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Preliminary studies on the culture of the pacu, Colossoma mitrei, in floating cages: Effect of stocking density and feeding rate on growth performance
Aquaculture, 68 (1988) 243-248 Elsevier Science Publishers B.V.. Amsterdam
243 -
Printed
in The Netherlands
Preliminary Studies on the Culture of the Pacu, Colossoma mitrei, in Floating Cages: Effect of Stocking Density and Feeding Rate on Growth Performance NINO MEROLA’
and J. HENRIQUE
DE SOUZA”
‘FAO-Project GCPfRLAI075IITA Support to the Regional Aquaculture Activities for Latin America and the Caribbean, C.P. 07-1058, Brasilia 70.330, DF (Brazil) ‘Centro de Pesquisa e Treinamento em Aqiiicultura (CEPTA), SUDEPE (MA), Pirassununga 13.630, SP (Brazil) Address for correspondence: DF (Brazil) (Accepted
Nino Merola, FAO/ITA, c/o FAOR, C.P. 07-1058, Brasilia 70.330,
27 July 1987)
ABSTRACT Merola, N. and De Souza, J.H., 1988. Preliminary studies on the culture of the pacu. Colossoma mitrei, in floating cages: effect of stocking density and feeding rate on growth performance. Aquaculture, 68: 243-248. Two experiments were carried out with pacu (Colossoma mitrei) in 0.5-m” cages. In the first experiment, pacu were stocked at three densities: 50,100 and 150 fish/O.5 mR. Fish were fed a 30% protein diet of 5% body weight per day, twice daily. Mean individual fish growth per day was 1.15, 0.99 and 0.92 g for the three densities, respectively. Food conversion ranged from 2.85 to 3.25, and mortality was nil in all treatments. In the second experiment, pacu were stocked at a density of 100 fish/O.5 m” and segregated by weight into three size groups: loo-150 g, 150-200 g, and > 200 g. Treatments consisted of three different feeding regimes of 4,3 and 2% body weight per day. The mean individual daily gain for all treatments was 0.75 & (SD) 0.12 g/day. A standing crop of about 22.5-25.0 kg/O.5 m3 seems to be approximating a critical level for growth. Conversion values were high in all groups, ranging from 4.0 to 6.0. Preliminary data would seem to indicate the use of a feeding rate of 3% body weight for fish up to 150-180 g and 2% for larger fish. No mortality occurred in any cage.
INTRODUCTION
In Latin America, cage culture has great potential because of the large number of lakes and reservoirs. The introduction of cage culture technology could lead to the wide use of these waters, which are estimated to be more than 10.5
0044-8486/88/$03.50
0 1988 Elsevier Science Publishers
B.V.
244
million ha ( Cardenas, 1981) , and provide farmers with a new source of income and consumers with a high quality food fish. Fish species of the genus Colossoma are becoming important in the development of fresh-water aquaculture in Latin America. The important species with a potential for aquaculture are tambaqui (Colossoma macropomum) , pirapitinga (Colossoma brachypomum) and pacu ( Colossoma mitrei) . The first two are indigenous to the Amazon Basin while the pacu is indigenous to the Parana-Uruguay Basin. Both pacu and tambaqui are omnivorous in feeding habit and have been reported to reach weights of 18 kg and 30 kg, respectively, in the wild (Goulding, 1980; Barbosa, 1987). In cage culture, where natural food is limited or unavailable, artificial feeding is essential for good growth (Cache, 1978, 1979). In general, the food requirements of fish per unit weight decrease as the fish increase in size. Little work has been done on this relationship for tropical fish (Cache, 1982). There is no published information on the growth and optimum feeding rate for pacu in cages. Only tambaqui has been tried in cages, at densities of 10,20, 25 and 50 tish/m3, by researchers in Venezuela (Darmont and Salaya, 1984; Nuiiez and Salaya, 1984). This paper presents the results of two preliminary studies conducted to determine the response of pacu in intensive cage culture under three different stocking densities and three different feeding rates. MATERIALS AND METHODS
Six 0.5-m3 cylindrical cages (80 x 100 cm) made of 20-mm mesh plastic net were used for each experiment. The cages were tied to a raft positioned in a 5ha reservoir with an average depth of 3 m. The reservoir is fed by small springs. Water quality parameters were taken every 2 weeks during the experimental period and varied as follows: pH 6.2-6.5; dissolved oxygen rarely fell below 6 ppm; temperature 22-28”C, and alkalinity 7-10 mg/l. Temperatures and oxygen levels were measured with a YSI oxygen meter, pH with a Fisher digital meter, and alkalinity with an Hach kit model DR-EL/B. Primary production within the reservoir was low; Secchi disk transparency varied from 0.4 to 1 m, and was due primarily to suspended inorganics and not plankton. Fish used in all experiments came from artificial reproduction and were grown to stocking size (50-200 g, depending on the experiment) in earthen ponds. After stocking they were acclimatized to the feed and cages for 2 weeks. A sinking pelleted diet (30% protein) was fed to the fish in a submerged bucket, 6 days a week, twice daily (half at 8.00 a.m. and half at 4.00 p.m. ) . The composition of the diet was: fish meal, 15%; meat and bone meal, 30%; wheat flour, 40%; corn grain, 14%; poultry vitamin/mineral premix, 1%; and bentonite, 1.5%. All fish from each cage were batch weighed at intervals of 2 weeks.
