Effects of the dietary protein : lipid ratio on growth and nutrient utilization in gilthead seabream (Sparus aurata L.)

E�ects of the dietary protein : lipid ratio on growth andnutrient utilization in gilthead seabream (Sparus aurata L.)

P.J.M. SANTINHA1, F. MEDALE2, G. CORRAZE1 & E.F.S. GOMES11Instituto Ciencias BiomeÂdicas `Abel Salazar', Universidade do Porto, Porto, Portugal2Unite mixte INRA-IFREMER de Nutrition des Poissons, Station d'Hydrobiologie INRA, Saint PeÂe-sur-Nivelle, France

Abstract

Gilthead seabream Sparus aurata L. (initial mean body

weight: 42.5 g) were fed four experimental diets containing

either 47 or 51% of dry matter (DM) as protein and either 15

or 21% as lipid for 12 weeks. Each diet was hand-distributed

to triplicate groups of 60 ®sh, three times a day until

satiation. The digestibility coe�cients of the dietary compo-

nents were determined using chromic oxide as a marker.

The levels of protein or lipid in the diets did not a�ect the

digestibility. Fish regulated their feed intake and attained the

same weight at the end of the experiment. However, feed

e�ciency varied between diets, with best values obtained with

both diets containing 21% lipid. When diets contained only

15% lipid, feed e�ciency increased with dietary protein level.

Nitrogen retention was signi®cantly higher with high fat diets

regardless of dietary protein level. Neutral lipid deposition

was signi®cantly higher in liver for diets rich in lipids. It was

elevated in muscle only in ®sh fed the diet containing 47%

protein and 21% lipid and this deposition in muscle

contributed to a signi®cant increase in body fat content.

Phosphorus load to the environment, measured as percentage

retention of ingested or digestible phosphorus, was signi®-

cantly lower with both diets higher in lipids.

KEYKEY WORDSWORDS: dietary protein/lipid, feed e�ciency, gilthead

seabream, phosphorus, retention, Sparus aurata L.

Received 4 February 1997, accepted 14 May 19981

Correspondence: E.F.S. Gomes, Instituto Ciencias BiomeÂdicas `Abel

Salazar', Universidade do Porto, Porto, Portugal.

Introduction

In many ®sh species, protein retention may be improved by

partly replacing dietary protein by lipids. Such protein-

sparing e�ects have been demonstrated in salmon (Garcia

et al. 1981; Johnsen et al. 1991), trout (Beamish & Medland

1986), carp (Watanabe et al. 1987), hybrid striped bass

(Nematipour et al. 1992), yellowtail (Shimeno et al. 1980),

red sea bream (Takeuchi et al. 1991) and more recently in

gilthead sea bream Sparus aurata L.3 (Vergara et al. 1996).

The data clearly suggest that nitrogen excretion, resulting

from protein catabolism for energy, can be reduced by

increasing the nonprotein energy content of the diet; this

means decreasing the digestible protein to digestible energy

ratio. However, despite the environmental interest, such

high-energy diets generally lead to greater fat deposition

(Watanabe 1982), which can reduce the commercial value of

the product when it occurs in viscera. The level of dietary

lipid that is acceptable in terms of obtaining both a reduction

of nitrogen load into the environment and a product of

desirable quality seems to vary with ®sh species. It depends

on their relative ability to use dietary lipid as an energy

source and on the main sites of lipid storage.

Despite developments in gilthead seabream production,

mainly in the Mediterranean area, few data on nutrition of

this species are available. The protein requirement for growth

of gilthead seabream was found to be around 40% of the dry

matter by Sabaut & Luquet (1973). However, this study was

conducted with semisynthetic diets and the growth response

was rather small compared with that obtained in nature at

similar temperatures. Santinha et al. (1996) showed that a

minimum of 45% of protein in the diet was necessary to

obtain high growth rates (2.3% per day), together with good

feed e�ciencies.

According to Marais & Kissil (1979), 9% of fat in a diet

containing 44% protein would represent the maximum

amount needed for optimum growth of S. aurata. This level

of dietary lipid corresponds to the 8±10% found for a closely

related species, the red sea bream (Yone et al. 1971).

However, Takeuchi et al. (19914 ) found for the same species

that suitable crude protein and crude lipid levels in the diets

were around 52% and 15%, respectively. Vergara et al.

147

Aquaculture Nutrition 1999 5;147^156. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Ó 1999 Blackwell Science Ltd

(1996) clearly demonstrated that 15% dietary lipids led to

better performance than 9%, but these authors did not

experiment with diets that had a lipid content higher than

15%. The values for dietary lipid content in both species of

seabream are lower than those obtained for salmonids

(Johnsen et al. 1991); as an example, commercial diets for

Atlantic salmon usually contain up to 30% fat.

