Almost total replacement of fish meal by plant...

Almost total replacement of fish meal by plant

protein sources in the diet of a marine teleost,

the European seabass, Dicentrarchus labrax

S.J. Kaushika,*, D. Covesb, G. Duttob, D. Blanca

aFish Nutrition Research Laboratory, Unite Mixte INRA-IFREMER, Unite d’Hydrobiolgie,

64310 St-Pee-sur-Nivelle, FrancebMediterranean Finfish Research Laboratory, Station Experimentale d’Aquaculture IFREMER,

34250 Palavas-les-Flots, France

Received 1 April 2003; received in revised form 28 May 2003; accepted 28 May 2003

Abstract

Five practical diets in which the supply of protein from fish meal was decreased gradually from

100% to about 2% and replaced by plant protein sources were formulated. European seabass

weighing about 190 g were fed these diets for 12 weeks at a water temperature of 22 jC. Feed was

dispensed using automatic self-feeders and voluntary feed intake (VFI) was closely monitored. We

did not find any significant difference among diets in the apparent digestibility coefficients (ADC) of

dry matter (80–82%), protein (94–96%), energy (88–92%) or phosphorus (49–58%). Replacement

of fish meal by plant protein ingredients did not influence VFI. All groups had very good growth

rates (DGI above 1.3%/day) and there were no significant differences in growth rate, feed efficiency

or in daily nitrogen gains among groups. There was, however, a slight increase in fat deposition in

fish fed diets with plant protein sources. Ammonia nitrogen and soluble phosphorus excretion rates

were measured. Nitrogen and phosphorus balance studies indicated that fish meal replacement by

plant ingredients led to a slight increase in nitrogen losses (from 83 to 103 g N/kg weight gain) but

led to a significant reduction in total phosphorus losses (from 13 to 5 g P/kg weight gain). These

results combined with the remarkable acceptability of diets containing high levels of plant protein

ingredients with identical growth performances of European seabass show clearly that dietary fish

meal levels can be considerably reduced without any adverse consequence in terms of somatic

growth or nitrogen utilisation.

D 2004 Elsevier B.V. All rights reserved.

Keywords: European sea bass; Fish meal replacement; Plant proteins; Growth; Nutrient utilisation

0044-8486/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/S0044-8486(03)00422-8

* Corresponding author. Tel.: +33-5-59-51-59-51; fax: +33-5-59-54-51-52.

E-mail address: [email protected] (S.J. Kaushik).

www.elsevier.com/locate/aqua-online

Aquaculture 230 (2004) 391–404

1. Introduction

Given the global needs for fish oil and fish meal for aquaculture (FAO, 2002), there is

an increasing demand for more insight on the potential of alternative protein sources in

aquafeeds (New and Wijkstrom, 2002). Under intensive farming conditions, in species

such as the rainbow trout, development of fish meal free diets has met with some success

(Kaushik et al., 1995; Watanabe et al., 1998). When working with marine species, the

latter authors, however, observed a number of adverse effects in yellowtail fed non-fish

meal diets (Watanabe et al., 1999). This interest for the substitution of fish meal by more

sustainable and renewable protein sources applies also for the culture of marine teleosts of

interest to the Mediterranean area (Alexis, 1997; Alexis and Nengas, 2001). Although

studies were initiated as far back as the late 1970s on the replacement of fish meal by other

protein sources in the diets of European seabass (Alliot et al., 1979), only limited

knowledge is available today on the nutritional value and the physiological consequences

of plant protein use in this species (Dias et al., 1997; da Silva and Oliva-Teles, 1998; Dias,

1999). The aim of this study was to evaluate the possibility of replacing a great portion of

fish meal in the diet of European seabass using practical ingredients readily available to the

aquafeed industry and to detect possible effects on carcass quality, body composition,

excretion and plasma metabolites.

2. Materials and methods

2.1. Diets

Five diets were formulated to contain different levels of fish meal incorporation,

ranging from 520 to 50 g kg� 1 (Table 1). They were formulated so as to meet the nutrient

requirements of European seabass (Kaushik, 2001) as well as to be least costly, using the

solver macro of Microsoft’s Excel software. The diets with fish meal replacement above

50% (diets FM25, FM 12 and FM5) were supplemented with L-lysine and dicalcium

phosphate. In the absence of specific data on vitamin, mineral and trace element require-

ments of European seabass, requirement data for other species were applied (NRC, 1993;

Kaushik et al., 1998). All diets contained yttrium oxide as an inert marker for determining

apparent digestibility coefficients (ADC). Extruded diets were manufactured by an

industrial fish feed manufacturer (Le Gouessant, 29 Lamballe, France) using a twin-screw

extruder (Buhler).

