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Carbohydrate in rainbow trout diets. II. Influence of carbohydrate levels on chemical composition and feed utilization of fish from different families
Aquaculture, 11 (1977) 39-50 o Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
39
CARBOHYDRATE IN RAINBOW TROUT DIETS. II. INFLUENCE OF CARBOHYDRATE LEVELS ON CHEMICAL COMPOSITION AND FEED UTILIZATION OF FISH FROM DIFFERENT FAMILIES
E. AUSTRENG*,
S. RISA**, D.J. EDWARDS**
*Depa;tment ofoPou1tt-y and Fur Animal Science, 1432 As-NLH, As (Norway) **Deptrtment 1432 As-NLH,
o,f Animal Genetics As (Norway)
and Breeding,
and H. HVIDSTEN* Agricultural Agricultural
University University
of Norway, of Norway,
(Received 14 December 1976; revised 11 March 1977)
ABSTRACT 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. Groups of rainbow trout fingerlings from ten different families were sampled after 12 and 24 weeks feeding on each of three diets which were similar in protein and energy content but had different percentages of metabolixable energy present as carbohydrate: 17, 25 and 38%. Fish fed different diets had the same dressed carcass weights, expressed as percentage of body weight, but fiih fed higher carbohydrate diets had heavier livers and a higher percentage of discoloured livers. Chemical analyses of fiih bodies showed lower fat and energy but higher protein and ash content for those fed higher carbohydrate diets. Livers contained more fat and carbohydrate but the same dry matter, and faeces showed less dry matter, more protein and the same energy content in fiih with higher dietary carbohydrate. Utilization of dietary energy and protein was poorer in fiih fed high levels of carbohydrate, but health of all fish appeared good. Little variation between different fiih families, and no interaction between fish family and diet was found for any of the above parameters, indicating that prospects for change through selective breeding are poor.
INTRODUCTION
Calories in fish diets can sometimes be provided more cheaply as carbohydrate than as protein or fat, and it is therefore of interest to try to formulate diets containing the maximum level of carbohydrate which the fish can efficiently utilize. Commercial salmonid diets often contain about 15% of their metabolizable energy as carbohydrate but some studies, e.g. that of Buhler and Halver (1961) on chinook salmon, have indicated that levels of some carbohydrates con-
40
siderably higher than this can be assimilated and result in a more efficient utilization of protein. However, Phillips et al. (1948) suggested that feeding trout with high levels of carbohydrate may result in poor health, in particular deleterious changes in the liver, and eventual increased mortality. They suggest, ed a safe maximum content of 12% of digestible carbohydrate in the diet of brook trout. Phillips et al. (1966) reported differences in chemical composition of the bodies and livers of fish fed diets differing in their content of carbohydrate. An experiment to determine the effect of feeding diets containing different proportions of their metabolizable energy as carbohydrate on the growth of rainbow trout was described by Edwards et al. (1977). During and after that experiment, fish were sampled for chemical analyses and veterinary examination to determine what effects the diets had had on rainbow trout health and the chemical compositions of the fish bodies, livers and faeces. The results of these analyses are presented here. MATERIALS
AND METHODS
The design of the experiment, numbers and weights of fish used, feeding regime, and composition of the diets were described in an earlier paper (Edwards et al., 1977). The composition and results of chemical analyses of the experimental diets are also shown in Tables I and II in the present paper. Rainbow trout from ten different families were grown for 12 weeks indoors in fresh water and fed on each of three diets containing 17,25 or 38% of TABLE
I
Composition of experimental trout diets differing in the percentage of their metabolizable energy present as carbohydrate (g/kg) Ingredients
Diet 1
Diet 2
Diet 3
Capelin meal Torula yeast Soya-bean meal Wheat meal Cooked wheat Sucrose Capelin oil Chalk meal Vitamin mixture II* salt Seaweed meal Methionine
300 100 100 380
300 100 100
300 100 200
*Austreng (1976).
410 90
10 10 5 4 1
60 10 10 5 4 1
355 15 10 10 5 4 1
41
TABLE
II
Chemical analyses and calculated content of metabolizable energy of experimental trout diets Chemical component Dry matter (%) Crude protein (%) HCl-ether extract (crude fat) (%) Crude fibre (W) Nitrogen-free extracts (%) Ash (%) Calcium (%) Phosphorus (%) Gross energy (k&kg) Calculated content of metabolizable energy (kcal/kg) Metabolizable energy from carbohydrates (%)
Diet 1 90.3 34.3
Diet 2 90.8 34.6
Diet 3 94.0 35.5
14.1 2.1 31.9 7.9 0.82 0.83 4 596
11.4 2.3 35.5 7.0 0.74 0.85 4 598
6.3 1.8 43.6 6.8 0.72 0.77 4 392
2 976
3 032
3 034
17
25
38
their metabolizable energy in the form of carbohydrate. One fish from each family was then removed from each of two duplicate tanks in which fish received the same diet, i.e. 60 fish were taken, for chemical analysis. In addition, two fish from each family on each diet, i.e. another 60 fish, were removed for veterinary examination. After a further 12 weeks’ growth in brackish water (15°/oo) in outdoor tanks, 120 more fish were sampled in the same way for chemical and veterinary examination, and faeces were taken from all fish remaining alive. Faeces were expelled by stroking the fish firmly down the belly, using much the same technique as for stripping eggs from ripe fish. All fish sampled were killed and weighed individually. Each fish intended for chemical analysis was immediately cut open along the belly and its viscera were removed. The dressed out carcass (body with viscera removed but head on) was weighed. The liver was separated from the other viscera and weighed alone. The carcass, liver and faeces were frozen separately, packed in ice, and sent from the Fish Breeding Experimental Station, Sunndals$ra, where the experiment was carried out, to the Central Chemical Research Laboratory and the Department of Poultry and Fur Animal Science of the Agricultural University of Norway at As for analysis. Fish for veterinary examination were immediately frozen after slaughter and sent to the National Veterinary Institute at Oslo. The carcasses of each pair of fish from the same family fed the same diet were pooled for analysis. The whole fish were homogenized, and the following analyses performed. (1) Percentage dry matter was measured by drying for 20 h in an oven at 105” c.