The objective of the present study was to determine the

e�ects of two levels of lipid (15 and 21% of dry matter) in

association with two levels of dietary protein on growth,

protein utilization and body composition of juvenile gilthead

seabream. The e�ect of the composition of the diet on

phosphorus utilization and excretion was also studied since

decreasing the dietary content of ®shmeal usually results in

decreasing the phosphorus load into the environment.

Materials and methods

Feed intake, digestibility, growth performance and weight-

gain composition were studied, allowing the evaluation of the

e�ciency of the various diets.

Experimental diets

Four experimental diets were formulated to contain either 47

or 51% crude protein combined with either 15 or 21% fat

(Table 1). They were denominated according to the expected

levels of protein and fat. Fish meal was the main protein

source and a soluble concentrate of ®sh protein was also

added. The two di�erent lipid levels were obtained by

changing the amount of cod liver oil by substitution with

wheat meal. For measurement of digestibility 10 g kg)1

chromic oxide was added to each diet as an inert indicator.

The diets were prepared according to the formula in Table 1

using experimental feed manufacture in the University of

TraÂ s-os-Montes e Alto Douro (UTAD5 , Vila Real, Portugal).

After pelleting (maximum temperature 70°C) the diets

were dried (35°C) then stored at 4°C until use. Feed were

distributed in the form of dry pellets of 2±3 mm diameter,

according to ®sh size.

Growth trial

Young gilthead seabream were obtained from a commercial

hatchery. The growth trial and the digestibility measurements

were performed in the experimental facilities of a ®sh farm

located in the South of Portugal (Aquamarim, OlhaÄ o,

Algarve).

Fish (mean individual body weight: 42.5(�0.2 g)) were

randomly divided into 12 groups of 60 ®sh which were placed

into 12 similar square ®breglass tanks of an open circuit.

Each tank (real capacity: 600 L) was supplied with ®ltered

sea water (38 g L±1) at an output of about 600 L h±1. Water

was continuously aerated by an air compressor. Dissolved

oxygen content and water temperature were measured in

each tank twice daily, before the morning meal and after the

evening meal. Dissolved oxygen showed an average value of

6.5 mg O2 L±1 during the experimental period. Water tem-

perature slowly increased during the course of the experiment

(4 May±27 July) from 19°C to 26°C. The pH of the water was

around 8.2±8.4 (normal values for the water supplying the

®sh farm). The natural photoperiod was, on an average, 8 h

dark and 16 h light.

Each experimental diet was given to three groups of ®sh

for 3 months. The ®sh were fed 3 meals per day (0930, 1430

and 1830 h), 7 days per week, except for the day before

weighing. Feed was hand-distributed ad libitum at each meal.

intakes were recorded and computed, on a dry matter basis,

every 3 weeks.

Every 3 weeks, ®sh were left unfed for a period of 36 h.

They were then anaesthetized with ethylene glycol monophe-

nyl ether (0.8 mg L±1) and weighed in groups.

Fifteen ®sh were sampled for whole-body composition

analysis at the start of the experiment. At the end of the trial,

six ®sh were drawn from each tank and individually weighed.

Table 1 Formulation and proximate composition of the experimen-

tal diets fed to seabream for 3 months (names indicate % protein-fat

levels)

Diets 47-15 47-21 51-15 51-21

Ingredients (g kg)1)Binder1 20 20 20 20CPSP2 100 110 130 140Fish meal 497 500 550 550Fish oil3 48 105 40 95Mineral premix4 10 10 10 10Vitamin premix5 5 5 5 5Wheat meal 320 250 245 180Proximate compositionAsh (g kg)1DM6) 81 81 88 86Dry matter (g kg)1WW) 923 930 924 914Energy (kJ g)1DM) 223 236 226 239Fat (g kg)1DM) 150 209 151 208Phosphorus (mg g)1DM) 121 121 124 120Protein (g kg)1DM) 469 470 516 515

1 Carboxymethyl cellulose.2 Fish protein concentrate (Soprop ªeche, France).3 Stabilized with ethoxyquin (Soprop ªeche, France).4 Minerals inmg kg)1diet: Co 0.4; Cu 5.0; Fe 40; F1.0; I 0.6;Mg100;Mn10.5 V|tamins inmg kg)1diet: E 20; K3 5; B15; B2 5; B310; B5100; B6 5; B9 2; B120.05; H 0.5; ascorbic acid26 200; p-aminobenzoic acid 50; inositol 500;choline chloride 500. In UI kg)1diet: A10 000; D3 2000.6 DM: dry matter.

