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Water quality and nutrient budget in closed shrimp (Penaeus monodon) culture systems
Aquacultural Engineering 27 (2003) 159 /176 www.elsevier.com/locate/aqua-online
Water quality and nutrient budget in closed shrimp (Penaeus monodon) culture systems Dhirendra Prasad Thakur a,b,*, C. Kwei Lin a a
Agriculture and Aquatic Systems and Engineering Program, Asian Institute of Technology, PO Box 4, Klong Luang, Pathumthani 12120, Thailand b Department of Aquaculture, Faculty of Agriculture, Kochi University, Nankoku, Kochi 783-8502, Japan Received 19 March 2002; accepted 9 October 2002
Abstract An experiment was conducted for intensive culture of shrimp (Penaeus monodon ) in concrete tanks for a period of 90 days without water exchange (closed system) to determine the effects of stocking density (25 and 50 juveniles per m2) and bottom substrate (soil and concrete) on water quality, shrimp growth performance, and nutrient distribution and budget. Total ammonia and nitrite /nitrogen concentrations in all the treatments remained low in the safe range for shrimp during the study period. Shrimp weight gain and production was higher in the treatment with higher stocking density. Shrimp survival and FCR were not significantly different among the treatments. Nutrient budget revealed that shrimp could assimilate only 23 /31% nitrogen and 10 /13% phosphorus of the total inputs. The major source of nutrient input was feed, shrimp feed accounted for 76 /92% nitrogen and 70 /91% phosphorus of the total inputs. The major sinks of nutrients were in the sediment, which accounted for 14 /53% nitrogen and 39 /67% phosphorus of the total inputs. The drained water at harvest contained 14 /28% nitrogen and 12 /29% phosphorus of the total inputs. The study has demonstrated that closed shrimp culture system can maintain acceptable water quality for shrimp growth and reduce nutrient loss through pond effluents. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Shrimp culture; Closed system; Water quality; Growth; Nutrient budget
* Corresponding author. Tel.: /81-88-864-5160; fax: /81-88-864-5197 E-mail address: [email protected] (D.P. Thakur). 0144-8609/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 4 4 - 8 6 0 9 ( 0 2 ) 0 0 0 5 5 - 9
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1. Introduction Intensive shrimp farming has been developed steadily over the last decade in response to increasing world market demand. The production system evolved from extensive toward intensive with increasing inputs of high quality feed and water supply. Consequently, waste loads from culture ponds as uneaten feed and metabolic wastes was increased (Lin, 1995). In traditional intensive shrimp culture, the deteriorated pond water is frequently exchanged with new external water supply to maintain desirable water quality for shrimp growth (Wang, 1990; Boyd and Musig, 1992; Hopkins et al., 1993). The nutrient laden effluent discharged from shrimp farms can cause eutrophication of coastal waters and its impact has been a major environmental concern (Phillips et al., 1993; Hopkins et al., 1995; Shang et al., 1998). Previous reports on nutrient budget reveal that in such open shrimp culture system as much as 90% of the nitrogen and phosphorous input is in the form of feed, out of which the major portion is lost to the system with only less than one sixth being assimilated in the shrimp biomass (Muthuvan, 1991; Briggs and Funge-Smith, 1994). Furthermore, nitrogen waste produced in the system (e.g. ammonia and nitrite) that exceed the assimilating capacity of receiving waters lead to deterioration of water quality ultimately making the environment toxic to shrimp. Shrimp aquaculture growth in Asia has suffered many problems in recent years, and the major factor contributing to the problem in sustaining shrimp aquaculture are disease outbreaks, environmental degradation and poor management practice (Primavera, 1998). There is a pressing need for the development and dissemination of a range of shrimp culture systems that are both environmentally and economically sustainable (Funge-smith and Briggs, 1998). To mitigate the environmental impacts of effluent discharge and to reduce the risk of disease contamination from externally polluted water supply, the intensive shrimp culture in recent years has evolved from ‘open system’ with frequent water discharge to ‘closed system’ with little or ‘zero’ water discharge. However, the major problem associated with closed system is the rapid eutrophication in ponds, resulting from increasing concentrations of nutrients and organic matters over the culture period. The super-eutrophic pond water can lead to the flash point of pond carrying capacity by adverse pond environment (Lin, 1995). Obviously, the balance between waste production and assimilation capacity in pond environment is of paramount importance for the success of closed system. The closed systems need to take full account of waste impact on growth of culture organisms, mortality, and the overall expansion of total biomass in the production system (Richard et al., 1995). Manipulation of the environment to favor greater production requires an understanding of basic physical, chemical and biological processes (Boyd, 1986). To understand the chemical processes, information on the fate of the added nutrient, particularly nitrogen and phosphorous, is essential. The establishment of the nutrient budget in the pond is the basic step for the quantitative study of food utilization efficiency, pond fertility, water quality and processes in the sediments (Avnimelech and Lacher, 1979). The present study aims to investigate water quality,
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shrimp growth and nutrient distribution in closed intensive shrimp culture tanksystem with muddy bottom or with base bottom.
2. Materials and methods The experiment was carried out at the Asian Institute of Technology (AIT) in Thailand, in 4 m3 concrete tank (2 /2 /1 m) with no water exchange throughout the 90-day culture period. The experiment was designed in a two-by-two factorial with two levels of stocking density (25 and 50 juveniles per m2) as one factor and two types of bottom substrate (soil and concrete) as second factor. Treatments with two levels of stocking density and two types of bottom substrate are abbreviated as C25, C50 (Concrete bottom tanks with 25 and 50 juveniles per m2), and S25, S50 (Soil bottom tanks with 25 and 50 juveniles per m2). All treatments had three replications; and tank allocation for each treatment was completely randomized. In treatment with soil substrate the tank bottom was lined with 5 cm clayey loam farm soil (pH 7.5) before filling water. Culture tanks were filled to 90 cm depth (volume 3.6 m3) with 20 ppt sea water, and each tank was aerated with five pieces of air stone suspended in mid-depth of water column. Tank water level was maintained at 90 cm and 20 ppt salinity throughout the culture period by adding new freshwater (tap water) weekly to make up the loss to evaporation. To stimulate phytoplankton bloom, culture tanks were fertilized with urea (N /27%) and triple super phosphate (P /20%) at the rate of 2.5 and 0.5 g/m2 during the first week prior to shrimp stocking. Fifteen-day-old post larvae of Penaeus monodon (PL15), purchased from a hatchery were nursed for 30 days in concrete tanks at a stocking density of 500 PL per m2. Uniformly sized juveniles were selected for stocking, and batch weights of the juveniles stocked were taken for each tank before being released to experimental tank. The mean shrimp individual weight at stocking ranged from 0.63 / 0.74 g in different treatments. Experimental shrimp were fed with commercial shrimp feed (crude protein content 42%, C.P. Shrimp Feed No. 3, Bangkok, Thailand) ad libitum four times daily at 06:00, 12:00, 18:00 and 22:00 h in a 50 /50 cm net feeding tray which was placed at bottom of each tank during a 2 h feeding time. Feeding was started at a rate of 8% body weight per day, and adjustment was made to actual consumption based on observations in feeding trays 2 h after each feeding. Uneaten feed left in the tray, 2 h after feeding, was not removed but based on any such observation feed amount supplied at the next feeding was readjusted. Total nutrient (N and P) input, output, uptake and accumulation in the culture system during the rearing cycle were measured. The nutrient budget of N and P were calculated based on inputs from water, fertilizer, stocked shrimp and feed; and outputs were calculated based on harvested shrimp, drained water and sediment. Nutrient input and output in the form of water was calculated by multiplying the nutrient concentration with total water volume. Nutrient input in the from of water represents nutrient contained in water on the day of shrimp stocking, as water sample was collected on the same day prior to shrimp stocking. This is to note that
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though the source water was same for all the tanks, after filling water the system went through preparation phase for 1 week before stocking shrimp. Nutrient output in the form of water represents nutrient contained in water on the harvest-day; column water sample was taken to measure final nutrient concentration in water prior to tank draining. Soil samples were collected by taking six cores from each tank, and mixed in a composite sample for analysis; final soil sample was taken on the harvest-day prior to tank draining. Nutrient concentration in the initial and final soil sample was measured to calculate nutrient surplus in the soil over the study period; the calculation was as follows: total nutrient content in sediment /nutrient concentration in the sediment /total mass, total mass was calculated from mean
Fig. 1. Fluctuation of TAN and nitrite-nitrogen concentrations in different treatments over 90-day trial.
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bulk density. In concrete bottom treatment, tank was drained, shrimp harvested, and the sediment deposited was left to dry before being collected to determine the total mass of sediment deposited; nutrient concentration in the sediment sample was determined to calculate total nutrient contained in the sediment. Soil sample taken before stocking shrimp and post harvest were analyzed for total nitrogen (Raive and Avnimelech, 1979) and followed by determination of total ammonia (Solozno, 1969); and total phosphorus content was analyzed by persulphate digestion method and followed by ascorbic acid method (APHA, 1989). Total Nitrogen and phosphorous concentration was also analyzed for shrimp feed, carcasses of juvenile (at stocking) and harvested shrimp following the methods used for sediment. Nutrient inputs in the form of feed was calculated as follows: nutrient (N/P) in feed /nutrient concentration in feed /total amount of feed supplied. Nutrient input and output in the form of shrimp was calculated as follows: nutrient (N/P) in shrimp /nutrient concentration in shrimp carcasses /total shrimp biomass. Mean shrimp weight (batch weight) was determined at initial and final harvest, as well as 20 shrimp from each tank was sampled randomly and batch weight was taken to assess shrimp growth at 2 week intervals throughout the culture period. Water quality of culture tanks measured biweekly at 10.00 h, included total ammonia nitrogen (TAN; Modified Phenate method; Parsons et al., 1984), nitrite nitrogen (Bendschneider and Robinson method; Parsons et al., 1984), total nitrogen (Raive and Avnimelech, 1979; Solozno, 1969), soluble reactive phosphate (SRP; Ascorbic acid method; APHA, 1989), and total phosphorus (APHA, 1989). Chlorophyll-a concentration was determined by flurometer with methanol extraction of the filter (Holm-Hansen and Riemann, 1978). Temperature, dissolve oxygen (DO), and pH (at 20 cm below the water surface) were measured weekly in situ. Water quality data were analyzed by repeated measure two-way ANOVA using ‘JMP statistical software’ (SAS institute Inc. Cary, NC, USA). Shrimp growth, survival, production and nutrient budget data were analyzed by two-way ANOVA using ‘STATGRAPHICS 7.0’ (Manugistics, Inc., Maryland, USA), significant differences between the treatment means were compared by LSD test. Differences were considered significant at an alpha level of 0.05.
