Growth of dentex fingerlings (Dentex dentex) fed diets containing different levels of protein and lipid

Aquaculture 218 (2003) 479 – 490 www.elsevier.com/locate/aqua-online

Growth of dentex fingerlings (Dentex dentex) fed diets containing different levels of protein and lipid F.J. Espino´s a, A. Toma´s a, L.M. Pe´rez a, S. Balasch b, M. Jover a,* a

Departamento de Ciencia Animal, Laboratorio de Acuicultura, Universidad Polite´cnica de Valencia, Camino de Vera, 14, Valencia 46071, Spain b Departamento de Estadı´stica e Investigacio´n Operativa Aplicadas y Calidad, Universidad Polite´cnica de Valencia, Camino de Vera, 14, Valencia 46071, Spain Received 28 September 2001; received in revised form 2 July 2002; accepted 2 July 2002

Abstract Dentex fingerlings weighing 2.5 g initial live weight were fed 12 extruded diets containing four crude protein levels (40%, 45%, 50% and 55%) and three crude lipid levels (12%, 17% and 22%) according to a factorial design, for 6 weeks. Survival was high (84 – 92%) and specific growth rate (SGR) values were 3.4%, 3.9%, 4.3% and 4.1% day 1 and food conversion ratio (FCR) were 2.2, 2.1, 1.5 and 1.9 for fish fed the diets containing 40%, 45%, 50% and 55% crude protein, respectively. With respect to lipid levels, SGR values were 4.2%, 4.0% and 3.6% day 1 and FCR were 1.7, 1.8 and 2.3 for 12%, 17% and 22% crude lipid, respectively. The effects of protein and lipid levels were significant; final live weight, SGR, FCR and PER values were best for fish fed 50% crude protein and poorest for those fed 22% lipid diets. The best growth performance was observed in fish fed diets containing 50/12 and 50/17 protein/lipid ratios. The results indicate that optimum growth and nutrient utilisation of dentex fingerlings can be obtained when they are fed a diet containing around 50% crude protein with a lipid level ranging from 12% to 17%, and having a crude protein/gross energy ratio from 22 to 25 g MJ 1. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Dentex dentex; Nutrition; Growth; Protein; Lipid; Cooking-extruded diets

1. Introduction Common dentex is a species of interest in aquaculture to complement those already under mass production, due to its rapid growth, consumer acceptance and ease of *

Corresponding author. Tel.: +34-6-3877434; fax: +34-6-3877439. E-mail address: [email protected] (M. Jover).

0044-8486/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 ( 0 2 ) 0 0 3 1 3 - 7

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reproduction in captivity, although at present there are some problems with larval production. Preliminary studies have shown that common dentex have rapid growth rates, both juveniles and larger fish (Riera et al., 1993, 1995; Efthimiou et al., 1994; Company et al., 1999; Katavic et al., 2000). Nutrient requirements for growth of dentex juvenile (10 – 30 g initial live weight) have been studied (Tibaldi et al., 1996; Cardenete et al., 1997a,b,c; Company et al., 1999) using pelleted diets, but little information exists concerning optimum dietary protein and lipid levels for fingerlings weighing less than 10 g (Jover et al., 1998). The aim of this work was to determine the growth and nutrient utilisation of dentex fingerlings (2– 3 g live weight) fed extruded diets containing several protein and lipid levels.

2. Materials and methods 2.1. Rearing system The trial was conducted in 36 plastic cylindrical tanks (80 l), set up in a recirculating marine water system (4 m3 capacity) with a first pressure biofilter of 60-l capacity and a second gravity biofilter of around 800-l capacity. All tanks were equipped with aeration and the water was heated by electric heaters installed in the gravity biofilter. Water temperature ranged from 25 to 29 jC, salinity was 36 F 1 g l 1, dissolved oxygen was 6.5 F 0.5 mg l 1 and pH ranged from 7.5 to 8.5. Photoperiod was natural in summer (16 L/8 D) and all tanks had similar light conditions. 2.2. Fish Dentex fingerlings (Dentex dentex) were transported from the Balearic Government Aquaculture Station (Mallorca Island, Spain) to the Aquaculture Laboratory of the Polytechnic University of Valencia (Spain). After a 10-day adaptation period, 648 fish were classified according to live weight and stocked into 36 tanks (18 fish per tank), three for each diet. Initial live weight was around 2.5 F 0.5 g, and all tanks had a similar initial

