘Sci., 53: 629-636.