3. Results 3.1. Water quality parameters TAN concentration fluctuated largely over the study period (Fig. 1). In the treatment S50 and C50, TAN concentration peaked between the fourth week and the eighth week followed by sharp decline at the end of the rearing cycle. TAN concentration in the treatments with lower stocking density showed different pattern for S25 and C25; in S25 a sharp increase was recorded from the fourth week of experiment up to the eighth week followed by decline at the end, however, in C25 TAN concentration remained almost flat in the later half of the experiment. Moreover, the TAN in all the treatments remained low ( B/1 mg/l) over the
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experimental period. In this study, though TAN concentration was not significantly affected by any of the treatment either stocking density or bottom substrate relatively, higher TAN concentration was recorded in soil bottom substrate treatments than concrete bottom (Table 1). NO2-N concentration in all treatments remained low below 0.003 mg/l up to the second week of the study (Fig. 1). The highest NO2-N concentration (0.648 mg/l) was recorded in the treatment S50 on the eighth week which dropped remarkably at the end of the rearing cycle; In rest of the treatments, NO2-N concentration remained below 0.01 mg/l till sixth week of rearing and then showed an increasing trend in the later part of the study. NO2-N concentration was significantly affected by the treatments stocking density, bottom substrate, and their interaction was also significant (Table 1). Total nitrogen (TN) concentration in all the treatments fluctuated without any definite trend up to the sixth week of the study, since thereafter, sharp increase was observed till the end of the rearing cycle (Fig. 2). The highest TN concentration (6.4 mg/l) was recorded in the treatment S50 during last sampling. Over the study period, mean TN concentration ranged from 3.0 to 3.7 mg/l, and was not significantly affected by the treatments either stocking density or bottom substrate (Table 1). Total phosphorus (TP) concentration in all the treatments decreased to a minimum in the second week of the study (Fig. 2). In the treatments with lower stocking density TP concentration showed alternate (biweekly) decline and surge throughout the study period. Among higher stocking density treatments, exponential increase in TP concentration was observed in the treatment C50 from the second week of culture till the end of the study; however, in S50 TP concentration increased till the sixth or 7 week of rearing followed by a decline in the later part of the study. Mean TP concentration ranged from 517 to 864 mg/l, and the value was significantly higher in the treatments with higher stocking density. There was no significant effect of the treatment bottom substrate on TP concentration. Mean SRP concentration ranged from 218 to 384 mg/l, and was significantly higher in the treatments with higher stocking density (Table 1). SRP concentration in different treatments fluctuated over the study period, fluctuations in SRP concentration showed similar trend as that observed for TP (Fig. 3). The high initial chlorophyll-a concentration in all the treatments, dropped to the lowest value on the second week of the study and then showed increasing trend till the end of the rearing; with the exception of the eighth week sampling when a slight drop in chlorophyll-a concentration was noticed in some treatments (Fig. 3). Chlorophyll-a concentration was significantly different with the treatment stocking density, however, the effect of the treatment bottom substrate on Chlorophyll-a concentration was not significant (Table 1). Over the study period, DO ranged from 4.4 to 7.4 mg/l in the morning and from 6.7 to 11.9 mg/l in the afternoon, water temperature ranged from 26.8 to 31 8C in the morning and 29 /34.5 8C in the afternoon (Table 1). Column water pH ranged from 7.0 to 8.9 in the morning and from 7.3 to 9.4 in the afternoon. No significant difference was observed in DO, water temperature and water pH among the treatments over the study period.
Variable
Water quality variable Temperature (6 h 8C) Temperature (16 h 8C) DO (6 h mg/l) DO (16 h mg/l) PH (6 h) PH (16 h) TAN (mg/l)b NO2-N (mg/l)b Total nitrogen (mg/l) SRP (mg/l)b Total Phosphorus (mg/l)b Chlorophyll-a (mg/l)b Pond management variable Total input of N (g/m2) Total input of P (g/m2)
Stocking density 25 PL per m2 bottom substrate
Stocking density 50 PL per m2 bottom substrate
Significancea
Soil (S25)
Concrete (C25)
Soil (S50)
Concrete (C50)
D
B
D /B
27.0 /30.8 29.3 /34.5 5.4 /7.2 7.0 /11.6 7.0 /8.9 7.3 /9.4 276.89/90.1 38.19/14.9 3.49/0.04 218.09/23.1 539.49/15.8 288.09/37.9
26.8 /30.8 29.0 /34.0 5.5 /7.2 7.1 /10.6 7.0 /8.9 7.7 /9.4 1989/53.5 16.79/9.8 3.09/0.27 311.19/47.5 517.39/70.4 199.89/34.8
26.8 /30.8 29.0 /33.5 4.7 /7.2 6.7 /11.9 7.0 /8.9 7.2 /9.4 519.19/92.9 232.69/52.1 3.79/0.1 363.79/46.0 666.59/46.3 301.19/50.9
26.8 /31 29.2 /34.5 4.4 /7.4 6.9 /11.0 7.1 /8.8 7.7 /9.3 281.99/97.3 50.89/40.2 3.39/0.43 384.39/76.9 864.09/157.1 415.99/15.8
ns ns ns ns ns ns ns xxx ns xx xx xx
ns ns ns ns ns ns ns xx ns ns ns ns
ns ns ns ns ns ns ns xx ns ns ns xx
16.29/0.8 2.39/0.1
14.09/0.8 2.29/0.1
40.69/1.0 5.89/0.1
23.49/7.8 3.89/1.3
xxx ns ns xxx ns ns
Values are mean9/S.E. (n /3 for each treatment). For each treatment, water quality data for all sampling times were averaged. The range (minimum and maximum) for temperature, DO and pH was given for each treatment. a Results from repeated measure two-way ANOVA for water quality data and two-way ANOVA for nutrient input data; D /stocking density; B /bottom substrate; DXB /stocking density X bottom substrate interaction. b Data transformed (log x ) prior to statistical analysis.
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Table 1 Effects of stocking density (25 and 50 PL per m2) and bottom substrate (soil and concrete) on water quality and pond management variables in tanks stocked with P. monodon during a 90-day trial
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Fig. 2. Fluctuation of total nitrogen and total phosphorus concentrations in different treatments over 90day trial.
3.2. Growth, survival and production of shrimp The results of shrimp performance indices are presented in Table 2. Shrimp growth (weight gain) was significantly higher in the treatments with higher stocking density but the treatment with bottom substrate had not significantly affected shrimp growth. Mean shrimp survival rate ranged from 50 to 78% in different treatments,
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Fig. 3. Fluctuation of SRP and chlorophyll-a concentrations in different treatments over 90-day trial.
and was not significantly affected by any of the treatments either stocking density or bottom substrate. Shrimp survival rate ranged widely due to the extremely low survival rate (35 and 38%) observed in the two tanks of treatment C50, while the third tank of the same treatment had fairly good survival rate (78%). Average daily weight gain ranged from 0.11 to 0.14 g per shrimp per day in all the treatments, and was not significantly different among the treatments. FCR was widely ranged from 1.3 to 2.1, and was not significantly different among the treatments.
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Variable
Weight gain (g per shrimp) Survival (%)b Net biomass gained (g per tank) Production (g/m2 per day) Growth rate (g per shrimp per day) Food conversion ratioc
Stocking density 25 PL per m2 bottom substrate
Stocking density 50 PL per m2 bottom substrate
Significancea
Soil (S25)
Concrete (C25)
Soil (S50)
Concrete (C50)
D
B
D /B
7.69/0.3 629/3 440.19/30.1 1.89/0.1 0.119/0.0 1.839/0.1
7.49/0.3 589/5 400.19/49.3 1.69/0.2 0.119/0.0 1.599/0.1
9.69/0.6 789/4 1461.19/68.9 5.89/0.3 0.149/0.0 1.529/0.1
8.69/1.1 509/14 853.19/396.0 3.49/1.6 0.129/0.0 1.539/0.2
xx ns xxx xxx ns ns
ns ns ns ns ns ns
ns ns xx xx ns ns
Values are mean9/S.E. (n /3 for each treatment). a Results from two-factor ANOVA; D /stocking density; B /bottom substrate; D /B /stocking density /bottom substrate interaction. b Data transformed (arcsine x05) prior to statistical analysis. c Data transformed (log x ) prior to statistical analysis.
