1. Introduction
Cobia is an important and popular sport fish in many parts of the world. Its aquaculture has become popular only recently with the success of artificial propagation
and larval production. The cobia is regarded as having the greatest potential among all candidate species for offshore cage culture in Taiwan and other tropical waters. Under
the present cultivating practice, the cobia can attain 4–6 kg in 1 year and survival is
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usually high 90 . About 500 tons of whole fish, averaging 6 kgrfish, were exported from Taiwan to Japan during 1999 at a wholesale price of around US4.8 to 5.7 per kg
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Fish Farming International, January 2000 . The mostly white meat of the fish is served in restaurants as raw fish, sashimi. The belly portion with high lipid content is highly
valued. The cobia is widely distributed in tropical, subtropical and warm temperate seas. A
survey of a natural population in the northeastern Gulf of Mexico revealed that the cobia ranged from age 0 to 9–11 and growth is fast through age 2, after which growth slows
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gradually Franks et al., 1999 . Cobia adults and juveniles mean 340 g were found to Ž
be carnivorous and feed exclusively on small fish, crustaceans and squid Franks et al., .
1996 . They are strong swimmers and aggressive feeders and quite capable of catching mobile prey. They often feed in the water column and possibly near the surface. Under
net cage culture conditions in Taiwan, the cobia are fed either trash fish, or sinking pellets that are modified from feed originally formulated for groupers or seabass. Feed
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conversion ratios FCRs range from 1.6 to1.8. Proximate analysis of a commercial cobia feed revealed a crude protein concentration of 45.3, crude lipid concentration of
16.0 and ash concentration of 11.0. Despite the rapid development of cage culture of cobia and a steady improvement in
feed formulation, little information has been published concerning the nutrient require- ments of this species. Although the preliminary success of commercial compounded
feeds points to a rough estimation of the nutrient requirements of this fish, basic nutritional research is still needed to lay the foundation for further feed development.
The present study investigated the most suitable concentrations of both protein and lipid for optimal growth of juvenile cobia raised in indoor tanks. Proteins and lipids are the
most important chemical ingredients in the diets of most carnivorous fish. The results offer us basic background knowledge on the nutrient requirements of this fish.
2. Materials and methods
2.1. Experiment 1 Seven experimental diets were formulated based on a basal diet in which fish meal
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Ž crude protein concentrations 67.8 gr100 g and casein crude protein concentrations
. 84.0 gr100 g were the protein sources. Alpha-starch was used to replace casein to
Ž render protein concentration gradation ranging between 36 and 60 g per 100 g diet 36
. and 60 . Cellulose was added accordingly so all experimental diets had an approxi-
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mate metabolizable energy ME content of 355 kcalr100 g Table 1 . Crude protein content was determined as Kjeldahl N = 6.25, whereas ME contents were estimated
Ž from published values for the dietary ingredients and reported ME Shiau and Huang,
. Ž
. Ž
. Ž
. 1989 for protein 4.5 kcalrg , carbohydrate 3.49 kcalrg and lipid 8.51 kcalrg . All
ingredients were thoroughly mixed and wet-extruded as pellets, and dried at 458C. The diets were refrigerated at 48C until fed.
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Cobia Rachycentron canadum juveniles from a single egg mass were obtained from the hatchery of the Tungkang Marine Laboratory and fed a commercial seabass feed
until they were 30–35 g. Eight fish were weighed and stocked into each of 21 fiberglass Ž
. Ž
tanks 1 m in diameter, water depth 30 cm supplied with continuous flow flow .
rate s 2–2.5 lrmin of sand-filtered seawater. Water temperature was maintained at 28 0.58C; and salinity was 32 1‰. Each dietary treatment was randomly assigned to
three tanks. The fish were fed twice daily by hand as much as they would consume in 30 min at 0900 and 1600 h. The feeding trials lasted for 8 weeks. Fish were individually
weighed biweekly and at the end of the experiment. A portion of lateral muscle of one fish from each tank was sampled, homogenized and frozen for subsequent proximate
analysis.
Table 1 Ingredient and proximate composition of the experimental diets investigated in Experiment 1
Diet P36
P40 P44
P48 P52
P56 P60
Designated protein level 360
400 440
480 520
560 600
w x
grkg dry diet [
] Ingredient composition g r kg dry diet
Casein 191
238 286
333 380
429 476
a-Starch 309
258 206
155 103
52 00
Cellulose 4
8 12
16 20
24
1
Others 500
500 500
500 501
500 500
2
[ ]
Proximate composition ns 3
g r100 g dry diet Crude protein
37.0 43.1
45.1 51.7
54.9 58.0
63.9 Crude lipid
5.8 6.6
7.1 7.5
7.5 7.9
8.8 Ash
9.5 9.6
9.6 9.4
9.5 9.7
9.5 Ž
. Mineral premix consisted of
grkg premix : KCl, 51.8; MgSO P7H 0, 68.5; NaH PO P2H O, 308.1;
4 2
2 4
2
Ca-lactate, 197.4; ferric citrate, 18.5; AlCl P6H O, 0.1; ZnSO P7H O, 1.8; CuCl, 0.1; MnSO P4–6H O, 0.4;
3 2
4 2
4 2
KI, 0.1; CoCl P6H O, 0.5 and a-cellulose, 362.3.
