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Carbohydrate in rainbow trout diets. III. Growth and chemical composition of fish from different families fed four levels of carbohydrate in the diet
35
Aquaculture, 25 (1981) 35-49 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
CARBOHYDRATE IN RAINBOW TROUT DIETS. III. GROWTH AND CHEMICAL COMPOSITION OF FISH FROM DIFFERENT FAMILIES FED FOUR LEVELS OF CARBOHYDRATE THE DIET
TERJE
REFSTIE*
*Department 1432 AS-NLH **Department 1432 &NLH (Accepted
and ERLAND
IN
AUSTRENG**
of Animal Genetics and Breeding, Agricultural University of Norway, (Norway) ofPoultry and Fur Animal Science, Agricultural University of Norway, (Norway)
24 November 1980)
ABSTRACT 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. Groups of rainbow trout (Salmo gairdneri) from five different families and five inbred groups were fed four diets similar in protein and energy content but differing in the percentage of metabolizable energy present as carbohydrate. The percentage of metabolizable energy from carbohydrate was 15, 26, 37 and 49%, respectively. Significant differences between fish families were found for growth, condition factor, chemical composition of the fish, relative liver weight, liver colour, digestibility of energy and N-free extracts, dressing percentage amount of intestinal fat and flesh colour. Interaction between diet and family was significant for relative liver weight and liver colour. There was no interaction between diet and family for growth, indicating that the prospects for selectively breeding a strain of rainbow trout specifically better able to utilize carbohydrate are not promising. Growth rate and condition factor increased with decreasing carbohydrate level in diet. The fish fed high levels of carbohydrate had less dry matter, fat and ash in the body, and higher percentage carbohydrate in the liver. They also had higher relative liver weights and more discoloured livers. The fish fed high levels of carbohydrate had significantly better apparent digestibility of energy and protein. There were no significant differences in mortality rate between the groups, and veterinary examination did not reveal any pathological differences in the fish fed different feeding regimes.
INTRODUCTION
The natural diet of salmonids contains a high proportion of protein and little carbohydrate, and a proportion of the dietary protein is utilized as an
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o 1981
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36
energy source. However, all fish studies have shown some ability to utilize carbohydrates as an energy source (Cowey and Sargent, 1972) and dietary energy can sometimes be provided more cheaply as carbohydrate than as protein. Edwards et al. (1977) and Austreng et al, (1977) considered it possible that genetic differences might occur between individuals of the same species in their ability to grow well on high carbohydrates diet. They fed diets containing 17-38% of metabolizable energy present as carbohydrate to different families of rainbow trout (Sulmo gairdneri). They concluded that the prospects of selectively breeding strains of rainbow trout better able to utilize carbohydrate were not promising. However, though the fish fed higher level of carbohydrate have a lower growth rate, their health appeared good. This indicates that 38% of metabolizable energy present as carbohydrate in the diet is not the upper limit, and fish fed higher levels of carbohydrate might show genetic variation in their ability to grow on these high carbohydrate levels. Therefore a second experiment was started to determine differences between families of rainbow trout in growth performance, carcass composition, liver carbohydrate, food digestibility and health, when the trout were fed diets containing 15-49% of metabolizable energy present as carbohydrate. This paper describes the results of this experiment. MATERIAL
AND METHODS
The experiment was carried out at the Research Station for Salmonids, Sunndals$ra, Norway. Rainbow trout from five full-sib groups (families) and five inbred lines (F = 0.375) were used in the experiment. One-hundred and twenty individuals were sorted from each family or inbred line and freeze-branded (Refstie and A&tad, 1975) for subsequent recognition. The experiment began in December 1975 (period I), and fish from each family or inbred line were each weighed and measured. The mean weight of the fish was 57 g and the coefficient of variation between individuals within a family was on average 26.8%. Fifteen fish from each family or inbred line were then sampled, weighed in bulk, and put into fresh water in eight indoor fiberglass tanks of the type described by Aulstad and Refstie (1975). After 26 weeks (period I) each fish was weighed and transfered to four outdoor circular concrete ponds, diameter 10 m and depth 0.75 m, and the experiment was continued for a further 13 weeks (period 11). The water supply during period II had a salinity of 15-18 “/QO. At the end of the experiment, in September 1976, each fish was weighed and measured. Three times during period I and once during period II all the fish from each family in each tank or pond were weighed in bulk and counted. This gives seven records of weight during the experiment. Water temperature varied between 8.0 and 12.O”C during period I and 8.0
37
and 14.O”C during period II, but was the same in all tanks at any one time. The oxygen concentration in the water never fell below 7.0 mg/l. During the experiment, the fish received dry pelleted feed in excess. This was so that the quantity of feed delivered should not be the limiting factor for growth. The feed was delivered by electrically driven automatic feeders and all tanks received the same amount of feed each day. Four diets were given, each diet being fed to fish in two of the tanks during period I, and one of the ponds during period II. The use of all fish families and inbred lines in each of two tanks receiving the same diet during period I gave duplicate sets of results for this period. To get different proportions of carbohydrate in the four diets, while keeping constant the protein and metabolizable energy content, diets with more carbohydrate were formulated with correspondingly less fat. Two diets, I and IV, were formulated. Diets II and III were mixtures of diet I and IV. The composition of the diets is shown in Table I. In period II the diets were supplemented with 50 mg canthaxanthin per kg, to give a red coloured flesh at slaughtering. Chemical analyses of the diets were carried out at the Central Chemical Research Laboratory at The Agricultural University of Norway. Results of the analyses and energy contents of the four diets are shown in Tables II and III. The method of Phillips and Brockway (1959) was used to calculate the metabolizable energy of dietary constituents. Thus it was assumed that raw TABLE
I
Composition of experimental diets containing different levels of carbohydrate Ingredients
Diet I
Diet II
Diet III
Diet IV
260.0 106.7 153.3 50.0 430.0
390.0 0.0 180.0 0.0 430.0
Wheat meal Soybean meal Capelin oil’ Constant ingredients’
320.0 100.0 150.0 430.0
130.0 213.3 126.7 100.0 430.0
’ Constant ingredients: Capelin meal Dried brewer’s yeast Seaweed meal Vitamin mixture*
300.0 100.0 3.5 10.0
Methionine Salt Chalk meal Microminerals**
GhCO.92
0.0
(g/kg)
1.0 5.0 10.0 0.5
*Supplying per kg of diet: vitamin A, 7500 i.u.; vitamin D,, 1500 i.u.; vitamin K,, 10 mg; a-tocopherol, 250 mg; thiamine, 25 mg; riboflavin, 150 mg; pyridoxin, 45 mg; Ca-pantothenate, 55 mg; niacin, 550 mg; folic acid, 10 mg; vitamin B,,, 0.01 mg; biotin, 0.45 mg; cholinecloride-HCl, 3.5 g; inositol, 550 mg; paraminobenzoic acid, 25 mg; ascorbic acid, 265 mg. **Supplying per kg of diet: Fe, 21.4 mg; Mn, 28.6 mg; Zn, 25.2 mg; Cu, 7.2 mg; Co, 0.5 mg; I, 1.0 mg. 2 Supplemented with 0.02% of the antioxidant EMQ.