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Table 2 Effects of stocking density (25 and 50 PL per m2) and bottom substrate (soil and concrete) on shrimp performance indices in tanks stocked with Peaneus monodon during a 90-day trial
Treatments
Inputs
Outputs
Water
Fertilizer
Shrimp
Feed
Nitrogen S25 (g per tank) (%) C25 (g per tank) (%) S50 (g per tank) (%) C50 (g per tank) (%)
5.79/1.9 8.89/2.4 8.59/0.8 15.29/1.9 6.19/2.1 3.89/1.4 4.69/1.5 4.99/2.5
2.79/0.0 4.29/0.2 2.79/0.0 4.89/0.3 2.79/0.0 1.79/0.04 2.79/0.0 2.99/0.9
2.19/0.4 3.29/0.5 2.09/0.2 3.69/0.2 3.59/0.5 2.19/0.3 4.39/0.4 4.69/1.5
54.39/1.48 83.89/2.1 42.79/3.7 76.49/2.2 150.29/5.3 92.49/1.4 81.99/31.03 87.69/3.6
Phosphorus S25 (g per tank) (%) C25 (g per tank) (%) S50 (g per tank) (%) C50 (g per tank) (%)
1.19/0.4 11.79/4.3 2.09/0.2 23.29/2.2 1.49/0.7 6.09/2.9 2.89/0.9 18.49/1.7
0.49/0.0 4.39/0.2 0.49/0.0 4.79/0.3 0.49/0.0 1.79/0.02 0.49/0.0 2.69/0.7
0.29/0.04 2.19/0.5 0.29/0.0 2.39/0.1 0.49/0.1 1.79/0.2 0.49/0.0 2.69/0.7
7.79/0.2 81.99/3.8 6.09/0.5 69.89/2.4 21.29/0.7 90.69/2.8 11.69/4.4 76.39/2.4
Total
Water
Shrimp
Sediment
Total
Unaccounted
64.89/3.3 100 55.99/3.4 100 162.59/3.8 100 93.59/31.4 100
18.49/1.2 28.49/3.2 13.89/1.4 24.79/2.9 22.99/1.1 14.19/0.7 17.99/3.7 19.19/3.9
14.89/0.8 22.89/0.5 13.49/1.6 23.99/2.0 45.49/2.4 27.99/1.4 28.79/11.5 30.79/2.9
26.59/3.5 40.99/3.8 15.89/1.4 28.39/4.5 85.89/0.0 52.89/1.3 13.29/1.3 14.19/3.0
59.79/2.9 92.19/0.9 43.09/2.5 76.99/7.2 154.19/1.6 94.89/1.7 59.89/15.7 64.09/6.3
5.19/0.7 7.99/0.9 12.99/4.4 23.19/7.2 8.49/2.9 5.29/1.7 33.79/15.7 36.09/5.2
9.49/0.3 100 8.69/0.6 100 23.49/0.2 100 15.29/5.3 100
2.29/0.4 23.49/4.0 1.49/0.1 16.39/1.2 2.99/0.4 12.49/1.8 4.49/0.98 28.99/9.1
1.09/0.1 10.69/0.4 0.99/0.1 10.59/0.6 3.09/0.2 12.89/0.6 1.99/0.8 12.59/1.1
5.79/0.3 60.69/2.7 4.99/0.1 57.09/3.2 15.69/0.5 66.79/1.6 5.99/1.7 38.89/8.3
8.99/0.3 94.79/1.3 7.29/0.3 83.79/4.1 21.59/0.5 91.99/2.3 12.29/2.9 80.29/6.9
0.59/0.14 5.39/1.3 1.49/0.5 16.39/4.1 1.99/0.5 8.19/2.3 3.09/2.4 19.79/9.3
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Table 3 Nutrient budget for different treatments during a 90-day trial
Values are mean9/S.E. (n /3).
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3.3. Nutrient budget Table 3 shows the nutrient budget for all the treatments over the experimental period. Nutrient budget revealed that shrimp feed was the major input of nitrogen and phosphorous. Nitrogen inputs in the form of feed ranged from 76.4 to 92.4%, and phosphorous input through feed ranged from 69.8 to 90.6% of the total inputs. In addition, nitrogen and phosphorous inputs through feed were significantly higher in the treatments with higher stocking density (Table 1). Nutrient budget showed that the major portion of the nutrient inputs were deposited in the sediment followed by nutrient contained in discharge water at harvest, and relatively smaller fraction was retained by the shrimp. Percentage nitrogen accumulated into sediments ranged from 14.1 to 52.8% and was significantly higher in the treatments with soil bottom substrate. Percentage phosphorous accumulated into sediment ranged from 38.8 to 66.7% and was also significantly higher in the treatments with soil bottom substrate. However, nitrogen and phosphorous accumulated into sediment were not significantly affected by stocking density. Nitrogen and phosphorous retained in the shrimp ranged from 22.8 to 30.7 and 10.5 /12.8% of the total inputs, and were not significantly different among the treatments either stocking density or bottom substrate. Nitrogen output in the discharged water during harvest ranged from 14.1 to 28.4%, and was significantly higher in the treatments with lower stocking density, although no significant effect of bottom substrate was observed. Phosphorus output in the discharged water during harvest ranged from 12.4 to 28.9% and was not significantly different among the treatments with factor either stocking density or bottom substrate. In addition, nutrient budgets revealed that during the rearing cycle some of the nitrogen and phosphorous went unaccounted. Unaccounted nitrogen ranged from 5.2 to 36.0% of the total inputs, and was significantly higher for the treatments with concrete bottom substrate. Unaccounted phosphorous ranged from 5.3 to 19.7%, and was also significantly higher for the treatments with concrete bottom substrate.
4. Discussion 4.1. Water quality parameters In the present study, shrimp growth was not limited by any of the water quality parameters. TAN and NO2-N concentrations were remained fluctuating but never went beyond safe level during the study period (Chen et al., 1990). Moreover, it was reported earlier that in P. monodon grow out system, even with frequent water exchange, ammonia may increase up to 6.5 mg/l (Chen and Tu, 1991). In contrast, ammonia and nitrite nitrogen in this study never exceeded the safe range indicating that during the study period shrimp biomass was very much below the carrying capacity of the system, and the deleterious nitrogenous waste was effectively removed by phytoplankton and microbial activity (Shilo and Rimon, 1982; Diab and Shilo, 1988). In this study, ammonia nitrogen peak was observed on the eighth
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week of rearing followed by increase in nitrite nitrogen on the tenth week, suggests that the system took 8 weeks to establish nitrification process (Mevel and Chamroux, 1981). Total nitrogen and total phosphorus concentration in water increased with the progress of rearing. Throughout the study, soluble reactive phosphorus concentration remained much lower than total phosphorous concentration, suggesting that a large portion of water phosphorus was contained in suspended solids as well. In addition, SRP concentration remained high throughout the rearing, the highest concentration observed was 1100 mg/l. The result is in contrast to the report of Boyd (1990) who mentioned that dissolved orthophosphate concentration are usually not greater than 5/20 mg/l, and seldom exceed 100 mg/l even in highly eutrophic water. In the present study water nutrient concentration was linearly correlated with the cumulative feed input (data not shown). Obviously, being a closed system nutrient kept on accumulating within the system over time; this might be an advantage of the closed system as the high nitrogen and phosphorus accumulated within the system could support the growth of natural food organisms contributing ultimately to the shrimp growth. This view is in agreement with the report of Allan et al. (1995) who observed that prawn grow faster in prepared ponds where meiofauna is abundant. Chlorophyll-a concentration increased with the progress of rearing, and at the end of the study chlorophyll-a reached as high as 808 mg/l which was higher than the range reported previously for shrimp ponds (Lin, 1986; Martin et al., 1998). Moreover, in the present study, chlorophyll-a concentration remained relatively high (/94 mg/l) in all the treatments from fourth week of the rearing till the end of the study, indicating that the system never became nutrient limiting, and thus, in turn, sustained high phytoplankton biomass. Seemingly, dissolve nutrients together with the high light intensity, and warm temperature supported active growth of phytoplankton; which helped to condition the water quality in the tank by the production of oxygen and uptake of dissolved nutrients (Krom and Neori, 1989). 4.2. Growth, survival and production of shrimp In this study higher weight gain of shrimp in the treatments with stocking density 50 juveniles per m2 than the treatments 25 juveniles per m2 suggested that the biomass of shrimp in the culture tanks were well within the carrying capacity of the system. Furthermore, though in the present study greater shrimp growth was observed in high stocking density treatment there are no apparent factors that show obvious difference to affect shrimp growth in the water quality data, except the chlrophyll-a concentration, which was significantly higher in high stocking density treatment. This may indicate that there was greater amount of natural food in detrial from in those high-density tanks and had contributed to the better shrimp growth. Shrimp growth rate obtained in the present study seems relatively slow as compared with the previous reports (Chen et al., 1989; Lumare et al., 1993). However, growth rate observed in this study is comparable to the report of Muthuvan (1991) for commercial ponds stocked with P. monodon. Sandifer et al. (1991) also reported similar growth rate 0.94 g per shrimp per week for supper intensive pond production
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system. Production rate (17 /58 kg/ha per day) observed in this study could be reasonably compared with production rate of most of the commercial systems (Chen and Wang, 1990; Lumare et al., 1993). In the present study, shrimp production increased significantly with the stocking density, this is in agreement with the report of Allan and Maguire (1992). Shrimp survival rate observed in the present study was lower than that reported by Liao (1987); however, there was no significant difference in shrimp survival rates among the treatments. This suggested that increasing stocking density up to 50 shrimp per m2 in closed system had not affected shrimp survival; and lower survival rate observed in the present study as compared with the report of Liao (1987) could be ascribed to any other reason rather over crowding. This contention is in agreement with the observation of Allan and Maguire (1992) who reported that increasing stocking densities from 5 to 40 shrimps/m2 had no effect on survival of P. monodon. Moreover, in the present study the exceptionally low survival rate (35 and 38%) was observed in two tanks could not be related to any of the water quality parameter measured. This is to mention here that TAN and NO2-N concentration in those tanks remained relatively low (TAN B/0.81 mg/l; NO2-N B/0.09), and therefore, could not explain the low survival rate. FCR value observed in this study could be reasonably compared with the previous reports on P. monodon culture (Chen et al., 1989; Lumare et al., 1993). Sandifer et al. (1991) reported that intensive shrimp ponds typically have a feed conversion ratio of 2.0 or above. The low FCR value obtained in this study may be ascribed to the strict control of feeding by trays as well as the build up of benthic population over time might have supported shrimp growth. This study showed the potential of closed system in producing shrimp with low FCR, as in the closed system nutrient and organic matter released to the system could be accounted well for the production of natural food organisms. Reymond and Lagardere (1990) reported that in low stocking density culture systems meiofauna may constitute an important part of the feeding. Avnimelech et al. (1994) also emphasized that in ponds operated with high water exchange rate, a large fraction of the feed and the organic matter accumulating in the pond is drained and often wasted. 4.3. Nutrient budget It had been reported previously that the predominant inputs of nitrogen and phosphorus in water exchange shrimp ponds are feed, which accounted 82 /95% nitrogen and 38/91% phosphorus of the total inputs (Muthuvan, 1991; Stapornvanit, 1993; Briggs and Funge-Smith, 1994). In the present study, percentage contribution of feed to the total nutrient inputs was in the same range as reported previously. In addition, percentage nitrogen and phosphorus inputs in the form of feed were significantly higher in the treatment with higher stocking density; the result is in agreement with the previous reports (Martin et al., 1998; Briggs and FungeSmith, 1994). In this study some of the potential source of nitrogen was not examined, such as inorganic nitrogen inputs through precipitation which was considered insignificant for the present study, and nitrogen fixation by blue /green