42
(2) Ash content was determined using an oven at 600” C for 15 h. (3) Energy content was determined by bomb calorimetry. (4) Total nitrogen was measured using the Kjeldahl method, and protein content calculated as N X 6.25. (5) Fat was determined by ether extraction after HCl hydrolysis. Principally, the methods were as described by Stoldt (1952) for foodstuffs and Hartfield (1960) for faeces. Livers from the pairs of fish from the same family fed the same diet were also pooled. Dry matter was measured by the above method, fat content determined by ether extraction, and carbohydrate content by the Anthrone method (Hodge and Hofreiter, 1962). During the preliminary treatment of tissues for analysis by this method glycogen is broken down into simple carbohydrates. It was decided to use this method rather than analysing only for glycogen in case any natural breakdown of glycogen to glucose had occurred during transport and preparation of livers. Faeces from all fish of the same family fed the same diet were pooled for analysis, and protein, dry matter and energy determined by the above methods. Fish sent for veterinary testing were examined for pathological changes. Special note was taken subjectively of the amount of visceral fat and the state and colour of the liver, as well as the general condition of the fish. RESULTS
Table III shows the mean dressed out carcass weights, expressed as a percentage of whole body weight, and results of the chemical analyses of rainbow trout fed on diets differing in the percentage of their metabolizable energy present as carbohydrate. Table IV shows the results of analyses of variance for carcass weight and the data from the chemical analyses. No significant difference was found between dressed out weights of fish fed different diets, but fish fed diets high in carbohydrate contained significantly more protein and ash, but less fat and total energy. No significant difference was found for any of these parameters between different families of fish, nor was there any significant interaction between diet and family. However, there were significant differences between the chemical composition of fish slaughtered after 12 weeks in fresh water and that of fish grown for an extra 12 weeks in brackish water, trends towards differences in chemical composition on different diets already apparent at 12 weeks being accentuated by 24 weeks. Preliminary analyses showed that the percentages of dry matter in fish fed different diets did not differ significantly, being always slightly over 30% (range 30.1-30.7%). Subsequently, fish homogenates were dried before chemical analysis, and the results of percentage chemical composition presented assume a 30~%dry matter content in the carcasses. In Table V the mean weights of livers (expressed as a percentage of body weight), results of chemical analyses of livers and the percentage of fish having
43
TABLE
III
Dressed carcass weight, expressed as percentage of body weight, and chemical composition * standard error (assuming 30% dry matter) of rainbow trout fed diets differing in the proportion of their metabolizable energy present as carbohydrate Percent metabolixable energy as carbohydrate 17 Dressing percentage After 12 weeks After 24 weeks Protein (%) After 12 weeks After 24 weeks Fat (%) After 12 weeks After 24 weeks Ash (%) After 12 weeks After 24 weeks Gross energy (kcaI/kg) After 12 weeks After 24 weeks
25
in diet
38
85.4 t 0.75 85.6 f 0.73
83.8 f 85.6 f
0.68 0.37
85.1 * 86.8 +
16.6 + 0.18 17.0 f 0.15
16.8 + 17.2 t
0.15 0.19
17.4 f 0.17 18.9 ?r 0.28
11.1 f 0.19 10.9 + 0.13
11.3 f 10.6 +
0.17 0.20
10.0 f 8.6 f
0.20 0.27
2.2 f 0.09 2.5 * 0.07
2.3 * 2.5 *
0.09 0.07
2.5 f 2.8 +
0.04 0.05
1 960 1910
+ 5.5 f 8.3
1950 1900
f 10.7 f 15.7
1880 1800
0.39 0.56
f 12.5 * 13.5
discoloured (grey or beige) livers are shown. Table VI shows the results of analyses of variance on these data. Liver weight was significantly correlated with diet and experimental time, the largest livers being found in fish fed the 38% carbohydrate diet after 12 weeks, but in fish fed the 25% diet after 24 weeks. Similarly, the livers of fish fed higher carbohydrate diets had the highest carbohydrate content when slaughtered after 12 weeks, but this relationship did not hold for fish slaughtered after 24 weeks. Correlation between liver percentage and carbohydrates in liver was r = 0.78***, otherwise there was no significant correlation between liver parameters. No significant difference was found in the percentage of dry matter in livers between groups of fish fed different diets, but the livers of fish fed high carbohydrate diets contained less fat and a higher proportion of fish fed high carbohydrate diets had discoloured livers (0.05 > P > 0.01). With the exception of liver fat content, no significant difference was found between fish families for these parameters, and no significant interaction between diet and fish family was found. Table VII shows the chemical composition of faeces removed from rainbow trout after 24 weeks’ feeding. Percentage of dry matter was significantly lower and protein (expressed as a percentage of dry matter) significantly higher in faeces from fish fed high carbohydrate diets (correlation between dry matter and protein was r = -0.94*** ), but the energy content of faeces was the same for all groups of fish. No significant difference was found between fish families for any of these parameters.