P. J. M. Santinha et al.

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Ó 1999 Blackwell Science Ltd Aquaculture Nutrition 5; 147^156

148

The liver, the whole digestive tract and a part of the white

muscle (one ventral and one dorsal ®llet) were quickly excised,

weighed and frozen in liquid nitrogen. Five other ®sh were

sampled from each tank for whole body composition analysis.

All of the samples were maintained at ±18°C until analysis.

Feed e�ciency (FE), speci®c growth rate (SGR) and

retention e�ciency (RE) were calculated using the following

formulae:

Feed efficiency (FE) � wet body weight gain

(g)/dry feed intake (g)

Specific growth rate (SGR) � 100�ln FBWÿ ln IBW)/T

where FBW is the ®nal mean body weight, IBW the initial

mean body weight and T the number of days between each

two weighing.

Retention efficiency (RE) � gain in fish (g or kJ)/digestible

intake (g or kJ)

Nitrogen load into the environment was calculated as the

di�erence between the nitrogen consumption and the nitro-

gen retention.

Digestibility measurement

For the digestibility trials, eight groups of 15 ®sh each

(individual mean body weight: 60 g) were placed in 50-L

tanks, similar to those described by Cho et al. (1982). The

tanks were supplied with ®ltered sea water at a rate of

90 L h±1 in an open circuit system. Fish were fed twice daily

(0900 and 1600 h) at a level of 2.5% of the initial weight of

the group. Each of the experimental diets containing 1% of

chromic oxide was distributed to two groups of ®sh over a

12-day period. The ®rst 4 days were used as an adaptation

period. Faeces collection was carried out over the last 8 days,

between the evening meal and the morning meal, according

to Cho et al. (1982). The tanks were cleaned out thoroughly

every evening, 15 min after the evening meal, thereby

ensuring that no uneaten food was incorporated into the

faeces. The daily collected fractions of faeces were centri-

fuged and stored at ±18°C until analyses.

Apparent digestibility coe�cients (ADC) were calculated

using the following formula:

ADC of nutrients�%� � 100ÿ �100� �% chromic oxide in diet=% chromic oxide in faeces�� % of nutrient in faeces=% of nutrient in diet��

Analytical methods

The ®sh sampled for composition analysis were ground and

an aliquot was analysed for dry matter content. Faeces and

whole-body samples were then lyophilized before the ana-

lytical procedures. Proximate analyses of the diet, whole

body and faeces were made following the usual procedures:

dry matter after drying in an oven at 104 � 1°C for 24 h, ash

by combustion in a mu�e furnace (about 600°C) for 15 h,

protein (N ´ 6.25) by the Kjeldahl method after acid

digestion, energy in an adiabatic bomb calorimeter (Parr6 ),

fat content by petroleum ether 40±60°C extraction (Soxhlet).

Total lipids in the samples of tissues were extracted according

to Folch et al. (1957). Neutral and polar fractions were

separated on small silica columns (Juaneda & Roquelin

19857 ). Chromium in the diets and faeces was measured after

acidic digestion according to Furukawa & Tsukahara (1966).

Total phosphorus was measured by spectrophotometric

determination at 430 nm after digestion using perchloric

acid and nitric acid (Organisation Internationale de Normal-

ization 1980).

Statistical analyses were made following methods outlined

by Snedecor & Cochran (1956) using STATISTICSSTATISTICS 4.0 for

Windows. Means were compared after analysis of variance

and signi®cance of di�erences between dietary treatments

(P < 0.05) were assessed by Tukey's test.

Results

Digestibility

No signi®cant di�erences were found for protein or lipid

digestibility coe�cients (Table 2). The values for both

components were high (between 92 and 95%). The digest-

ibility of energy was slightly lower for diet 47-15 than for the

three other diets but without any signi®cant di�erence. From

Table 2 Apparent digestibility coe�cients (%) of the dietary

components and composition of the experimental diets as digestible

nutrients or energy for gilthead seabream

Diets 47-15 47-21 51-15 51-21

Digestibility (%)Dry matter 79.8b 83.4a 83.7a 78.9b

Energy 86.9 89.8 89.6 88.5ns

Fat 92.4 93.6 94.6 94.7ns

Phosphorus 70.1 73.1 71.5 70.2ns

Protein 93.3 93.8 94.5 92.0ns

Digestible nutrients (%) or energy (kJ g)1DM) in the diets

Digestible energy (DE) 19.4 21.2 20.3 21.2ns

Digestible lipid 13.9b 19.6a 14.3b 19.7a

Digestible protein (DP) 43.8b 44.1b 48.8a 47.4a

DP/DE (mg kJ)1) 22.6b 20.8c 24.1a 22.5b

Numbers (means of duplicate groups) with different superscripts in thesame line are significantly different from each other (P < 0.05); ns: no

signi®cant differences.