2.2. Rearing facilities

The trial was conducted at the experimental facilities of IFREMER (34 Palavas les

Flots, France). The rearing system consisted of 15 individual tanks supplied with sand

filtered (15 Am) and UV-treated seawater (salinity 31–38x) in a flow-through system

(flow rate: 1 m3 h� 1 in each tank). Water temperature was maintained constant (21.8F 0.1

jC) using titanium heat exchangers and oxygenated to maintain oxygen concentrations

above 80% saturation. Each tank was covered and individually equipped with a lamp (75W)

S.J. Kaushik et al. / Aquaculture 230 (2004) 391–404392

about 70 cm above the surface of the water with a photoperiod of 16 h of light (0600 to 2200

h) and 8 h of darkness with twilights of 30 min each at dawn and dusk (Fig. 1). An

electronic demand feeding and monitoring system developed by Boujard et al. (1992) was

attached to each individual tank. Some adaptations were made as described by Coves et al.

(1998) in order to prevent any unintentional triggering of the feeders by fish. This system

allows for distribution of feed per demand as well as to closely monitor demands and

rewards and to store data directly in a computer.

2.3. Digestibility and growth trial

European seabass juveniles (Dicentrarchus labrax), obtained from a commercial farm

(Extramer, France), were acclimated to the experimental conditions before the beginning

of the trial. Fish were sorted and 15 homogenous groups (coefficient of variation of

body weight < 12%) of 50 fish each were randomly allotted to each tank. The five diets

Table 1

Ingredient and chemical composition of the practical diets (g/kg)

FM52 FM40 FM25 FM12 FM5

Fish meal LT 94 (CP, 70%) 520.0 400.0 250.0 125.0 50.0

Corn gluten meal 80.0 205.8 209.6 200.0

Wheat gluten 50.0 166.0 237.8

Extruded wheat 195.4 67.9 19.6

Soybean meal (CP 48%) 94.7 150.0 150.0 140.1 131.5

Rapeseed meal (primor 00) 100.0 100.0 100.0 100.0

L-Lysine 0.65 6.5 10.0

CaHPO4�2H2O (18%P) 6.7 22.5 32.8

Fish oil (Scandinavian) 158.9 171.2 186.3 199.4 206.9

Binder 10.0 10.0 10.0 10.0 10.0

Yttrium oxide 1.0 1.0 1.0 1.0 1.0

Mineral premixa 10.0 10.0 10.0 10.0 10.0

Vitamin premixb 10.0 10.0 10.0 10.0 10.0

Dry matter (DM), g/kg 902.8 896.9 895.5 894.2 889.8

Crude protein, g/kg DM 450.5 483.1 503.7 468.8 506.6

Crude fat, g/kg DM 216.2 217.1 199.6 227.6 183.0

Phosphorus, g/kg DM 11.6 12.5 9.3 7.2 5.7

Gross energy, MJ/kg DM 24.2 24.3 24.1 24.8 24.4

Theoretical cost (o/ton)c 651 610 604 633 649

Both mixtures were manufactured by Unite de preparation des aliments experimentaux (UPAE), INRA, Jouy-

en-Josas, France, according to the recommendations of NRC (1993).a Mineral mixture (g or mg/kg diet): calcium carbonate (40% Ca), 2.15 g; magnesium oxide (60% Mg), 1.24

g; ferric citrate, 0.2 g; potassium iodide (75% I), 0.4 mg; zinc sulphate (36% Zn), 0.4 g; copper sulphate (25%

Cu), 0.3 g; manganese sulphate (33% Mn), 0.3 g; dibasic calcium phosphate (20% Ca, 18%P), 5 g; cobalt

sulphate, 2 mg; sodium selenite (30% Se), 3 mg; KCl, 0.9 g; NaCl, 0.4 g.b Vitamin mixture (IU or mg/kg diet): DL-a tocopherol acetate, 60 IU; sodium menadione bisulphate, 5 mg;

retinyl acetate, 15,000 IU; DL-cholecalciferol, 3000 IU; thiamin, 15 mg; riboflavin, 30 mg; pyridoxine, 15 mg;

B12, 0.05 mg; nicotinic acid, 175 mg; folic acid, 500 mg; inositol, 1000 mg; biotin, 2.5 mg; calcium panthotenate,

50 mg; choline chloride, 2000 mg.c As per ingredient prices in June 2000.