38 TABLE
II
Chemical analyses of experimental diets containing different levels of carbohydrate Chemical component
Diet I
Diet III
Diet II
Diet IV
Dry matter (%) Crude protein (%) HCl-ether extract (crude fat) (%) Crude fibre (%) N-free extracts (%) Ash (%) Calcium (%) Phosphorus (%) Gross energy (MJ/kg) TABLE
91.7 35.5
89.3 35.3
89.8 35.7
89.1 34.9
16.4 2.1 30.6 7.1 1.07 0.80 20.2
12.8 1.8 32.4 7.0 1.10 0.77 19.3
9.1 1.7 36.3 7.0 1.13 0.77 18.5
4.3 1.4 41.6 6.9 1.26 0.75 17.1
III
Metabolizable energy’ content of experimental trout diets calculated using the results of chemical analyses (MJ/kg) Chemical component
Diet I
Diet II
Diet III
Diet IV
Protein Fat Raw starch Glucose Total Percent of metabolizable energy from carbohydrate
5.79 5.49 2.05 0.00 13.33
5.75 4.29 1.30 2.24 13.58
5.82 3.05 0.69 4.47 14.03
5.69 1.44 0.20 6.71 14.01
15
26
37
49
’ In calculating the metabolizable energy it was assumed that raw starch gives 6.7, glucose 17.2, protein 16.3 and fat 33.5 kJ metabolizable energy/g, respectively (Phillips and Brockway, 1959).
starch gives 6.7 (40% digestibility), glucose 17.2 (99% absorption), protein 16.3 (90% digestibility), and fat 33.5 kJ (85% digestibility) of metabolizable energy/g, respectively. At the end of period I faeces from all fish of the same family fed the same diet were pooled for analysis in order to determine digestibility. The faeces were stripped from the fish as described by Austreng (1978a), and analyzed for crude fibre (AOAC, 1975) as a digestion indicator, as described by Austreng (1978b). Protein, fat, ash and energy were analyzed as given below. Digestibility was calculated as follows. Digestion coefficient = (a-b)
X 100/a
a = protein in feed/crude fibre in feed b = protein in faecesjcrude fibre in faeces
39
At the end of period I two fish from each family were removed from each of the two duplicate tanks in which fish received the same diet for chemical analysis. The carcasses of the four 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. 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 a micro-Kjeldahl method, and protein calculated as N X 6.25. 5. Fat was determined by Soxhlet diethyl ether extraction. At the end of the experiment all the fish were slaughtered. The dressing percentage (dressed out carcass weight X loo/body weight) and the relative liver weight (liver weight X loo/body weight) were determined. The colour of the liver, the colour of the flesh and the amount of intestinal fat were judged visually. For the liver colour the following scores was used: 0 for normal, 1 for partly discoloured, and 2 for totally discoloured. For the colour of the flesh a scale from 0 to 5 was used, where 0 is without colour and 5 is very red. For the intestinal fat the following scores was used: 0 for little, 1 for some, and 2 for much. Five livers from each family on each diet were analysed separately. Dry matter was measured by the above method 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 storing and preparation of livers. At the end of the experiment 25 fish from each of the four diets were removed for veterinary examination of pathological changes. Statistical analysis was carried out using the following model, in which the effect of diets was regarded as fixed and the effect of family as random: Yijk
= p +
Ui
+
bj
+
(Ub)ij
+
ck
+ eijk
the actual observation, effect of diet i, ai = effect of rainbow trout family j, bi (ab)ij = interaction between diet i and family j, Ck = effect of replicate k, = random effect for the ijkth recording. eijk Yijk
= =
RESULTS
Weight, length and condition factors after 282 days of the experiment are shown in Table IV for each of the 10 families fed each of the four diets. Re-
IV
III
x 100
592.2 35.7 1.27
975.0 40.6 1.44
1045.8 42.0 1.38
1219.0 43.2 1.49
length (cm)”
weight(g)
(g) (cm) factor
* Condition factor =
Mean weight Mean length Mean condition
Diet IV
Mean weight (9) Mean length (cm) Mean condition factor
Diet
Mean weight (g) Mean length (cm) Mean condition factor
Diet II
Mean weight (g) Mean length (cm) Mean condition factor’
Diet I
415.4 30.7 1.38
596.5 33.6 1.49
802.7 37.0 1.48
1004.4 39.3 1.58
644.5 36.9 1.28
927.8 40.1 1.41
1239.8 43.9 1.44
1297.0 44.4 1.46
517.4 33.5 1.35
781.1 37.7 1.40
939.9 39.9 1.43
1040.8 40.4 1.54
4
683.7 36.4 1.41
978.3 40.0 1.48
1144.0 42.3 1.44
1269.8 42.8 1.59
5
8
447.9 32.1 1.31
663.0 36.2 1.36
809.8 38.8 1.35
399.8 30.0 1.44
636.8 34.4 1.52
796.8 36.9 1.52
486.9 32.9 1.35
835.4 38.1 1.47
1119.2 41.8 1.47
860.0 868.3 1112.5 38.8 37.2 41.6 i.42 1.64 1.52
7
6
3
1
2
Inbred line
495.7 33.4 1.30
805.2 37.9 1.43
1015.1 40.7 1.47
1043.9 41.2 1.45
9
480.4 32.4 1.39
801.3 37.4 1.49
1107.7 41.2 1.54
1033.7 39.3 1.64
10
factor for each of 10 rainbow trout families after 282 days growth on each of four diets
Family
Mean individual weight, length and condition containing different levels of carbohydrate
TABLE
41
lative growth rates (7%per day; Fisher, 1958) during period I and II for each of the 10 families fed each of the four diets are shown in Fig.1 (duplicate tanks combined for period I). The differences found between families in weight, length and condition factor at the end of the experiment, were significant (P < O.Ol), as it was for relative growth rate during the two periods (P < 0.01). No significant interaction between diet and family was found for these traits.
46 n
114 8?0
Fig.1. Relative growth rate (% per day) in rainbow trout in each of 10 families fed four diets containing different proportions of metabolizable energy as carbohydrate.
Fig.2 shows results of chemical analysis of fish from each of 10 families fed each of the four diets. (Two fish from each family were sampled at the end of period I, and replicate tanks were pooled.) The differences between families were significant for percentage dry matter (P < O.Ol), percentage protein (P < 0.05) and percentage fat (P < 0.05). The differences between families for gross energy content, and percentage ash were not significant, and are not shown in Fig.2. The mean values for each diet are shown in Table V. Relative liver weight, percentage carbohydrate in liver, and score for liver colour, are shown in Fig.3. The differences between families were significant for relative liver weight, and score for liver colour (P < O.Ol), and close to significant for percentage carbohydrate (P = 0.06). There were also significant interactions between diet fed and family for relative liver weight (P <0.05), and score for liver colour (P < 0.01). There were no significant differences between families for percentage dry matter in liver. The differences between families for digestibility were significant for energy (P < 0.05) and N-free extracts (NFE) (P < O.Ol), but not for fat and protein. Digestibility coefficient for energy for each family fed each of the four diets are shown in Fig.4.