245
Stocking density experiment Three stocking densities were tested: 50, 100 and 150 fish/O.5 m3. The rate of feeding was 5% body weight, and the experiment lasted for 91 days. Each treatment was duplicated. Growth and production data were analysed statistically by ANOVA, and means compared using Duncan’s Multiple Range Test. Feeding rate experiment The experimental design included the use of predetermined rates of feeding to groups of pacu, each of known mean weight. Fish were segregated by size into three groups: loo-150 g, 150-200 g, and > 200 g. Fish in the 100-150-g size group were fed at 3% and 4% body weight/day; fish in the 150-200-g group were fed at 2% and 3%; and fish in the > 200-g group were fed at the 2% rate only. Fish in ail size groups were stocked at a density of 100 fish/O.5 m3 and the experiment was conducted for a total of 104 days. Instead of the 89 scheduled feeding days, fish were only fed 81 days due to logistic problems. The unfed days occurred within each experimental period. RESULTS
Stocking density The averages of two replicates per treatment are summarized in Table 1. The observed daily gain decreased with increasing biomass and fish size. The highest value (1.15 g/day) was obtained at the lowest density ( 50 fish/O.5 m’) , although no significant difference (P> 0.05 ) was detected between the three treatments. TABLE I Growth, densities
feed conversion,
Stocking density (fish/O.5
Average
and production
weight
of pacu
in cages
Growth
(0.5 m”) after
Standing
crop
(kg/O.5 m”
(9)
)
91 days,
at three
Net production (kg/O.5 ma
)
m”) Stocking
Harvest
Daily
Relative
(g/day)
f%)
Stocking
50
54.2
159.3
1.15”
193.0
2.7
100 150
54.8 52.2
144.7 135.9
0.99” 0.92”
163.9 161.3
5.45 7.8
Values with the same superscript
are not significantly
different
Harvest
7.95 14.45 20.4
Total
5.25” g.gb 12.5’
at the 5% level.
stocking
Feed conversion
Monthly
1.7
2.85
2.9 4.1
3.25 3.25
2
rate
(g)
4
4 3
3
2 2
of 66 days only.
100-150
100-150 100-150
150-200
150-200 > 200’
*Results
(%o)
Feeding
Group size
166.4 + 3.6
162.7 + 3.3 234.2 f 8.7
growth,
236.9 +_11.2 277.0 & 18.8
253.6 f 24.9
184.1? 22.1 180.9 k 23.9
211.1~21.2
Harvest
(g)
between
weight
109.5 + 3.8 115.2 + 4.5
114.343.5
Stocking
Average
Mean weights ( & SE) and relationship a density of 100 fish/O.5 m3
TABLE
0.62 0.71
1.01
1.22 0.86
1.13
O-30
Days
0.69 0.60
0.79
0.59 0.60
0.79
31-66
Daily gain (g/d)
feed conversion
0.81 -
0.76
0.44 0.49
0.91
67-104
0.71 0.65
0.84
0.72 0.63
0.93
4.08 5.10
3.92
2.99 3.28
3.34
O-30
Days
4.30 6.94
6.12
7.99 5.88
6.10
31-66
Feed conversion
crop of pacu in cages
Mean
and standing
3.77 -
4.38
8.30 4.90
4.30
67-104
4.0 6.0
4.8
5.6 4.6
4.5
crop
18.1 25.5
197
14.6 14.1
14.8
O-30
Days
20.6 27.7
22.5
16.7 16.25
17.7
31-66
(kg/O.5 m”)
Standing
three experimental
Mean
(0.5 ms) during
at
23.7 -
25.4
18.4 18.1
21.1
67-104
periods,
247
