water quality and partial mass budget in extensive shrimp ponds in bangladesh

Water quality and partial mass budget in extensive

shrimp ponds in Bangladesh

Md. Abdul Wahaba, Asbjorn Bergheimb,*, Bjorn Braatenc

aFaculty of Fisheries, Bangladesh Agricultural University (BAU), Mymensingh 2202, BangladeshbRF-Rogaland Research, P.O. Box 8046, 4068 Stavanger, Norway

cNorwegian Institute for Water Research (NIVA), P.O. Box 173, 0411 Oslo, Norway

Received 7 August 2002; received in revised form 3 January 2003; accepted 6 January 2003

Abstract

In a Bangladeshi–Norwegian project, water quality analyses have been conducted in some

brackish water shrimp ponds stocked with Penaeus monodon in Bangladesh. The water-sampling

program included five extensively improved farms in the Khulna region and four improved extensive

farms in the Cox’s Bazar region, annually producing 100–500 kg ha� 1. In addition to water

sampling, data of input of manure, fertiliser and feed; shrimp stocking density, mortality and

production; and water exchange rate data were collected.

Measured pH, salinity and concentrations of dissolved oxygen and gaseous ammonia were usually

within acceptable ranges for shrimp farming. However, sub-lethal levels of ammonia can be present

and create stressing conditions. Low salinity could also create unfavourable environmental

conditions.

In one Gher, frequent sampling of inlet–outlet water was carried out during one production

cycle. On average, the concentrations of total phosphorus and total nitrogen were reduced by 70%

and 40% through the pond (inlet–outlet), respectively. Apparently, 80–90% of the supplied

nutrients in fertiliser and inlet water was trapped in the pond. Consequently, the extensive shrimp

culture in Bangladesh seems to act as a sink for solids and nutrients in the water that led to the

system.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Water quality; Partial mass budget; Extensive shrimp ponds; Bangladesh

0044-8486/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0044-8486(03)00009-7

* Corresponding author. Tel.: +47-51-87-5000.

E-mail address: [email protected] (A. Bergheim).

www.elsevier.com/locate/aqua-online

Aquaculture 218 (2003) 413–423

1. Introduction

Until the early 1990s, brackish water shrimp culture in Bangladesh was mainly based on

traditional extensive systems. According to the Marine Fisheries Department (Choudhury,

1997), the total number of shrimp farms in 1992–1993 was 6540 with an average pond area

of 16.5 ha (total pond area: 108,300 ha). Hussain (1994) estimated that 70% of the total area

under cultivation was ‘extensive’ culture (100–200 kg shrimp ha� 1), 25% was ‘semi-

intensive’ culture (500–1000 kg shrimp ha� 1) and 5% of the area was ‘intensive’ culture

systems (1000–2000 kg shrimp ha� 1). The total production in 2001 was 26,240 tons in a

pond area of ca. 150,000 ha, i.e. an average production rate of 100–400 kg ha� 1 year� 1

(Md. Karim, personnel communications). Today, very few semi-intensive farms exist as a

result of severe disease problems.

The extensive Bangladeshi shrimp production is based on the so-called Gher systems

characterised as the following:

Two crops of shrimp/year (January–April and May–September), followed by one crop

of rice (October–December). In autumn, the salinity is too low for shrimp production

( < 10x). Shrimp cum salt production is also commonly practiced in the SE Cox’s

Bazar area.

The production is based on a stocking density of 1–2 PL m� 2, producing 100–400 kg

ha� 1 year� 1.

Mortality is normally in the range of 85–90% from stocking to harvest.

The traditional grow-out pond (Gher) is large (30–100 ha) with a depth of 0.3–1 m.

Water intake is tidal based with no aeration.

No supplemental feeding, possibly liming and fertilising.

In recent years, diseases have been a regular occurrence in shrimp farms in Bangladesh.