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algae. In addition, tap-water added to the tanks to make up the loss to evaporation was also not accounted for nutrient budget. Nutrient budget showed that only 22.8 /30.7% N and 10.5 /12.8% P of the total inputs were incorporated into harvested shrimp (Table 3); the remainder in the system as uneaten feed, excreted material went to support high levels of phytoplankton and heterotrophic activity. Nitrogen and phosphorus percentage removed from the system via shrimp harvest is comparable to the previous reports (Muthuvan, 1991; Stapornvanit, 1993; Briggs and Funge-Smith, 1994). Nutrient retained by shrimp was not significantly different among the treatments; implying that in terms of the proportional recovery of the nutrient from closed shrimp culture system, efficiency was not affected by the treatments either stocking density or bottom substrate. Moreover, further accounting of the nutrients budget revealed that rearing 1 kg shrimp resulted into 98 g N and 18 g P loss at stocking density 25 juveniles per m2, whereas, at stocking density 50 juveniles per m2 it resulted into loss of 70 g N and 13 g P; calculation was as follows: Nutrient (N/P) loss /(Mean total nutrient inputs /mean nutrient retained in harvested shrimp)/(Mean shrimp biomass at harvest). This suggests that in closed system rearing shrimp at stocking density 50 juveniles per m2 is more nutrients efficient than rearing at stocking density 25 juveniles per m2. However, Martin et al. (1998) reported that, in water exchange system, the quantity of waste produced per unite shrimp production was proportional to the stocking density as stocking density increased from 1 to 30 shrimp per m2. Nitrogen waste generated in producing one kg of shrimp in the present study was about half of the value, at stocking density 30 shrimp per m2, reported by Martin et al. (1998), while to estimate the nitrogen loss they used nitrogen input from feed only in their calculation. In the present study, the lower amount of nitrogen waste generated with per unit shrimp production could be due to the study being a closed system; In closed shrimp culture system excess nutrient inputs, especially originated from uneaten feed, keep on accumulating within the system which in turn may support the growth of natural food organisms and ultimately the shrimp growth. It was reported previously that the major output of nutrients in water exchange shrimp ponds were in the discharge water (Muthuvan, 1991; Stapornvanit, 1993). In contrast, the present study showed that in closed shrimp culture system loss of nutrient through sediment is higher than the water borne loss. The result is in agreement with the view of Briggs and Funge-Smith (1994) who emphasized that, in culture system with low water exchange, water borne loss of nutrient is less important than loss through the sediment, due to rapid accumulation of sediments in shrimp ponds. Furthermore in the present study, significantly higher nutrients sink into sediments in the treatments with soil bottom substrate, emphasizes the importance of soil bottom in minimizing the water born loss from the system. Chen et al. (1989) mentioned that sediments play an important role in the balance of an aquaculture system, it can act as buffer in water nutrient concentration. Enell and Ackefors (1991) mentioned that approximately 50% of the nitrogen and phosphorous that settle on the bottom is translocated back into water column. Once all the measurable outputs of nutrients had been quantified, 5.2 /36.0% N and 5.3 /19.7% P of the total inputs went unaccounted. We assume that the nitrogen
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might have lost from the system either by ammonia gas volatilization and/or denitrification in the sediment. Daniels and Boyd (1988) mentioned that the most probably loss of nitrogen is by ammonia gas volatilization, which is further enhanced by vigorous aeration and high pH in the tanks. Unaccounted phosphorus is likely to the result of muds adsorption, as muds were known to have a strong affinity for phosphorous (Boyd, 1985). However, from the nutrient budget data (Table 3) it is apparent that larger percentage of the total nutrient inputs went unaccounted in the treatments with concrete bottom than the muddy bottom treatments; procedural difference (as explained in the Section 2) in estimating nutrient output in the form of sediment between the two treatments might have caused this discrepancies. It is likely that some of the sediment deposited in the concrete bottom tank could have washed out of the tank with the drained water during harvest, and thus could have went unaccounted for the nutrient budget. In conclusion, this study provides valuable information on the management practice for closed shrimp culture systems, as a viable alternative to the current water exchange systems. The study demonstrated that closed culture system could maintain acceptable water quality for shrimp culture, and shrimp could be grown well at stocking density of 50 juveniles per m2. In addition, the study showed that closed shrimp culture system can reduce the nutrient loss through pond effluents and thus minimize the environment impacts of shrimp culture. Nevertheless, the total quantity of nutrient released in closed shrimp culture system may be same to the water exchange system, the small volumes of concentrated effluent produced at harvest in closed system should be easier for the shrimp grower to treat before being discharged to the external environment.
Acknowledgements The authors wish to thank Dr J.B. Hambrey for his suggestions and encouragement during all phase of the study. Thanks extended to the staffs of Aquaculture Program, Asian Institute of Technology, for their field and lab assistance. First author, would like to thank D.D. Kassam, Department of Aquaculture, Kochi University for his help in statistical analysis of data. Funding from the Government of Denmark through DANIDA, AIT, grant is gratefully acknowledged.
References Allan, G.L., Maguire, G.B., 1992. Effect of stocking density on production of Penaeus monodon fabricius in model farming ponds. Aquaculture 107, 49 /66. Allan, G.L., Moriarty, D.J.W., Maguire, G.B., 1995. Effects of pond preparation and feeding rate on production of Penaeus monodon fabricius, water quality, bacteria and benthos in model farming ponds. Aquaculture 130, 329 /349. APHA, 1989. Standard Methods for the Examination of Water and Waste Water, 17th ed.. American Public Health Association, American Water Works Association, Water Pollution Control Federation, Washington, DC.
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Avnimelech, V., Lacher, M., 1979. A tentative nutrient balance for intensive fish ponds. Bamidgeh 31, 3 / 8. Avnimelech, Y., Kochaba, M., Diab, S., 1994. Development of controlled intensive aquaculture with a limited water exchange and adjusted carbon to nitrogen ratio. Bamidgeh 46, 119 /131. Boyd, C.E., 1985. Chemical budgets for channel catfish ponds. Trans. Am. Fish. Soc. 114, 291 /298. Boyd, C.E., 1986. Component of development techniques for management of environment quality in aquaculture. Aquacult. Eng. 5, 135 /146. Boyd, C.E., 1990. Water Quality in Ponds for Aquaculture, 473p. Alabama Agriculture Experiment Station, Auburn University, Alabama, pp. 379 /380. Boyd, C.E., Musig, Y., 1992. Shrimp pond effluents: observation of nature of the problem on commercial farms. In: Wyban, J. (Ed.), Proceedings of The Special Session on Shrimp Farming. World Aquaculture Society, Baton Rouge, LA, pp. 195 /197. Briggs, M.R.P., Funge-Smith, S.J., 1994. A nutrient budget of some intensive marine ponds in Thailand. Aquacult. Fish. Manage. 24, 789 /811. Chen, J.C., Wang, T.C., 1990. Culture of Tiger shrimp and Red tailed shrimp in a semi-static system. In: Hirano, R., Hanyu, I. (Eds.), The Second Asian Fisheries Forum. 991p. Asian Fisheries Society, Manila, Philippines, pp. 77 /79. Chen, J.C., Tu, C.C., 1991. Influence of ammonia on growth of Penaeus monodon Fabricius post-larvae. Aquacult. Fish. Manage. 22, 457 /462. Chen, J.C., Liu, P.C., Lin, Y.T., 1989. Culture of Penaeus monodon in an intensified system in Taiwan. Aquaculture 77, 319 /328. Chen, J.C., Liu, P.C., Lie, S.C., 1990. Toxicity of ammonia and nitrite to Penaeus monodon adolescents. Aquaculture 89, 127 /137. Daniels, H.V., Boyd, C.E., 1988. Pond bottom degradation. Coastal Aquacult. 5, 2 /5. Diab, S., Shilo, M., 1988. Effect of light on the activity and survival of Nitrosomonas sp. and Nitrobacter sp. isolated from fish pond. Bamidgeh 40, 50 /56. Enell, M., Ackefors, H., 1991. Nutrient discharges from aquaculture operations in Nordic countries into adjacent sea water. In: The International Council for Exploration of the Sea Committee Meeting. 1991/ F 56, p. 17. Funge-smith, S.J., Briggs, M.R.P., 1998. Nutrient budgets in intensive shrimp ponds: implications for sustainability. Aquaculture 164, 117 /133. Holm-Hansen, O., Riemann, B., 1978. Chlorophyll-a determination: improvement in methodology. Oikos 30, 438 /447. Hopkins, J.S., Hamilton, R.D., II, Sandifer, P.A., 1993. Effect of water exchange rate on production, water quality, effluent characteristics and nitrogen budget of intensive shrimp ponds. J. World Aquacult. Soc. 24, 304 /320. Hopkins, J.S., Sandifer, P.A., Browdy, C.L., 1995. A review of water management regimes which abate the environmental impact of shrimp farming. In: Browdy, C.L., Hopkins, J.S. (Eds.), Swimming Through Troubled Water. World Aquaculture Society, Baton Rouge, LA, pp. 13 /22. Krom, M.D., Neori, A., 1989. A total nutrient budget for an experimental intensive fish pond with circulatory moving seawater. Aquaculture 15, 185 /193. Liao, C., 1987. Prawn culture from Taiwan’s experience. Round Table Panel, 18th annual meeting of World Aquaculture Society, Contrib. B-52 Tungkang Marine Laboratory, Guayaquil, Ecuador, 18 /23 January 1987, p. 24. Lin, C.K., 1986. Biological principles of pond culture: phytoplankton and macrophytes. In: Lannan, J.E., Smitherman, R.O., Tchobanoglous, G. (Eds.), Principles and Practices of Pond Aquaculture, Oregon, p. 252. Lin, C.K., 1995. Progression of Intensive marine shrimp culture in Thailand. In: Browdy, C., Hopkins, J.S. (Eds.), Swimming Through Troubled Water. World Aquaculture Society, Baton Rouge, LA, pp. 13 / 22. Lumare, F., Muro, P.D., Tenderini, L., Zupo, V., 1993. Experimental intensive culture of Penaeus monodon in the cold-temperate climate of the North-East Coast of Italy. Aquaculture 113, 231 /241.