Protein to lipid ratio in seabream diet

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Ó 1999 Blackwell Science Ltd Aquaculture Nutrition 5;147^156

149

Table 2 it can be seen that dry matter digestibility varied

signi®cantly among the four diets, with diets 47-21 and 51-15

having the highest values.

The composition of the experimental diets in terms of

digestible nutrients or energy is also presented in Table 2.

Digestible energy in the four diets did not show any

signi®cant di�erence. Consequently, the balance between

digestible protein and digestible energy was lowest for diet

47-21 (20.8 mg kJ±1), highest for diet 51-15 (24.1 mg kJ±1)

and similar for the two other diets (22.6 mg kJ±1).

Intake and growth performance

Voluntary feed intake of seabream changed signi®cantly with

the composition of the diet (Table 3). Fish fed diet 47-15

consumed the largest amount of food (daily average of 22.7 g

dry matter kg)1 of mean body weight). The increase in

dietary lipid content led to a signi®cant decrease in feed

intake at both protein levels. However, this di�erence in feed

intake was more pronounced in ®sh fed diets containing 47%

crude protein than in ®sh fed 51% crude protein (di�erence

of 35% and 9%, respectively). The increase in dietary protein

content resulted in a decrease in feed intake only when the

diets contained 15% lipid. Digestible energy intake was, on

average 320 kJ kg)1 body weight day)1 except for ®sh fed

diet 47-15 which daily ingested about 440 kJ kg)1 body

weight day)18 . Despite the variations in feed intake, no

di�erence was found in the speci®c growth rate of the

experimental ®sh. At the end of the growth trial (12 weeks),

®sh of the four groups had a similar weight irrespective of the

dietary treatment (Table 3).

Feed efficiency, energy and nitrogen utilization

Feed e�ciency (Table 3) was the highest for both diets

containing 21% lipid (diets 47-21 and 51-21) regardless of the

protein content. Increase in dietary protein content led to an

improvement in feed e�ciency only when associated with the

`low' lipid level (15%). The amount of digestible energy

required to produce 1 kg weight gain was signi®cantly lowest

with both diets containing 21% lipid. Among the two diets

that had 15% lipid, the highest energy need for growth was

observed with the lowest protein level. Coe�cients of energy

retention e�ciency, as percentage of digestible intake

(Fig. 1), were also signi®cantly (P < 0.05) di�erent, the

highest being with diets containing 21% lipid.

Table 3 Feed intake, growth performance and feed e�ciency of seabream fed to satiation the experimental diets for 12 weeks

Diets 47-15 47-21 51-15 51-21

27Feed intake (g or kJ kg)1 mean body weight day)1)

Digestible energy 439.6 þ 13.8a 309.1 þ 8.3b 337.2 þ 8.3b 320.3 þ 11.6b

Digestible lipid 3.1 þ 0.1a 2.9 þ 0.1b 2.3 þ 0.0c 3.0 þ 0.1ab

Digestible protein 9.9 þ 0.3a 6.4 þ 0.2d 8.1 þ 0.2b 7.2 þ 0.3c

Dry matter 22.7 þ 0.7a 14.6 þ 0.4c 16.7 þ 0.4b 15.2 þ 0.6c

Growth performance

Initial body weight (g) 42.2 þ 0.3 42.6 þ 0.7 42.6 þ 0.7 42.4 þ 1.5 ns

Final body weight (g) 143.5 þ 5.6 140.3 þ 7.7 143.6 þ 1.6 146.3 þ 4.2 ns

Specific growth rate (% day)1) 1.65 þ 0.1 1.61 þ 0.1 1.67 þ 0.1 1.67 þ 0.0 ns

Feed efficiency 0.64 þ 0.02c 0.79 þ 0.02a 0.71 þ 0.01b 0.79 þ 0.02a

Requirements kg gain)128

Digestible energy (MJ) 30.2 þ 0.9a 26.9 þ 0.6c 28.7 þ 0.5b 26.7 þ 0.8c

Digestible protein (g) 681.9 þ 21.2a 559.8 þ 11.5c 691.5 þ 11.9a 600.0 þ 9.4b

Numbers (means of triplicate groups þ SD) with different superscripts in the same line are significantly different from each other (P < 0.05); ns: no


Figure 1 E�ect of dietary treatment on retention e�ciency of

digestible protein. Numbers with di�erent superscripts are signi®-

cantly di�erent from each other (P < 0.05).