S.J. Kaushik et al. / Aquaculture 230 (2004) 391–404 393

were randomly allotted to triplicate tanks and self feeders were filled with the respective

diets each day and the feed intake (quantity dispensed minus eventual refusals)

monitored daily.

At the beginning of the growth trial, a representative sample of six fish was withdrawn

and kept frozen (� 20 jC). Growth trial was conducted for 12 weeks (Sept–Dec 2000),

and every third weeks fish in each tank were bulk-weighed. At the end of the growth study

and after an overnight fast, six fish from each tank were sampled and frozen. Fifteen fish

per treatment were anesthetised (ethylene glycol monophenyl-ether 1:2500). They were

individually weighed and total length measured; blood was collected from the caudal vein

with a heparinised syringe. Plasma was recovered after centrifugation and stored at

� 20jC for total cholesterol and triacylglycerol analysis.

Fish were ground and moisture content was determined (110 jC, 24 h) and

subsequently freeze-dried before further analyses. For digestibility measurements, fecal

matter was collected in a modified version of the decantation chamber described by Cho

and Slinger (1979) and directly fitted to the circular rearing tanks (Fig. 1). Almost at the

end of the growth trial, faeces were collected over five consecutive days and frozen.

ADC of the dietary nutrients and energy were calculated according to Maynard et al.

(1979):

ADC ð%Þ ¼ 100� 1� dietary Y2O level

faecal Y2O level� faecal nutrient or energy level

dietary nutrient or energy level

� �

In order to quantify soluble nitrogen (N) and phosphorus (P) losses, outlet water

from the same rearing tanks was automatically sampled using a peristaltic pump over

three consecutive 24-h cycles and collected daily into bottles containing a small

Fig. 1. Diagram of a tank with photoperiod control, attached demand feeder and fecal trap used for rearing

European seabass.


amount of chloroform following procedures used by Kaushik (1980) and Dosdat et al.

(1996).

2.4. Analyses

Fecal samples were freeze dried before analyses of dry matter, nitrogen, energy,

phosphorus and yttrium oxide. Diets, freeze-dried feces and whole body samples were

analysed for dry matter (DM, 110 jC, 24 h), ash (550 jC, 18 h) crude protein (Kjeldahl

nitrogen� 6.25) and lipids (dichloromethane extraction by Soxlhet method) were

performed following AOAC (1984) procedures. Gross energy was determined by

calorimetry (IKA Adiabatic Calorimeter C4000A). Yttrium concentrations were deter-

mined in diet and fecal samples by atomic absorption spectrophotometry using a nitrous

oxide-acetylene flame, after acid digestion (2% nitric acid and 2 g l� 1 KCl). Concen-

trations of ammonia-N and urea-N were analysed by the indophenol blue (Treguer and

Le Corre, 1975) and diacetylmonoxime methods (Aminot and Kerouel, 1982), respec-

tively, using an autoanalyser. Soluble P (PO4) in the water was determined by the

ammonium molybdate method (Treguer and Le Corre, 1975). Based on diet and

comparative carcass analysis, daily N, fat and P gains and retention (as % intake) were

calculated. Concentrations of plasma triacylglycerols and cholesterol were determined

spectrophotometrically using commercial enzymatic kits (BioMerieux and Boehringer

Mannheim).

2.5. Statistical analysis

Data were analysed using ANOVA using the GLM procedure of SAS (1993). The

means were subsequently compared using Duncan’s multiple range test (significance level

P < 0.05). Data presented are meansF standard deviations with superscript letters indi-

cating differences between groups.

