Water quality and nutrient budget in closed shrimp (Penaeus monodon) culture systems

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.

Full text

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Water quality and nutrient budget in closed
shrimp (Penaeus monodon) culture systems
Dhirendra Prasad Thakura,b,*, C. Kwei Lina
aAgriculture and Aquatic Systems and Engineering Program, Asian Institute of Technology, PO Box 4,
Klong Luang, Pathumthani 12120, Thailand
bDepartment 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: dpthakur@hotmail.com (D.P. Thakur).
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0144-8609/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
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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 tank-
system 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 m3concrete 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/m2during 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 experi-
mental tank. The mean shrimp individual weight at stocking ranged from 0.63?/
0.74 g in different treatments. Experimental shrimp were fed with commer-
cial 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 differ-
ences 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 S50and 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 S25and C25; in S25a 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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