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150

An average of 685 g of digestible protein was required to

produce 1 kg weight gain when diets contained 15% lipid,

irrespective of dietary protein level. Conversely, protein

e�ciency was slightly better with the lowest level of protein

when the diets contained 21% lipid. Nitrogen gain (Table 4)

was highest for ®sh fed diet 47-15 which had signi®cantly

increased their voluntary feed intake, but both soluble and

solid wastes were also the highest when expressed as mg per

kg mean body weight per day (Table 4). The lowest values

for nitrogen load into the environment were obtained with

the diet 47-21. However, the amount of nitrogen recovered as

soluble wastes was signi®cantly (P < 0.05) lower in ®sh fed

diet 51-21 (Fig. 2).9 Consequently, protein retention e�ciency

was signi®cantly (P < 0.05) higher with both high fat diets,

regardless of the dietary protein level (Fig. 3).

Body composition and lipid content of liver,digestive tract andmuscle

The only signi®cant di�erence in whole-body composition

(Table 5) was for fat content, which was higher in ®sh fed

diet 47-21 than in the other three groups. These ®sh had a

body protein content slightly lower than ®sh fed diet 51-21,

so the average energy content of the whole body was similar

for all four groups. A signi®cant increase in lipid content and

a decrease in protein content of the dry matter was observed

compared with the ®sh sampled at the beginning of the

experiment.

No signi®cant di�erences (P > 0.05) were found in

hepatosomatic index (Table 6). However, dry matter, total

lipid and neutral lipids in liver changed signi®cantly with

dietary treatment. The values found for those parameters

were signi®cant higher with both diets containing 21% lipid

without any e�ect of the dietary protein level10 . Polar lipid did

not change signi®cantly throughout the experiment.

With respect to the digestive tract, no signi®cant di�eren-

ces were found in viscera-somatic index, lipid content and

lipid composition.

The muscle of seabream represented an average of 48% of

body weight. Total lipid in muscle was signi®cantly higher

only in animals fed diet 47-21 compared with those fed the

diets containing 15% lipid. This increase in muscle lipid

Table 4 Nitrogen balance estimated from carcass analysis and digestibility coe�cients

Diets 47-15 47-21 51-15 51-21

Nitrogen (mg kg)1mean body weight29 day)1)N intake 1701.3 þ 53.1a 1097.1 þ 29.2d 1377.6 þ 32.1b 1251.7 þ 47.6c

Digestible N intake 1587.2 þ 49.8a 1029.3 þ 27.5d 1302.4 þ 30.5b 1152.0 þ 34.4c

N gain 353.6 þ 25.1a 290.8 þ 13.6b 302.4 þ 17.5b 314.8 þ 5.7b

Solid wastes 114.1 þ 3.3a 67.7 þ 1.9d 75.2 þ 1.6b 99.7 þ 4.3c

Soluble wastes 1233.6 þ 42.1a 738.6 þ 15.4d 1000.0 þ 38.9b 837.2 þ 38.3c

Soluble wastes 77.72 þ 2.65a 71.76 þ 1.49b 76.78 þ 2.99a 72.67 þ 2.32b

(% digestible N intake)

Numbers (means of triplicate groups þ SD) with different superscripts in the same line are significantly different from each other (P < 0.05).

Figure 2 E�ect of dietary treatment on retention e�ciency of

digestible energy. Numbers23 with di�erent superscripts are signi®-

cantly di�erent from each other (P < 0.05).

Figure 3 E�ect of dietary treatment on crude nitrogen intake

excreted as solid and soluble wastes: h solid wastes; soluble

wastes.24 Numbers with di�erent superscripts are signi®cantly di�erent

from each other (P < 0.05).


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content was due to an increase in neutral lipids. Polar lipid

content varied neither in the muscle nor in the liver nor in the

digestive tract, whose lipid composition was una�ected by

dietary treatment.

Phosphorus utilization

Whole body of the gilthead seabream contained 27 mg P g±1

dry matter at the start of the experiment and an average of

23 � 1 mg g±1 at the end of the growth trial, without no

signi®cant di�erence between groups. For phosphorus avail-

ability, the values obtained were in the range 70±73% and

slightly higher for diet 47-21, but the di�erence in ADC

between experimental diets was not signi®cant (Table 2).

The daily intakes of total and digestible phosphorus

(Table 7) were signi®cantly higher for ®sh fed the low fat

diets regardless of the dietary protein level. Solid wastes as

percentage of phosphorus intake were signi®cantly higher

with diets 47-15 and 51-21, the lowest being with diet 47-21.

Soluble wastes were higher with low-fat diets (Fig. 4),

although no signi®cant di�erences (P > 0.05) were found

between diet 47-15 and 47-21. Retention as percentage of

crude phosphorus intake was signi®cantly better with the

high fat diets, the lowest value was obtained with the diet

47-15 (Table 7).