Feed conversion ratio (FCR) was high (2.85-3.25) for all treatments. Net production and mean monthly production varied with density, showing an increase directly proportional to the stocking density. Differences in mean net production were significant at the 5% level. Mortality was nil for all treatments. Feeding rate Fish in the 100-150-g group fed 4% and 3% body weight had average weight increases of 85.7 g and 65.7 g and average conversion ratios of 5.0 and 4.6, respectively. The daily weight gain decreased with an increase in biomass, as shown in Table 2, but a clear correlation between the two factors was not observed. Fish in the 150-200-g group fed 3% and 2% body weight showed similar weight increases (87.2 g and 74.2 g) and conversion ratios of 4.8 and 4.0, respectively (Table 2). The daily weight gain did not show any real differences except during the first feeding period (days O-30). Fish in the > 200-g group fed at 2% body weight had gains of 42.8 g after 66 days. Feed conversion was the lowest. In the third period (days 67-104) the feeding rate of this group was reduced to l%, but there was a considerable loss in fish weight (18% ) . No mortality occurred during the experimental period. DISCUSSION
The average daily gains achieved, especially during the stocking density study, are encouraging when compared with gains of 1.0 g/day obtained by Nufiez and Salaya (1984 ) with C. macropomum at a much lower density ( 50 fish/m” ) . In general, weight gains appeared to decrease as a function of biomass (standing crop) of fish. Preliminary analysis showed a daily absolute growth in the range 1.0-1.2 g until a standing crop of 28-30 kg/m” was reached. After this, weight gains for all size classes declined to around 0.8-1.0 g with a standing crop of 40-50 kg/m3. Additional increases in standing crop decreased the growth to below 1 g/day. Although the observations are preliminary, the results indicate that standing crop values of 45-50 kg/m3, under the given experimental conditions, might be approximating a critical level for growth. Feed conversion was poor in both experiments. Particularly high values were obtained in all groups at the various feeding rates, regardless of total biomass. This may have been due to the composition and quality of the dietary ingredients used; the diet contained a high level of meat and bone meal, the protein of which is known to be deficient in the essential amino acid methionine, and the ash component containing a very high percentage of calcium, and a poultry vitamin premix. Nuiiez and Salaya (1984) fed caged tambaqui a diet containing 26.5% protein and achieved a growth of 1.0 g/day and a feed conversion of 1.23. However, no data were provided on the composition of the diet used.
248
Tentatively, a feeding rate of 3% could be used for pacu up to 150-180 g, but for fish > 200 g the rate should be lowered to 2%. Further reduction in the rate of feeding was not tested but still could favour good growth, since tambaqui in Colombian cages are fed at a rate of 1.5% body weight (Hernandez and Paza, 1986 ) . A 100% survival of fish in all treatments suggests that the pacu are very amenable to intensive cage culture conditions. The high degree of variability in the size of the caged fish at harvest might be genetic, but this remains to be determined since the fish available were in their first generation of captivity. ACKNOWLEDGEMENTS
We are most grateful to Mr. Mario Pedini, Dr. F.A. Pagan-Font and Dr. Colin Nash for their useful critique and overall suggestions of the manuscript. Special thanks are also due to the research, administrative and technical personnel of CEPTA. This investigation was suppported by UNDP-FAO through Project RLA/76/010, by IDRC through Project Aquaculture-Brazil-3-p-76-0001 and by SUDEPE (Ministry of Agriculture, Brazil).