IV
23.6* 1.4 3.3 2.7 4.3 .p
1 2 9 18 29
Dressing percentage
freedom
*p< 0.05; **p < 0.01; ***p< 0.001.
Time (12 and 24 weeks) Diet Fish family Interaction diet/ family Error ----
Mean squares
Degrees of
0.2 0.4
0:s
1;*;:::
Protein in fish
Analyses of variance for dressed carcass weights and chemical composition ____~ _
TABLE
0.2 0.5
___~
8.3 *** 18.8*** 0.6
Fat in fish
-
0.02 0.07 _
1.14*** 0.42** 0.02
Ash in fish
-
1 218 1 488
52 392*** 55 380*** 1 029
Gross energy in fiih
of rainbow trout presented in Table III
45
TABLE
V
Weight, expressed as percentage of body weight, chemical composition, and percentage of discoloured livers of rainbow trout fed diets differing in the proportion of their metabolizable energy present as carbohydrate; results in percent f standard error Percent metabolizable energy as carbohydrate 25
17 Liver percentage After 12 weeks After 24 weeks Dry matter After 12 weeks After 24 weeks Fat in dry matter After 12 weeks After 24 weeks Carbohydrate in dry matter After 12 weeks After 24 weeks Discoloured livers (four fish/family) After 24 weeks
1.08 + 0.06 1.14 f 0.06
in diet
38
1.69 c 1.41 +
0.07 0.06
1.87 i 0.10 1.25 * 0.08
24.4 24.1
* 0.94 f 0.41
24.0 23.5
f *
0.32 0.39
23.1 23.1
+ 0.39 f 1.30
17.1 10.8
* 2.01 r 1.91
17.3 10.4
f f
1.62 1.21
12.6 6.1
+ 1.11 r 0.46
1.3 2.7
fr 0.16 + 0.16
5.8 3.4
f *
0.47 0.27
6.6 2.1
* 0.53 + 0.35
25.0
+ 9.1
37.5
f 10.0
55.0
r 7.3
Knowledge of the amount and chemical composition of food given, and the weight (Edwards et al., 1977) and chemical composition of fish at the start of the experiment, after 12 weeks and after 24 weeks allowed the efficiency of utilization of dietary components to be calculated (percent utilization = (content in growth/content in food) X 100). Utilization of dietary gross energy, metabolizable energy and protein is shown in Table VIII. Utilization of energy and protein was usually poorer in fish fed the diet containing 38% of its metabolizable energy as carbohydrate than in those fed the 17 or 25% diets. Other than the occurrence of discoloured livers (Table V), there were no apparent pathological differences between fish fed different diets. DISCUSSION
Phillips et al. (1966) analysed the bodies of trout fed different diets. Their results, in common with those of the present study, showed that dry matter content does not vary according to the carbohydrate level in the diet, but that fish fed higher carbohydrate diets tend to have a higher ash content. Phillips et al. (1966) found no significant difference in fat or protein content of fish fed different amounts of carbohydrate but, in the present study, high carbohydrate diets tended to produce fish with more protein and less fat in their bodies (Table III). However, these differences were probably not great enough to affect the commercial value of the flesh and, since the dressed out weights
VI
< 0.001.
*PC 0.05; **PC
0.01; ***p
1 2 9 18 29
Time (12 and 24 weeks) Diet Fish family Interaction diet/family Error
Degrees of freedom
0.10 0.05 0.07
Liver percentage
Mean squares Fat in liver 616.3*** 111.3*** 67.6*** 14.7 12.0
27.3 41.5 20.9 22.1 22.8
47.0** 42.7*** 0.6 0.6 4.1
Glucose in liver
of livers presented in Table V
Dry matter in liver
Analyses of variance for liver percentage and chemical composition
TABLE
6
VII
*p<
3 470
0.25 0.26
+ 25.3
18.9 f 11.8 f
0.05; **p < 0.01; *** P < 0.001; NS - not significant.