3. Results

3.1. Apparent digestibility

ADC of dry matter ranged between 80% and 82%, that of protein between 94% and

96% and that of energy between 88% and 92%, little affected by dietary fish meal

Table 2

Apparent digestibility coefficients (ADC, %) of the different experimental diets

FM52 FM40 FM25 FM12 FM5

Dry matter 82.4F 1.2 78.9F 1.2 80.3F 0.8 80.3F 1.4 79.7F 0.3

Protein 94.9F 0.4 94.2F 0.8 95.5F 0.6 96.1F 0.3 96.1F 0.3

Energy 91.6F 0.6 88.8F 0.8 89.4F 0.6 89.4F 0.5 88.5F 0.5

Phosphorus 49.2F 0.1 55.3F 0.3 54.5F 3.9 57.9F 4.3 52.3F 1.5

Values are meansF SD. Absence of superscript indicates no significant difference between treatments.


replacement levels (Table 2). Availability of phosphorus was lower in all diets (49% to

58%) but given the variability, there was no significant difference among diets.

3.2. Growth performance and nutrient utilisation

At the end of the 12 weeks of the growth trial, all groups of European seabass had

reached mean individual body weights ranging from 313 to 333 g (Table 3) with no

significant differences among groups. Analysis of length–weight relationships did not

show any differences either between groups described by a common relation of

y = 0.027X 2.808 with an R2 of 0.682, where X and Y represent total length (cm) and body

weight (g), respectively. Over the 12-week period of the growth trial, the daily mean

voluntary feed intake (VFI) was slightly higher in fish fed FM40 (10 g/kg/day) than in the

other groups (9.1 to 9.4 g/kg/day) which did not differ among themselves. The values of

daily growth index (DGI) were high for seabass of this size, grown at 22 jC, reflecting the

good rearing conditions and physico-chemical quality of water, with no significant

differences among treatments (Table 3). While there was no difference between groups

in feed efficiency (FE =wet weight gain/dry feed intake), the protein efficiency ratios

(PER=wet weight gain/crude protein intake) varied and the lowest value was observed in

seabass fed diet FM5. Whole body protein or phosphorus contents did not vary among

groups but body fat content increased with increasing level of fish meal replacement

(Table 4).

3.3. Nitrogen and phosphorus losses

Measurements of total ammonia-N rates showed that ammonia-N excretion rates were

lower in FM52-fed fish than in all other groups (Table 5), reflecting possible amino acid

imbalance or more probably an excess supply in the latter groups, which, however, did not

affect N-gains differently. Urea N excretion rates differed between treatments ranging

between 35 and 50 mg urea-N/kg BW/day. The relative proportion of urea-N to

ammonia + urea N decreased with decreasing levels of dietary fish meal levels. Daily P

Table 3

Growth performance and nutrient utilisation in European seabass (initial body weight: 190F 2 g) fed diets with

graded levels of fish meal replacement over 12 weeks at 22 jC

Diet FM52 FM40 FM25 FM12 FM5

Final body weight, g 330.8F 12.9 333.2F 10.9 317.2F 12.0 327.3F 8.9 313.9F 11.6

Final total length, cm 28.9F 0.9 28.1F1.2 27.9F 0.8 28.0F 0.9 28.7F 1.0

Voluntary feed intake, g/kg BW/day 9.25F 0.09b 10.02F 0.20a 9.17F 0.52b 9.42F 0.40b 9.07F 0.08b

Daily growth index (DGI)1 1.36F 0.11ab 1.48F 0.12a 1.34F 0.09ab 1.38F 0.09ab 1.25F 0.07b

Feed efficiency (FE)2 0.68F 0.04 0.68F 0.04 0.69F 0.01 0.68F 0.01 0.65F 0.02

Protein efficiency ratio (PER)3 1.51F 0.09a 1.41F 0.07ab 1.36F 0.02bc 1.46F 0.03ab 1.28F 0.05c

Values are meanF SD. Within a row, means with different superscript letters differ significantly (P < 0.05).

Absence of superscript indicates no significant difference between treatments.1 DGI: (FBW1/3� IBW1/3)/84 days)� 100.2 FE, Weight gain/dry feed intake.3 PER, Wet weight gain/crude protein intake.


Table 5

Daily mean NH4–N and PO4–P excretion rates in European seabass fed the different diets

Dietary Treatment FM52 FM40 FM25 FM12 FM5

Ammonia-N excretion,

mg N/kg/d

187.7F 25.5 280.6F 18.6 258.8F 18.4 288.6F 18.1 312.8F 50.9

Urea N-excretion,

mg N/kg/d

37.1F 5.1 49.7F 2.8 40.1F 6.2 38.6F 1.4 34.5F 5.1

Urea N/[Ammonia +

Urea N, %

16.5F 0.8 15.1F1.2 13.4F 1.0 11.8F 0.3 9.9F 0.4

[Ammonia + urea]�N

excreted as % of

digestible N intake

44.2F 0.7b 48.0F 1.8b 52.7F 3.1b 45.7F 3.1b 53.3F 2.3a

PO4–P excreted,

mg P/kg/d

23.1F 6.6 29.4F 4.6 11.6F 0.9 4.4F 1.5 � 2.6F 0.9

PO4–P excretion as %

of available P intake

41.8F 6.7a 40.3F 5.4a 23.6F 4.8b 6.6F 2.1c 0


Absence of superscript indicates no significant difference between treatments.

