Cage culture of tambaqui (Colossoma macropomum) in a central Amazon floodplain lake

Aquaculture 253 (2006) 374 – 384 www.elsevier.com/locate/aqua-online

Cage culture of tambaqui (Colossoma macropomum) in a central Amazon floodplain lake Levy de Carvalho Gomes a,⁎, Edsandra Campos Chagas a , Heitor Martins-Junior b , Rodrigo Roubach b , Eduardo Akifumi Ono b , José Nestor de Paula Lourenço a a

b

Embrapa Amazônia Ocidental-C.P. 319, Manaus, AM, 69011-970, Brazil Instituto Nacional de Pesquisas da Amazônia/INPA, C.P. 478, Manaus, AM, 69069-001, Brazil Received 31 January 2005; received in revised form 6 August 2005; accepted 11 August 2005

Abstract Tambaqui, Colossoma macropomum, a native fish species of the Amazon basin, has been tested in cages placed in floodplain lakes with promising results. The present study evaluated culture performance and economic feasibility of tambaqui raised at different stocking densities in cages placed in a central Amazon floodplain lake. Fish were stocked in triplicate 6-m3 cages at 20, 30, 40 and 50 fish/m3. Fish were fed a 34% crude protein (CP) extruded feed six days a week during the first two months and a 28% CP extruded feed for six months. The experiment lasted for 240 days. Mean survival rates were over 97% and were not significantly affected by fish density. Tambaqui growth rate and weight gain were not affected by the tested densities. Fish growth rate did not decline during the culture period, indicating that fish carrying capacity per cage was not reached. Fish hematological parameters, as well as, glucose, cortisol and ions did not show significant differences among the four tested densities. Fish raised at 40 and 50 fish/m3 had significantly lower feed conversion ratio (FCR) than fish stocked at 20 or 30 fish/m3. FCR presented an inverse relationship to stocking density, with lower FCR at higher densities. Economic sensitivity analysis showed that economic performance of tambaqui raised in cages is more sensitive to sale price then to feed cost. To increase fish yield/m3, higher stocking densities should be tested. © 2005 Elsevier B.V. All rights reserved. Keywords: Cage; Tambaqui; Growth; Fish physiology; Yield; Economic feasibility

1. Introduction The gap between food fish demand and supply in the Amazon basin is increasing, as a result of population growth and over-exploitation of the main commercial fish stocks (Batista et al., 1998). To reduce the gap between supply and demand, local authorities are considering new technologies, such as cage fish culture. ⁎ Corresponding author. Tel.: +55 92 3621 0381; fax: +55 92 3621 0300. E-mail address: [email protected] (L.C. Gomes). 0044-8486/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2005.08.020

The immense number of rivers and lakes in the Amazon region presents potential for cage fish culture, a technology that demands less capital investment than the traditional pond fish production (Beveridge, 1996), and may allow the development of family scale production for extra income in local communities. Cage fish culture as a family activity, has been largely applied in Asia (Huchette and Beveridge, 2003; Liao et al., 2004), Africa (Outtara et al., 2003) and South American countries, such as Bolivia, Chile, Ecuador, Peru, and Venezuela (Beveridge, 1996; Araujo-Lima and Goulding, 1997; Alcántara et al., 2003). Tambaqui

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(Colossoma macropomum) cage culture has shown promising results in trials carried out in Amazon floodplain lakes (Chagas et al., 2003). Along with good cage culture performance, tambaqui presents additional advantages for culturing in the central Amazon: year round juvenile supply; high tolerance to low dissolved oxygen concentration; and good market value (AraujoLima and Goulding, 1997). In order to develop a production technology for a fish species, one of the primary steps is to determine the ideal stocking density, which will also define the optimum fish yield per unit of volume or area. Jobling (1994) reported that the stocking density has an effect on fish survival and growth. Normally, fish raised at lower densities present good growth rates and high survival, but total yield is low (Gomes et al., 2000). On the other hand, fish kept at high densities normally have slower growth (El-Sayed, 2002) and become stressed resulting in higher mortality (Wedemeyer, 1997). Density has also an effect on production,

375

normally with lower costs at higher stocking rates (Huguenin, 1997). This study aimed to evaluate the effect of tambaqui stocking density in cages placed in a central Amazon floodplain lake on culture performance and economic feasibility. The evaluation was done by monitoring fish growth efficiency, as well as, physiological, environmental and economical parameters.