42
370 1
Perrenfuge
dry matfer in fhe body
36.0 -
3503YO33032031.0 30.0 290
2fO MO 190 78.0 IZO
16.0
466
6 8 ?O
Fig.2. Dry matter, protein and fat content in rainbow trout from 10 families fed four diets containing different proportions of metabolizable energy as carbohydrate.
Fig.5 shows the dressing percentage, amount of intestinal fat (score) and flesh colour (score) for each of the 10 families fed each of the four diets. The differences found between families were significant (P < 0.05) for these traits, and significant interaction between diet and family were found for flesh colour (P < 0.05). Mean weights of fish during 282 days of growth on each of the four diets fed are shown in Fig.6. (The two replicate tanks are combined in period I.) Fish fed the lowest level of carbohydrate (diet I) showed the best growth for all periods in the experiment. Table V shows mean results for each of the four diets for relative growth rate during the experiment, weight, length and condition factor at the end
43
of the experiment, chemical content of the fish at the end of period I, dressing percentage, meat colour (score), and intestinal fat (score) at slaughtering, relative liver weight and liver colour (score) and chemical content of liver at the end of the experiment. The differences between fish fed different diets were significant for relative growth rate during the two periods (P < 0.01) and for weight, length and condition factor at the end of the experiment (P < 0.01). The fish fed the lowest level of carbohydrate had the highest condition factor at the end of the experiment and their weights were double that of fish fed the highest level of carbohydrate. The differences in chemical content in the body were significant for percentage dry matter (P < 0.05), percentage fat (P < 0.01) and percentage ash (P < 0.01). The fish fed high levels of carbohydrate had less dry matter, fat and ash. They also had less energy and slightly more protein in the carcass, but the differences between diets were not significant for these traits. The fish fed high levels of carbohydrate have better meat colour (score) and more intestinal fat (score) and the differences between diet were signifiTABLE
V
Mean results of an experiment with rainbow trout fed four diets differing in content of carbohydrate Diet I Body measurements R’ in period I (% per day) R’ in period II (% per day) Weight (g) Length (cm) Condition factor Chemical content of fish Dry matter (%) Protein (%) Fat (Ether extract) (%) Ash (%) Energy (MJ/W Observations at slaughter Dressing percentage Meat colour (score) Intestinal fiit (score) Relative liver weight Liver colour (score) Chemical content of liver Dry matter (%) Carbohydrate (%)
’ R = Relative growth =
Diet II ______.
1.15 0.87 1074.9 40.8 1.53
Diet III
1.13 0.86 1002.1 40.5 1.45
Diet IV
Mean _
1.02 0.80 800.0 37.6 1.45
0.85 0.70 516.4 33.4 1.35
1.04 0.81 848.3 38.1 1.45
36.3 20.0 16.4 1.6 29.6
35.0 20.1 13.4 1.4 28.9
32.8 21.5 11.9 1.3 27.9
28.9 21.6 6.3 1.5 25.8
33.3 20.8 12.0 1.5 28.1
86.2 4.20 1.40 1.21 0.30
85.7 4.20 1.30 1.57 0.20
85.8 4.10 1.00 1.85 0.60
86.0 3.60 0.80 2.57 1.20
86.0 4.03 1.13 1.80 0.58
21.6 1.1
21.2 2.2
19.5 2.6
20.5 3.0
20.7 2.1
In weight at end - In weight at start days
X 100 (Fisher, 1958)
liver percenk of bodywe@t
YO t
Fig.3. Relative liver weight, percentage carbohydrate in liver and score for liver colour in rainbow trout from 10 families fed four diets containing different proportions of metabolizable energy as carbohydrate.
Fig.4. Digestibility of energy in rainbow trout from 10 families fed four diets containing different proportions of metabolizable energy as carbohydrate.
cant (P < 0.01) for these traits. There were no significant differences in dressing percentage between diets. The fish fed high levels of. carbohydrate had significantly higher relative liver weight (P < O.Ol), more discoloured liver (P < 0.01) lower percentage dry matter in the liver (P< 0.05) and higher percentage carbohydrate (P< 0.01) in the liver. Table VI shows the digestibility coefficients for the different diets. The fish fed high levels of carbohydrate had significantly better digestibility of energy, protein and NFE (P< 0.01). The differences in digestibility for fat
45 Dressing percentage t38,0t
870
86.0
4II
85.0
i.
111 J+
Fomdy 2
h 1IIJ ‘f6
6
m
i
8 f0
2
66
r
I-
Fig.5. Dressing percentage, intestine fat and flesh colour in rainbow trout from 10 families fed four diets containing different proportion of metabolizable energy as carbohydrate.
BOO-
-
600-
--e-m
-
I m
YOO-
Fig. 6. Increase in mean weight of rainbow trout fed four diets containing different propor tions of metabolizable energy as carbohydrate.
46 TABLE
VI
Calculated digestibility (%) for crude protein, crude fat, crude carbohydrate energy in four diets differing in content of carbohydrate
Crude protein Crude fat Crude carbohydrate Gross energy
Diet I
Diet II
Diet III
Diet IV
82.2 91.2 7.2 63.3
85.2 77.9 44.0 71.2
84.5 84.4 48.2 72.8
86.4 77.5 72.3 81.8
were not significant. Veterinary examination did not show any pathological fish which could be a result of feeding, and no significant tality rate were found between fish fed different diets.