Disease outbreaks like mass mortality for unidentified reasons in the wet season, gill

parasites, black spot disease, etc. were not detrimental for the industry until the outbreak of a

new disease, white spot syndrome virus (WSSV) (e.g. reviewed by Sangamaheswaran and

Jeyaseelan, 2001). WSSV accounted for about 90% losses in the SE and about 40% in the

SW-situated farms of Bangladesh since the initial outbreak (Department of Fisheries,

personnel communications).

Water quality problems such as low salinity, fluctuating temperature and high turbidity

were reported on 23% of extensive farms and on 39% of semi-intensive farms (Department

of Fisheries, personnel communications). High mortality caused by virulent shrimp

pathogens is often precipitated by transient conditions of stress, such as sudden changes

in pH and low dissolved oxygen concentration (Flegel, 1996).

Sparse information is available on effluent load from extensive systems. There are,

however, published several reports describing the effluent load from both semi-intensive

(Teichert-Coddington et al., 2000; Paez-Osuna et al., 1997) and intensive systems (Briggs

and Funge-Smith, 1994).

This paper deals with water quality measurements performed in brackish water shrimp

ponds in traditional extensive systems in the Khulna region (SW Bangladesh). In addition, a

nutrient mass budget study performed in a large extensive Gher is described.

M.A. Wahab et al. / Aquaculture 218 (2003) 413–423414

2. Materials and methods

2.1. Culture systems

All involved farms in the Khulna region were traditional extensive systems, so-called

Ghers (ponds), 40–100 ha in size; water exchange, tidal based (rate, 0–10% day� 1);

stocking rate, 1–2 PL m� 2; cumulative mortality, 40–50% cycle� 1 (WSSVoutbreak: 85–

90% mortality); production, 200–300 kg ha� 1 year� 1 (2 cycles year� 1); no artificial

feeding, fertiliser, 1000–3000 kg cow dung ha� 1 cycle� 1 + 100 kg inorganic (N, P) ha� 1

cycle� 1; lime, 500–1000 kg CaO ha� 1 cycle� 1.

Improved extensive farms, both in the Khulna region and Cox’s Bazar: introduction of

‘nursery pond practice’ within the Gher, stocking rate grow-out ponds: 2–4 PL m� 2,

cumulative mortality: 40–50% cycle� 1 (WSSVoutbreak frequency reduced), production:

300–500 kg ha� 1 year� 1 (2 cycles year� 1). Further management as described for

traditional systems (water exchange, fertiliser input).

The involved farms were typical for the extensive culture systems in the two

regions regarding size and management. The Khulna and Cox’s Bazar regions are

situated in the south–western and in the south–eastern part of the country, respectively

(Fig. 1).

Fig. 1. Map of Bangladesh indicating the two regions with sampling of shrimp farms, Khulna and Cox’s Bazar.

M.A. Wahab et al. / Aquaculture 218 (2003) 413–423 415

2.2. Sampling and analyses

Manual sampling of pond inlets within ponds and pond outlets was carried out with

fortnightly intervals at five extensive farms (Ghers) in the Khulna region in 1997. The

planned sampling at farms in Cox’s Bazar failed in 1997 due to a cyclone in April

followed by a severe flooding episode in July. In 1998, within pond sampling was

confined to four farms in Cox’s Bazar. Sample collection and on-site measurements

(1997–1998) were carried out in daylight, between 10 am and 2 pm.

Temperature, salinity (Refractometer, mod. 4200/REV A/05-95, Conductivity meter),

pH (pH Meter, mod. EC 10 portable pH meter, Hach) and dissolved oxygen (D.O. Meter,

mod. DO 175, Hach) were measured on the spot. After acid preservation, water samples

were brought to Water Quality and Pond Dynamics Laboratory, BAU, Mymensingh for

measurement of total ammoniacal nitrogen (TAN) method (Stirling, 1985). Un-ionised

ammonia (NH3) was calculated from measurements of TAN, pH, temperature and salinity

(Fivelstad, 1988).