2. Material and methods Tambaqui (Colossoma macropomum) with mean weight of 55.0 ± 14.2 g were obtained from Balbina fish culture station, Presidente Figueiredo-AM, Brazil. Fish were transported to lake Ariauzinho, Iranduba-AM (Fig. 1), a typical central Amazon floodplain lake. Lake Ariauzinho has a dendritic shape and muddy water type according to the classification of Esteves (1998). Water levels can fluctuate 8 m over the year, depending on the

Fig. 1. Study area at lake “Ariauzinho” (3° 14′ 16.40″ S and 60° 14′ 18.20″ W), Iranduba, AM, Brazil. This map corresponds to the low water season, when there is no communication between the Amazon River and lake. During the high water season, the Amazon river water invades the lake, and the vegetation between the river and the lake is flooded, forming the floodplain. Source: Cartographic base from the Instituto Brasileiro de Geografia e Estatística (IBGE).

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water level of the Amazon River. The lake is supplied by small streams year around and by the Amazon River, during the rainy season from March to July when water level raises. The floodplain lake was chosen for the trial because: 1) tambaqui commonly frequent these lakes; 2) among several habitats available in central Amazon, floodplain lakes are the most desirable to install cages, according to Beveridge (1996); and 3) most rural communities in the central Amazon are located around floodplain lakes. At the moment there are approximately 18 families living around Lake Ariauzinho which make a livelihood from agriculture and subsistence fisheries. The experiment was carried out using twelve 6-m3 (2.0 × 2.0 × 1.5 m) cages floated over the water surface with low density native wood. Cages were enclosed with a 25-mm galvanized wired mesh coated with UV resistant PVC and were installed at a distance of 10 m from the lake margin, where water depth ranged from 3 to 8 m according to water season. The experiment lasted for 240 days (8 months), between August 2003 and May 2004. Fish were randomly distributed in triplicate cages at four densities: 20, 30, 40 and 50 fish/m3. Fish were fed six days a week, twice a day to their apparent satiation, with commercial purchased extruded fish feeds with 34% crude protein (CP) in the first two months and 28% CP in the remaining six months. Water temperature and dissolved oxygen were monitored three times a week with a YSI 55 probe, and every two weeks pH was monitored with a digital pH meter, hardness and alkalinity were determined by titration and total ammonia by the endophenol method. Water samples were collected at 7 AM from inside of all the cages and from two different points in the lake at a distance of 100 m from the cages.

2.1. Growth and yield Monthly, 10% of fish were captured from each cage, anesthetized with benzocaine (100 mg/L; Gomes et al., 2001) individually weighted and measured. The following parameters were evaluated: growth in weight (g) and standard length (cm), coefficient of variation of the length (CV = standard deviation/mean * 100) and specific growth rate (SGR = [ln final mean weight − ln initial mean weight / time] * 100). At the end of the experiment, fish survival (%), yield per volume (kg/m3), weight gain (WG = final weight − initial weight) and

feed conversion ratio (FCR = feed consumption / weight gain) were calculated. 2.2. Physiological analyses Three fish from each cage (nine per treatment), were sampled at the beginning, and after 6 and 8 months of culture to evaluate physiological parameters. Fish were anesthetized with 100 mg/L benzocaine and blood collected from the caudal vein with heparized syringes. Hematocrit values (Ht) were obtained with a microhematocrit centrifuge. Hemoglobin (Hb) was determined colorimetrically (Kampen and Zijlstra, 1964). Red blood cells (RBC) were counted after blood dilution with formol citrate. Hematimetric equations (Brow, 1976) were used to determine the mean corpuscular volume (MCV), the mean corpuscular hemoglobin (MCH) and the mean corpuscular hemoglobin concentration (MCHC). Glucose levels were measured with a blood glucose monitor (Advantage™, Germany). Plasma was obtained through blood centrifugation at 3500 g for 5 min, and then stored at − 20 °C until analyzed. Cortisol levels were determined by enzymatic immunoassay in ELISA plate using a commercial kit (Human, cat. 55050). Sodium (Na+) and potassium (K+) levels were measured with a flame photometer (Micronal B462). 2.3. Economic analysis The mean harvest fish weight was used to value the fish. In the local retail market, whole fresh tambaqui between 800 and 1000 g are sold at US$ 1.18/kg (US $1 = R$2.80). Marketing expenses represented US$ 0.06/kg of fish and included the ice used to chill fish and transport by truck to the market. Stocked tambaqui juveniles (55 g mean weight) cost US$ 0.11 per fish. The feed cost US$ 0.53/kg (35% CP) and US$ 0.37/kg (28% CP). The following assumptions were made for the economic analysis: cage cost included material and assembly; the caretaker can handle routine work of forty 6-m3 cages and labor cost included working benefits in compliance with the law. The maintenance and repair costs included cleaning of cage mesh after harvest and the repair of a wooden canoe. Annual depreciation was calculated by the straight-line method and considered the economic life and salvage value of the equipment (Jolly and Clonts, 1993). The investments were classified as capital and operating costs. Interest on investment and operating capital