Table 1 Chemical composition of ingredients (international feed number between parentheses) used in experimental diets Ingredient

Dry matter (%)

Crude protein (% DM)

Crude lipid (% DM)

Crude fiber (% DM)

Ash (% DM)

Carbohydrate (% DM)

Fish meal, herring (5-02-000) Blood meal (5-02-381) Full-fat soybean (5-04-597) Extracted soybean (5-04-604) Wheat meal (4-05-268)

93.1 91.9 91.1 88.3 88.2

73.4 93.5 41.1 45.1 12.4

8.0 0.6 23.7 3.5 2.7

1.0 0.5 7.2 6.2 2.5

15.5 2.3 5.5 7.9 9.5

2.1 3.1 22.5 37.3 72.9

F.J. Espino´s et al. / Aquaculture 218 (2003) 479–490

density of fish (0.5 kg m September 1998).

1

481

). The experimental period was 42 days (from August to

2.3. Diets and feeding Twelve diets containing one of four crude protein (40%, 45%, 50% and 55% CP) and three crude lipid levels (12%, 17% and 22% CL), according to a factorial design, were formulated using commercial ingredients (Table 1). Different experimental diets were designed according to crude protein/crude lipid ratio (Table 2). Diets were prepared by cooking extrusion processing with a semi-industrial twin-screw extruder (Clextral BC-45). The processing conditions were as follows: 100 rpm speed screw, 110 jC temperature, 30 –40 atm pressure and 2 mm diameter pellets.

Table 2 Ingredient content and proximate composition of diets (dry weight basis) Ingredients (g kg

1

) 40/12 40/17 40/22 45/12 45/17 45/22 50/12 50/17 50/22 55/12 55/17 55/22

Fish meal, herring 280 265 (5-02-000) Blood meal 40 40 (5-02-381) Extracted soybean 55 60 (5-04-604) Full-fat soybean 230 270 (5-04-597) Wheat meal 322.5 250 (4-05-268) Fish oil (7-08-048) 32.5 75 Vitamin – mineral mixa 40 40

300

370

355

40

40

40

40

40

40

60

55

60

70

102.5

220

205

250

200

205

267.5 180

135 40

32.5 40

75 40

387.5 440

185

127.5 160 135 40

32.5 40

442.5 450

470

465

485

60

80

85

100

70

65

100

70

60

230

190

182.5 230

180

102.5

60

75 40

135 40

95

35

0

32.5 40

75 40

135 40

Analyzed composition (%) Crude protein (CP) 40.20 40.03 39.98 44.95 45.00 45.00 49.99 49.48 49.95 55.00 55.00 54.95 Crude lipid (CL) 12.04 16.94 21.91 11.99 16.99 21.95 11.97 17.00 22.02 11.99 17.00 21.91 N-free extract (NFE) 34.44 29.88 24.91 28.56 23.68 18.50 22.05 17.27 12.56 16.61 11.80 7.23 Crude fiber 3.12 3.24 2.80 2.86 3.00 2.60 2.83 2.81 2.40 2.68 2.68 2.19 Ash 9.19 9.00 8.37 9.83 9.15 8.96 10.25 9.74 9.24 10.16 9.55 9.21 Calculated values GE (MJ kg 1)b DE (MJ kg 1)c CP/GE (g kJ 1)b CP/DE (g kJ 1)c