and gross
______
differences differences
in the in mor-
DISCUSSION
Large differences in growth rate were found between different families of rainbow trout kept on the same diet (Table IV and Fig.1). This is in agreement with earlier results (A&tad et al., 1972; Nevdal et al., 1975; Refstie et al., 1977; Edwards et al., 1977; Gunnes and Gjedrem, 1978; Refstie and Steine, 1978; Austreng and Refstie, 1979) and shows the possibility of improving growth rate through selective breeding. However, in the present experiment, where energy from carbohydrate ranged from 15% to 49% of the metabolizable energy in the diet, there was no interaction for growth rate between carbohydrate level in the diet and family. Thus the prospects for breeding a strain of rainbow trout which can better utilize high levels of carbohydrate are not good. This is in agreement with Edwards et al. (1977). There were significant differences between families for percentage dry matter, percentage protein, and percentage fat in the fish body (Fig.2). Gjedrem (1976), Austreng et al. (1977) and Austreng and Refstie (1979) found significant differences between families of rainbow trout for chemical composition of the body. Ayles et al. (1979) found significant additive genetic differences in lipid content between strains of rainbow trout. They also found small but significant differences between strains for percentage dry matter. These findings indicate that there are possibilities for changing the carcass composition through selective breeding. The differences between families and interaction between diet and family found for relative liver weight and liver colour, together with the close to significant differences (P = 0.06) between families for carbohydrate content in liver may indicate that some families are better able to tolerate high levels of carbohydrate. Any possible long term effect on carbohydrate metabolism did not influence the health of the fish over 282 days of the experiment as there were no pathological changes evident from liver exam-
inations. Austreng et al. (1977) found no significant differences between rainbow trout families, or interaction between diet and family for liver percentage, percentage dry matter and fat and carbohydrate in liver after feeding diets containing different carbohydrate levels for 24 weeks. In the present study there were significant differences between families for energy digestibility (Fig.4), indicating genetic differences between families for this trait. There were no significant differences between families in digestibility of protein and fat. Austreng and Refstie (1979) found significant differences between families for protein digestibility when each family was fed different levels of protein. Austreng et al. (1977) found no differences between families for chemical composition of faeces when each family was fed a different level of carbohydrate. We also found significant differences between families for dressing percentage, amount of intestinal fat and flesh colour (Fig.5). Gjedrem (1976) found significant differences between rainbow trout families for flesh colour, and though this trait depends mainly on nutrition and food quality, there should be potential for improvement through selective breeding. The present investigation shows that though the prospects for selectively breeding a strain of fish specifically better able to utilize carbohydrate do not seem good, there are possibilities for improving traits such as carcass quality and digestibility of food components through selective breeding. However, genetic parameters and relative economic values have to be estimated before it is possible to state if these traits are worth including in a breeding scheme. In the present study high levels of carbohydrate reduced growth rate and condition factor, and at the end of the experiment the group fed the lowest level of carbohydrate were twice the weight of the group fed the highest carbohydrate level. However, even the group with highest level of carbohydrate in the diet had as high a survival rate as the other groups. This is in agreement with Pieper (1977) and Austreng et al. (1977). McLaren et al. (1946) recommended less than 20% carbohydrate in the diet and Phillips et al. (1948) recommended a maximum level of digestible carbohydrate in the diet of 12% to be consistent with good health. Buhler and Halver (1961) found that 20% carbohydrate in the diet was optimal for chinook salmon (Oncorhynchiu
tshawytscha).
It is well known that chemical content of the diet influences the chemical content in the fish body (Phillips et al., 1966; Pieper, 1977; Austreng et al., 1977; Austreng, 197813; Austreng and Refstie, 1979). In the present study high levels of carbohydrate produced less dry matter and fat in the body, and there were also differences in ash content (Table V). The fish fed high levels of carbohydrate also had less intestinal fat (Table V), which partly explains the differences in fat content of the body. The fish fed higher levels of carbohydrate also had less colour in the flesh (Table V), even though all diets had the same amount of canthaxanthin. At the end of the experiinent fish fed higher levels of carbohydrate had
48
significantly larger livers, and a higher amount of carbohydrate in the liver (Table V). This is in agreement with Phillips et al. (1966) and Pieper (1977). Austreng et al. (1977) found the same effect after a period of 12 weeks in fresh water, but after a further 12 weeks in brackish water this effect had almost disappeared. We also found a higher incidence of discoloured livers in fish fed high levels of carbohydrate. This is in agreement with Phillips et al. (1948) and Austreng et al. (1977). There were no differences in mortality between the groups, and the biological significance of the differences found in the livers are not clear. All groups were fed the same amount of food each day. Fish on high carbohydrate diets with poorer growth were therefore overfed and no meaningful feed conversion efficiency can be estimated, but the apparent digestibility of energy, protein and nitrogen-free extracts increased with increasing carbohydrate levels in the diet (Table VI, Fig.4). This means that better growth in groups fed lower levels of carbohydrate in the diets are not due to better digestibility or absorption of energy, but probably due to better appetite and food consumption. On the other hand, Austreng et al. (1977) found that utilization of gross energy, metabolizable energy, and protein tended to be less efficient, with higher levels of carbohydrate in the diets, and Kitamikado et al. (1964) found that digestibility of protein was poorer in rainbow trout fed high levels of starch. In the present study glucose was the main carbohydrate source in the diets, and glucose has a high digestibility compared with other carbohydrate sources (Phillips et al., 1948). This study does not support the theory that high levels of carbohydrate in the diets lead to higher mortality and reduced utilization of energy and protein. When low food costs are more important than rapid growth, a higher level of carbohydrate in the diet may give better economic results.
ACKNOWLEDGEMENT
This work was financially supported by the Norwegian Agricultural Research Council. The-authors are indebted to Lit. Sverre Ola Roald of the National Veterinary Institute, Oslo for veterinary examinations of fish. REFERENCES AOAC, 1975. Official Methods of Analysis of the Association of Official Analytical Chemists, twelfth edition. Association of Official Analytical Chemists, Washington, DC, 1094 pp. Au&ad, D. and Refstie, T., 1975. The effect of water depth in rearing tanks on growth and mortality of salmon and rainbow trout fingerlings. Prog. Fish Cult., 37: 113-114. Aulstad, D., Gjedrem. T. and Skjervold, H., 1972. Genetic and environmental sources of variation in length and weight of rainbow trout (Salmo gairdneri). J. Fish. Res. Board Can., 29: 237-241.