377

6

34

5

33 32

4

31 3 30 2

29

1

DO

Water temperature

0

28

Water temperature (ºC)

DO (mg/L)

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27 1

2

3

5

4

6

7

8

Month Fig. 2. In-cage morning dissolved oxygen (DO) and water temperature during tambaqui, Colossoma macropomum, culture in cages for 8 months at Lake Ariauzinho, Iranduba, AM, Brazil. No significant differences among densities (treatments) were found and data from different densities were pooled.

were calculated by multiplying the total investment and the total operating capital by the annual interest rate of 8%, which represents the current rate for loans destined to small agribusiness enterprises, multiplied by 0.67 (1.5 crops/year). Enterprise budgets were prepared for each stocking density tested according to Jolly and Clonts (1993). Based on the parameters of the best stocking density, a scenario was created to project the cash flow of an enterprise with twelve 6-m3 cages, over a 10-year period (Kubitza and Ono, 2004). The internal rate of return (IRR), net present value (NPV) and payback period of the enterprise cash flow were calculated according to Lapponi (2000) and Kubitza and Ono (2004). NPV was calculated considering a discount rate of 12%/year based on the current interest rate paid by bank savings for investments under US$18,000 and the same discount rate was applied in the cash flow analysis. Payback Alkalinity (mg/L)

30

period was calculated as the time required for the sum of yearly net cash flow to equal the investment (Kubitza and Ono, 2004). An economic sensitivity analysis was modeled simulating changes in fish sales price, feed cost (which produces the same effect as changing FCR), culture period and production at higher stocking densities based on the same production efficiency of fish raised at 50 fish/m3.

2.4. Statistical analysis Results were expressed as mean ± standard deviation. Significant differences among means from the treatments were established through analyses of variance (ANOVA) and Tukey test used for post hoc mean comparisons (P < 0.05). Regression analysis was used to evaluate the stocking density effect on FCR and yield.

Hardness (mg/L)

Ammonia (mg/L)

pH (units)

27 24 21 18 15 12 9 6 3 0 1

2

3

4

5

6

7

8

Month Fig. 3. In-cage pH, alkalinity, hardness and ammonia during tambaqui, Colossoma macropomum, growth in cages for 8 months at Lake Ariauzinho, Iranduba, AM, Brazil. No significant differences among densities (treatments) were found and data from different densities were pooled.

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Density (fish/m3)

3. Results

4

3.1. Water quality

20

A

30

40

50

SGR (%)

3

2

1

0 1 11

2

3

4

5

6

7

8

B

10

CV (%)

Water quality parameters monitored during the study did not present any significant difference among fish densities or the two points monitored in the lake. Therefore, the results were pooled to characterize the water quality of the lake. Dissolved oxygen presented a strong variation during the study, reaching its lowest values (0.4 mg/L) during the first 45 days and the highest values (5.8 mg/L) between the sixth and seventh month (Fig. 2). In the last month dissolved oxygen concentration reached 3.0 mg/L. Water temperature ranged between 28.3 and 33.1 °C (Fig. 2). Total ammonia and pH were constant throughout the studied period (Fig. 3). Total alkalinity and water hardness, as expected, had values similar to each other and were not affect by fish densities. Total alkalinity and hardness concentrations decreased during the experiment and in the last month their values

9 8 7 6

Density (fish/m3) 20

1200

30

5

40

0

50

2

3

4

5

6

7

8

Month

1000

Weight (g)

1

Fig. 5. Tambaqui, Colossoma macropomum, specific growth rate (SGR) (A) and coefficient of variation of the length (CV) (B) during 8 months rearing at different stocking densities in cages at Lake Ariauzinho, Iranduba, AM, Brazil. No significant differences among densities were found during the experimental months.