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Martin, J.L.M., Veran, Y., Guelorget, O., Pham, D., 1998. Shrimp rearing: Stocking density, growth, impact on sediment, waste output and their relationships studied through the nitrogen budget in rearing ponds. Aquaculture 164, 135 /149. Mevel, G., Chamroux, S., 1981. A study on nitrification in the presence of prawn (P. Japonicus ) in marine closed systems. Aquaculture 23, 29 /43. Muthuvan, V., 1991. Nutrient budget and water quality in intensive marine shrimp culture ponds. AIT master thesis AE 91 /40. Parsons, T.R., Yoshiaki, M., Carol, M.L., 1984. A Manual of Chemical and Biological Methods for Seawater Analysis. Pergamon Press, Oxford, p. 173. Phillips, M.J., Lin, C.K., Bereridge, M.C.M., 1993. Shrimp culture and the environmental-lessons from the World’s most rapidly expanding warm water aquaculture sector. In: Pullin, R.S.V., Rosenthal, H., Maclean, J.L. (Eds.) Environment and Aquaculture in Developing Countries. ICLARM Conference Proceeding, vol. 31, pp. 171 /197. Primavera, J.H., 1998. Tropical shrimp farming and its sustainability. In: De Silva, S.S. (Ed.), Tropical Mariculture. Academic Press, San Diego, pp. 257 /289. Raive, A., Avnimelech, Y., 1979. Total nitrogen analysis in water, soil and plant material with persulphate oxidation. Water Res. 13, 911 /912. Reymond, H., Lagardere, J.P., 1990. Feeding rhythms and food of Penaeus japonicus Bate (crustacea, Penaeidae) in salt marsh ponds: role of halophilic entomofaunaphona. Aquaculture 84, 125 /143. Sandifer, P.A., Hopkins, J.S., Stokes, A.D., Pruder, G.D., 1991. Technological advances in intensive pond culture of shrimp in the United States. In: DeLoach, P., Dougherty, W.J., Davidson, M.A. (Eds.), Frontiers in Shrimp Research. Elsevier, Amsterdam, pp. 241 /256. Sandifer, P.A., Hopkins, J.S., Stokes, A.D., Pruder, G.D., 1991. Technological advances in intensive pond culture of shrimp in the United States. In: DeLoach, P., Dougherty, W.J., Davidson, M.A. (Eds.), Frontiers in Shrimp Research. Elsevier, Amsterdam, pp. 241 /256. Shang, Y.C., Leung, P., Ling, B.H., 1998. Comparative economics of shrimp farming in Asia. Aquaculture 164, 183 /200. Shilo, M., Rimon, A., 1982. Ammonia transformation in intensive fish ponds. Bamidgeh 34, 101 /114. Solozno, L., 1969. Determination of ammonia in natural water by the phenolhypochlorite method. Limnol. Oceanogr. 5, 799 /801. Stapornvanit, K., 1993. The environmental impact of shrimp farm effluent. AIT thesis AE-93-30. Wang, J.K., 1990. Managing shrimp pond water to reduce discharge problems. Aquacult. Eng. 9, 61 /73.
Water quality and nutrient budget in closed shrimp (Penaeus monodon) culture systems Dhirendra Prasad Thakur a,b,*, C. Kwei Lin a a
Agriculture and Aquatic Systems and Engineering Program, Asian Institute of Technology, PO Box 4, Klong Luang, Pathumthani 12120, Thailand b Department of Aquaculture, Faculty of Agriculture, Kochi University, Nankoku, Kochi 783-8502, Japan Received 19 March 2002; accepted 9 October 2002
Abstract An experiment was conducted for intensive culture of shrimp (Penaeus monodon ) in concrete tanks for a period of 90 days without water exchange (closed system) to determine the effects of stocking density (25 and 50 juveniles per m2) and bottom substrate (soil and concrete) on water quality, shrimp growth performance, and nutrient distribution and budget. Total ammonia and nitrite /nitrogen concentrations in all the treatments remained low in the safe range for shrimp during the study period. Shrimp weight gain and production was higher in the treatment with higher stocking density. Shrimp survival and FCR were not significantly different among the treatments. Nutrient budget revealed that shrimp could assimilate only 23 /31% nitrogen and 10 /13% phosphorus of the total inputs. The major source of nutrient input was feed, shrimp feed accounted for 76 /92% nitrogen and 70 /91% phosphorus of the total inputs. The major sinks of nutrients were in the sediment, which accounted for 14 /53% nitrogen and 39 /67% phosphorus of the total inputs. The drained water at harvest contained 14 /28% nitrogen and 12 /29% phosphorus of the total inputs. The study has demonstrated that closed shrimp culture system can maintain acceptable water quality for shrimp growth and reduce nutrient loss through pond effluents. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Shrimp culture; Closed system; Water quality; Growth; Nutrient budget
* Corresponding author. Tel.: /81-88-864-5160; fax: /81-88-864-5197 E-mail address: [email protected] (D.P. Thakur). 0144-8609/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 4 4 - 8 6 0 9 ( 0 2 ) 0 0 0 5 5 - 9
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1. Introduction Intensive shrimp farming has been developed steadily over the last decade in response to increasing world market demand. The production system evolved from extensive toward intensive with increasing inputs of high quality feed and water supply. Consequently, waste loads from culture ponds as uneaten feed and metabolic wastes was increased (Lin, 1995). In traditional intensive shrimp culture, the deteriorated pond water is frequently exchanged with new external water supply to maintain desirable water quality for shrimp growth (Wang, 1990; Boyd and Musig, 1992; Hopkins et al., 1993). The nutrient laden effluent discharged from shrimp farms can cause eutrophication of coastal waters and its impact has been a major environmental concern (Phillips et al., 1993; Hopkins et al., 1995; Shang et al., 1998). Previous reports on nutrient budget reveal that in such open shrimp culture system as much as 90% of the nitrogen and phosphorous input is in the form of feed, out of which the major portion is lost to the system with only less than one sixth being assimilated in the shrimp biomass (Muthuvan, 1991; Briggs and Funge-Smith, 1994). Furthermore, nitrogen waste produced in the system (e.g. ammonia and nitrite) that exceed the assimilating capacity of receiving waters lead to deterioration of water quality ultimately making the environment toxic to shrimp. Shrimp aquaculture growth in Asia has suffered many problems in recent years, and the major factor contributing to the problem in sustaining shrimp aquaculture are disease outbreaks, environmental degradation and poor management practice (Primavera, 1998). There is a pressing need for the development and dissemination of a range of shrimp culture systems that are both environmentally and economically sustainable (Funge-smith and Briggs, 1998). To mitigate the environmental impacts of effluent discharge and to reduce the risk of disease contamination from externally polluted water supply, the intensive shrimp culture in recent years has evolved from ‘open system’ with frequent water discharge to ‘closed system’ with little or ‘zero’ water discharge. However, the major problem associated with closed system is the rapid eutrophication in ponds, resulting from increasing concentrations of nutrients and organic matters over the culture period. The super-eutrophic pond water can lead to the flash point of pond carrying capacity by adverse pond environment (Lin, 1995). Obviously, the balance between waste production and assimilation capacity in pond environment is of paramount importance for the success of closed system. The closed systems need to take full account of waste impact on growth of culture organisms, mortality, and the overall expansion of total biomass in the production system (Richard et al., 1995). Manipulation of the environment to favor greater production requires an understanding of basic physical, chemical and biological processes (Boyd, 1986). To understand the chemical processes, information on the fate of the added nutrient, particularly nitrogen and phosphorous, is essential. The establishment of the nutrient budget in the pond is the basic step for the quantitative study of food utilization efficiency, pond fertility, water quality and processes in the sediments (Avnimelech and Lacher, 1979). The present study aims to investigate water quality,
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shrimp growth and nutrient distribution in closed intensive shrimp culture tanksystem with muddy bottom or with base bottom.