49
Austreng, E., 1978a. Digestibility determination in fish using chromic oxide marking and analysis of contents from different segments of the gastrointestinal tract. Aquaculture, 13: 265-272. Austreng, E., 1973b. Fett og protein i for til laksefisk. V. Proteininnhold i tdrrfor til regnbueaure (Salmo gairdneri, Richardson). Meld. Norg. Landbrukshdgsk., 57( 22), I2 PP. 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 fiih from different families. Aquaculture, 11: 39-50. Ayles, G.B., Bernard, D. and Hendzel, M., 1979. Genetic differences in lipid and dry matter content between strains of rainbow trout (Salmo gairdneri) and their hybrids. Aquaculture, 18: 253-262. Buhler, D.R. and Halver, J.E., 1961. Nutrition of salmonoid fishes. IX. Carbohydrate requirements of chinook salmon. J. Nutr., 74: 307-318. Cowey, C.B. and Sargent, J.R., 1972. Fish nutrition. Adv. Mar. Biol., 10: 383-492. 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. Fisher, R.A., 1958. Statistical Methods for Research Workers, thirteenth edition. Oliver and Boyd, Edinburgh and London, 356 pp. Gjedrem, T., 1976. Possibilities for genetic improvements in salmonids. J. Fish. Res. Board Can., 33: 1094-1099. Gunnes, K. and Gjedrem, T., 1978. Selection experiments with salmon. IV. Growth of Atlantic salmon during two years in the sea. Aquaculture, 15: 19-33. 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 content in diets, and size of fish. Bull. Jpn. Sot. Sci. Fish., 30: 50-54. McLaren, B.A., Herman, E.F. and Elvehjem, C.A., 1946. Nutrition of rainbow trout; Studies with purified rations. Arch. Biochem. Biophys., 10: 433-441. Naevdal, G., Holm, M., Mdller, D. and @thus, O.D., 1975. Experiments with selective breeding of Atlantic salmon. In: Count. Explor. Sea. Comm. Meet. 22, Anadromus and Catadromus Fish Committee, Fisheries Improvement, 9 pp. Phillips, A.M. and Brockway, D.R., 1959. Dietary calories and the production of trout in hatcheries. Prog. Fish Cult., 21: 3-16. Phillips, A.M., Tunison, A.V. and Brockway, D.R., 1948. The utilization of carbohydrates by trout. Fish. Res. Bull., 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., 29: 6-14. Pieper, A., 1977. Untersuchungen i.iber die Verwertung einiger Kohlenhydrate durch Regenbogenforellen (Salmo gairdnerii Richardson). Dissertation zur Erlangung des Doktorgrad der Landwirtschaftlichen FakultHt der Georg-August-Universittt zu Gottingen, 94 pp. Refstie, T. and Austad, D., 197 5. Tagging experiment with salmonids. Aquaculture, 5 : 367-374. Refstie, T. and Steine, T.A., 1978. Selection experiments with salmon. III. Genetic and environmental sources of variation in length and weight of Atlantic salmon in the freshwater phase. Aquaculture, 14: 221-234. Refstie, T., Steine, T.A. and Gjedrem, T., 1977. Selection experiments with salmon, II. Proportion of Atlantic salmon smoltifying at 1 year of age. Aquaculture, 10: 231242.
Aquaculture, 25 (1981) 35-49 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
CARBOHYDRATE IN RAINBOW TROUT DIETS. III. GROWTH AND CHEMICAL COMPOSITION OF FISH FROM DIFFERENT FAMILIES FED FOUR LEVELS OF CARBOHYDRATE THE DIET
TERJE
REFSTIE*
*Department 1432 AS-NLH **Department 1432 &NLH (Accepted
and ERLAND
IN
AUSTRENG**
of Animal Genetics and Breeding, Agricultural University of Norway, (Norway) ofPoultry and Fur Animal Science, Agricultural University of Norway, (Norway)
24 November 1980)
ABSTRACT 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. Groups of rainbow trout (Salmo gairdneri) from five different families and five inbred groups were fed four diets similar in protein and energy content but differing in the percentage of metabolizable energy present as carbohydrate. The percentage of metabolizable energy from carbohydrate was 15, 26, 37 and 49%, respectively. Significant differences between fish families were found for growth, condition factor, chemical composition of the fish, relative liver weight, liver colour, digestibility of energy and N-free extracts, dressing percentage amount of intestinal fat and flesh colour. Interaction between diet and family was significant for relative liver weight and liver colour. There was no interaction between diet and family for growth, indicating that the prospects for selectively breeding a strain of rainbow trout specifically better able to utilize carbohydrate are not promising. Growth rate and condition factor increased with decreasing carbohydrate level in diet. The fish fed high levels of carbohydrate had less dry matter, fat and ash in the body, and higher percentage carbohydrate in the liver. They also had higher relative liver weights and more discoloured livers. The fish fed high levels of carbohydrate had significantly better apparent digestibility of energy and protein. There were no significant differences in mortality rate between the groups, and veterinary examination did not reveal any pathological differences in the fish fed different feeding regimes.
INTRODUCTION
The natural diet of salmonids contains a high proportion of protein and little carbohydrate, and a proportion of the dietary protein is utilized as an
0044~8466/81/0000-0000/$02.50
o 1981
Elsevier Scientific
Publishing Company
36
energy source. However, all fish studies have shown some ability to utilize carbohydrates as an energy source (Cowey and Sargent, 1972) and dietary energy can sometimes be provided more cheaply as carbohydrate than as protein. Edwards et al. (1977) and Austreng et al, (1977) considered it possible that genetic differences might occur between individuals of the same species in their ability to grow well on high carbohydrates diet. They fed diets containing 17-38% of metabolizable energy present as carbohydrate to different families of rainbow trout (Sulmo gairdneri). They concluded that the prospects of selectively breeding strains of rainbow trout better able to utilize carbohydrate were not promising. However, though the fish fed higher level of carbohydrate have a lower growth rate, their health appeared good. This indicates that 38% of metabolizable energy present as carbohydrate in the diet is not the upper limit, and fish fed higher levels of carbohydrate might show genetic variation in their ability to grow on these high carbohydrate levels. Therefore a second experiment was started to determine differences between families of rainbow trout in growth performance, carcass composition, liver carbohydrate, food digestibility and health, when the trout were fed diets containing 15-49% of metabolizable energy present as carbohydrate. This paper describes the results of this experiment. MATERIAL
AND METHODS
The experiment was carried out at the Research Station for Salmonids, Sunndals$ra, Norway. Rainbow trout from five full-sib groups (families) and five inbred lines (F = 0.375) were used in the experiment. One-hundred and twenty individuals were sorted from each family or inbred line and freeze-branded (Refstie and A&tad, 1975) for subsequent recognition. The experiment began in December 1975 (period I), and fish from each family or inbred line were each weighed and measured. The mean weight of the fish was 57 g and the coefficient of variation between individuals within a family was on average 26.8%. Fifteen fish from each family or inbred line were then sampled, weighed in bulk, and put into fresh water in eight indoor fiberglass tanks of the type described by Aulstad and Refstie (1975). After 26 weeks (period I) each fish was weighed and transfered to four outdoor circular concrete ponds, diameter 10 m and depth 0.75 m, and the experiment was continued for a further 13 weeks (period 11). The water supply during period II had a salinity of 15-18 “/QO. At the end of the experiment, in September 1976, each fish was weighed and measured. Three times during period I and once during period II all the fish from each family in each tank or pond were weighed in bulk and counted. This gives seven records of weight during the experiment. Water temperature varied between 8.0 and 12.O”C during period I and 8.0
37
and 14.O”C during period II, but was the same in all tanks at any one time. The oxygen concentration in the water never fell below 7.0 mg/l. During the experiment, the fish received dry pelleted feed in excess. This was so that the quantity of feed delivered should not be the limiting factor for growth. The feed was delivered by electrically driven automatic feeders and all tanks received the same amount of feed each day. Four diets were given, each diet being fed to fish in two of the tanks during period I, and one of the ponds during period II. The use of all fish families and inbred lines in each of two tanks receiving the same diet during period I gave duplicate sets of results for this period. To get different proportions of carbohydrate in the four diets, while keeping constant the protein and metabolizable energy content, diets with more carbohydrate were formulated with correspondingly less fat. Two diets, I and IV, were formulated. Diets II and III were mixtures of diet I and IV. The composition of the diets is shown in Table I. In period II the diets were supplemented with 50 mg canthaxanthin per kg, to give a red coloured flesh at slaughtering. Chemical analyses of the diets were carried out at the Central Chemical Research Laboratory at The Agricultural University of Norway. Results of the analyses and energy contents of the four diets are shown in Tables II and III. The method of Phillips and Brockway (1959) was used to calculate the metabolizable energy of dietary constituents. Thus it was assumed that raw TABLE
I
Composition of experimental diets containing different levels of carbohydrate Ingredients
Diet I
Diet II
Diet III
Diet IV
260.0 106.7 153.3 50.0 430.0
390.0 0.0 180.0 0.0 430.0
Wheat meal Soybean meal Capelin oil’ Constant ingredients’
320.0 100.0 150.0 430.0
130.0 213.3 126.7 100.0 430.0
’ Constant ingredients: Capelin meal Dried brewer’s yeast Seaweed meal Vitamin mixture*
300.0 100.0 3.5 10.0
Methionine Salt Chalk meal Microminerals**
GhCO.92
0.0
(g/kg)
1.0 5.0 10.0 0.5
*Supplying per kg of diet: vitamin A, 7500 i.u.; vitamin D,, 1500 i.u.; vitamin K,, 10 mg; a-tocopherol, 250 mg; thiamine, 25 mg; riboflavin, 150 mg; pyridoxin, 45 mg; Ca-pantothenate, 55 mg; niacin, 550 mg; folic acid, 10 mg; vitamin B,,, 0.01 mg; biotin, 0.45 mg; cholinecloride-HCl, 3.5 g; inositol, 550 mg; paraminobenzoic acid, 25 mg; ascorbic acid, 265 mg. **Supplying per kg of diet: Fe, 21.4 mg; Mn, 28.6 mg; Zn, 25.2 mg; Cu, 7.2 mg; Co, 0.5 mg; I, 1.0 mg. 2 Supplemented with 0.02% of the antioxidant EMQ.