REFERENCES Barbosa, J.M., 1987. Especies do gcnero Colossoma (Pisces, Characidae ) importantes para piscicultura em regides tropicais. In: Sfntese dos trabalhos realizados corn especies do gcnero Colossoma, Marco 82 a Abril86. Centro de Pesquisa e Treinamento em Aquicultura, Pirassununga, SP, Brazil, p. 8 (abstract). Cardenas, J., 1981. La acuicultura en Ameria Latina. Memorias de1 II Simposio sobre desarrollo de la Acuicultura en el Peru. Universidad National Agraria y Ministerio de Pesqueria, Peru, pp. 225-235. Cache, A.G., 1978. Revue des pratiques d’elkvage des poissons en cages dans les eaux continentales. Aquaculture, 13: 157-189. Cache, A.G., 1979. A review of cage fish culture and its application in Africa. In: T.V.R. Pillay and W.A. Dill (Editors), Advances in Aquaculture. Fishing News Books Ltd., Farnham, Surrey, pp. 428-441. Cache, A.G., 1982. Cage culture of tilapias. In: R.S.V. Pullin and R.H. Lowe-McConnel (Editors), The Biology and Culture of Tilapias. ICLARM Conference Proceedings 7, International Centre for Living Aquatic Resources Management, Manila, Philippines, pp. 205-246. Darmont, M. and Salaya, J.J., 1984. Ensayo de cultivo de la cachama Colossoma macropomum Cuvier 1818, en jaulas flotantes rigidas. Mem. Asoc. Latino-Americana Acuicultura, 5: 465-479. Goulding, M., 1980. The Fishes and the Forest. Exploration in Amazonian Natural History. University of California Press, Berkeley, CA, 280 pp. Hernandez, C. and Paza, A., 1986. Cultivo intensivo de cachama (Colossoma macropomum) en jaulas flotantes a densidades de 10 y 20 peces/m” en la cienaga Matapalma. Informe final, Corporation fondo de apoyo a empresas asociativas “CORFAS”, Bogota, Colombia, 155 pp. Nuiiez, J.M. and Salaya, J.J., 1984. Cultivo de cachama, Colossoma macropomum, en jaulas flotantes no rigidas en la represa de Guanapito, Edo. Guarico, Venezuela. Mem. Asoc. LatinoAmericana Acuicultura, 5: 481-494.
243 -
Printed
in The Netherlands
Preliminary Studies on the Culture of the Pacu, Colossoma mitrei, in Floating Cages: Effect of Stocking Density and Feeding Rate on Growth Performance NINO MEROLA’
and J. HENRIQUE
DE SOUZA”
‘FAO-Project GCPfRLAI075IITA Support to the Regional Aquaculture Activities for Latin America and the Caribbean, C.P. 07-1058, Brasilia 70.330, DF (Brazil) ‘Centro de Pesquisa e Treinamento em Aqiiicultura (CEPTA), SUDEPE (MA), Pirassununga 13.630, SP (Brazil) Address for correspondence: DF (Brazil) (Accepted
Nino Merola, FAO/ITA, c/o FAOR, C.P. 07-1058, Brasilia 70.330,
27 July 1987)
ABSTRACT Merola, N. and De Souza, J.H., 1988. Preliminary studies on the culture of the pacu. Colossoma mitrei, in floating cages: effect of stocking density and feeding rate on growth performance. Aquaculture, 68: 243-248. Two experiments were carried out with pacu (Colossoma mitrei) in 0.5-m” cages. In the first experiment, pacu were stocked at three densities: 50,100 and 150 fish/O.5 mR. Fish were fed a 30% protein diet of 5% body weight per day, twice daily. Mean individual fish growth per day was 1.15, 0.99 and 0.92 g for the three densities, respectively. Food conversion ranged from 2.85 to 3.25, and mortality was nil in all treatments. In the second experiment, pacu were stocked at a density of 100 fish/O.5 m” and segregated by weight into three size groups: loo-150 g, 150-200 g, and > 200 g. Treatments consisted of three different feeding regimes of 4,3 and 2% body weight per day. The mean individual daily gain for all treatments was 0.75 & (SD) 0.12 g/day. A standing crop of about 22.5-25.0 kg/O.5 m3 seems to be approximating a critical level for growth. Conversion values were high in all groups, ranging from 4.0 to 6.0. Preliminary data would seem to indicate the use of a feeding rate of 3% body weight for fish up to 150-180 g and 2% for larger fish. No mortality occurred in any cage.
INTRODUCTION
In Latin America, cage culture has great potential because of the large number of lakes and reservoirs. The introduction of cage culture technology could lead to the wide use of these waters, which are estimated to be more than 10.5
0044-8486/88/$03.50
0 1988 Elsevier Science Publishers
B.V.