2. Materials and methods The experiment was carried out at the Asian Institute of Technology (AIT) in Thailand, in 4 m3 concrete tank (2 /2 /1 m) with no water exchange throughout the 90-day culture period. The experiment was designed in a two-by-two factorial with two levels of stocking density (25 and 50 juveniles per m2) as one factor and two types of bottom substrate (soil and concrete) as second factor. Treatments with two levels of stocking density and two types of bottom substrate are abbreviated as C25, C50 (Concrete bottom tanks with 25 and 50 juveniles per m2), and S25, S50 (Soil bottom tanks with 25 and 50 juveniles per m2). All treatments had three replications; and tank allocation for each treatment was completely randomized. In treatment with soil substrate the tank bottom was lined with 5 cm clayey loam farm soil (pH 7.5) before filling water. Culture tanks were filled to 90 cm depth (volume 3.6 m3) with 20 ppt sea water, and each tank was aerated with five pieces of air stone suspended in mid-depth of water column. Tank water level was maintained at 90 cm and 20 ppt salinity throughout the culture period by adding new freshwater (tap water) weekly to make up the loss to evaporation. To stimulate phytoplankton bloom, culture tanks were fertilized with urea (N /27%) and triple super phosphate (P /20%) at the rate of 2.5 and 0.5 g/m2 during the first week prior to shrimp stocking. Fifteen-day-old post larvae of Penaeus monodon (PL15), purchased from a hatchery were nursed for 30 days in concrete tanks at a stocking density of 500 PL per m2. Uniformly sized juveniles were selected for stocking, and batch weights of the juveniles stocked were taken for each tank before being released to experimental tank. The mean shrimp individual weight at stocking ranged from 0.63 / 0.74 g in different treatments. Experimental shrimp were fed with commercial shrimp feed (crude protein content 42%, C.P. Shrimp Feed No. 3, Bangkok, Thailand) ad libitum four times daily at 06:00, 12:00, 18:00 and 22:00 h in a 50 /50 cm net feeding tray which was placed at bottom of each tank during a 2 h feeding time. Feeding was started at a rate of 8% body weight per day, and adjustment was made to actual consumption based on observations in feeding trays 2 h after each feeding. Uneaten feed left in the tray, 2 h after feeding, was not removed but based on any such observation feed amount supplied at the next feeding was readjusted. Total nutrient (N and P) input, output, uptake and accumulation in the culture system during the rearing cycle were measured. The nutrient budget of N and P were calculated based on inputs from water, fertilizer, stocked shrimp and feed; and outputs were calculated based on harvested shrimp, drained water and sediment. Nutrient input and output in the form of water was calculated by multiplying the nutrient concentration with total water volume. Nutrient input in the from of water represents nutrient contained in water on the day of shrimp stocking, as water sample was collected on the same day prior to shrimp stocking. This is to note that
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though the source water was same for all the tanks, after filling water the system went through preparation phase for 1 week before stocking shrimp. Nutrient output in the form of water represents nutrient contained in water on the harvest-day; column water sample was taken to measure final nutrient concentration in water prior to tank draining. Soil samples were collected by taking six cores from each tank, and mixed in a composite sample for analysis; final soil sample was taken on the harvest-day prior to tank draining. Nutrient concentration in the initial and final soil sample was measured to calculate nutrient surplus in the soil over the study period; the calculation was as follows: total nutrient content in sediment /nutrient concentration in the sediment /total mass, total mass was calculated from mean
Fig. 1. Fluctuation of TAN and nitrite-nitrogen concentrations in different treatments over 90-day trial.
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bulk density. In concrete bottom treatment, tank was drained, shrimp harvested, and the sediment deposited was left to dry before being collected to determine the total mass of sediment deposited; nutrient concentration in the sediment sample was determined to calculate total nutrient contained in the sediment. Soil sample taken before stocking shrimp and post harvest were analyzed for total nitrogen (Raive and Avnimelech, 1979) and followed by determination of total ammonia (Solozno, 1969); and total phosphorus content was analyzed by persulphate digestion method and followed by ascorbic acid method (APHA, 1989). Total Nitrogen and phosphorous concentration was also analyzed for shrimp feed, carcasses of juvenile (at stocking) and harvested shrimp following the methods used for sediment. Nutrient inputs in the form of feed was calculated as follows: nutrient (N/P) in feed /nutrient concentration in feed /total amount of feed supplied. Nutrient input and output in the form of shrimp was calculated as follows: nutrient (N/P) in shrimp /nutrient concentration in shrimp carcasses /total shrimp biomass. Mean shrimp weight (batch weight) was determined at initial and final harvest, as well as 20 shrimp from each tank was sampled randomly and batch weight was taken to assess shrimp growth at 2 week intervals throughout the culture period. Water quality of culture tanks measured biweekly at 10.00 h, included total ammonia nitrogen (TAN; Modified Phenate method; Parsons et al., 1984), nitrite nitrogen (Bendschneider and Robinson method; Parsons et al., 1984), total nitrogen (Raive and Avnimelech, 1979; Solozno, 1969), soluble reactive phosphate (SRP; Ascorbic acid method; APHA, 1989), and total phosphorus (APHA, 1989). Chlorophyll-a concentration was determined by flurometer with methanol extraction of the filter (Holm-Hansen and Riemann, 1978). Temperature, dissolve oxygen (DO), and pH (at 20 cm below the water surface) were measured weekly in situ. Water quality data were analyzed by repeated measure two-way ANOVA using ‘JMP statistical software’ (SAS institute Inc. Cary, NC, USA). Shrimp growth, survival, production and nutrient budget data were analyzed by two-way ANOVA using ‘STATGRAPHICS 7.0’ (Manugistics, Inc., Maryland, USA), significant differences between the treatment means were compared by LSD test. Differences were considered significant at an alpha level of 0.05.
3. Results 3.1. Water quality parameters TAN concentration fluctuated largely over the study period (Fig. 1). In the treatment S50 and C50, TAN concentration peaked between the fourth week and the eighth week followed by sharp decline at the end of the rearing cycle. TAN concentration in the treatments with lower stocking density showed different pattern for S25 and C25; in S25 a sharp increase was recorded from the fourth week of experiment up to the eighth week followed by decline at the end, however, in C25 TAN concentration remained almost flat in the later half of the experiment. Moreover, the TAN in all the treatments remained low ( B/1 mg/l) over the
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experimental period. In this study, though TAN concentration was not significantly affected by any of the treatment either stocking density or bottom substrate relatively, higher TAN concentration was recorded in soil bottom substrate treatments than concrete bottom (Table 1). NO2-N concentration in all treatments remained low below 0.003 mg/l up to the second week of the study (Fig. 1). The highest NO2-N concentration (0.648 mg/l) was recorded in the treatment S50 on the eighth week which dropped remarkably at the end of the rearing cycle; In rest of the treatments, NO2-N concentration remained below 0.01 mg/l till sixth week of rearing and then showed an increasing trend in the later part of the study. NO2-N concentration was significantly affected by the treatments stocking density, bottom substrate, and their interaction was also significant (Table 1). Total nitrogen (TN) concentration in all the treatments fluctuated without any definite trend up to the sixth week of the study, since thereafter, sharp increase was observed till the end of the rearing cycle (Fig. 2). The highest TN concentration (6.4 mg/l) was recorded in the treatment S50 during last sampling. Over the study period, mean TN concentration ranged from 3.0 to 3.7 mg/l, and was not significantly affected by the treatments either stocking density or bottom substrate (Table 1). Total phosphorus (TP) concentration in all the treatments decreased to a minimum in the second week of the study (Fig. 2). In the treatments with lower stocking density TP concentration showed alternate (biweekly) decline and surge throughout the study period. Among higher stocking density treatments, exponential increase in TP concentration was observed in the treatment C50 from the second week of culture till the end of the study; however, in S50 TP concentration increased till the sixth or 7 week of rearing followed by a decline in the later part of the study. Mean TP concentration ranged from 517 to 864 mg/l, and the value was significantly higher in the treatments with higher stocking density. There was no significant effect of the treatment bottom substrate on TP concentration. Mean SRP concentration ranged from 218 to 384 mg/l, and was significantly higher in the treatments with higher stocking density (Table 1). SRP concentration in different treatments fluctuated over the study period, fluctuations in SRP concentration showed similar trend as that observed for TP (Fig. 3). The high initial chlorophyll-a concentration in all the treatments, dropped to the lowest value on the second week of the study and then showed increasing trend till the end of the rearing; with the exception of the eighth week sampling when a slight drop in chlorophyll-a concentration was noticed in some treatments (Fig. 3). Chlorophyll-a concentration was significantly different with the treatment stocking density, however, the effect of the treatment bottom substrate on Chlorophyll-a concentration was not significant (Table 1). Over the study period, DO ranged from 4.4 to 7.4 mg/l in the morning and from 6.7 to 11.9 mg/l in the afternoon, water temperature ranged from 26.8 to 31 8C in the morning and 29 /34.5 8C in the afternoon (Table 1). Column water pH ranged from 7.0 to 8.9 in the morning and from 7.3 to 9.4 in the afternoon. No significant difference was observed in DO, water temperature and water pH among the treatments over the study period.
Variable
Water quality variable Temperature (6 h 8C) Temperature (16 h 8C) DO (6 h mg/l) DO (16 h mg/l) PH (6 h) PH (16 h) TAN (mg/l)b NO2-N (mg/l)b Total nitrogen (mg/l) SRP (mg/l)b Total Phosphorus (mg/l)b Chlorophyll-a (mg/l)b Pond management variable Total input of N (g/m2) Total input of P (g/m2)
Stocking density 25 PL per m2 bottom substrate
Stocking density 50 PL per m2 bottom substrate
Significancea
Soil (S25)
Concrete (C25)
Soil (S50)
Concrete (C50)
D
B
D /B
27.0 /30.8 29.3 /34.5 5.4 /7.2 7.0 /11.6 7.0 /8.9 7.3 /9.4 276.89/90.1 38.19/14.9 3.49/0.04 218.09/23.1 539.49/15.8 288.09/37.9
26.8 /30.8 29.0 /34.0 5.5 /7.2 7.1 /10.6 7.0 /8.9 7.7 /9.4 1989/53.5 16.79/9.8 3.09/0.27 311.19/47.5 517.39/70.4 199.89/34.8
26.8 /30.8 29.0 /33.5 4.7 /7.2 6.7 /11.9 7.0 /8.9 7.2 /9.4 519.19/92.9 232.69/52.1 3.79/0.1 363.79/46.0 666.59/46.3 301.19/50.9
26.8 /31 29.2 /34.5 4.4 /7.4 6.9 /11.0 7.1 /8.8 7.7 /9.3 281.99/97.3 50.89/40.2 3.39/0.43 384.39/76.9 864.09/157.1 415.99/15.8
ns ns ns ns ns ns ns xxx ns xx xx xx
ns ns ns ns ns ns ns xx ns ns ns ns
ns ns ns ns ns ns ns xx ns ns ns xx
16.29/0.8 2.39/0.1
14.09/0.8 2.29/0.1
40.69/1.0 5.89/0.1
23.49/7.8 3.89/1.3
xxx ns ns xxx ns ns
Values are mean9/S.E. (n /3 for each treatment). For each treatment, water quality data for all sampling times were averaged. The range (minimum and maximum) for temperature, DO and pH was given for each treatment. a Results from repeated measure two-way ANOVA for water quality data and two-way ANOVA for nutrient input data; D /stocking density; B /bottom substrate; DXB /stocking density X bottom substrate interaction. b Data transformed (log x ) prior to statistical analysis.