38 TABLE
II
Chemical analyses of experimental diets containing different levels of carbohydrate Chemical component
Diet I
Diet III
Diet II
Diet IV
Dry matter (%) Crude protein (%) HCl-ether extract (crude fat) (%) Crude fibre (%) N-free extracts (%) Ash (%) Calcium (%) Phosphorus (%) Gross energy (MJ/kg) TABLE
91.7 35.5
89.3 35.3
89.8 35.7
89.1 34.9
16.4 2.1 30.6 7.1 1.07 0.80 20.2
12.8 1.8 32.4 7.0 1.10 0.77 19.3
9.1 1.7 36.3 7.0 1.13 0.77 18.5
4.3 1.4 41.6 6.9 1.26 0.75 17.1
III
Metabolizable energy’ content of experimental trout diets calculated using the results of chemical analyses (MJ/kg) Chemical component
Diet I
Diet II
Diet III
Diet IV
Protein Fat Raw starch Glucose Total Percent of metabolizable energy from carbohydrate
5.79 5.49 2.05 0.00 13.33
5.75 4.29 1.30 2.24 13.58
5.82 3.05 0.69 4.47 14.03
5.69 1.44 0.20 6.71 14.01
15
26
37
49
’ In calculating the metabolizable energy it was assumed that raw starch gives 6.7, glucose 17.2, protein 16.3 and fat 33.5 kJ metabolizable energy/g, respectively (Phillips and Brockway, 1959).
starch gives 6.7 (40% digestibility), glucose 17.2 (99% absorption), protein 16.3 (90% digestibility), and fat 33.5 kJ (85% digestibility) of metabolizable energy/g, respectively. At the end of period I faeces from all fish of the same family fed the same diet were pooled for analysis in order to determine digestibility. The faeces were stripped from the fish as described by Austreng (1978a), and analyzed for crude fibre (AOAC, 1975) as a digestion indicator, as described by Austreng (1978b). Protein, fat, ash and energy were analyzed as given below. Digestibility was calculated as follows. Digestion coefficient = (a-b)
X 100/a
a = protein in feed/crude fibre in feed b = protein in faecesjcrude fibre in faeces
39
At the end of period I two fish from each family were removed from each of the two duplicate tanks in which fish received the same diet for chemical analysis. The carcasses of the four 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. 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 a micro-Kjeldahl method, and protein calculated as N X 6.25. 5. Fat was determined by Soxhlet diethyl ether extraction. At the end of the experiment all the fish were slaughtered. The dressing percentage (dressed out carcass weight X loo/body weight) and the relative liver weight (liver weight X loo/body weight) were determined. The colour of the liver, the colour of the flesh and the amount of intestinal fat were judged visually. For the liver colour the following scores was used: 0 for normal, 1 for partly discoloured, and 2 for totally discoloured. For the colour of the flesh a scale from 0 to 5 was used, where 0 is without colour and 5 is very red. For the intestinal fat the following scores was used: 0 for little, 1 for some, and 2 for much. Five livers from each family on each diet were analysed separately. Dry matter was measured by the above method 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 storing and preparation of livers. At the end of the experiment 25 fish from each of the four diets were removed for veterinary examination of pathological changes. Statistical analysis was carried out using the following model, in which the effect of diets was regarded as fixed and the effect of family as random: Yijk
= p +
Ui
+
bj
+
(Ub)ij
+
ck
+ eijk
the actual observation, effect of diet i, ai = effect of rainbow trout family j, bi (ab)ij = interaction between diet i and family j, Ck = effect of replicate k, = random effect for the ijkth recording. eijk Yijk
= =
RESULTS
Weight, length and condition factors after 282 days of the experiment are shown in Table IV for each of the 10 families fed each of the four diets. Re-
IV
III
x 100
592.2 35.7 1.27
975.0 40.6 1.44
1045.8 42.0 1.38
1219.0 43.2 1.49
length (cm)”
weight(g)
(g) (cm) factor
* Condition factor =
Mean weight Mean length Mean condition
Diet IV
Mean weight (9) Mean length (cm) Mean condition factor
Diet
Mean weight (g) Mean length (cm) Mean condition factor
Diet II
Mean weight (g) Mean length (cm) Mean condition factor’
Diet I
415.4 30.7 1.38
596.5 33.6 1.49
802.7 37.0 1.48
1004.4 39.3 1.58
644.5 36.9 1.28
927.8 40.1 1.41
1239.8 43.9 1.44
1297.0 44.4 1.46
517.4 33.5 1.35
781.1 37.7 1.40
939.9 39.9 1.43
1040.8 40.4 1.54
4
683.7 36.4 1.41
978.3 40.0 1.48
1144.0 42.3 1.44
1269.8 42.8 1.59
5
8
447.9 32.1 1.31
663.0 36.2 1.36
809.8 38.8 1.35
399.8 30.0 1.44
636.8 34.4 1.52
796.8 36.9 1.52
486.9 32.9 1.35
835.4 38.1 1.47
1119.2 41.8 1.47
860.0 868.3 1112.5 38.8 37.2 41.6 i.42 1.64 1.52
7
6
3
1
2
Inbred line
495.7 33.4 1.30
805.2 37.9 1.43
1015.1 40.7 1.47
1043.9 41.2 1.45
9
480.4 32.4 1.39
801.3 37.4 1.49
1107.7 41.2 1.54
1033.7 39.3 1.64
10
factor for each of 10 rainbow trout families after 282 days growth on each of four diets
Family
Mean individual weight, length and condition containing different levels of carbohydrate
TABLE
41
lative growth rates (7%per day; Fisher, 1958) during period I and II for each of the 10 families fed each of the four diets are shown in Fig.1 (duplicate tanks combined for period I). The differences found between families in weight, length and condition factor at the end of the experiment, were significant (P < O.Ol), as it was for relative growth rate during the two periods (P < 0.01). No significant interaction between diet and family was found for these traits.