244
million ha ( Cardenas, 1981) , and provide farmers with a new source of income and consumers with a high quality food fish. Fish species of the genus Colossoma are becoming important in the development of fresh-water aquaculture in Latin America. The important species with a potential for aquaculture are tambaqui (Colossoma macropomum) , pirapitinga (Colossoma brachypomum) and pacu ( Colossoma mitrei) . The first two are indigenous to the Amazon Basin while the pacu is indigenous to the Parana-Uruguay Basin. Both pacu and tambaqui are omnivorous in feeding habit and have been reported to reach weights of 18 kg and 30 kg, respectively, in the wild (Goulding, 1980; Barbosa, 1987). In cage culture, where natural food is limited or unavailable, artificial feeding is essential for good growth (Cache, 1978, 1979). In general, the food requirements of fish per unit weight decrease as the fish increase in size. Little work has been done on this relationship for tropical fish (Cache, 1982). There is no published information on the growth and optimum feeding rate for pacu in cages. Only tambaqui has been tried in cages, at densities of 10,20, 25 and 50 tish/m3, by researchers in Venezuela (Darmont and Salaya, 1984; Nuiiez and Salaya, 1984). This paper presents the results of two preliminary studies conducted to determine the response of pacu in intensive cage culture under three different stocking densities and three different feeding rates. MATERIALS AND METHODS
Six 0.5-m3 cylindrical cages (80 x 100 cm) made of 20-mm mesh plastic net were used for each experiment. The cages were tied to a raft positioned in a 5ha reservoir with an average depth of 3 m. The reservoir is fed by small springs. Water quality parameters were taken every 2 weeks during the experimental period and varied as follows: pH 6.2-6.5; dissolved oxygen rarely fell below 6 ppm; temperature 22-28”C, and alkalinity 7-10 mg/l. Temperatures and oxygen levels were measured with a YSI oxygen meter, pH with a Fisher digital meter, and alkalinity with an Hach kit model DR-EL/B. Primary production within the reservoir was low; Secchi disk transparency varied from 0.4 to 1 m, and was due primarily to suspended inorganics and not plankton. Fish used in all experiments came from artificial reproduction and were grown to stocking size (50-200 g, depending on the experiment) in earthen ponds. After stocking they were acclimatized to the feed and cages for 2 weeks. A sinking pelleted diet (30% protein) was fed to the fish in a submerged bucket, 6 days a week, twice daily (half at 8.00 a.m. and half at 4.00 p.m. ) . The composition of the diet was: fish meal, 15%; meat and bone meal, 30%; wheat flour, 40%; corn grain, 14%; poultry vitamin/mineral premix, 1%; and bentonite, 1.5%. All fish from each cage were batch weighed at intervals of 2 weeks.
245
Stocking density experiment Three stocking densities were tested: 50, 100 and 150 fish/O.5 m3. The rate of feeding was 5% body weight, and the experiment lasted for 91 days. Each treatment was duplicated. Growth and production data were analysed statistically by ANOVA, and means compared using Duncan’s Multiple Range Test. Feeding rate experiment The experimental design included the use of predetermined rates of feeding to groups of pacu, each of known mean weight. Fish were segregated by size into three groups: loo-150 g, 150-200 g, and > 200 g. Fish in the 100-150-g size group were fed at 3% and 4% body weight/day; fish in the 150-200-g group were fed at 2% and 3%; and fish in the > 200-g group were fed at the 2% rate only. Fish in ail size groups were stocked at a density of 100 fish/O.5 m3 and the experiment was conducted for a total of 104 days. Instead of the 89 scheduled feeding days, fish were only fed 81 days due to logistic problems. The unfed days occurred within each experimental period. RESULTS
Stocking density The averages of two replicates per treatment are summarized in Table 1. The observed daily gain decreased with increasing biomass and fish size. The highest value (1.15 g/day) was obtained at the lowest density ( 50 fish/O.5 m’) , although no significant difference (P> 0.05 ) was detected between the three treatments. TABLE I Growth, densities
feed conversion,
Stocking density (fish/O.5
Average
and production
weight
of pacu
in cages
Growth
(0.5 m”) after
Standing
crop
(kg/O.5 m”
(9)
)
91 days,
at three
Net production (kg/O.5 ma
)
m”) Stocking
Harvest
Daily
Relative
(g/day)
f%)
Stocking
50
54.2
159.3
1.15”
193.0
2.7
100 150
54.8 52.2
144.7 135.9
0.99” 0.92”
163.9 161.3
5.45 7.8
Values with the same superscript
are not significantly
different
Harvest
7.95 14.45 20.4
Total
5.25” g.gb 12.5’
at the 5% level.
stocking
Feed conversion
Monthly
1.7
2.85
2.9 4.1
3.25 3.25
2
rate
(g)
4
4 3
3
2 2
of 66 days only.