20.5 16.5 19.6 24.4

21.6 17.5 18.6 22.8

22.7 18.6 17.6 21.5

20.5 16.8 21.9 26.8

21.7 17.9 20.7 25.2

22.7 18.9 19.8 23.8

20.6 17.0 24.3 29.3

21.6 18.1 22.9 27.4

22.9 19.3 21.8 25.9

20.8 17.5 26.4 31.5

22.0 18.6 25.0 29.6

23.1 19.7 23.8 27.9

a Vitamin and mineral mix (values are g kg 1 except those in parenthesis): Premix: 25; Choline, 10; DL-atocopherol, 5; ascorbic acid, 5; (PO4)2Ca3, 5. Premix composition: retinol acetate, 1,000,000 IU kg 1; calciferol, 500 IU kg 1; DL-a-tocopherol, 10; menadione sodium bisulphite, 0.8; thiamin hydrochloride, 2.3; riboflavin, 2.3; pyridoxine hydrochloride, 15; cyanocobalamin, 25; nicotinamide, 15; pantothenic acid, 6; folic acid, 0.65; biotin, 0.07; ascorbic acid, 75; inositol, 15; betaine, 100; polypeptides, 12; Zn, 5; Se, 0.02; I, 0.5; Fe, 0.2; CuO, 15; Mg, 5.75; Co, 0.02; Met, 1.2; Cys, 0.8; Lys, 1.3; Arg, 0.6; Phe, 0.4; Trcp, 0.7; excpt. to 1000 g (Dibaq-Diproteg). b Calculated using: 23.9 kJ g 1 protein, 39.8 kJ g 1 lipid and 17.6 kJ g 1 carbohydrate. c Calculated using: 20.9 kJ g 1 protein, 33.5 kJ g 1 lipid and 11.7 kJ g 1 carbohydrate (Tibaldi et al., 1996).

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Fish were fed by hand four times a day (0900, 1300, 1700 and 2000 h) to apparent satiation. Pellets were distributed slowly, allowing all fish to eat. All fish were individually weighed at 21 and 42 days. At the end of the trial, three fish per tank were sampled to determine biometric parameters and body chemical composition (AOAC, 1990). 2.4. Statistical analysis The trial was planned according to a two-way factorial design with four protein levels and three lipid levels, and consequently, the results were submitted to a two-way analysis of variance (ANOVA), introducing the initial live weight as covariate in growth indexes. All variables were checked for normality and variance homogeneity, and Newman– Keuls test was used for comparisons between mean at 0.05 significant level (Statgraphics 1997, Statistical Graphics System, Version Plus 3.1, USA). The quantitative character of variables (protein and lipid levels) allowed the global effect to be split into linear, quadratic and cubic components. In order to determine the optimum levels of protein and lipid for maximum growth and food efficiency, the relationships between dietary protein or lipid and specific growth rate (SGR) or food conversion ratio (FCR) were fitted using a second-order polynomial regression analysis (Hidalgo and Alliot, 1988), where SGR or FCR was a function of protein or lipid level, after the formula Y = a + bX + cX2. The maximum or minimum of such curves corresponds to the optimum levels for SGR or FCR, respectively.

3. Results All indexes of growth and nutrient utilisation were affected by both protein and lipid levels (Table 3) and, in most of them, linear, quadratic and cubic components were significant for protein, but only linear and quadratic components for lipid. The effect of protein  lipid interaction was not significant, so the one-way analysis of variance for diets was not necessary, as the effect of dietary protein on different variables was similar for several lipid levels (Fig. 1). Survival at the end of trial was acceptable in most of the diets, ranging from 84% to 92%, and statistical differences were found for protein level; 40% and 45% CP resulted in a lower survival rate (Table 4). The best growth results (final live weight and SGR) were obtained in fish fed diets containing 50% protein (4.26% day 1), and 12% or 17% lipid (4.16% and 3.96% day 1) (Table 4). With respect to nutritive parameters, fish fed 50% protein had a lower daily feed intake (DFI = 5.07% day 1), daily protein intake (DPI = 2.55% day 1) and food conversion ratio (FCR = 1.55) (Table 4). On the other hand, DFI, DPI and FCR were similar for 12% and 17% lipid, but 22% gave a higher value. The same trend was observed for PER; the best ratios were obtained in fish fed 50% CP and 12% or 17% CL. Specific growth rate (SGR) and food conversion ratio (FCR) were a quadratic function of protein level (CP), of the form SGR = 12.067 + 0.637CP 0.0062CP 2 and

Source

df SGR

Crude protein 3 (CP) Linear 1 component Quadratic 1 component Cubic 1 component Crude lipid 2 (CL) Linear 1 component Quadratic 1 component CP  CL 6 Error 22

1.03 * *

Survival FI

FCR

PER

PI

CF

125.79

2.02 * *

0.74 * *

0.12 *

1.27 * *

0.42 4.03 0.21

0.29

0.76 * (

2.27 * (+)