800 600 400 200

started to increase as a result of water influx from the Amazon river into the lake (Fig. 3).

A

0 0

1

2

3

4

5

6

7

8

3.2. Growth

Standard lenght (cm)

30

25

20

15

B 10 0

1

2

3

4

5

6

7

8

Fish growth was not significantly influenced by densities (Fig. 4). Mean final weight were 1052.3, 899.1, 923.4 and 945.3 g at 20, 30, 40 and 50 fish/m3, respectively. Fish density did not affect SGR (Fig. 5A), with higher weight gain during the second month, when SGR was approximately 2.6 %, and lower during the first month (0.5%). CV was always lower than 10% (Fig. 5B) with no density effect. CV decreased in all densities during the experiment with lowest value (5.4 %) in the last month.

Month Fig. 4. Tambaqui, Colossoma macropomum, weight (A) and standard length (B) during 8 months rearing at different stocking densities in cages at Lake Ariauzinho, Iranduba, AM, Brazil. No significant differences among densities were found during the experimental months.

3.3. Physiological analyses Hematocrit (30.71 ± 3.79 %), Hb (13.05 ± 1.09 g/dL), RBC (2.45 ± 0.40 106/mm3), MCV (134.01 ± 32.51 μm3),

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Table 1 Tambaqui, Colossoma macropomum, production parameters during 8 months of rearing at different stocking densities in cages in lake Ariauzinho, Iranduba, AM, Brazil Parameter

Stocking density (fish/m3) 20

Survival (%) Weight gain (g) Feed conversion Yield (kg/m3)

30 a

99.7 ± 0.5 997.1 ± 50.6a 2.85 ± 0.12a 21.0 ± 0.92d

40 a

50 a

98.7 ± 1.4 844.1 ± 95.4a 2.50 ± 0.17b 26.6 ± 2.44c

97.0 ± 2.3a 890.3 ± 58.5a 1.88 ± 0.13c 45.8 ± 2.46a

97.6 ± 2.3 868.4 ± 43.0a 2.07 ± 0.05c 36.0 ± 1.48b

Values are means (±SD) of 3 cages for each density. Means followed by different letters are significantly different at P < 0.05 by Tukey's test.

Mean survival for all densities was over 97% with no treatment effect (Table 1). Mean weight gain for all treatments was around 900 g with no significant difference among densities (Table 1). FCR was significantly more efficient for fish at densities of 40 and 50 fish/m3 than fish raised at 20 or 30 fish/m3. FCR was significantly lower for fish at 30 fish/m3 than for fish at 20 fish/m3. No significant difference in FCR was observed for fish at 40 and 50 fish/m3. FCR presented an inverse relationship with stocking density, with lower FCR at higher densities (Fig. 6A and Table 1). Yield was significantly affected by stocking density, reaching 45.8 kg/m3, at the highest density (50 fish/m3) (Fig. 6B and Table 1). During harvest some small fish were found in the cages, species like “matrinxã” (Brycon sp.), “piau” (Leporinus sp.), “sardinha” (Triportheus sp.), “mandı” (Pimelodus sp.), “tucunaré” (Cichla sp.) and “piranha” (Serrasalmus sp.), with no more than three specimens per cage. The occurrence of school of small characids (2–3 cm length) around the cages was common, with access to the inside of the cages. During harvest fish from all densities presented an adequate color pattern for market and no signs of diseases or bruises were found.