46 n
114 8?0
Fig.1. Relative growth rate (% per day) in rainbow trout in each of 10 families fed four diets containing different proportions of metabolizable energy as carbohydrate.
Fig.2 shows results of chemical analysis of fish from each of 10 families fed each of the four diets. (Two fish from each family were sampled at the end of period I, and replicate tanks were pooled.) The differences between families were significant for percentage dry matter (P < O.Ol), percentage protein (P < 0.05) and percentage fat (P < 0.05). The differences between families for gross energy content, and percentage ash were not significant, and are not shown in Fig.2. The mean values for each diet are shown in Table V. Relative liver weight, percentage carbohydrate in liver, and score for liver colour, are shown in Fig.3. The differences between families were significant for relative liver weight, and score for liver colour (P < O.Ol), and close to significant for percentage carbohydrate (P = 0.06). There were also significant interactions between diet fed and family for relative liver weight (P <0.05), and score for liver colour (P < 0.01). There were no significant differences between families for percentage dry matter in liver. The differences between families for digestibility were significant for energy (P < 0.05) and N-free extracts (NFE) (P < O.Ol), but not for fat and protein. Digestibility coefficient for energy for each family fed each of the four diets are shown in Fig.4.
42
370 1
Perrenfuge
dry matfer in fhe body
36.0 -
3503YO33032031.0 30.0 290
2fO MO 190 78.0 IZO
16.0
466
6 8 ?O
Fig.2. Dry matter, protein and fat content in rainbow trout from 10 families fed four diets containing different proportions of metabolizable energy as carbohydrate.
Fig.5 shows the dressing percentage, amount of intestinal fat (score) and flesh colour (score) for each of the 10 families fed each of the four diets. The differences found between families were significant (P < 0.05) for these traits, and significant interaction between diet and family were found for flesh colour (P < 0.05). Mean weights of fish during 282 days of growth on each of the four diets fed are shown in Fig.6. (The two replicate tanks are combined in period I.) Fish fed the lowest level of carbohydrate (diet I) showed the best growth for all periods in the experiment. Table V shows mean results for each of the four diets for relative growth rate during the experiment, weight, length and condition factor at the end
43
of the experiment, chemical content of the fish at the end of period I, dressing percentage, meat colour (score), and intestinal fat (score) at slaughtering, relative liver weight and liver colour (score) and chemical content of liver at the end of the experiment. The differences between fish fed different diets were significant for relative growth rate during the two periods (P < 0.01) and for weight, length and condition factor at the end of the experiment (P < 0.01). The fish fed the lowest level of carbohydrate had the highest condition factor at the end of the experiment and their weights were double that of fish fed the highest level of carbohydrate. The differences in chemical content in the body were significant for percentage dry matter (P < 0.05), percentage fat (P < 0.01) and percentage ash (P < 0.01). The fish fed high levels of carbohydrate had less dry matter, fat and ash. They also had less energy and slightly more protein in the carcass, but the differences between diets were not significant for these traits. The fish fed high levels of carbohydrate have better meat colour (score) and more intestinal fat (score) and the differences between diet were signifiTABLE
V
Mean results of an experiment with rainbow trout fed four diets differing in content of carbohydrate Diet I Body measurements R’ in period I (% per day) R’ in period II (% per day) Weight (g) Length (cm) Condition factor Chemical content of fish Dry matter (%) Protein (%) Fat (Ether extract) (%) Ash (%) Energy (MJ/W Observations at slaughter Dressing percentage Meat colour (score) Intestinal fiit (score) Relative liver weight Liver colour (score) Chemical content of liver Dry matter (%) Carbohydrate (%)
’ R = Relative growth =
Diet II ______.
1.15 0.87 1074.9 40.8 1.53
Diet III
1.13 0.86 1002.1 40.5 1.45
Diet IV
Mean _
1.02 0.80 800.0 37.6 1.45
0.85 0.70 516.4 33.4 1.35
1.04 0.81 848.3 38.1 1.45
36.3 20.0 16.4 1.6 29.6
35.0 20.1 13.4 1.4 28.9
32.8 21.5 11.9 1.3 27.9
28.9 21.6 6.3 1.5 25.8
33.3 20.8 12.0 1.5 28.1
86.2 4.20 1.40 1.21 0.30
85.7 4.20 1.30 1.57 0.20
85.8 4.10 1.00 1.85 0.60
86.0 3.60 0.80 2.57 1.20
86.0 4.03 1.13 1.80 0.58
21.6 1.1
21.2 2.2
19.5 2.6
20.5 3.0
20.7 2.1
In weight at end - In weight at start days
X 100 (Fisher, 1958)
liver percenk of bodywe@t
YO t
Fig.3. Relative liver weight, percentage carbohydrate in liver and score for liver colour in rainbow trout from 10 families fed four diets containing different proportions of metabolizable energy as carbohydrate.
Fig.4. Digestibility of energy in rainbow trout from 10 families fed four diets containing different proportions of metabolizable energy as carbohydrate.
cant (P < 0.01) for these traits. There were no significant differences in dressing percentage between diets. The fish fed high levels of. carbohydrate had significantly higher relative liver weight (P < O.Ol), more discoloured liver (P < 0.01) lower percentage dry matter in the liver (P< 0.05) and higher percentage carbohydrate (P< 0.01) in the liver. Table VI shows the digestibility coefficients for the different diets. The fish fed high levels of carbohydrate had significantly better digestibility of energy, protein and NFE (P< 0.01). The differences in digestibility for fat
45 Dressing percentage t38,0t
870
86.0
4II
85.0
i.
111 J+
Fomdy 2
h 1IIJ ‘f6
6
m
i
8 f0
2
66
r
I-
Fig.5. Dressing percentage, intestine fat and flesh colour in rainbow trout from 10 families fed four diets containing different proportion of metabolizable energy as carbohydrate.
BOO-
-
600-
--e-m
-
I m
YOO-
Fig. 6. Increase in mean weight of rainbow trout fed four diets containing different propor tions of metabolizable energy as carbohydrate.