2.3. Partial mass budget

The sampled Gher Soladana has a surface area of 94 ha with one main water inlet point

and three different outlet points. In addition, there is a smaller combined inlet–outlet point

(Fig. 2). A part of the water volume of the Gher is routinely exchanged during 4–6 days at

full and new moon every fortnight. Besides, the frequency of inflow–outflow within the

Fig. 2. Sketch of Gher Soladana in Paikgacha, Khulna selected for sampling of effluent load during the period of

March–September 2001 (2nd production cycle). The Gher has two flow inlets (squared gates) and three outlets

(circular pipes of 1.5-m diameter). Sampling points: main inlet and main outlet (indicated).


exchange period is changed from pre-harvest towards the harvest period. On average, the

water level fluctuates between 10 and 20 cm every fortnight, which indicates an exchange

rate in the range of 0–10% volume day� 1.

Unlike systems solely producing shrimp, there is no final complete drainage of theGher;

at the end of the cycle, a combined harvest of the remaining shrimp and planting of rice

takes place. To keep the rice fields wet, the outlet gates are locked to keep a few inches of

water within the Gher.

Stocking rate (1–2 PL m� 2) and input of manure, fertiliser and lime followed the

procedure for extensive ponds (Ghers) formerly described. Despite no accurate available

figures, the estimated input of manure and fertiliser corresponded to about 40 kg N ha� 1

and 10 kg P ha� 1. No artificial feeding was applied.

There is no peak harvest period at the end of each cycle but frequently harvesting at

water exchange over a long period. Final harvest and planting of rice were carried out in

early September.

Sampling period: one production cycle, March–September 2001 (5.5 months).

Sampling frequency: every 2nd week (2 samplings month� 1).

Sampling procedure: diurnal sampling (24 h). Mixed inlet sample, six separate outlet

samples (4-h intervals).

Analytical parameters: total nitrogen (TN), total phosphorus (TP). TN was deter-

mined with Skalar auto analyzer (based on Norwegian Standard, NS 4745) and TP

was determined with Skalar auto analyzer after digestion with peroxodisulphate (NS

4725).

Analyses: Lab., Norwegian Institute for Water Research, Oslo.

Monitoring on spot: daily reading of water level (dipstick).

3. Results

3.1. Water quality

Water quality measurements from extensive culture systems (Ghers) in Khulna and

improved extensive culture systems in Cox’s Bazar are shown in Table 1. The results

obtained from the second production cycle (July–September 1997) within the large

extensive Ghers indicate a relatively high concentration of total ammonia nitrogen (TAN)

and, in spite of sampling solely in daylight, a strongly fluctuating pH level. Comprising all

extensive system sites, un-ionised ammonia (NH3) peak concentrations of 0.29–0.86 mg

N l� 1 were found. In improved extensive culture systems, no sub-critical ammonia (NH3)

concentrations were detected at midday.

3.2. Partial mass budget

A total crop of 7338-kg shrimp at an average individual size of 21.2 g was caught.

The quantified crop constituted ca. 78 kg ha� 1.

Despite less irregular level fluctuations between the water exchange phases

(smaller inlets–outlets, leakage, evaporation), about 1/3 of the total pond volume


seemed to be replaced every fortnight. Daily readings of water level in June are

shown in Fig. 3. Obviously, the exchange peaks occurred when the gates were

opened during the 4–6-day periods twice a month. In addition, the volume of the

Gher was gradually reduced throughout the cycle (mean water level reduced from ca.

70 to 50 cm).

Fig. 3. Daily monitoring of water level in Gher Soladana in June 2001. Diurnal sampling of inlet and outlet water

on June 7–8 and on June 24–25 marked with bold lines.

Table 1

Within-pond water quality range at brackish water shrimp farms in two Bangladeshi regions, in the Khulna region

and in Cox’s Bazar 1997–1998

Site Production

system

Temperature

(jC)pH Salinity

(ppt)

DO

(mg l� 1)

TAN

(mg l� 1)

NH3-N

(mg l� 1)

Khulna region, July–September 1997

Soladana Extensive 29.5–32.7 6.5–8.3 1.2–8.9 4.5–4.7 1.7–2.5 0.01–0.41

Horikhali, station 1 Extensive 29.3–30.0 7.1–8.3 1.6–9.8 3.8–5.0 1.1–2.3 0.01–0.33