Gross energy in dry matter (k&kg)
Dry matter (W) Protein in dry matter (%)
17
Percent of metabolizable
0.36 0.12
f 25.9
14.0 f 14.8 * 3 440
25
0.40 0.19 f. 46.2
10.5 + 17.3 f
in diet
3 480
38
energy as carbohydrate
NS
NS NS
Between families
NS
*** ***
Between diets
Level of significance
Chemical composition of the faeces of rainbow trout fed for 24 weeks on diets differing in the proportion of their metabolizable energy present aa carbohydrate ____
TABLE
48
TABLE
VIII
Utilization of dietary energy and protein by rainbow trout fed diets containing different proportions of their metabolizable energy as carbohydrate; results in percent Experimental period; conversion efficiency
Percent metabolizable energy as carbohydrate in diet 17
25
38
40 62 42
42 64 44
36 53 34
12-24 weeks in brackish water; Of gross energy Of metaboiizable energy Of protein
30 46 39
21 32 29
9 13 32
O-24 Of Of Of
35 54 40
33 50 38
26 38 34
O-12 Of Of Of
weeks in fresh water; gross energy metabolizable energy protein
weeks - whole experiment; gross energy metabolizable energy protein
(expressed as a percentage of body weight) did not differ between fish fed on different diets (Table III), it is unlikely that the overall sale quality of the carcasses was significantly affected by diet. Phillips et al. (1966) found that trout fed high carbohydrate diets had significantly larger livers than fish fed control diets. The present study confirmed this (Table V) for the indoor, freshwater period of the experiment but not for the period when the fish were held in brackish water outdoors. Phillips et al. (1948) and Phillips et al. (1966) also found higher glycogen content in the livers of fish fed high carbohydrate diets. In the present study the amount of carbohydrate (determined as glucose) present in the liver was higher with high carbohydrate diets during the indoor freshwater period but after 12 more weeks outside in brackish water the livers of fish fed the highest (38%) and lowest (17%) carbohydrate diets contained almost the same amount of carbohydrate (Table V). It is possible that the trout had adapted to the high carbohydrate diet by the end of the experiment, that being kept in brackish water affected the metabolism of carbohydrate, or merely that carbohydrate is more efficiently metabolized by larger fish. The results of Phillips et al. (1948) and Kitamikado et al. (1964) favour the last explanation. Contrary to the results of the present experiment (Table V), Phillips et al. (1948) found no difference in liver fat content of trout fed different amounts of carbohydrate. In the present study, utilization of gross energy, metabolizable energy and protein tended to be less efficient with higher carbohydrate diets (Table VIII).
49
Kitamikado et al. (1964) found that digestibility of protein was poorer in rainbow trout fed high levels of starch, and the results reported here both for utilization of protein (Table VIII) and content of protein in the faeces (Table VII) would seem to support this. Alternatively, the higher proportion of organic matter present in high carbohydrate diets may increase the secretion of digestive fluids, resulting in a higher nitrogen content in the faeces. That the utilization of energy tended to be less efficient with high carbohydrate diets might also be a result of poorer digestibility of the carbohydrates. Singh and Nose (1967) showed a negative correlation between concentration and digestibility of dextrin and potato a-starch in the diet. Phillips et al. (1948) found a higher incidence of pale livers in brook trout fed high carbohydrate diets, and the results presented here showed a higher proportion of rainbow trout having discoloured (grey or beige) livers in those groups fed high carbohydrate diets (Table V). The cause of this is not clear, but mortality in fish groups with discoloured livers was not significantly higher than in those with normal livers during the 24-week experimental period (Edwards et al., 1977). By the end of this time most of the fish in all feeding groups had passed or were approaching the usual commercial sale size for rainbow trout in Europe of around 200 g. However, in Norway rainbow trout are usually grown in the sea for over 1 year more and to a size of several kilos, and the consequences on survival of feeding high carbohydrate diets for so long are still unknown. The fact that, with the exception of liver fat content, no significant difference was found between fish families in chemical composition of their bodies or livers, and that no interaction was found between diet and fish family (Tables IV and VI), indicates that there is little prospect for selectively breeding strains of trout specifically better able to metabolize high levels of carbohydrate in their diets. As far as can be ascertained from the results of the present study, the proportion of the metabolizable energy of the diet provided as carbohydrate makes no difference to the health of the fish. However, the reduced conversion efficiency of energy and protein reported here (Table VIII), and the inferior growth of fish (Edwards et al., 1977) on high carbohydrate diets may make the use of large amounts of carbohydrate uneconomic in diets for commercial trout culture. ACKNOWLEDGEMENT
The authors are indebted to Dr Tore H&stein of the National Institute, Oslo, for veterinary examinations of fish.
Veterinary
50
REFERENCES Austreng, E., 1976. Fat and protein in diets for salmonid fishes. I. Fat content in dry diets for salmon parr (Solmo salar, L.). Meld. Nor. Landbrukshdgsk., 55 (5): 16 pp. (In Norwegian with English summary). Buhler, D.R. and Halver, J.E., 1961. Nutrition of salmonid fishes. IX. Carbohydrate requirements of chinook salmon. J. Nutr., 74: 307-318. Edwards, D.J., Austreng, E., Risa, S. and Gjedrem, T., 1977. Carbohydrate in rainbow trout diets I. Growth of fiih of different families fed diets containing different proportions of carbohydrate. Aquaculture, 11: 31-38. Hartfield, W., 1960. Stoffwechselversuche mit Klilbern und Probleme bei der Fettbestimmung in anfallenden Kot. Arch. Tiererniihr., 10: 264-274. Hodge, J.E. and Hofreiter, B.T., 1962. Determination of reducing sugars and carbohydrates Methods Carbohydr. Chem., 1: 380-394. Kitamikado, M., Morishita, T. and Tachino, S., 1964. Digestibility of dietary protein in rainbow trout - II. Effect of starch and oil contents in diets, and size of fish. Bull. Jpn. Sot. Sci. Fish., 30: 50-54. Phillips, A.M., Tunison, A.V. and Brockway, D.R., 1948. The utilization of carbohydrates by trout. Fish. Res. Bull. N.Y., 11: 44 pp. Phillips, A.M., Livingston, D.L. and Poston, H.A., 1966. The effect of changes in protein quality, calorie sources, and calorie levels upon the growth and chemical composition of brook trout. Fish. Res. Bull. N.Y., 29: 6-14. Singh, R.P. and Nose, T., 1967. Digestibility of carbohydrates in young rainbow trout. Bull. Freshwater Fish. Res. Lab. Tokyo, 17 : 21-25. Stoldt, W., 1952. Vorschlag zur Vereinheitlichung der Fettbestimmung in Lebensmitteln. Fette, Seifen, Anstrichm., 54: 206-207.