Discussion

The results from the digestibility trial (Table 2) showed high

coe�cients of apparent digestibility (ADC) for protein, fat

and energy, indicating the high quality of the dietary

ingredients. The values are in the same range as the ADC

Table 5 E�ect of dietary treatment on body composition of seabream

Diets Initial 47-15 47-21 51-15 51-21

Ash (g kg)1 DM) 14.1 þ 1.0a 9.9 þ 0.5b 9.0 þ 0.6b 9.5 þ 0.2b 9.7 þ 0.4b

Dry matter (g kg)1) 327 þ 2 325 þ 10 340 þ 6 333 þ 10 334 þ 3Energy (kJ g)1 DM) 23.6 þ 0.4b 26.2 þ 0.4a 26.2 þ 0.3a 25.5 þ 0.1a 25.9 þ 0.2a

Fat (g kg)1 DM) 349 þ 10c 388 þ 17b 440 þ 4a 396 þ 25b 394 þ 3b

Phosphorus (g kg)1 DM) 27 þ 2a 21 þ 1b 23 þ 1b 23 þ 1b 23 þ 1b

Protein1 (g kg)1 DM) 526 þ 4a 486 þ 5bc 477 þ 5c 496 þ 14bc 499 þ 5b

Values are means of triplicate groups þ SD. Numbers with different superscripts in the same line are significantly different from each other (P < 0.05).1 Protein = N ´ 6.25.

Table 6 Relative size and lipid content (mg g)1 wet weight) of liver, digestive tract and muscle of seabream fed the experimental diets to

satiation for 3 months

Diets 47-15 47-21 51-15 51-21

Liver

HSI1 (%) 1.3 þ 0.3 1.2 þ 0.1 1.1 þ 0.1 1.1 þ 0.1ns

Dry matter (g kg)1) 320 þ 13b 347 þ 14a 324 þ 12b 344 þ 14a

Neutral lipid 85.1 þ 9.2b 130.1 þ 22.2a 94.7 þ 19.1b 131.9 þ 20.3a

Polar lipid 24.7 þ 6.3 28.1 þ 2.5 27.2 þ 4.5 23.5 þ 3.2ns

Total lipid 109.8 þ 8.0b 158.3 þ 21.9a 122.0 þ 18.5b 155.4 þ 19.0a

Digestive tract

VSI2 (%) 3.9 þ 0.7 4.5 þ 1.1 3.9 þ 0.7 4.1 þ 0.6ns

Dry matter (g kg)1) 484 þ 79 519 þ 55 531 þ 62 548 þ 60ns

Neutral lipid 329.5 þ 77.9 353.6 þ 48.0 374.6 þ 65.2 389.4 þ 64.4ns


Total lipid 336.5 þ 78.3 365.5 þ 47.2 387.3 þ 65.1 401.3 þ 63.8ns

Muscle

Dry matter (g kg)1) 281 þ 7 281 þ 5 282 þ 5 277 þ 13ns

Neutral lipid 66.3 þ 7.8b 80.2 þ 8.8a 68.6 þ 6.3b 70.2 þ 9.3b


Total lipid 75.5 þ 8.4b 89.3 þ 10.3a 78.1 þ 6.7b 79.7 þ 9.0ab

Values aremeans of18 values per dietary treatment for relative size of the tissues andmeans of six values for drymatter and lipid content. Numbers withdifferent superscripts in the same line are significantly different from each other (P < 0.05); ns: no signi®cant differences.1HSI = (liver weight/body weight) ´ 100.2 VSI = (weight of the whole digestive tract/body weight) ´ 100.


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152

coe�cients obtained in rainbow trout fed diets containing

protein sources of high quality (Cho & Kaushik 1990; Gomes

et al. 1993). This con®rms the ability of the gilthead seabream

to digest protein and fat (Santinha et al. 1996). No signi®cant

di�erences in protein or lipid ADC were found among the

experimental diets. This means that an increase in dietary

protein or dietary lipid does not a�ect digestibility, as has been

already shown for other ®sh species (Cho & Kaushik 1990).

Although no di�erences in ADCof protein, lipid and energy

were observed between the four diets, the digestibility of dry

matter was lower for diets 47-15 and 51-21 than for the other

two. The lower dry matter ADC in diet 47-15 could be

explained by the greater amount of wheat included in this diet,

since many ®sh species do not digest this component as

e�ciently as lipids or proteins (Shimeno et al. 1979). The lower

value of dry matter ADC in the diet 51-21 cannot be explained

in the same way. In any case, values of dry matter digestibility

obtained in the present study were slightly higher than those

reported by Santinha et al. (1996) for the same species.

At the end of the 3-month experimental period, ®sh

attained the same weight irrespective of dietary treatment.