Table 4

Whole body composition, nutrient retention and gain in European seabass (IBW: 190F 2 g) fed diets with graded

levels of fish meal replacement over 12 weeks at 22 jC

Diet FM52 FM40 FM25 FM12 FM5

HSI1, % 2.1F 0.4 2.1F 0.5 2.2F 0.5 2.3F 0.6 2.5F 0.6

VSI2, % 11.0F 1.6 11.2F 1.5 11.3F 2.3 11.9F 3.7 10.7F 1.5

GSI3, % 0.52F 0.36 0.72F 0.39 1.41F 0.50 1.05F 0.65 1.01F 0.75

Final body composition4

Moisture, % 62.3F 1.9ab 60.5F 0.4b 60.2F 0.5b 60.8F 0.1ab 59.6F 0.2b

Protein, % 16.1F 0.8 16.4F 0.1 16.4F 0.8 16.6F 0.3 15.9F 0.5

Fat, % 18.4F 1.0c 19.8F 0.3b 19.9F 0.5b 20.0F 0.2b 21.8F 1.0a

Phosphorus, % 0.6F 0.05 0.6F 0.3 0.6F 0.1 0.6F 0.03 0.6F 0.1

Ash, % 3.1F 0.3 3.3F 0.6 3.6F 0.7 2.7F 0.2 2.7F 0.7

Energy, kJ/g 10.8F 0.7c 11.5F 0.2ab 11.6F 0.3ab 11.4F 0.2bc 12.1F 0.2a

Retention, % of intake

Dry matter 26.8F 4.4 29.7F 0.9 30.6F 0.6 29.4F 0.5 30.1F1.2

Protein 22.3F 4.3 22.1F1.6 21.0F 3.0 23.2F 0.6 17.9F 2.2

Energy 34.2F 6.0b 38.2F 1.2ab 39.5F 1.5ab 36.8F 0.8ab 40.9F 1.6a

Phosphorus 23.1F 7.0c 25.8F 3.5bc 36.1F11.9abc 46.5F 7.8a 44.2F 16.0ab

Daily nutrient gain (mg or g/kg/d)

Nitrogen (mg) 148.9F 30.1 171.3F 15.7 155.2F 25.3 163.5F 7.2 132.0F 17.0

Fat (g) 1.5F 0.2b 1.8F 0.1ab 1.7F 0.1ab 1.8F 0.1ab 1.9F 0.2a

Phosphorus (mg) 25.1F 7.8 32.7F 4.8 31.3F 11.2 32.0F 6.5 23.1F 8.5


Absence of superscript indicates no significant difference between treatments.1 HSI = hepato-somatic index, 100�weight of the liver/whole body weight.2 VSI = viscero-somatic index, 100�weight of digestive tract/whole body weight.3 GSI = gonado-somatic index, 100�weight of gonads/whole body weight.4 Initial body composition was: moisture 63.6, protein 17.1, fats 14.7, phosphorus 0.78, ash 4.7, and energy 9.9.


gain did not differ among groups, P retention values increased with increasing levels of

plant protein incorporation. Based on data on VFI, digestibility and on measured soluble

nutrient losses, combined with comparative carcass analysis, total nitrogen and phoshorus

fluxes were calculated and reported in Figs. 2 and 3.

Fig. 2. Nitrogen budget in European seabass (mg N/kg BW/day) based on digestibility, ammonia + urea N

excretion and comparative carcass analyses. The fraction corresponding to ‘‘Other N’’ represents that not

accounted for by measurements of fecal and metabolic losses.

Fig. 3. Phosphorus budget in European seabass (mg P/kg BW/day) based on digestibility, total soluble PO4

excretion and comparative carcass analyses.