Considering the current fish sales price and production costs, net incomes were directly related to stocking density. However, at current sales price, capital investment and operating cost, only cages stocked at 50 fish/m3 showed positive net income (Table 2) and acceptable IRR, NPV and payback values (Table 3). The cost analysis showed that even cages stocked at the highest density nearly broke even. Relative participation of the variable cost over the total cost was directly related to stocking density while fixed cost was inversely related. At the highest stocking density, variable cost and fixed cost represented 88.1% and 11.9% of total cost, respectively (Table 2). Feed represented the most costly production item and its relative participation over the total cost was directly 4

A

3

FCR

3.4. Yield

3.5. Economic analysis

2 1

FCR = - 0.033 * density + 3.49 n= 12 R2 = 0.914 p= 0.000

0 10

Yield (kg/m3)

MCH (57.17 ± 12.73 pg) and MCHC (44.33± 6.59 %) did not vary at any density among the different sampling times. Higher cortisol values were observed at densities of 30 (88.41 ± 10.22 ng/dL) and 40 fish/m3 (93.28 ± 11.75 ng/dL) in the initial sample, without statistical differences. Overall cortisol mean value was 75.68 ± 9.58 ng/dL. Glucose and Na+ and K+ among densities were significantly similar for all samplings. Mean values were 54.46 ± 8.16 mg/dL; 117.23 ± 11.88 mEq/L; 6.96 ± 1.04 mEq/L, respectively.

60 50

20

30

40

50

60

B

40 30 20 Yield = 0,84 * density + 2.97 n= 12 R2 = 0.961 p= 0.000

10 0 10

20

30

40

50

60

Stocking density (fish/m3) Fig. 6. Linear relation between density and feed conversion rate (FCR) (A) and yield (B), after 8 months of tambaqui, Colossoma macropomum, rearing at different stocking densities in cages at Lake Ariauzinho, Iranduba, AM, Brazil.

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Table 2 Cost and return analysis of tambaqui, Colossoma macropomum, during 8 months of rearing at different stocking densities in cages in Lake Ariauzinho, Iranduba, AM, Brazil Parameter

Unit cost Stocking density (fish/m3) (US$) 20 30 40

Total harvest (kg) Revenue (tambaqui) Revenue per kg of fish Capital investment Wooden canoe 53.57 Wire mesh cage 142.86 Plastic bucket 10.71 Nylon net 28.57 Scale 89.29 Variable cost Variable cost (% of total cost) Tambaqui 0.11 juvenile (unit) 34% CP feed for 0.54 juveniles (kg) 28% CP feed for 0.38 grow-out (kg) Labor (month) 10.85 Marketing 0.06 expenses (kg) 8% Interest on operation capital (year) Variable cost per kg of fish Income after variable cost Fixed cost Fixed cost (% of total cost) Licenses (year) 14.29 Maintenance 24.08 8% and repair (year) Interest on investment (year) Depreciation 94.87 (year) Fixed cost per kg of fish Total cost Total cost per kg of fish Net income Net income per kg of fish

50

377.60 445.05 1.18

479.40 649.20 826.00 564.96 765.11 973.53 1.18 1.18 1.18

481.70 13.39 428.57 21.43 7.14 11.16 583.40 83.56

481.70 481.70 481.70 13.39 13.39 13.39 428.57 428.57 428.57 21.43 21.43 21.43 7.14 7.14 7.14 11.16 11.16 11.16 654.10 744.53 852.37 85.07 86.64 88.13

38.57

57.86

77.14

96.43

80.36

91.61 106.07 109.29

325.13

355.88 398.25 468.00

86.79 21.58

86.79 27.39

86.79 37.10

86.79 47.20

30.99

34.58

39.18

44.67

1.55

1.36

1.15

1.03

−138.35

− 89.15

114.77 16.44

20.58 121.16

114.77 114.77 114.77 14.93 13.36 11.87

Analysis of cash flow projected for an enterprise with twelve 6-m3 cages stocked at 50 fish/m3 resulted in IRR of 14.4%, NPV of US$ 565.18 and payback period of 6.1 years. The economic sensitivity analysis modeled for the highest stocking density showed that one percent change in sales price has greater effect over the IRR, NPV and payback period than one percent change in feed cost or FCR (Table 3). Also, a 10% decrease in sales price or a 10% increase in feed cost (or in FCR) results in unacceptable IRR and negative NPV. In contrast, an increase of 10% in sales price or reduction of 10% in feed cost (or in FCR) results in great improvement to the enterprise economic performance. A one-month shortage or extension on the culture period also has an important effect on the enterprise budget result (Table 3). Raising tambaqui in cages at higher stocking densities maintaining the production efficiency obtained at a density of 50 fish/m3 greatly enhances the economic results of the enterprise (Table 3). 4. Discussion 4.1. Water quality Tambaqui is a low dissolved oxygen tolerant (AraujoLima and Goulding, 1997), confirmed by the survival rate over 97% in this study. Early morning dissolved oxygen concentrations during the initial 45 days of the study were lower than 1 mg/L (Fig. 1), and contributed to tambaqui low growth rate during this period. Tambaqui under hypoxia present physiological and behavioral adaptations with high energy costs (Val and Almeida-Val, 1995). According to Jobling (1994) the main