39
CARBOHYDRATE IN RAINBOW TROUT DIETS. II. INFLUENCE OF CARBOHYDRATE LEVELS ON CHEMICAL COMPOSITION AND FEED UTILIZATION OF FISH FROM DIFFERENT FAMILIES
E. AUSTRENG*,
S. RISA**, D.J. EDWARDS**
*Depa;tment ofoPou1tt-y and Fur Animal Science, 1432 As-NLH, As (Norway) **Deptrtment 1432 As-NLH,
o,f Animal Genetics As (Norway)
and Breeding,
and H. HVIDSTEN* Agricultural Agricultural
University University
of Norway, of Norway,
(Received 14 December 1976; revised 11 March 1977)
ABSTRACT 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. Groups of rainbow trout fingerlings from ten different families were sampled after 12 and 24 weeks feeding on each of three diets which were similar in protein and energy content but had different percentages of metabolixable energy present as carbohydrate: 17, 25 and 38%. Fish fed different diets had the same dressed carcass weights, expressed as percentage of body weight, but fiih fed higher carbohydrate diets had heavier livers and a higher percentage of discoloured livers. Chemical analyses of fiih bodies showed lower fat and energy but higher protein and ash content for those fed higher carbohydrate diets. Livers contained more fat and carbohydrate but the same dry matter, and faeces showed less dry matter, more protein and the same energy content in fiih with higher dietary carbohydrate. Utilization of dietary energy and protein was poorer in fiih fed high levels of carbohydrate, but health of all fish appeared good. Little variation between different fiih families, and no interaction between fish family and diet was found for any of the above parameters, indicating that prospects for change through selective breeding are poor.
INTRODUCTION
Calories in fish diets can sometimes be provided more cheaply as carbohydrate than as protein or fat, and it is therefore of interest to try to formulate diets containing the maximum level of carbohydrate which the fish can efficiently utilize. Commercial salmonid diets often contain about 15% of their metabolizable energy as carbohydrate but some studies, e.g. that of Buhler and Halver (1961) on chinook salmon, have indicated that levels of some carbohydrates con-
40
siderably higher than this can be assimilated and result in a more efficient utilization of protein. However, Phillips et al. (1948) suggested that feeding trout with high levels of carbohydrate may result in poor health, in particular deleterious changes in the liver, and eventual increased mortality. They suggest, ed a safe maximum content of 12% of digestible carbohydrate in the diet of brook trout. Phillips et al. (1966) reported differences in chemical composition of the bodies and livers of fish fed diets differing in their content of carbohydrate. An experiment to determine the effect of feeding diets containing different proportions of their metabolizable energy as carbohydrate on the growth of rainbow trout was described by Edwards et al. (1977). During and after that experiment, fish were sampled for chemical analyses and veterinary examination to determine what effects the diets had had on rainbow trout health and the chemical compositions of the fish bodies, livers and faeces. The results of these analyses are presented here. MATERIALS
AND METHODS
The design of the experiment, numbers and weights of fish used, feeding regime, and composition of the diets were described in an earlier paper (Edwards et al., 1977). The composition and results of chemical analyses of the experimental diets are also shown in Tables I and II in the present paper. Rainbow trout from ten different families were grown for 12 weeks indoors in fresh water and fed on each of three diets containing 17,25 or 38% of TABLE
I
Composition of experimental trout diets differing in the percentage of their metabolizable energy present as carbohydrate (g/kg) Ingredients
Diet 1
Diet 2
Diet 3
Capelin meal Torula yeast Soya-bean meal Wheat meal Cooked wheat Sucrose Capelin oil Chalk meal Vitamin mixture II* salt Seaweed meal Methionine
300 100 100 380
300 100 100
300 100 200
*Austreng (1976).
410 90
10 10 5 4 1
60 10 10 5 4 1
355 15 10 10 5 4 1
41
TABLE
II
Chemical analyses and calculated content of metabolizable energy of experimental trout diets Chemical component Dry matter (%) Crude protein (%) HCl-ether extract (crude fat) (%) Crude fibre (W) Nitrogen-free extracts (%) Ash (%) Calcium (%) Phosphorus (%) Gross energy (k&kg) Calculated content of metabolizable energy (kcal/kg) Metabolizable energy from carbohydrates (%)
Diet 1 90.3 34.3
Diet 2 90.8 34.6
Diet 3 94.0 35.5
14.1 2.1 31.9 7.9 0.82 0.83 4 596
11.4 2.3 35.5 7.0 0.74 0.85 4 598
6.3 1.8 43.6 6.8 0.72 0.77 4 392
2 976
3 032
3 034
17
25
38
their metabolizable energy in the form of carbohydrate. One fish from each family was then removed from each of two duplicate tanks in which fish received the same diet, i.e. 60 fish were taken, for chemical analysis. In addition, two fish from each family on each diet, i.e. another 60 fish, were removed for veterinary examination. After a further 12 weeks’ growth in brackish water (15°/oo) in outdoor tanks, 120 more fish were sampled in the same way for chemical and veterinary examination, and faeces were taken from all fish remaining alive. Faeces were expelled by stroking the fish firmly down the belly, using much the same technique as for stripping eggs from ripe fish. All fish sampled were killed and weighed individually. Each fish intended for chemical analysis was immediately cut open along the belly and its viscera were removed. The dressed out carcass (body with viscera removed but head on) was weighed. The liver was separated from the other viscera and weighed alone. The carcass, liver and faeces were frozen separately, packed in ice, and sent from the Fish Breeding Experimental Station, Sunndals$ra, where the experiment was carried out, to the Central Chemical Research Laboratory and the Department of Poultry and Fur Animal Science of the Agricultural University of Norway at As for analysis. Fish for veterinary examination were immediately frozen after slaughter and sent to the National Veterinary Institute at Oslo. The carcasses of each pair of fish from the same family fed the same diet were pooled for analysis. The whole fish were homogenized, and the following analyses performed. (1) Percentage dry matter was measured by drying for 20 h in an oven at 105” c.