100-150
100-150 100-150
150-200
150-200 > 200’
*Results
(%o)
Feeding
Group size
166.4 + 3.6
162.7 + 3.3 234.2 f 8.7
growth,
236.9 +_11.2 277.0 & 18.8
253.6 f 24.9
184.1? 22.1 180.9 k 23.9
211.1~21.2
Harvest
(g)
between
weight
109.5 + 3.8 115.2 + 4.5
114.343.5
Stocking
Average
Mean weights ( & SE) and relationship a density of 100 fish/O.5 m3
TABLE
0.62 0.71
1.01
1.22 0.86
1.13
O-30
Days
0.69 0.60
0.79
0.59 0.60
0.79
31-66
Daily gain (g/d)
feed conversion
0.81 -
0.76
0.44 0.49
0.91
67-104
0.71 0.65
0.84
0.72 0.63
0.93
4.08 5.10
3.92
2.99 3.28
3.34
O-30
Days
4.30 6.94
6.12
7.99 5.88
6.10
31-66
Feed conversion
crop of pacu in cages
Mean
and standing
3.77 -
4.38
8.30 4.90
4.30
67-104
4.0 6.0
4.8
5.6 4.6
4.5
crop
18.1 25.5
197
14.6 14.1
14.8
O-30
Days
20.6 27.7
22.5
16.7 16.25
17.7
31-66
(kg/O.5 m”)
Standing
three experimental
Mean
(0.5 ms) during
at
23.7 -
25.4
18.4 18.1
21.1
67-104
periods,
247
Feed conversion ratio (FCR) was high (2.85-3.25) for all treatments. Net production and mean monthly production varied with density, showing an increase directly proportional to the stocking density. Differences in mean net production were significant at the 5% level. Mortality was nil for all treatments. Feeding rate Fish in the 100-150-g group fed 4% and 3% body weight had average weight increases of 85.7 g and 65.7 g and average conversion ratios of 5.0 and 4.6, respectively. The daily weight gain decreased with an increase in biomass, as shown in Table 2, but a clear correlation between the two factors was not observed. Fish in the 150-200-g group fed 3% and 2% body weight showed similar weight increases (87.2 g and 74.2 g) and conversion ratios of 4.8 and 4.0, respectively (Table 2). The daily weight gain did not show any real differences except during the first feeding period (days O-30). Fish in the > 200-g group fed at 2% body weight had gains of 42.8 g after 66 days. Feed conversion was the lowest. In the third period (days 67-104) the feeding rate of this group was reduced to l%, but there was a considerable loss in fish weight (18% ) . No mortality occurred during the experimental period. DISCUSSION
The average daily gains achieved, especially during the stocking density study, are encouraging when compared with gains of 1.0 g/day obtained by Nufiez and Salaya (1984 ) with C. macropomum at a much lower density ( 50 fish/m” ) . In general, weight gains appeared to decrease as a function of biomass (standing crop) of fish. Preliminary analysis showed a daily absolute growth in the range 1.0-1.2 g until a standing crop of 28-30 kg/m” was reached. After this, weight gains for all size classes declined to around 0.8-1.0 g with a standing crop of 40-50 kg/m3. Additional increases in standing crop decreased the growth to below 1 g/day. Although the observations are preliminary, the results indicate that standing crop values of 45-50 kg/m3, under the given experimental conditions, might be approximating a critical level for growth. Feed conversion was poor in both experiments. Particularly high values were obtained in all groups at the various feeding rates, regardless of total biomass. This may have been due to the composition and quality of the dietary ingredients used; the diet contained a high level of meat and bone meal, the protein of which is known to be deficient in the essential amino acid methionine, and the ash component containing a very high percentage of calcium, and a poultry vitamin premix. Nuiiez and Salaya (1984) fed caged tambaqui a diet containing 26.5% protein and achieved a growth of 1.0 g/day and a feed conversion of 1.23. However, no data were provided on the composition of the diet used.
248
Tentatively, a feeding rate of 3% could be used for pacu up to 150-180 g, but for fish > 200 g the rate should be lowered to 2%. Further reduction in the rate of feeding was not tested but still could favour good growth, since tambaqui in Colombian cages are fed at a rate of 1.5% body weight (Hernandez and Paza, 1986 ) . A 100% survival of fish in all treatments suggests that the pacu are very amenable to intensive cage culture conditions. The high degree of variability in the size of the caged fish at harvest might be genetic, but this remains to be determined since the fish available were in their first generation of captivity. ACKNOWLEDGEMENTS
We are most grateful to Mr. Mario Pedini, Dr. F.A. Pagan-Font and Dr. Colin Nash for their useful critique and overall suggestions of the manuscript. Special thanks are also due to the research, administrative and technical personnel of CEPTA. This investigation was suppported by UNDP-FAO through Project RLA/76/010, by IDRC through Project Aquaculture-Brazil-3-p-76-0001 and by SUDEPE (Ministry of Agriculture, Brazil).
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