9.57 16.14

9.57 16.14

9.57 16.14

9.57 16.14

25.82

25.82

25.82

25.82

Table 3 Economic sensitivity analysis of tambaqui, Colossoma macropomum, production in twelve 6-m3 cages stocked at a density of 50 fish/m3, and simulation at 75 and 100 fish/m3

63.24

63.24

63.24

63.24

Situations

0.30

0.24

0.18

0.14

Internal rate of return (IRR, %)

Net present value (NPV, US$)

Payback period (years)

Current conditions Sales price—10% increase Sales price—10% decrease Feed cost — 10% increase Feed cost — 10% decrease Culture period — 1 month shorter Culture period — 1 month longer Stocking density — 75 fish/m3 Stocking density — 100 fish/m3

14.4 26.6 1.5 6.5 22.8 17.0

565.18 3511.97 − 2381.61 − 1322.00 2452.36 1171.09

6.1 3.6 9.8 9.2 4.2 5.4

12.4

93.91

6.9

23.3

3514.14

4.1

28.7

6464.05

3.4

698.18 1.85

768.87 859.30 967.14 1.60 1.32 1.17

−253.12 − 203.92 − 94.19 − 0.67 − 0.43 − 0.15

6.39 0.01

Values are in US Dollars (US$1 = R$ 2.80).

related to stocking density. In cages stocked at the highest density feed accounted for 59.7% of total cost.

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consequences of these adaptations are lower feed consumption and growth, even when there is a feed surplus. Fish cage culture in natural lakes and rivers requires careful site selection, since control or management of water quality is impossible (Beveridge, 1996; Huguenin, 1997). During our study, water temperature, alkalinity, hardness, pH and total ammonia were within the optimum ranges for tambaqui culture. Water quality of lake “Ariauzinho” is similar to most floodplain lakes in the region (Melack and Fisher, 1983) and is also the natural habitat of tambaqui. Therefore, the main limitation in floodplain lakes is dissolved oxygen availability, which can reach undesirable low levels for optimum fish performance. Site selection is fundamental when installing cages in floodplain lakes, with special attention given to evaluating dissolved oxygen concentration in the lake.

rate observed was adequate, but lower than data from Chagas et al. (2003). Specific growth rate (SGR) on the first month was low due to low dissolved oxygen concentrations (< 1 mg/L) and adaptation of the fish to the new environment. SGR was highest in the second month because dissolved oxygen levels increase and possibly due to a compensatory growth, which has been reported for several fish species (Ali et al., 2003) and tambaqui (Ituassú et al., 2004). From the second month, SGR values gradually decreased until the 5th and 6th months, when SGR presented a slight increase, reflecting the highest weight gain during this period. According to Jobling (1994), CV values lower than 10% indicate group homogeneity and it is desirable. In our study CV were lower than 10% during all period and presented decreased values, resulting in a more uniform fish size and a favorable situation for fish sale.

4.2. Growth

4.3. Physiology

Mean weight gain was twice that reported for tambaqui by Chellapa et al. (1995), using the same rearing period at a density of 35 fish/m3 and three times the growth reported by Alcántara et al. (2003), at a density of 20 fish/m3 in the same rearing period. Improved tambaqui growth observed in this study was due to fish feed. Feed used at the present study was extruded and formulated for tambaqui nutritional requirements. Fish were fed two types of feed according to the manufacturer’s directions. First phase feed had 34% CP recommended for fish up to 200 g, and second phase feed had 28% CP, recommended for fish with more than 200 g. Chellapa et al. (1995) reported the use of a sinking feed, which presents lower digestibility and results in greater loss from the cage. Alcántara et al. (2003) used a balanced feed and regional fruits to feed the fish, which is known to lower tambaqui growth performance (Roubach and Saint-Paul, 1994). Chagas et al. (2003) in a 4-month study at the same floodplain lake with similar cages, feed management, and a density of 50 fish/m3 reported a final fish mean weight of 465 g, twice that observed (261 g) in the same time period and density in the present study. The greatest weight gain occurred after the fourth month, when highest dissolved oxygen levels were measured in the lake. Final mean weight (850–1000 g) was greater than the one projected by Araujo-Lima and Goulding (1997), which was 500 g for an eight month cage rearing. Projection was based on several data from papers with tambaqui growth in cage. Our result shows that growth