42
(2) Ash content was determined using an oven at 600” C for 15 h. (3) Energy content was determined by bomb calorimetry. (4) Total nitrogen was measured using the Kjeldahl method, and protein content calculated as N X 6.25. (5) Fat was determined by ether extraction after HCl hydrolysis. Principally, the methods were as described by Stoldt (1952) for foodstuffs and Hartfield (1960) for faeces. Livers from the pairs of fish from the same family fed the same diet were also pooled. Dry matter was measured by the above method, fat content determined by ether extraction, and carbohydrate content by the Anthrone method (Hodge and Hofreiter, 1962). During the preliminary treatment of tissues for analysis by this method glycogen is broken down into simple carbohydrates. It was decided to use this method rather than analysing only for glycogen in case any natural breakdown of glycogen to glucose had occurred during transport and preparation of livers. Faeces from all fish of the same family fed the same diet were pooled for analysis, and protein, dry matter and energy determined by the above methods. Fish sent for veterinary testing were examined for pathological changes. Special note was taken subjectively of the amount of visceral fat and the state and colour of the liver, as well as the general condition of the fish. RESULTS
Table III shows the mean dressed out carcass weights, expressed as a percentage of whole body weight, and results of the chemical analyses of rainbow trout fed on diets differing in the percentage of their metabolizable energy present as carbohydrate. Table IV shows the results of analyses of variance for carcass weight and the data from the chemical analyses. No significant difference was found between dressed out weights of fish fed different diets, but fish fed diets high in carbohydrate contained significantly more protein and ash, but less fat and total energy. No significant difference was found for any of these parameters between different families of fish, nor was there any significant interaction between diet and family. However, there were significant differences between the chemical composition of fish slaughtered after 12 weeks in fresh water and that of fish grown for an extra 12 weeks in brackish water, trends towards differences in chemical composition on different diets already apparent at 12 weeks being accentuated by 24 weeks. Preliminary analyses showed that the percentages of dry matter in fish fed different diets did not differ significantly, being always slightly over 30% (range 30.1-30.7%). Subsequently, fish homogenates were dried before chemical analysis, and the results of percentage chemical composition presented assume a 30~%dry matter content in the carcasses. In Table V the mean weights of livers (expressed as a percentage of body weight), results of chemical analyses of livers and the percentage of fish having
43
TABLE
III
Dressed carcass weight, expressed as percentage of body weight, and chemical composition * standard error (assuming 30% dry matter) of rainbow trout fed diets differing in the proportion of their metabolizable energy present as carbohydrate Percent metabolixable energy as carbohydrate 17 Dressing percentage After 12 weeks After 24 weeks Protein (%) After 12 weeks After 24 weeks Fat (%) After 12 weeks After 24 weeks Ash (%) After 12 weeks After 24 weeks Gross energy (kcaI/kg) After 12 weeks After 24 weeks
25
in diet
38
85.4 t 0.75 85.6 f 0.73
83.8 f 85.6 f
0.68 0.37
85.1 * 86.8 +
16.6 + 0.18 17.0 f 0.15
16.8 + 17.2 t
0.15 0.19
17.4 f 0.17 18.9 ?r 0.28
11.1 f 0.19 10.9 + 0.13
11.3 f 10.6 +
0.17 0.20
10.0 f 8.6 f
0.20 0.27
2.2 f 0.09 2.5 * 0.07
2.3 * 2.5 *
0.09 0.07
2.5 f 2.8 +
0.04 0.05
1 960 1910
+ 5.5 f 8.3
1950 1900
f 10.7 f 15.7
1880 1800
0.39 0.56
f 12.5 * 13.5
discoloured (grey or beige) livers are shown. Table VI shows the results of analyses of variance on these data. Liver weight was significantly correlated with diet and experimental time, the largest livers being found in fish fed the 38% carbohydrate diet after 12 weeks, but in fish fed the 25% diet after 24 weeks. Similarly, the livers of fish fed higher carbohydrate diets had the highest carbohydrate content when slaughtered after 12 weeks, but this relationship did not hold for fish slaughtered after 24 weeks. Correlation between liver percentage and carbohydrates in liver was r = 0.78***, otherwise there was no significant correlation between liver parameters. No significant difference was found in the percentage of dry matter in livers between groups of fish fed different diets, but the livers of fish fed high carbohydrate diets contained less fat and a higher proportion of fish fed high carbohydrate diets had discoloured livers (0.05 > P > 0.01). With the exception of liver fat content, no significant difference was found between fish families for these parameters, and no significant interaction between diet and fish family was found. Table VII shows the chemical composition of faeces removed from rainbow trout after 24 weeks’ feeding. Percentage of dry matter was significantly lower and protein (expressed as a percentage of dry matter) significantly higher in faeces from fish fed high carbohydrate diets (correlation between dry matter and protein was r = -0.94*** ), but the energy content of faeces was the same for all groups of fish. No significant difference was found between fish families for any of these parameters.