46 TABLE
VI
Calculated digestibility (%) for crude protein, crude fat, crude carbohydrate energy in four diets differing in content of carbohydrate
Crude protein Crude fat Crude carbohydrate Gross energy
Diet I
Diet II
Diet III
Diet IV
82.2 91.2 7.2 63.3
85.2 77.9 44.0 71.2
84.5 84.4 48.2 72.8
86.4 77.5 72.3 81.8
were not significant. Veterinary examination did not show any pathological fish which could be a result of feeding, and no significant tality rate were found between fish fed different diets.
and gross
______
differences differences
in the in mor-
DISCUSSION
Large differences in growth rate were found between different families of rainbow trout kept on the same diet (Table IV and Fig.1). This is in agreement with earlier results (A&tad et al., 1972; Nevdal et al., 1975; Refstie et al., 1977; Edwards et al., 1977; Gunnes and Gjedrem, 1978; Refstie and Steine, 1978; Austreng and Refstie, 1979) and shows the possibility of improving growth rate through selective breeding. However, in the present experiment, where energy from carbohydrate ranged from 15% to 49% of the metabolizable energy in the diet, there was no interaction for growth rate between carbohydrate level in the diet and family. Thus the prospects for breeding a strain of rainbow trout which can better utilize high levels of carbohydrate are not good. This is in agreement with Edwards et al. (1977). There were significant differences between families for percentage dry matter, percentage protein, and percentage fat in the fish body (Fig.2). Gjedrem (1976), Austreng et al. (1977) and Austreng and Refstie (1979) found significant differences between families of rainbow trout for chemical composition of the body. Ayles et al. (1979) found significant additive genetic differences in lipid content between strains of rainbow trout. They also found small but significant differences between strains for percentage dry matter. These findings indicate that there are possibilities for changing the carcass composition through selective breeding. The differences between families and interaction between diet and family found for relative liver weight and liver colour, together with the close to significant differences (P = 0.06) between families for carbohydrate content in liver may indicate that some families are better able to tolerate high levels of carbohydrate. Any possible long term effect on carbohydrate metabolism did not influence the health of the fish over 282 days of the experiment as there were no pathological changes evident from liver exam-
inations. Austreng et al. (1977) found no significant differences between rainbow trout families, or interaction between diet and family for liver percentage, percentage dry matter and fat and carbohydrate in liver after feeding diets containing different carbohydrate levels for 24 weeks. In the present study there were significant differences between families for energy digestibility (Fig.4), indicating genetic differences between families for this trait. There were no significant differences between families in digestibility of protein and fat. Austreng and Refstie (1979) found significant differences between families for protein digestibility when each family was fed different levels of protein. Austreng et al. (1977) found no differences between families for chemical composition of faeces when each family was fed a different level of carbohydrate. We also found significant differences between families for dressing percentage, amount of intestinal fat and flesh colour (Fig.5). Gjedrem (1976) found significant differences between rainbow trout families for flesh colour, and though this trait depends mainly on nutrition and food quality, there should be potential for improvement through selective breeding. The present investigation shows that though the prospects for selectively breeding a strain of fish specifically better able to utilize carbohydrate do not seem good, there are possibilities for improving traits such as carcass quality and digestibility of food components through selective breeding. However, genetic parameters and relative economic values have to be estimated before it is possible to state if these traits are worth including in a breeding scheme. In the present study high levels of carbohydrate reduced growth rate and condition factor, and at the end of the experiment the group fed the lowest level of carbohydrate were twice the weight of the group fed the highest carbohydrate level. However, even the group with highest level of carbohydrate in the diet had as high a survival rate as the other groups. This is in agreement with Pieper (1977) and Austreng et al. (1977). McLaren et al. (1946) recommended less than 20% carbohydrate in the diet and Phillips et al. (1948) recommended a maximum level of digestible carbohydrate in the diet of 12% to be consistent with good health. Buhler and Halver (1961) found that 20% carbohydrate in the diet was optimal for chinook salmon (Oncorhynchiu
tshawytscha).
It is well known that chemical content of the diet influences the chemical content in the fish body (Phillips et al., 1966; Pieper, 1977; Austreng et al., 1977; Austreng, 197813; Austreng and Refstie, 1979). In the present study high levels of carbohydrate produced less dry matter and fat in the body, and there were also differences in ash content (Table V). The fish fed high levels of carbohydrate also had less intestinal fat (Table V), which partly explains the differences in fat content of the body. The fish fed higher levels of carbohydrate also had less colour in the flesh (Table V), even though all diets had the same amount of canthaxanthin. At the end of the experiinent fish fed higher levels of carbohydrate had
48
significantly larger livers, and a higher amount of carbohydrate in the liver (Table V). This is in agreement with Phillips et al. (1966) and Pieper (1977). Austreng et al. (1977) found the same effect after a period of 12 weeks in fresh water, but after a further 12 weeks in brackish water this effect had almost disappeared. We also found a higher incidence of discoloured livers in fish fed high levels of carbohydrate. This is in agreement with Phillips et al. (1948) and Austreng et al. (1977). There were no differences in mortality between the groups, and the biological significance of the differences found in the livers are not clear. All groups were fed the same amount of food each day. Fish on high carbohydrate diets with poorer growth were therefore overfed and no meaningful feed conversion efficiency can be estimated, but the apparent digestibility of energy, protein and nitrogen-free extracts increased with increasing carbohydrate levels in the diet (Table VI, Fig.4). This means that better growth in groups fed lower levels of carbohydrate in the diets are not due to better digestibility or absorption of energy, but probably due to better appetite and food consumption. On the other hand, Austreng et al. (1977) found that utilization of gross energy, metabolizable energy, and protein tended to be less efficient, with higher levels of carbohydrate in the diets, and Kitamikado et al. (1964) found that digestibility of protein was poorer in rainbow trout fed high levels of starch. In the present study glucose was the main carbohydrate source in the diets, and glucose has a high digestibility compared with other carbohydrate sources (Phillips et al., 1948). This study does not support the theory that high levels of carbohydrate in the diets lead to higher mortality and reduced utilization of energy and protein. When low food costs are more important than rapid growth, a higher level of carbohydrate in the diet may give better economic results.
ACKNOWLEDGEMENT
This work was financially supported by the Norwegian Agricultural Research Council. The-authors are indebted to Lit. Sverre Ola Roald of the National Veterinary Institute, Oslo for veterinary examinations of fish. REFERENCES AOAC, 1975. Official Methods of Analysis of the Association of Official Analytical Chemists, twelfth edition. Association of Official Analytical Chemists, Washington, DC, 1094 pp. Au&ad, D. and Refstie, T., 1975. The effect of water depth in rearing tanks on growth and mortality of salmon and rainbow trout fingerlings. Prog. Fish Cult., 37: 113-114. Aulstad, D., Gjedrem. T. and Skjervold, H., 1972. Genetic and environmental sources of variation in length and weight of rainbow trout (Salmo gairdneri). J. Fish. Res. Board Can., 29: 237-241.
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