2 2
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Vitamin premix supplied the diet with mgrkg dry diet : retinyl acetate 0.1; all-rac-a-tocopherol, 400; menadione, 40; thiamin hydrochloride, 60; riboflavin, 200; pyridoxine hydrochloride, 40; nicotinic acid, 800;
D
-Ca pantothenate, 280; inositol, 400; biotin, 6.0; folic acid, 15; PABA, 400; choline chloride, 8000; cyanocobalamin, 0.1; ascorbic acid, 2000; b-carotene, 12.
1
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Other ingredients grkg dry diet : fish meal, 301; squid oil, 30; fish oil, 46; mineral premix, 70; vitamin premix, 30 and carboxymethylcellulose, 23.
2 y1
Ž
y1
Metabolizable energy: 355 kcal 100 g diet based on protein s 4.5 kcal g
; carbohydrates 3.49 kcal
y1 y1
. g
and lipid s8.51 kcal g ; Shiau and Huang, 1989 .
2.2. Experiment 2 Ž
Seven dietary treatments with various concentrations of lipid 2.3–18.9 per 100 g .
Ž diet were examined. The composition of the diets is shown in Table 2. Fish meal crude
. protein concentration s 67.8 gr100 g; crude lipid concentrations 14.7 gr100 g , casein
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Ž crude protein concentrations 84.0 gr100 g and soy protein crude protein concentra-
. tion s 62.6 gr100 g; crude lipid concentrations 0.9 gr100 g supplied the proteins.
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Combinations of an oil mixture cod liver oil: soybean oil s 2: 1 , a-starch and cellulose rendered all experimental diets isoenergetic. The crude protein concentration was
approximately 39. The procedures for diet preparation and storage were the same as the first experiment.
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Juvenile cobia 41 g average size were obtained and stocked in indoor fiberglass tanks and fed a commercial seabass feed prior to the start of the experiment. Eight fish
were individually weighed and placed into each of the 21 fiberglass tanks. The tanks Ž
. were supplied with flowing seawater 28 0.58C and 33 1 ppt . Each diet was
randomly assigned to three tanks. The feeding scheme was the same as the first experiment. The fish were fed the experimental diets for 8 weeks. At the end of the
experiment, fish from each tank were counted and weighed.
Table 2 Ingredient and proximate composition of the experimental diet investigated in Experiment 2
Diet L3
L6 L9
L12 L15
L18 Designated lipid level
30 60
90 120
150 180
w x
grkg dry diet [
] Ingredient composition g r kg dry diet
1
Oil mixture 25
55 85
115 145
175 a-Starch
365 292
219 146
73 Cellulose
43 86
129 172
215
2
Other 610
610 610
610 610
610 [
] Proximate composition ns 3
g r100 g dry diet Crude protein
40.0 39.0
39.4 39.1
38.7 39.7
Crude lipid 2.3
5.4 8.4
12.3 16.6
18.9 Ash
7.8 7.6
7.6 7.6
7.5 7.4
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Mineral premix consisted of grkg premix : MgSO .7H 0, 133; Ca H PO , 1; AlCl P6H O, 7; ZnSO P
4 2
2 4 2
3 2
4
7H O, 20; KI, 0.05; K HPO , 0.2; FeSO P7H O, 35; CuSO P5H O, 10 and NaCl, 50.
2 2
4 4
2 4
2
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Vitamin premix supplied the diet with mgrkg dry diet : retinyl acetate 0.1; all-rac-a-tocopherol, 400; menadione, 40; thiamin hydrochloride, 60; riboflavin, 200; pyridoxine hydrochloride, 40; nicotinic acid, 800;
D
-Ca pantothenate, 280; inositol, 400; biotin, 6.0; folic acid, 15; PABA, 400; choline chloride, 8000; cyanocobalamin, 0.1; ascorbic acid, 2000; b-carotene, 12.
1
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Oil mixture was cod liver oil : soybean oil s 2:1 vrv .
2
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Other ingredients grkg dry diet : fish meal, 310; casein, 111; soy protein crude protein s62.2 , 115; wheat flour, 30; mineral premix, 27; vitamin premix, 15 and polyphosphorylated ascorbic acid, 2.
2.3. Satistical analyses Ž
. Final weight, percent weight gain, feed conversion ratio FCR , protein efficiency
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ratio PER , net protein utilization NPU and body composition data were analyzed for Ž
. statistical significance
P - 0.05 by analysis of variance, and individual differences between dietary treatments were determined by Duncan’s new multiple range test.
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Growth responses in terms of average weight gain in gram
to dietary protein concentrations in Experiment 1 were estimated by the polynomial regression method
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Zeitoun et al., 1976 . The broken-line analysis technique Robbins, 1986 was used to Ž
. examine growth response mean weight gain in gram to lipid supplementation in
Experiment 2. The breakpoints of the regression equations are regarded as the most suitable levels for optimal growth.
3. Results
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