IV
23.6* 1.4 3.3 2.7 4.3 .p
1 2 9 18 29
Dressing percentage
freedom
*p< 0.05; **p < 0.01; ***p< 0.001.
Time (12 and 24 weeks) Diet Fish family Interaction diet/ family Error ----
Mean squares
Degrees of
0.2 0.4
0:s
1;*;:::
Protein in fish
Analyses of variance for dressed carcass weights and chemical composition ____~ _
TABLE
0.2 0.5
___~
8.3 *** 18.8*** 0.6
Fat in fish
-
0.02 0.07 _
1.14*** 0.42** 0.02
Ash in fish
-
1 218 1 488
52 392*** 55 380*** 1 029
Gross energy in fiih
of rainbow trout presented in Table III
45
TABLE
V
Weight, expressed as percentage of body weight, chemical composition, and percentage of discoloured livers of rainbow trout fed diets differing in the proportion of their metabolizable energy present as carbohydrate; results in percent f standard error Percent metabolizable energy as carbohydrate 25
17 Liver percentage After 12 weeks After 24 weeks Dry matter After 12 weeks After 24 weeks Fat in dry matter After 12 weeks After 24 weeks Carbohydrate in dry matter After 12 weeks After 24 weeks Discoloured livers (four fish/family) After 24 weeks
1.08 + 0.06 1.14 f 0.06
in diet
38
1.69 c 1.41 +
0.07 0.06
1.87 i 0.10 1.25 * 0.08
24.4 24.1
* 0.94 f 0.41
24.0 23.5
f *
0.32 0.39
23.1 23.1
+ 0.39 f 1.30
17.1 10.8
* 2.01 r 1.91
17.3 10.4
f f
1.62 1.21
12.6 6.1
+ 1.11 r 0.46
1.3 2.7
fr 0.16 + 0.16
5.8 3.4
f *
0.47 0.27
6.6 2.1
* 0.53 + 0.35
25.0
+ 9.1
37.5
f 10.0
55.0
r 7.3
Knowledge of the amount and chemical composition of food given, and the weight (Edwards et al., 1977) and chemical composition of fish at the start of the experiment, after 12 weeks and after 24 weeks allowed the efficiency of utilization of dietary components to be calculated (percent utilization = (content in growth/content in food) X 100). Utilization of dietary gross energy, metabolizable energy and protein is shown in Table VIII. Utilization of energy and protein was usually poorer in fish fed the diet containing 38% of its metabolizable energy as carbohydrate than in those fed the 17 or 25% diets. Other than the occurrence of discoloured livers (Table V), there were no apparent pathological differences between fish fed different diets. DISCUSSION
Phillips et al. (1966) analysed the bodies of trout fed different diets. Their results, in common with those of the present study, showed that dry matter content does not vary according to the carbohydrate level in the diet, but that fish fed higher carbohydrate diets tend to have a higher ash content. Phillips et al. (1966) found no significant difference in fat or protein content of fish fed different amounts of carbohydrate but, in the present study, high carbohydrate diets tended to produce fish with more protein and less fat in their bodies (Table III). However, these differences were probably not great enough to affect the commercial value of the flesh and, since the dressed out weights
VI
< 0.001.
*PC 0.05; **PC
0.01; ***p
1 2 9 18 29
Time (12 and 24 weeks) Diet Fish family Interaction diet/family Error
Degrees of freedom
0.10 0.05 0.07
Liver percentage
Mean squares Fat in liver 616.3*** 111.3*** 67.6*** 14.7 12.0
27.3 41.5 20.9 22.1 22.8
47.0** 42.7*** 0.6 0.6 4.1
Glucose in liver
of livers presented in Table V
Dry matter in liver
Analyses of variance for liver percentage and chemical composition
TABLE
6
VII
*p<
3 470
0.25 0.26
+ 25.3
18.9 f 11.8 f
0.05; **p < 0.01; *** P < 0.001; NS - not significant.
Gross energy in dry matter (k&kg)
Dry matter (W) Protein in dry matter (%)
17
Percent of metabolizable
0.36 0.12
f 25.9
14.0 f 14.8 * 3 440
25
0.40 0.19 f. 46.2
10.5 + 17.3 f
in diet
3 480
38
energy as carbohydrate
NS
NS NS
Between families
NS
*** ***
Between diets
Level of significance
Chemical composition of the faeces of rainbow trout fed for 24 weeks on diets differing in the proportion of their metabolizable energy present aa carbohydrate ____
TABLE
48
TABLE
VIII
Utilization of dietary energy and protein by rainbow trout fed diets containing different proportions of their metabolizable energy as carbohydrate; results in percent Experimental period; conversion efficiency
Percent metabolizable energy as carbohydrate in diet 17
25
38
40 62 42
42 64 44
36 53 34
12-24 weeks in brackish water; Of gross energy Of metaboiizable energy Of protein
30 46 39
21 32 29
9 13 32
O-24 Of Of Of
35 54 40
33 50 38
26 38 34
O-12 Of Of Of
weeks in fresh water; gross energy metabolizable energy protein
weeks - whole experiment; gross energy metabolizable energy protein
(expressed as a percentage of body weight) did not differ between fish fed on different diets (Table III), it is unlikely that the overall sale quality of the carcasses was significantly affected by diet. Phillips et al. (1966) found that trout fed high carbohydrate diets had significantly larger livers than fish fed control diets. The present study confirmed this (Table V) for the indoor, freshwater period of the experiment but not for the period when the fish were held in brackish water outdoors. Phillips et al. (1948) and Phillips et al. (1966) also found higher glycogen content in the livers of fish fed high carbohydrate diets. In the present study the amount of carbohydrate (determined as glucose) present in the liver was higher with high carbohydrate diets during the indoor freshwater period but after 12 more weeks outside in brackish water the livers of fish fed the highest (38%) and lowest (17%) carbohydrate diets contained almost the same amount of carbohydrate (Table V). It is possible that the trout had adapted to the high carbohydrate diet by the end of the experiment, that being kept in brackish water affected the metabolism of carbohydrate, or merely that carbohydrate is more efficiently metabolized by larger fish. The results of Phillips et al. (1948) and Kitamikado et al. (1964) favour the last explanation. Contrary to the results of the present experiment (Table V), Phillips et al. (1948) found no difference in liver fat content of trout fed different amounts of carbohydrate. In the present study, utilization of gross energy, metabolizable energy and protein tended to be less efficient with higher carbohydrate diets (Table VIII).