This probably results from feeding to satiation. When ®sh are

fed to satiation, growth might not depend on diet compo-

sition but on the ability of ®sh to regulate food consumption.

According to this data, gilthead seabream appear to be able

to adjust voluntary feed intake in order to meet needs for

maximal growth, the greatest increase in feed intake being

observed for ®sh given the diet with the lowest levels of

protein and lipid. Voluntary feed intake of these animals was

not regulated by the amount of digestible energy or digestible

protein supplied by the diet, while feed intake of ®sh fed the

three other experimental diets appears to be based on

digestible energy. The amount of digestible energy required

to produce 1 kg gain was signi®cantly higher with diet 47-15

than with the other experimental diets. However, an e�ect of

the level of dietary carbohydrates (which is high in the diet

47-15) on appetite cannot be excluded and requires further

investigation.

Speci®c growth rate, although similar with all four diets,

was lower than the 2.3% per day obtained by Santinha et al.

(1996). This is probably due to higher initial weight of the ®sh

in the present experiment (42 g vs. 20 g in the previous

experiment) since bigger ®sh usually grow more slowly.

The lowest feed e�ciency was noted with diet 47-15 and

was exactly the same as that obtained by Vergara et al.

(1996) with a similar diet (46-15). This value is higher than

those reported by PeÂ rez-SaÂ nchez et al. (1995) who obtained

the best value of feed e�ciency (0.54) with a diet containing

55% of crude protein. Since feed intake varied according to

the dietary treatments and allowed the ®sh to have the same

growth performance, feed e�ciency was consequently di�er-

ent between the four experimental diets. It was signi®cantly

better with the highest lipid level. Pereira et al. (1987) also

reported a bene®cial e�ect of an increased dietary lipid

Table 7 Phosphorus balance estimated from carcass analysis and digestibility coe�cients

Diets 47-15 47-21 51-15 51-21

Phosphorus (mg kg)1mean body weight30 day)1)P intake 274.9 þ 8.6a 176.0 þ 4.7c 206.0 þ 4.9b 181.8 þ 6.6c

Digestible P intake 192.7 þ 6.0a 128.6 þ 3.5c 147.3 þ 3.5b 127.6 þ 4.6c

P gain 90.1 þ 2.3 78.5 þ 9.4 83.9 þ 6.9 86.5 þ 1.6ns

Solid wastes 82.2 þ 2.6a 47.3 þ 1.3d 58.7 þ 1.4b 54.2 þ 2.0c

Soluble wastes 102.6 þ 4.0a 50.1 þ 6.0c 63.4 þ 4.9b 41.2 þ 3.5d

Retention (% P intake) 16.4 þ 0.4c 23.2 þ 1.4a 20.3 þ 1.4b 23.8 þ 0.4a

Retention (% digestible P) 23.4 þ 0.6c 31.7 þ 2.0ab 28.5 þ 2.0b 33.9 þ 0.5a

Numbers (means of triplicate groups þ SD) with different superscripts in the same line are significantly different from each other (P < 0.05); ns: no


Figure 4 E�ect of dietary treatment on crude phosphorus intake lost

as solid and soluble wastes: h solid wastes; soluble wastes25 .

Numbers with di�erent superscripts are signi®cantly di�erent from

each other (P < 0.05).


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153

content on growth of gilthead seabream and feed utilization

as in sea bass (Metailler et al. 1981) yellowtail (Shimeno

et al. 1980) and carp (Watanabe et al. 1987). When diets

contained 15% lipid, increasing protein level from 47 to 51%

led to an improvement in feed e�ciency. In contrast, there

was no e�ect of protein level in 21% lipid diets.

The amount of digestible energy required to produce 1 kg

®sh was a�ected in the same manner by the dietary

treatments. The values are high compared with current

recommendations (National Research Council 199311 ) and the

values obtained for trout (Kim & Kaushik 1992). Conse-

quently, e�ciency of energy retention was quite low (<40%

of digestible energy intake) but signi®cantly improved by the

increase in dietary lipid from 15 to 21% of the dry matter.

Higher dietary lipid content also led to a higher protein

retention e�ciency which was increased by 26 and 18% with

diets containing 47 and 51% crude protein, respectively.

Protein needs for growth were also reduced by the increase in

dietary lipid. From an experiment on gilthead seabream fed

diets with graded levels of crude protein (42±58%) Vergara

et al. (1996) concluded that dietary protein level could be

decreased from 58 to 46% when increasing the lipid content

of dry matter from 9 to 15%. Despite this protein-sparing

e�ect, these authors did not ®nd any signi®cant e�ects of

dietary lipid level on the protein e�ciency ratio (PER).