0.73 * (+)

1.90 * * (+) 1.16 * * (

)

0.03 0.89 * * 1.86 * * (

17.11 )

)

0.55 * (+)

3.66 * * (+) 0.46

0.15 * (

)

0.88 * * (+)

3.13 * *

0.26 * *

1.03 * *

33.19 46.65

CP

EE

2.09 20.13

2.88 * * (+)

0.21 * (

5.29 * * (+) 1.93 * * (+) 0.48 * * (

0.04 0.05 0.06

) 0.02

VSI HSI

0.86 * *

0.28

0.07

0.44 * (+)

0.65 0.42

0.13 0.12

0.02 0.03

0.25 0.10

38.13 *

12.25

29.38

8.33 6.15 0.35 0.25

6.91 49.73 141.81 * * 282.17 * * (

3.32 5.63 0.61 * 24.16 12.46 1.82 3.04 0.21 17.49 17.25

GEE

56.38 *

148.55 * * (

) 1.38 * * (+)

1.46

CPE

)

78.23 * (

)

6.79 87.53 * * ) 173.88 * * (+)

1.44

1.18

16.63 12.78

5.03 11.14

F.J. Espino´s et al. / Aquaculture 218 (2003) 479–490

Table 3 ANOVA table showing the mean squares of the main effects and interactions of the treatments (dietary protein and lipid level) on growth and nutritive parameters, body composition and nutrient retention of dentex fingerlings

* P < 0.05. ** P < 0.01.

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Fig. 1. Second-order polynomial fitting of SGR (a) and FCR (b) to dietary protein level (CP).

FCR = 16.653 0.593CP + 0.0059CP2 (Table 5), and optimum protein levels for these indexes were 51.4% and 50.2% CP, respectively. Dietary lipid level (CL) only had a linear effect on SGR and FCR, of the form SGR = 4.842 0.053CL and FCR = 0.872 + 0.062CL, but in this case, the optimum dietary lipid content would be the lower level of the considered interval. Condition factor (CF) and viscerosomatic index (VSI) were affected only by lipid level; fish fed 22% lipid presented the lowest value (Table 6). In relation with body composition, all fish increased dry matter and lipid level, and reduced protein and ash with respect to initial content (Table 6). Only body lipid composition was affected by both dietary protein and lipid; fish fed 55% CP or 12% gave the lowest values. Nutrient efficiency retention was affected by both dietary protein and lipid (Table 6), gross energy efficiency (GEE) was

Table 4 Effect of protein and lipid level on growth and nutritive parameters of common dentex fingerlings Parameters

Protein level (%)

Final weight (g), n = 3 SGRv (% day 1), n = 3 Survival (%), n = 3 DFIw (g 100 g fish 1 day 1), n=3 DPIx (g 100 g fish 1 day 1), n=3 FCRy, n = 3 PERz, n = 3

Lipid level (%)

45 c

50 b

55 a

Significance 12 a,b

17 a

22 a

Significance b

10.3 F 0.6 3.41c F 0.12 84.6b F 2.1 6.12a F 0.27

12.8 F 0.7 14.6 F 0.6 13.7 F 0.7 3.92b F 0.12 4.26a F 0.12 4.09a,b F 0.12 83.9b F 2.1 89.5a,b F 2.1 91.7a F 2.3 6.03a F 0.27 5.07b F 0.27 5.92a F 0.28

** ** * **

14.1 F 0.6 13.1 F 0.6 11.2 F 0.7 ** 4.16a F 0.12 3.96a F 0.12 3.60b F 0.12 ** 88.4 F 2.0 88.0 F 2.0 85.3 F 2.1 n.s. 5.44b F 0.23 5.56b F 0.23 6.40a F 0.24