49
Kitamikado et al. (1964) found that digestibility of protein was poorer in rainbow trout fed high levels of starch, and the results reported here both for utilization of protein (Table VIII) and content of protein in the faeces (Table VII) would seem to support this. Alternatively, the higher proportion of organic matter present in high carbohydrate diets may increase the secretion of digestive fluids, resulting in a higher nitrogen content in the faeces. That the utilization of energy tended to be less efficient with high carbohydrate diets might also be a result of poorer digestibility of the carbohydrates. Singh and Nose (1967) showed a negative correlation between concentration and digestibility of dextrin and potato a-starch in the diet. Phillips et al. (1948) found a higher incidence of pale livers in brook trout fed high carbohydrate diets, and the results presented here showed a higher proportion of rainbow trout having discoloured (grey or beige) livers in those groups fed high carbohydrate diets (Table V). The cause of this is not clear, but mortality in fish groups with discoloured livers was not significantly higher than in those with normal livers during the 24-week experimental period (Edwards et al., 1977). By the end of this time most of the fish in all feeding groups had passed or were approaching the usual commercial sale size for rainbow trout in Europe of around 200 g. However, in Norway rainbow trout are usually grown in the sea for over 1 year more and to a size of several kilos, and the consequences on survival of feeding high carbohydrate diets for so long are still unknown. The fact that, with the exception of liver fat content, no significant difference was found between fish families in chemical composition of their bodies or livers, and that no interaction was found between diet and fish family (Tables IV and VI), indicates that there is little prospect for selectively breeding strains of trout specifically better able to metabolize high levels of carbohydrate in their diets. As far as can be ascertained from the results of the present study, the proportion of the metabolizable energy of the diet provided as carbohydrate makes no difference to the health of the fish. However, the reduced conversion efficiency of energy and protein reported here (Table VIII), and the inferior growth of fish (Edwards et al., 1977) on high carbohydrate diets may make the use of large amounts of carbohydrate uneconomic in diets for commercial trout culture. ACKNOWLEDGEMENT
The authors are indebted to Dr Tore H&stein of the National Institute, Oslo, for veterinary examinations of fish.
Veterinary
50
REFERENCES Austreng, E., 1976. Fat and protein in diets for salmonid fishes. I. Fat content in dry diets for salmon parr (Solmo salar, L.). Meld. Nor. Landbrukshdgsk., 55 (5): 16 pp. (In Norwegian with English summary). Buhler, D.R. and Halver, J.E., 1961. Nutrition of salmonid fishes. IX. Carbohydrate requirements of chinook salmon. J. Nutr., 74: 307-318. Edwards, D.J., Austreng, E., Risa, S. and Gjedrem, T., 1977. Carbohydrate in rainbow trout diets I. Growth of fiih of different families fed diets containing different proportions of carbohydrate. Aquaculture, 11: 31-38. Hartfield, W., 1960. Stoffwechselversuche mit Klilbern und Probleme bei der Fettbestimmung in anfallenden Kot. Arch. Tiererniihr., 10: 264-274. Hodge, J.E. and Hofreiter, B.T., 1962. Determination of reducing sugars and carbohydrates Methods Carbohydr. Chem., 1: 380-394. Kitamikado, M., Morishita, T. and Tachino, S., 1964. Digestibility of dietary protein in rainbow trout - II. Effect of starch and oil contents in diets, and size of fish. Bull. Jpn. Sot. Sci. Fish., 30: 50-54. Phillips, A.M., Tunison, A.V. and Brockway, D.R., 1948. The utilization of carbohydrates by trout. Fish. Res. Bull. N.Y., 11: 44 pp. Phillips, A.M., Livingston, D.L. and Poston, H.A., 1966. The effect of changes in protein quality, calorie sources, and calorie levels upon the growth and chemical composition of brook trout. Fish. Res. Bull. N.Y., 29: 6-14. Singh, R.P. and Nose, T., 1967. Digestibility of carbohydrates in young rainbow trout. Bull. Freshwater Fish. Res. Lab. Tokyo, 17 : 21-25. Stoldt, W., 1952. Vorschlag zur Vereinheitlichung der Fettbestimmung in Lebensmitteln. Fette, Seifen, Anstrichm., 54: 206-207.