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Table 1 Effects of stocking density (25 and 50 PL per m2) and bottom substrate (soil and concrete) on water quality and pond management variables in tanks stocked with P. monodon during a 90-day trial
165
166
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Fig. 2. Fluctuation of total nitrogen and total phosphorus concentrations in different treatments over 90day trial.
3.2. Growth, survival and production of shrimp The results of shrimp performance indices are presented in Table 2. Shrimp growth (weight gain) was significantly higher in the treatments with higher stocking density but the treatment with bottom substrate had not significantly affected shrimp growth. Mean shrimp survival rate ranged from 50 to 78% in different treatments,
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Fig. 3. Fluctuation of SRP and chlorophyll-a concentrations in different treatments over 90-day trial.
and was not significantly affected by any of the treatments either stocking density or bottom substrate. Shrimp survival rate ranged widely due to the extremely low survival rate (35 and 38%) observed in the two tanks of treatment C50, while the third tank of the same treatment had fairly good survival rate (78%). Average daily weight gain ranged from 0.11 to 0.14 g per shrimp per day in all the treatments, and was not significantly different among the treatments. FCR was widely ranged from 1.3 to 2.1, and was not significantly different among the treatments.
168
Variable
Weight gain (g per shrimp) Survival (%)b Net biomass gained (g per tank) Production (g/m2 per day) Growth rate (g per shrimp per day) Food conversion ratioc
Stocking density 25 PL per m2 bottom substrate
Stocking density 50 PL per m2 bottom substrate
Significancea
Soil (S25)
Concrete (C25)
Soil (S50)
Concrete (C50)
D
B
D /B
7.69/0.3 629/3 440.19/30.1 1.89/0.1 0.119/0.0 1.839/0.1
7.49/0.3 589/5 400.19/49.3 1.69/0.2 0.119/0.0 1.599/0.1
9.69/0.6 789/4 1461.19/68.9 5.89/0.3 0.149/0.0 1.529/0.1
8.69/1.1 509/14 853.19/396.0 3.49/1.6 0.129/0.0 1.539/0.2
xx ns xxx xxx ns ns
ns ns ns ns ns ns
ns ns xx xx ns ns
Values are mean9/S.E. (n /3 for each treatment). a Results from two-factor ANOVA; D /stocking density; B /bottom substrate; D /B /stocking density /bottom substrate interaction. b Data transformed (arcsine x05) prior to statistical analysis. c Data transformed (log x ) prior to statistical analysis.
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Table 2 Effects of stocking density (25 and 50 PL per m2) and bottom substrate (soil and concrete) on shrimp performance indices in tanks stocked with Peaneus monodon during a 90-day trial
Treatments
Inputs
Outputs
Water
Fertilizer
Shrimp
Feed
Nitrogen S25 (g per tank) (%) C25 (g per tank) (%) S50 (g per tank) (%) C50 (g per tank) (%)
5.79/1.9 8.89/2.4 8.59/0.8 15.29/1.9 6.19/2.1 3.89/1.4 4.69/1.5 4.99/2.5
2.79/0.0 4.29/0.2 2.79/0.0 4.89/0.3 2.79/0.0 1.79/0.04 2.79/0.0 2.99/0.9
2.19/0.4 3.29/0.5 2.09/0.2 3.69/0.2 3.59/0.5 2.19/0.3 4.39/0.4 4.69/1.5
54.39/1.48 83.89/2.1 42.79/3.7 76.49/2.2 150.29/5.3 92.49/1.4 81.99/31.03 87.69/3.6
Phosphorus S25 (g per tank) (%) C25 (g per tank) (%) S50 (g per tank) (%) C50 (g per tank) (%)
1.19/0.4 11.79/4.3 2.09/0.2 23.29/2.2 1.49/0.7 6.09/2.9 2.89/0.9 18.49/1.7
0.49/0.0 4.39/0.2 0.49/0.0 4.79/0.3 0.49/0.0 1.79/0.02 0.49/0.0 2.69/0.7
0.29/0.04 2.19/0.5 0.29/0.0 2.39/0.1 0.49/0.1 1.79/0.2 0.49/0.0 2.69/0.7
7.79/0.2 81.99/3.8 6.09/0.5 69.89/2.4 21.29/0.7 90.69/2.8 11.69/4.4 76.39/2.4
Total
Water
Shrimp
Sediment
Total
Unaccounted
64.89/3.3 100 55.99/3.4 100 162.59/3.8 100 93.59/31.4 100
18.49/1.2 28.49/3.2 13.89/1.4 24.79/2.9 22.99/1.1 14.19/0.7 17.99/3.7 19.19/3.9
14.89/0.8 22.89/0.5 13.49/1.6 23.99/2.0 45.49/2.4 27.99/1.4 28.79/11.5 30.79/2.9
26.59/3.5 40.99/3.8 15.89/1.4 28.39/4.5 85.89/0.0 52.89/1.3 13.29/1.3 14.19/3.0
59.79/2.9 92.19/0.9 43.09/2.5 76.99/7.2 154.19/1.6 94.89/1.7 59.89/15.7 64.09/6.3
5.19/0.7 7.99/0.9 12.99/4.4 23.19/7.2 8.49/2.9 5.29/1.7 33.79/15.7 36.09/5.2
9.49/0.3 100 8.69/0.6 100 23.49/0.2 100 15.29/5.3 100
2.29/0.4 23.49/4.0 1.49/0.1 16.39/1.2 2.99/0.4 12.49/1.8 4.49/0.98 28.99/9.1
1.09/0.1 10.69/0.4 0.99/0.1 10.59/0.6 3.09/0.2 12.89/0.6 1.99/0.8 12.59/1.1
5.79/0.3 60.69/2.7 4.99/0.1 57.09/3.2 15.69/0.5 66.79/1.6 5.99/1.7 38.89/8.3
8.99/0.3 94.79/1.3 7.29/0.3 83.79/4.1 21.59/0.5 91.99/2.3 12.29/2.9 80.29/6.9
0.59/0.14 5.39/1.3 1.49/0.5 16.39/4.1 1.99/0.5 8.19/2.3 3.09/2.4 19.79/9.3
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Table 3 Nutrient budget for different treatments during a 90-day trial
Values are mean9/S.E. (n /3).
169
170
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3.3. Nutrient budget Table 3 shows the nutrient budget for all the treatments over the experimental period. Nutrient budget revealed that shrimp feed was the major input of nitrogen and phosphorous. Nitrogen inputs in the form of feed ranged from 76.4 to 92.4%, and phosphorous input through feed ranged from 69.8 to 90.6% of the total inputs. In addition, nitrogen and phosphorous inputs through feed were significantly higher in the treatments with higher stocking density (Table 1). Nutrient budget showed that the major portion of the nutrient inputs were deposited in the sediment followed by nutrient contained in discharge water at harvest, and relatively smaller fraction was retained by the shrimp. Percentage nitrogen accumulated into sediments ranged from 14.1 to 52.8% and was significantly higher in the treatments with soil bottom substrate. Percentage phosphorous accumulated into sediment ranged from 38.8 to 66.7% and was also significantly higher in the treatments with soil bottom substrate. However, nitrogen and phosphorous accumulated into sediment were not significantly affected by stocking density. Nitrogen and phosphorous retained in the shrimp ranged from 22.8 to 30.7 and 10.5 /12.8% of the total inputs, and were not significantly different among the treatments either stocking density or bottom substrate. Nitrogen output in the discharged water during harvest ranged from 14.1 to 28.4%, and was significantly higher in the treatments with lower stocking density, although no significant effect of bottom substrate was observed. Phosphorus output in the discharged water during harvest ranged from 12.4 to 28.9% and was not significantly different among the treatments with factor either stocking density or bottom substrate. In addition, nutrient budgets revealed that during the rearing cycle some of the nitrogen and phosphorous went unaccounted. Unaccounted nitrogen ranged from 5.2 to 36.0% of the total inputs, and was significantly higher for the treatments with concrete bottom substrate. Unaccounted phosphorous ranged from 5.3 to 19.7%, and was also significantly higher for the treatments with concrete bottom substrate.