2.42a F 0.11

2.71a F 0.11

3.34b F 0.11

**

2.59a F 0.09

3.08b F 0.10

**

1.55b F 0.14 1.87a,b F 0.15 1.30a F 0.07 1.03b F 0.08

** **

1.70b F 0.12 1.83b F 0.12 2.28a F 0.13 1.26a F 0.06 1.19a F 0.06 0.97b F 0.06

** **

2.23a F 0.14 2.06a F 0.15 a,b 1.12 F 0.07 1.12a,b F 0.08

2.55a F 0.11

2.60a F 0.09

Mean F S.E., means followed by the same superscripts do not differ at P < 0.05 (Newman – Keuls test; covariate: initial weight or initial weight). v Specific growth rate (% day) 1, SGR = 100  ln (final weight/initial weight)/days. w Daily feed intake (g 100 g fish 1 day 1), DFI = 100  feed offered (g)/average total weight (g)  days. x Daily protein intake (g 100 g fish 1 day 1), DPI = 100  protein offered (g)/average total weight (g)  days. y Feed conversion ratio, FCR = feed offered (g)/weight gain (g). z Protein efficiency ratio, PER = weight gain (g)/protein offered (g). * < 0.05. ** < 0.01.

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485

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Table 5 Second-order polynomial fitting of SGR and FCR to dietary protein and lipid level Variable X: crude protein (%) Variable Y: SGR (% day 1) Y = 12.067 + 0.637X 0.0062X2 Significance < * * SGR maximum = 4.29% day 1 at CP = 51.37%

Variable X: crude lipid (%) Variable Y: SGR (% day 1) Y = 4.842 0.053X Significance < * *

Variable X: crude protein (%) Variable Y: FCR Y = 16.653 0.593X + 0.0059X2 Significance < * * FCR minimum = 1.75 at CP = 50.25%

Variable X: crude lipid (%) Variable Y: FCR Y = 0.872 + 0.062X Significance < * *

* < 0.05. ** < 0.01.

lower for fish fed 40% CP or 22% lipid and crude protein efficiency (CPE) was lower for fish fed 55% CP or 22% lipid.

4. Discussion Growth of dentex fingerlings in this trial was very high (SGR ranged from 3.41% to 4.26% day 1) and similar to results cited by Efthimiou et al. (1994) with dentex (2.4 g) fed a dry diet, but lower than fish fed a moist diet (SGR 6.1% day 1). SGR values were higher than obtained by Cardenete et al. (1997a,b) and Company et al. (1999), around 3% day 1, although these authors worked with bigger fish (8– 10 g initial live weight) and some of them with lower temperatures (20 jC). Other authors (Tibaldi et al., 1996; Cardenete et al., 1997c) reported lower SGR values, ranging from 0.8% 1.3% day 1, but initial live weight of dentex was somewhat higher (20 – 30 g) and consequently the SGR had to be lower. Likewise, SGR values were higher than cited by Sabault and Luquet (1973) and Vergara et al. (1996) for gilthead seabream (Sparus aurata) fingerlings, around 1.06– 1.62% and 1.41 – 2.40% day 1, respectively, and reported by Perez et al. (1997) for seabass (Dicentrarchus labrax), 1.62– 2.05% day 1, but similar to those obtained by Takeuchi et al. (1991) for red seabream (Pagrus major), ranging from 3.30% to 4.12% day 1. Mortalities of fish obtained in this experiment (around 15%) were lower than cited by Efthimiou et al. (1994) in fish of similar size (ranging from 27% to 40%), which was mainly due to biting attacks on the smallest fish, but mortality was higher than reported by Tibaldi et al. (1996), Cardenete et al. (1997a,b,c) or Company et al. (1999) (ranged from 2% to 13%) in bigger fish of 10 –20 g. With respect to feed conversion ratio, values in this trial (range 1.5– 2.2) were in the same interval with that observed by Tibaldi et al. (1996), but slightly worse than cited by Cardenete et al. (1997a,b,c) or Company et al. (1999) (around 1.1 –1.3). FCR values obtained in other species were very similar (Sabault and Luquet, 1973; Takeuchi et al., 1991; Vergara et al., 1996; Perez et al., 1997).