3.4. Other metabolic consequences

Very few fish (less than 3%) were found to be mature and the gonadosomatic index

varied between 0.5% and 1.4% with no differences between sexes. We did not find any

differences between groups in the liver to body weight ratio (HSI) or in the viscero-

somatic index (VSI; Table 4). Concentrations of plasma triacylglycerols ranged between

100 and 400 mg/dl (Fig. 4) with much variability among treatments. Plasma cholesterol

levels decreased with increasing levels of fish meal replacement, the lowest level being

found in fish fed diet FM5 (Fig. 5).

Fig. 4. Concentrations of plasma triacylglycerols in European seabass fed the different diets. Values are

meanF SD. Means with different superscript letters differ significantly ( P< 0.05).

Fig. 5. Plasma cholesterol concentrations in European seabass fed the different diets. Values are meanF SD.

Means with different superscript letters differ significantly ( P< 0.05).


4. Discussion

Some earlier studies with rainbow trout as well as European seabass have suggested

that the major problem connected with poor growth of fish fed fish meal-free, plant-

protein-based diets is caused by poor feed intake (Gomes et al., 1995; Dias, 1999). In

European seabass fed diets containing very high levels of single protein sources such as

soy protein concentrate or corn gluten meal, there was a decrease in VFI, which was

improved by supplementation with an attractant mix (Dias et al., 1997). But other data

indicate that when the same protein sources replaced about 60% of fish meal, adequately

supplemented with limiting amino acids such as lysine or methionine, there was no need

for an attractant mix such as squid extract (Tibaldi et al., 1999). It is thus of interest to note

that the diets used here and made commercially did not lead to any reduction in voluntary

feed intake.

The growth rates observed here were higher than those reported earlier by Dias (1999)

for seabass grown at 18 jC and even higher than those reported by Ballestrazzi et al.

(1994) for fish reared at similar temperature levels. Earlier data have shown that partial

replacement of fish meal by plant protein sources (Tibaldi et al., 1999; Tulli et al., 1999) or

single cell proteins (Oliva-Teles and Goncalves, 2001) is feasible in European seabass.

Dias (1999) observed that inclusion of corn gluten meal or soy protein concentrates as the

sole protein source led to significant growth reduction. This is the first ever demonstration

of an almost total replacement of fish meal and soybean meal by a mixture of other plant

protein sources in European seabass. Diets were formulated to be least costly and it is

apparent that considerable reduction in fish meal levels can be achieved without

comprising fish performance under similar economic terms.

We observed, however, a significant increase in fat content with increasing levels of

fish meal replacement. This consequently resulted in a similar increase in whole body

energy content. The high fat and energy retention values in this group clearly suggest that

there was increased lipogenesis with increasing levels of fish meal replacement, without

any effect on nitrogen utilisation. Indeed, daily N gains or N retention expressed as a

percentage of unit N intake did not vary among groups. The HSI values of above 2 as

found here are common in European seabass (Ballestrazzi et al., 1998; Dias, 1999), where

hepatic fat deposition indeed is very high (Dias, 1999). There is evidence that replacement

of fish meal by plant protein sources such as corn gluten meal or soy protein concentrates

affects hepatic lipogenic enzyme activities variably in seabass (Dias, 1999): while the

activity of malic enzyme decreased, that of fatty acid synthetase increased significantly

with high levels of corn gluten meal in the diet.

The data on the relative proportion of urea-N to ammonia + urea N which decreased

with decreasing levels of dietary fish meal levels have to be treated with caution, since the

daily excretion rates measured here in well growing fish were relatively lower than what

has been found earlier (Dosdat et al., 1996). Expressed per unit digestible N intake,

measured (ammonia + urea) N excretion rates ranged between 43% and 53%, higher than

values reported earlier by Robaina et al. (1999), but close to other data (Ballestrazzi et al.,

1994). Reactive phosphorus losses decreased with increasing levels of fish meal replace-

ment, with practically no measurable soluble phosphorus in the effluents in fish fed diet

FM5. The sensitivity of the analytical method, albeit low, was not sufficient to detect


differences between inlet and outlet water. Indeed, measurements of low levels of soluble

P in fish farm effluent water is difficult due to the low concentrations generally found.