4. Discussion 4.1. Water quality parameters In the present study, shrimp growth was not limited by any of the water quality parameters. TAN and NO2-N concentrations were remained fluctuating but never went beyond safe level during the study period (Chen et al., 1990). Moreover, it was reported earlier that in P. monodon grow out system, even with frequent water exchange, ammonia may increase up to 6.5 mg/l (Chen and Tu, 1991). In contrast, ammonia and nitrite nitrogen in this study never exceeded the safe range indicating that during the study period shrimp biomass was very much below the carrying capacity of the system, and the deleterious nitrogenous waste was effectively removed by phytoplankton and microbial activity (Shilo and Rimon, 1982; Diab and Shilo, 1988). In this study, ammonia nitrogen peak was observed on the eighth
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week of rearing followed by increase in nitrite nitrogen on the tenth week, suggests that the system took 8 weeks to establish nitrification process (Mevel and Chamroux, 1981). Total nitrogen and total phosphorus concentration in water increased with the progress of rearing. Throughout the study, soluble reactive phosphorus concentration remained much lower than total phosphorous concentration, suggesting that a large portion of water phosphorus was contained in suspended solids as well. In addition, SRP concentration remained high throughout the rearing, the highest concentration observed was 1100 mg/l. The result is in contrast to the report of Boyd (1990) who mentioned that dissolved orthophosphate concentration are usually not greater than 5/20 mg/l, and seldom exceed 100 mg/l even in highly eutrophic water. In the present study water nutrient concentration was linearly correlated with the cumulative feed input (data not shown). Obviously, being a closed system nutrient kept on accumulating within the system over time; this might be an advantage of the closed system as the high nitrogen and phosphorus accumulated within the system could support the growth of natural food organisms contributing ultimately to the shrimp growth. This view is in agreement with the report of Allan et al. (1995) who observed that prawn grow faster in prepared ponds where meiofauna is abundant. Chlorophyll-a concentration increased with the progress of rearing, and at the end of the study chlorophyll-a reached as high as 808 mg/l which was higher than the range reported previously for shrimp ponds (Lin, 1986; Martin et al., 1998). Moreover, in the present study, chlorophyll-a concentration remained relatively high (/94 mg/l) in all the treatments from fourth week of the rearing till the end of the study, indicating that the system never became nutrient limiting, and thus, in turn, sustained high phytoplankton biomass. Seemingly, dissolve nutrients together with the high light intensity, and warm temperature supported active growth of phytoplankton; which helped to condition the water quality in the tank by the production of oxygen and uptake of dissolved nutrients (Krom and Neori, 1989). 4.2. Growth, survival and production of shrimp In this study higher weight gain of shrimp in the treatments with stocking density 50 juveniles per m2 than the treatments 25 juveniles per m2 suggested that the biomass of shrimp in the culture tanks were well within the carrying capacity of the system. Furthermore, though in the present study greater shrimp growth was observed in high stocking density treatment there are no apparent factors that show obvious difference to affect shrimp growth in the water quality data, except the chlrophyll-a concentration, which was significantly higher in high stocking density treatment. This may indicate that there was greater amount of natural food in detrial from in those high-density tanks and had contributed to the better shrimp growth. Shrimp growth rate obtained in the present study seems relatively slow as compared with the previous reports (Chen et al., 1989; Lumare et al., 1993). However, growth rate observed in this study is comparable to the report of Muthuvan (1991) for commercial ponds stocked with P. monodon. Sandifer et al. (1991) also reported similar growth rate 0.94 g per shrimp per week for supper intensive pond production
172
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system. Production rate (17 /58 kg/ha per day) observed in this study could be reasonably compared with production rate of most of the commercial systems (Chen and Wang, 1990; Lumare et al., 1993). In the present study, shrimp production increased significantly with the stocking density, this is in agreement with the report of Allan and Maguire (1992). Shrimp survival rate observed in the present study was lower than that reported by Liao (1987); however, there was no significant difference in shrimp survival rates among the treatments. This suggested that increasing stocking density up to 50 shrimp per m2 in closed system had not affected shrimp survival; and lower survival rate observed in the present study as compared with the report of Liao (1987) could be ascribed to any other reason rather over crowding. This contention is in agreement with the observation of Allan and Maguire (1992) who reported that increasing stocking densities from 5 to 40 shrimps/m2 had no effect on survival of P. monodon. Moreover, in the present study the exceptionally low survival rate (35 and 38%) was observed in two tanks could not be related to any of the water quality parameter measured. This is to mention here that TAN and NO2-N concentration in those tanks remained relatively low (TAN B/0.81 mg/l; NO2-N B/0.09), and therefore, could not explain the low survival rate. FCR value observed in this study could be reasonably compared with the previous reports on P. monodon culture (Chen et al., 1989; Lumare et al., 1993). Sandifer et al. (1991) reported that intensive shrimp ponds typically have a feed conversion ratio of 2.0 or above. The low FCR value obtained in this study may be ascribed to the strict control of feeding by trays as well as the build up of benthic population over time might have supported shrimp growth. This study showed the potential of closed system in producing shrimp with low FCR, as in the closed system nutrient and organic matter released to the system could be accounted well for the production of natural food organisms. Reymond and Lagardere (1990) reported that in low stocking density culture systems meiofauna may constitute an important part of the feeding. Avnimelech et al. (1994) also emphasized that in ponds operated with high water exchange rate, a large fraction of the feed and the organic matter accumulating in the pond is drained and often wasted. 4.3. Nutrient budget It had been reported previously that the predominant inputs of nitrogen and phosphorus in water exchange shrimp ponds are feed, which accounted 82 /95% nitrogen and 38/91% phosphorus of the total inputs (Muthuvan, 1991; Stapornvanit, 1993; Briggs and Funge-Smith, 1994). In the present study, percentage contribution of feed to the total nutrient inputs was in the same range as reported previously. In addition, percentage nitrogen and phosphorus inputs in the form of feed were significantly higher in the treatment with higher stocking density; the result is in agreement with the previous reports (Martin et al., 1998; Briggs and FungeSmith, 1994). In this study some of the potential source of nitrogen was not examined, such as inorganic nitrogen inputs through precipitation which was considered insignificant for the present study, and nitrogen fixation by blue /green
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algae. In addition, tap-water added to the tanks to make up the loss to evaporation was also not accounted for nutrient budget. Nutrient budget showed that only 22.8 /30.7% N and 10.5 /12.8% P of the total inputs were incorporated into harvested shrimp (Table 3); the remainder in the system as uneaten feed, excreted material went to support high levels of phytoplankton and heterotrophic activity. Nitrogen and phosphorus percentage removed from the system via shrimp harvest is comparable to the previous reports (Muthuvan, 1991; Stapornvanit, 1993; Briggs and Funge-Smith, 1994). Nutrient retained by shrimp was not significantly different among the treatments; implying that in terms of the proportional recovery of the nutrient from closed shrimp culture system, efficiency was not affected by the treatments either stocking density or bottom substrate. Moreover, further accounting of the nutrients budget revealed that rearing 1 kg shrimp resulted into 98 g N and 18 g P loss at stocking density 25 juveniles per m2, whereas, at stocking density 50 juveniles per m2 it resulted into loss of 70 g N and 13 g P; calculation was as follows: Nutrient (N/P) loss /(Mean total nutrient inputs /mean nutrient retained in harvested shrimp)/(Mean shrimp biomass at harvest). This suggests that in closed system rearing shrimp at stocking density 50 juveniles per m2 is more nutrients efficient than rearing at stocking density 25 juveniles per m2. However, Martin et al. (1998) reported that, in water exchange system, the quantity of waste produced per unite shrimp production was proportional to the stocking density as stocking density increased from 1 to 30 shrimp per m2. Nitrogen waste generated in producing one kg of shrimp in the present study was about half of the value, at stocking density 30 shrimp per m2, reported by Martin et al. (1998), while to estimate the nitrogen loss they used nitrogen input from feed only in their calculation. In the present study, the lower amount of nitrogen waste generated with per unit shrimp production could be due to the study being a closed system; In closed shrimp culture system excess nutrient inputs, especially originated from uneaten feed, keep on accumulating within the system which in turn may support the growth of natural food organisms and ultimately the shrimp growth. It was reported previously that the major output of nutrients in water exchange shrimp ponds were in the discharge water (Muthuvan, 1991; Stapornvanit, 1993). In contrast, the present study showed that in closed shrimp culture system loss of nutrient through sediment is higher than the water borne loss. The result is in agreement with the view of Briggs and Funge-Smith (1994) who emphasized that, in culture system with low water exchange, water borne loss of nutrient is less important than loss through the sediment, due to rapid accumulation of sediments in shrimp ponds. Furthermore in the present study, significantly higher nutrients sink into sediments in the treatments with soil bottom substrate, emphasizes the importance of soil bottom in minimizing the water born loss from the system. Chen et al. (1989) mentioned that sediments play an important role in the balance of an aquaculture system, it can act as buffer in water nutrient concentration. Enell and Ackefors (1991) mentioned that approximately 50% of the nitrogen and phosphorous that settle on the bottom is translocated back into water column. Once all the measurable outputs of nutrients had been quantified, 5.2 /36.0% N and 5.3 /19.7% P of the total inputs went unaccounted. We assume that the nitrogen
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might have lost from the system either by ammonia gas volatilization and/or denitrification in the sediment. Daniels and Boyd (1988) mentioned that the most probably loss of nitrogen is by ammonia gas volatilization, which is further enhanced by vigorous aeration and high pH in the tanks. Unaccounted phosphorus is likely to the result of muds adsorption, as muds were known to have a strong affinity for phosphorous (Boyd, 1985). However, from the nutrient budget data (Table 3) it is apparent that larger percentage of the total nutrient inputs went unaccounted in the treatments with concrete bottom than the muddy bottom treatments; procedural difference (as explained in the Section 2) in estimating nutrient output in the form of sediment between the two treatments might have caused this discrepancies. It is likely that some of the sediment deposited in the concrete bottom tank could have washed out of the tank with the drained water during harvest, and thus could have went unaccounted for the nutrient budget. In conclusion, this study provides valuable information on the management practice for closed shrimp culture systems, as a viable alternative to the current water exchange systems. The study demonstrated that closed culture system could maintain acceptable water quality for shrimp culture, and shrimp could be grown well at stocking density of 50 juveniles per m2. In addition, the study showed that closed shrimp culture system can reduce the nutrient loss through pond effluents and thus minimize the environment impacts of shrimp culture. Nevertheless, the total quantity of nutrient released in closed shrimp culture system may be same to the water exchange system, the small volumes of concentrated effluent produced at harvest in closed system should be easier for the shrimp grower to treat before being discharged to the external environment.
Acknowledgements The authors wish to thank Dr J.B. Hambrey for his suggestions and encouragement during all phase of the study. Thanks extended to the staffs of Aquaculture Program, Asian Institute of Technology, for their field and lab assistance. First author, would like to thank D.D. Kassam, Department of Aquaculture, Kochi University for his help in statistical analysis of data. Funding from the Government of Denmark through DANIDA, AIT, grant is gratefully acknowledged.
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