Horikhali, station 2 Extensive 29.7–32.4 6.2–8.9 1.3–11.2 3.3–4.9 1.4–2.1 0.00–0.86

Sorlgram Extensive 27.0–30.6 7.7–8.3 1.1–6.8 4.5–5.4 0.5–2.1 0.02–0.29

Melek-purikati Extensive 29.7–32.5 7.7–8.7 2.6–8.5 4.5–4.8 1.9–2.6 0.07–0.58

Cox’s Bazar, April –October 1998

Chakhuria, station 1 Improved

extensive

27.1–31.1 6.2–7.7 5–26 5.4–6.7 0.5–1.1 0.00–0.02


extensive

27.1–32.2 6.8–7.9 5–24 6.2–6.9 0.4–1.0 0.00–0.04


extensive

27.0–32.0 6.6–7.3 5–25 6.6–7.3 0.4–1.1 0.00–0.07


extensive

27.0–32.1 6.2–8.2 5–25 6.2–8.9 0.2–1.8 0.00–0.09

Biweekly routine sampling.


Except for the final sampling, the concentrations of phosphorus and nitrogen were

consistently reduced in theGher, i.e. Conc.INLET>Conc.OUTLET (Fig. 4). On average, TPwas

reduced from 206 to 62 Ag l� 1 (70% reduction) and TN from 984 to 589 Ag l� 1 (40%

reduction). Inlet water at sampling in July 8–9 might have been influenced by nutrient-

enriched outlets from local sources.

Only a rough estimate of the nutrient mass balance of the Gher can be made. Sug-

gesting an average exchange rate of 2/3 of the total volume month� 1, the exchanged

volume was in the order of 1.9 million m3 over a cycle of 150 days. The total input of

nutrients to the Gher was around 14 kg TP ha� 1 and 60 kg TN ha� 1 (Table 2). Conse-

Fig. 4. Concentrations of total phosphorous and total nitrogen of inlet and outlet, Gher Soladana during March–

September 2001. Inlet water: composite samples. Outlet water: averageF S.D.


quently, about 79% of supplied TP and 87% of supplied TN were trapped in the pond

system.

4. Discussion

In most cases, the observed variations in studied water quality parameters were within

the acceptable range for Penaeus monodon culture. The optimal conditions for this species

are summarized by NACA (1994): dissolved oxygen, 3.8–5.0 mg l� 1; pH, 7.5–8; salinity,

10–20 ppt; and ammonia, < 0.1 mg N l� 1.

The salinity level during the second production cycle (July–September) decreased,

however, gradually to 1–2 ppt in the Khulna-based Ghers, far below the recommended

lower limit. However, freshwater will stunt the shrimp only after 100 days of culture period

(Kongkeo, 1997). The viability of P. monodon in low-saline water has, however, been

indicated by Saha et al. (1999): when salinity decreased after 60 days of culture (>5 ppt) to

freshwater level (0.16 ppt), the growth rate and the feed utilisation of the shrimp stock were

still satisfactory in semi-intensive pond.

Stress inducing temperature levels, up to 38–39 jC, were observed in three Ghers

in the Khulna region in May 2000. According to Chanratchakool et al. (1998),

the feeding response may be reduced by 30–50% at temperatures higher than

32–33 jC.Except for occasionally sub-critical un-ionised ammonia concentration levels within the

traditional culture systems (Ghers), no obvious signs of stress-inducing water quality

parameters were found. The observed ammonia concentrations at pH> 8.5 (maximum

0.86NH3-N l� 1) may be a potential environmental stressor contributing to disease outbreak.

In an experimental infection trial, addition of 2 ppm of total ammonia nitrogen (TAN) led to

40% mortality among P. monodon caused by WSSV (Wang et al., 1997). However, no

epidemic WSSV-caused mortality was reported at the traditional farms in 1997 (40–50%

losses cycle� 1).