Data of Ballestrazzi et al. (1994) also provide evidence that seabass fed diets containing

corn gluten meal had lower reactive phosphate concentration in the effluent water than in

those fed herring meal based diets. It is clear from Fig. 2 that while fecal N losses are

relatively small and that daily N gains are similar among groups (Tables 4 and 6),

metabolic N losses are high. We also observed that measurement of ammonia-N + urea N

as measured here does not, at least in our case, reflect total metabolic losses, confirming

earlier suggestions that in fish, N balance measurement based on comparative carcass

analysis is more reliable than measurement of excretory rates (Cho and Kaushik, 1990).

Earlier data have shown that phosphorus availability was reduced in European seabass

fed soy protein concentrates (Dias, 1999). Given that the raw materials used here did not

contain detectable levels of phytic acid, addition of phytase, known to improve phytic P

availability even in European seabass (Oliva-Teles et al., 1998), was not found necessary.

Although daily P gain did not differ among groups, P retention values increased with

increasing levels of plant protein incorporation, reflecting that the inorganic phosphorus

supplementation was efficient. The data on phosphorus budget (Fig. 3) show that quite a

significant portion of soluble P was not accounted for by measured soluble PO4 in

effluents from fish tanks. It is, however, interesting to note that total P losses were reduced

in fish fed plant-protein-rich diets (Table 6).

The decrease in plasma cholesterol levels in fish fed diets with plant proteins (Fig. 5)

has already been reported in rainbow trout (Kaushik et al., 1995) and in European seabass

fed different plant protein sources (Dias, 1999; Tulli et al., 1999) in replacement of fish

meal. In terrestrial animals, plant products are generally considered to have a hypocho-

lesteromic effect (de Schrijver, 1990), mainly due to the relatively high levels of

estrogeno-mimetic isoflavones (Setchell and Cassidy, 1999). Given that the diets used

here contained small amounts of soy bean meal, it appears that the hypocholesterolemic

effects seen in such studies have more to do with the withdrawal of fish meal rather than

with a direct effect of plant proteins. Besides, measurements of phytoestrogens in several

plant products do show that there was considerable variability in the flavonoid contents of

soybean products and that other plant products used here contain very low or no genistein

Table 6

Nitrogen and phosphorus budget per unit body weight gain in European seabass fed the different diets

Dietary treatment FM52 FM40 FM25 FM12 FM5

Nitrogen, g N/kg BW gain

Intake 106.3F 6.3 113.3F 5.8 117.4F 1.5 109.9F 2.2 125.2F 4.8

Gain 23.6F 3.3 25.0F 0.6 24.6F 3.2 25.4F 1.0 22.4F 2.0

Losses 82.8F 9.3 88.3F 6.3 92.8F 4.7 84.4F 1.4 102.8F 6.3

Phosphorus, g P/kg BW gain

Intake 17.1F1.0 18.3F 0.9 13.5F 0.2 10.5F 0.2 8.8F 0.3

Gain 3.9F 1.2 4.7F 0.6 4.9F 1.6 4.9F 0.7 3.9F 1.4

Losses 13.2F 1.7 13.6F 1.2 8.7F 1.7 5.7F 0.9 4.9F 1.5

Values are meanF SD. Absence of superscript indicates no significant difference between treatments.


or daidzein (Bennetau-Pelissero et al., in press). In an earlier study with rainbow trout

(Kaushik et al., 1995), we also observed that casein had a stronger hypocholesterolemic

effect than soy by-products. From a comparative perspective, it will be of interest to gather

more insight on the specific effects of plant proteins on lipid and cholesterol synthesis and

metabolism, in as much as these factors might affect, among others, flesh quality.

5. Conclusions

Data obtained here show that development of diets based mainly on commonly

available practical ingredients of plant origin with very low levels of fish meal

incorporation and appropriate supplementation of essential amino acids does not affect

growth or nitrogen utilisation in European seabass. The low levels of phosphorus in these

diets can be easily supplemented with inorganic phosphorus to maintain body phosphorus

status while having a significant reduction in phosphorus discharges.

Acknowledgements

We express our thanks to Le Gouessant Aquaculture, 29 Lamballe, France, for the

manufacture of extruded diets used in this study. Recognition is given to all the technical

staff of Ifremer, Palavas and Inra, St-Pee sur Nivelle for their assistance in conducting

growth trials, samplings and chemical analyses. Special thanks are due to A. Charrier,

(IFREMER) for her help in excretion measurements and to C. Vachot (INRA) for analysis

of plasma metabolites. This study is a part of the Eureka project A! 1960: Aqua-Maki 2.

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