In intensive culture, fish kept at high stocking densities are exposed to several stressing factors, which can overload the physiological system (Wedemeyer, 1997). Under stress, fish respond in several ways to maintain homeostasis, and the physiological changes include hormone release, energetic metabolism and electrolytic balance (Barton, 2002). The analysis of the physiological changes has proven useful in detecting disturbances that result in detrimental effects to fish growth. One of the most accepted primary responses to stress includes hormone release, as cortisol (Barton, 2002). In tambaqui, higher cortisol values were registered at densities of 30 and 40 fish/m3 in the initial sample and appear related to induced stress due to handling during the fish anesthesia. Cortisol levels observed in later samplings (6 and 8 months) at all densities were similar to basal values previously registered for tambaqui (80 ng/ml) (Gomes et al., 2003). Glucose mobilization during stress situations provides extra energy, enabling the animal to resist throughout the disturbance period (Morgan and Iwama, 1997). However, in the present experiment no significant differences were observed in blood glucose. Tambaqui raised in cages at different stocking densities presented a normal glucose range, 50–70 mg/dL (Gomes et al., 2003; Chagas et al., 2003). Tambaqui sodium and potassium ion concentrations and hematology did not show any significant changes during this study. Responses were similar to the ones reported by Chagas et al. (2003). Results showed that at tested fish

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densities, no adverse effect on fish physiological condition was found, allowing tambaqui to use all their growth potential when raised in cages.

tambaqui during an eight month production cycle was not reached, since the relation between fish density and yield remained linear.

4.4. Yield 4.5. Economic evaluation Fish survival was higher than 97%, confirming that tambaqui is well adapted to the cage culture system (Chellapa et al., 1995). Survival was also similar to the one observed for tambaqui reared in ponds according to Araujo-Lima and Goulding (1997). Survival over 95% was also observed for cobia (Rachycentron canadum) reared in sea cages (Liao et al., 2004) and for Nile tilapia (Oreochromis niloticus) reared in cages inside ponds (Yi and Lin, 2001). High survival is fundamental for an economically viable cage rearing system (Kam et al., 2003). Feed conversion ratio had a strong correlation with stocking densities. Fish stocked at 40 and 50 fish/m3 were significantly more efficient in their feed conversion than fish stocked at 20 and 30 fish/m3. Cage reared tambaqui have a higher FCR (1.8) than fish reared in ponds (1.3) (Jiménez-Montealegre et al., 2005). As tambaqui is a life time plankton consumer, the main explanation for the best FCR in pond systems is the greater availability of natural food. FCR observed at the highest density was similar to the one obtained with Nile tilapia fed with processed feed (1.64) (Yi and Lin, 2001) and lower for the same species when reared with periphyton (2.94) (Huchette and Beveridge, 2003). Studies reporting density effect over FCR present conflicting results among different fish species. For species such as black-chinned tilapia, Sarotherodon melanotheron (Outtara et al., 2003) and pacific threadfin, Polydactylus sexfilis (Kam et al., 2003), fish were more efficient in converting feed when stocked at lower densities in cages. Jundiá, Rhamdia quelen (Barcellos et al., 2004) and tambaqui converted feed more efficiently when held at higher densities. Increased competition for the available diet in the high-density treatment may have led to more efficient consumption and explain the better FCR over the low-density treatment. Yield at 50 fish/m3 (45.8 kg/m3) in this study is one of the highest reported for tambaqui reared in cages. Merola and Souza (1988) obtained a yield of 32.5 kg/ m3, Chellapa et al. (1995) harvest was 14.4 kg/m3, Chagas et al. (2003) obtained 34.0 kg/m3 and Alcántara et al. (2003) observed a mean yield of 5.0 kg/m3. A positive linear relation between density and production was found, yield increased with increased stocking density. This results show that the maximum yield for