According to other reports, incidents of dissolved oxygen deficit in extensive systems in

Khulna might have contributed to stressing conditions. In Indonesian tambaks, early

morning dissolved oxygen concentrations between 3 and 5 mg l� 1 were considered

Table 2

Brief estimate of the nutrient budget in Gher Soladana March–September 2001

Criterion Total phosphorus Total nitrogen

kg ha� 1 cycle� 1 % kg ha� 1 cycle� 1 %

Input

Fertiliser 10 71 40 67

Water 4 29 20 33

Total 14 100 60 100

Effluent 3 21 8 13

Trapped in the system 11 79 52a 87

a A significant part of TN probably got lost to the atmosphere.


favourable for growth without causing stress in P. monodon (Hariati et al., 1996).

According to Hall and Van Hamm (1998), dissolved oxygen concentrations need to be

maintained ‘‘above 4 mg l� 1 during the grow-out period if repeated bouts of metabolic

stress to the prawns are to be avoided’’ (juvenile P. monodon).

This study supports the view that extensive shrimp culture is not expected to

represent any significant load of nutrients to the surrounding waters (Phillips et al.,

1993). Extensive systems rely on natural feed, moderately stimulated by input of

manure and fertilisers and may be net removers of nutrients from coastal environment.

Beveridge (1984) reported that extensive finfish and mollusc farming significantly

reduced nutrients in aquatic ecosystems. In an extensive mixed shrimp–mangrove

forestry farm in Vietnam, Alongi et al. (2000), however, estimated a higher nutrient

output than input.

Extensive shrimp culture ponds are supplied with small amounts of nutrients. The

average input of nitrogen and phosphorus to intensive shrimp culture in Thailand was 858

kg TN ha� 1 cycle� 1 and 291 kg TP ha� 1 cycle� 1 (Briggs and Funge-Smith, 1994) or 14–

21 times the estimated input to the Gher Soladana.

Eighty to ninety percent of the estimated nutrient input to the Gher was apparently

trapped in the system. A significant part of the removed nutrients must have been

trapped by settling of solids, algae and detritus on the bottom of the Gher (Edwards,

1993). Nitrogen was probably also lost to the atmosphere by denitrification in the

sediments and ammonia diffusion at high pH (Briggs and Funge-Smith, 1994). Only a

minor part of accumulated sediments is resuspended and flushed out of shrimp-cum-rice

producing systems during the careful, not complete, drainage at the end of the shrimp

cycle. The accumulation of sediments year by year results in the rise in the bottom

level. Consequently, the Gher has to be dug out after some years and this operation

may enhance the erosion loss to the surrounding waters.

Unlike the significant effluent load from artificial feed-supplied intensive and semi-

intensive shrimp ponds with high water exchange rate (Hopkins et al., 1993), the

extensive shrimp culture in Bangladesh seems to act as a sink for solids and nutrients

in water led to the system.

5. Conclusions

The brackish water shrimp industry in Bangladesh is dominated by traditional

and extensive culture systems with an average production of 200–300 kg ha� 1

year� 1. A main reason for the low outcome is high mortality losses due to disease,

often amounting to 70–90% of the stocked shrimp. Observed increased ammonia

concentrations at high afternoon pH in the ponds (Ghers) may be a potential

environmental stressor contributing to disease outbreak. Seasonal unfavourable

temperature and salinity levels for the optimal growth of shrimp were also

indicated.

A partial mass budget in a large Gher indicates that the Bangladesh extensive shrimp

systems producing less than 1000 kg ha� 1 year� 1 do not pollute surrounding water

and rather act as a sink for nutrients and solids.


Acknowledgements

The authors are grateful to the Bangladesh Fisheries Research institute (BFRI) for their

kind cooperation during the sampling period. We also acknowledge the contribution of

research fellows at BAU for their efforts in data collection. Thanks are due to all involved

shrimp farmers. We are especially grateful to the staff at Soladana for their kind

cooperation. The contribution of the analytical staffs of NIVA and RF is duly

acknowledged. Mr. P. Wang at NIVA prepared the map of Bangladesh (Fig. 1).

The present study was funded by a grant from the Norwegian Agency for Development

Cooperation (NORAD).

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