Table 6 Effect of protein and lipid level on biometric parameters, body composition and nutrient retention of common dentex fingerlings

CFu, n = 9 VSIv, n = 9 HSIw, n = 9 Dry matter (%), n = 3 CP (% dm), n = 3 CL (% dm), n = 3 Ash (% dm), n = 3 GEEx (%), n = 3 CPEy (%), n = 3

Lipid level (%)

Initial

40

45

50

55

Significance

12

17

22

Significance

– – – 23.78 68.81 9.44 21.39 – –

18.1 F 0.5 7.1 F 0.4 2.1 F 0.1 26.2 F 0.4 61.3 F 1.3 16.3a,b F 1.3 17.6b F 0.6 12.6b F 1.3 19.2a,b F 1.5

17.6 F 0.5 6.9 F 0.4 1.8 F 0.1 26.7 F 0.4 62.0 F 1.4 18.1a F 1.3 17.9b F 0.7 16.0a,b F 1.3 21.5a,b F 1.5

17.8 F 0.5 7.7 F 0.4 2.0 F 0.1 26.5 F 0.4 64.9 F 1.3 14.9a,b F 1.3 20.3a F 0.6 17.5a F 1.3 22.8a F 1.5

17.9 F 0.5 7.5 F 0.4 2.1 F 0.1 25.6 F 0.4 63.7 F 1.3 13.3b F 1.3 20.4a F 0.6 15.4a,b F 1.3 17.5b F 1.6

n.s. n.s. n.s. n.s. n.s. * ** * **

18.1a,b F 0.4 7.4a,b F 0.3 2.0 F 0.1 25.8b F 0.3 64.4 F 1.2 12.7b F 1.1 19.9 F 0.6 18.0a F 1.0 23.5a F 1.2

18.4a F 0.4 7.8a F 0.3 2.1 F 0.1 25.9a,b F 0.3 62.5 F 1.1 18.2a F 1.1 18.1 F 0.6 15.3a,b F 1.0 20.5a F 1.2

17.1b F 0.4 6.7b F 0.3 1.9 F 0.1 26.9a F 0.3 62.2 F 1.1 15.6a,b F 1.1 19.4 F 0.6 12.6b F 1.1 16.8b F 1.3

** ** n.s. * n.s. ** n.s. ** **

Mean F S.E., means followed by the same superscripts do not differ at P < 0.05 (Newman – Keuls test). u Condition factor, CF = 100  total weight (g)/total length3 (cm). v Viscerosomatic index (%), VSI = 100  viscera weight (g)/empty fish weight (g). w Hepatosomatic index (%), HSI = 100  liver weight (g)/empty fish weight (g). x Gross energy efficiency, GEE=(fish energy gain, kJ)  100/(energy intake, kJ). y Crude protein efficiency, CPE=(fish protein gain, g)  100/(protein intake, g). * < 0.05. ** < 0.01.

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Protein level (%)

487

488

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The optimum growth results were obtained with 50% dietary protein level, in spite of lipid level (Fig. 1), because 40% and 45% CP gave poorer growth, and 55% CP originated a similar final live weight and SGR than 50% CP. These results were in agreement with Tibaldi et al. (1996) who obtained the best growth results with 55% CP, although without differences with respect to a diet containing 50%, and the worst with 45% CP. Likewise, Cardenete et al. (1997a) did not report growth differences with dietary protein levels higher than 50%, although these authors did not test any protein content of less than 50%. On the contrary, Company et al. (1999) cited a similar growth of fish fed two diets containing 46% or 55% protein, and Jover et al. (1998) did not obtain growth differences in dentex, growing from 1.5 to 93 – 98 g, fed extruded diets containing 45% or 50% CP. In relation with the dietary lipid level, results in this experiment did not show differences between 12% and 17% EE, but growth was poor with 22% EE, which disagrees with Cardenete et al. (1997a,c) because these authors obtained similar results with diets containing 17% or 20 –22% EE, although diet 54/22 gave a slight smaller final live weight. Tibaldi et al. (1996) only tried two lipid levels, 12% and 17%, and results differed in line with protein content; thus, 17% EE gave a better growth than 12% in diets with 45% and 50% CP, but both dietary lipid levels gave similar growth with 55% protein. On the contrary, Jover et al. (1998) cited a higher growth with 14% EE than 17% EE, although these differences appeared when water temperature was under 20 jC and fish growth reduced slightly. In other species, Vergara et al. (1996) obtained the best growth results in gilthead seabream with 55% protein, although this higher protein requirement could be explained because the initial fish size was smaller (0.8 g), whereas Sabault and Luquet (1973) reported a similar growth of fingerlings (2.3 –3.0 g initial weight) fed 38% and 53% protein, although fish fed 61% protein had a higher final weight. The optimum protein level for red seabream (1.6 g) was 52% CP (Takeuchi et al., 1991) and for seabass (2.8 g) was 45% CP and 12 –14% lipid (Perez et al., 1997). Although comparison between several trials is difficult because the experimental conditions and fish origin are generally different, a diet containing 50% protein and 17% lipid gave an optimum growth in the present trial, and in the experiments carried out by Tibaldi et al. (1996) and Cardenete et al. (1997a). Regarding the nutritive parameters, the best results of feed intake, food conversion ratio (FCR), PER and energy and protein retention (GEE and CPE) were also obtained with 50% protein and 12% or 17% lipid. It has already been commented that FCR obtained in the present trial was similar to that cited by Tibaldi et al. (1996) and worse than those cited by Cardenete et al (1997a,b,c) and Company et al. (1999), indicating that protein retention was similar to that of Tibaldi et al. (1996) and lower than that of Company et al. (1999). The explanation would be that feed intake was higher in the present trial, and although all fish fed to apparent satiety, the smallest dentex in the current experiment were fed small crumbles (0.5 mm), which could give rise to a higher quantity of uneaten feed. The protein-sparing effect of lipids observed by Tibaldi et al. (1996) and Company et al. (1999) with diets containing 17% EE with respect to 12% or 9% EE, respectively, was also ratified here. Nevertheless, the protein-sparing effect of lipid with a 20% EE reported by Cardenete et al. (1997c) is partially at odds with the present results because all diets containing 22% EE gave the worst growth and food conversion rate results. A protein-