Conversely, they observed a decrease in PER with the

increase in protein in the diet. In the present study, such an

e�ect was found only with diets containing 21% lipid. This

di�erence could be explained by the variation in feeding

regime, ®sh having been fed to satiation in the present study

and a restricted ration of 3% of body weight in the

experiment of Vergara et al. (1996).

Nitrogen load to the environment was obviously elevated

with diet 47-15. This resulted from both a high feed intake

and an increase in catabolism of dietary protein. As in other

species, the lower the nitrogen intake the lower the soluble

nitrogen wastes. However, when expressed as percentage of

digestible nitrogen intake, soluble nitrogen wastes were also

signi®cantly reduced in ®sh fed diets containing 21% lipid.

This corresponds to the protein-sparing e�ect induced by the

increase in dietary lipids in gilthead seabream. Reduced

protein utilization for energy production decreases the

deamination of amino acids and hence the excretion of

ammonia through the gills. However, levels of soluble

nitrogen wastes do not appear to be related to ratios of

digestible protein and digestible energy.

Dietary treatments, which had little e�ect on body protein

content, modi®ed the body fat content of sea bream. Both

diets containing 21% lipid led to a signi®cant increase in liver

fat. Lipid deposition in muscle increased signi®cantly only

when the high level of lipid was associated with 47% of

protein in the diet, which led to high body fat content.

Similar results have been reported for another marine ®sh,

the juvenile dentex (Tibaldi et al. 1996), and for carp

(Takeuchi et al. 1989). In ®sh fed diet 47-15, the increase in

body lipid content results from lipid deposition in muscle and

liver, which contained up to 9 and 16% lipid, respectively.

Muscular mass, representing about 48% of the body mass,

plays the most important contribution to higher body fat

content. In liver as well as in muscle the increase in lipid

content resulted from a greater deposition of neutral lipids

without any signi®cant variation in polar lipids.

Gilthead seabream stores between 11 and 15.8% of wet

weight of liver as fat, which is in the same range as the values

reported by Morris & Davies (1995) for the same species and

by Furuichi & Yone (1980) for the red seabream. Liver lipid

content was higher than that commonly found in freshwater

®sh, especially rainbow trout (Henderson & Tocher 1987).

The same observation has been reported for sea bass (Roche

et al. 1984; McClelland et al. 1995). The liver seems to be a

more important site of lipid storage in seawater ®sh than in

freshwater. Moreover, hepatic lipids are highly sensitive to

dietary lipid level. In contrast, there was no e�ect of dietary

lipid level on perivisceral fat, which is also quite di�erent

from salmonids. Indeed, numerous studies have shown a

great increase in visceral lipids in rainbow trout fed high lipid

diets (Watanabe 1982; Shearer 199412 ).

The ADC of phosphorus appears to be high with no

di�erences among diets. The values obtained are higher than

those observed in rainbow trout fed commercial diets with

graded levels of phosphorus (Robert et al. 1991). The

availability of phosphorus to ®sh is a�ected by two major

factors: the chemical form and the digestibility of ingredients

(Kaushik 1992). The bioavailability of phosphorus from

animal byproducts (®sh meal, meat meal) is generally higher

than from plant products. Recent ®sh diets made with high-

quality ®sh meals have been shown to decrease total

phosphorus loss per unit production (Kaushik 1992).

A lower dietary supply increases digestibility and improves

bioavailability in rainbow trout (Robert et al. 1991). These

two factors working together could explain the high digest-

ibility values obtained in the present experiment.

Phosphorus wastes clearly decrease from diet 47-15

compared to the others.13 This fact can be attributed to the

increment of ®shmeal and ®sh protein concentrate (CPSP)

with the consequent decrease of wheat content of the diets.

Under practical farming conditions, particulate forms of

phosphorus discharge are relatively higher (>60%) than of


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154

soluble forms (<40%) (Kaushik 1992). However, the results

obtained in this experiment shows an inverse relation with

higher soluble wastes, except for diet 51-21 which followed

the normal pattern. Further studies are needed in this ®eld

because of the importance phosphorus excretion in the

quality of aquatic environment.

Acknowledgements

This work was partially subsidised by a PhD grant (BD-502/

92) by the Junta Nacional de InvestigacË aÄ o CientõÂ ®ca e

TecnoloÂ gica of Portugal. The authors are greatly indebted to

the ®sh farm Aquamarim Lda., which provided the gilthead

sea bream and the experimental installations necessary to

carry out this work. The scienti®c and technical assistance

provided by P. Noronha (UCTRA, Portugal) and P. Rema

and A. Pinto (UTAD, Portugal) is sincerely appreciated.

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Documents

Effects of the dietary protein : lipid ratio on growth and nutrient utilization in gilthead seabream (Sparus aurata L.)