The scarcity of published data on the economic performance of fish raised in cages stocked under different densities made the thorough comparison of our data difficult. Even so, our findings that a higher stocking density returns a greater gross income and net profit are in accordance with the observation of Hengsawat et al. (1997) in cage production of Clarias gariepinus; although expenses with feed and juveniles are greater. The inverse relation between stocking density and relative feed participation on production cost found in our study was in conformity with the observation of Hengsawat et al. (1997). Huguenin (1997) states that feed cost in cage production of various fish species represents 30–60% of total costs, which is in agreement with our results (58.1– 59.7%). Bjorndal (1990) showed that variable cost dominates total production cost of caged salmon (74– 79%) while fixed cost are smaller, which was confirmed by our study (variable costs were 83.6–88.1% of total cost). The sensitivity analysis showed that the economic performance of tambaqui raised in cages is more sensitive to sales price then to feed cost, confirming the findings of Aiken (1989). The IRR of 14.4% per year and NPV of US$ 565.18 were quite low considering that during the year 2004, interest rate paid by a bank on medium risk investments over US$ 18,000 was between 16% and 20% per year. This 16% to 20 % return usually represents the minimum IRR required in the analysis of urban investments. However, considering the current inflation rate of 6.5% per year and the fact that small rural investors have access to loans at an interest rate of 8% per year, raising tambaqui in cages could provide a complementary income to small farmers and riparian communities. In addition, an increase in production scale can reduce costs with feed, juvenile fish (stockers) and labor, improving economic performance of tambaqui cultured in cages as demonstrated in the sensitivity analysis (Table 3). Raising tambaqui at a higher stocking density may improve economic performance, as demonstrated in the sensitivity analysis, by reducing the relative participation of the fixed cost and depreciation on the total fish cost. As a result, IRR, NPV and payback period would become more attractive even to larger investors.

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4.6. Final remarks There were no differences among tambaqui growth parameters at tested densities, fish growth was continuous until harvest, showing that cage carrying capacity was not reached during this eight-month trial. Therefore, yield/m3 should increase at higher densities. Results show that a density higher than 50 fish/m3 could favor a better feed conversion, better yield per cage volume and lower production costs. At a density of 50 fish/m3, tambaqui cage culture could be economically feasible for direct sale to the public, but this would limit the number of families that could adopt the technology, since the local market is restricted due to a high offer from capture fisheries and low tambaqui market acceptance of fish smaller than 1.5 kg. An increase in the number of producers would result in a harvest, which could be sold only to fish processors, and under these conditions the results would not be profitable with the prices paid to the producers by processors. An increase in yield per cage volume and lowering production costs are the possible solutions for a profitable system for the aquaculture industry. Acknowledgements This study was supported by project “TANRE/ FINEP/FUCAPI” and project “tanque-rede BASA”. The authors are thankful to the student staff at Embrapa Amazônia Ocidental and the technicians: Márcia Pessoa, Marcus Brito, Mario Kokay, Gil Viana and José Pereira for the assistance during the study. We also thank L.L. Lovshin for revising the manuscript. R. Roubach is a research fellowship recipient from CNPq. References Aiken, D., 1989. The economics of salmon farming in the bay of Fundy. World Aquac. 20 (3), 11–19. Alcántara, F.B., Chávez, C.V., Rodrıguez, L.C., Kohler, C.C., Kohler, S.T., Camargo, W.C., Colace, M., Tello, S., 2003. Gamitana (Colossoma macropomum) and paco (Piaractus brachypomus) culture in floating cages in the Peruvian Amazon. World Aquac. 34 (4), 22–24. Ali, M., Nicieza, A., Wootton, R.J., 2003. Compensatory growth in fishes: a response to growth depression. Fish Fish. 4, 147–190. Araujo-Lima, C.R.M., Goulding, M., 1997. So Fruitful Fish: Ecology, Conservation, and Aquaculture of the Amazon's Tambaqui. Columbia University Press, New York. 157 pp. Barcellos, L.J.G., Kreutz, L.C., Quevedo, R.M., Fagundes, M., Conrad, J., Baldissera, R.K., Bruschi, A., Ritter, F., 2004. Nursery rearing of jundiá, Rhamdia quelen (Quoy and Gaimard) in cages: cage type, stocking density and stress response to confinement. Aquaculture 232, 383–394.

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