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sparing effect of carbohydrate was also noted in the present trial; diet 45/12 gave as good an SGR as 50/12, 50/17, 55/12 and 55/17 (Fig. 1), which have carbohydrate levels of 28.5%, 22.0%, 17.3%, 16.6% and 11.8% NFE, respectively. This result improves upon the optimum carbohydrate level cited by Cardenete et al. (1997b) who obtained a good growth with 22% dextrin, although they did not test any high level of carbohydrate. The different results cited by several authors about optimum protein and lipid levels could be explained in terms of dietary energy levels and protein/energy ratio. As for optimum energy content, in the present trial, the best results were obtained with diets containing between 20.5 and 22.0 MJ kg 1 GE (16.8 – 18.6 MJ kg 1 DE), but an evident effect of protein/energy ratio was observed because only diets with a CP/GE higher than 22 g MJ 1 (45/12, 50/12, 50/17, 55/12 and 55/17) gave the best results of growth and nutritive efficiency (Fig. 1), whereas diets with CP/GE ratio lower than 21 g MJ 1 (40/12, 40/17, 40/22, 45/17, 45/22) presented a worse growth. Nevertheless, some diets with an optimum CP/GE ratio (50/22 and 55/22) also demonstrated poorer results, although their dietary energy content was higher than optimum interval. The best growth results were obtained by Tibaldi et al. (1996) with diets containing a gross energy content from 21.9 to 22.5 MJ kg 1 and a CP/GE ratio higher than 23.1 g MJ 1, whereas the good results reported by Cardenete et al. (1997a,b) with all experimental diets could be due to high CP/ GE ratio (>23 g MJ 1) and energy content (20.2 – 22.9 MJ kg 1). Although optimum dietary protein level is 50 – 51% CP, it would be possible to reduce this protein level to 45% CP by maintaining an adequate CP/GE ratio, as demonstrated by Company et al. (1999) with diet 46/17, and in the present experiment with diet 45/12, although Tibaldi et al. (1996) disagree because diet 44/12 or 44/17 gave lower results. Thus, diets with a minimum crude protein of around 50% could be adequate in order to ensure an optimum growth in dentex fingerlings. Moreover, the higher economic cost of feeding fish a diet containing 50% protein compared with 45% CP would be irrelevant because the feed intake during the fingerling period is very small, compared to growing phase.

Acknowledgements This work was supported by funds from JACUMAR (Ministry of Agriculture, Fishery and Food, Spain). The authors wish to thank the collaboration of Aquaculture Station of the Govern Balear. English text version revised by N. Macowan, BA (nmacowan@yahoo. co.uk).

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