Growth performance of Barbodes gonionotus (Bleeker) in intensively cultivated rice fields

Aquaculture 212 (2002) 167 – 178 www.elsevier.com/locate/aqua-online

Growth performance of Barbodes gonionotus (Bleeker) in intensively cultivated rice fields Nico Vromant a,*, Cao Quoc Nam b, Frans Ollevier a a

Laboratory of Aquatic Ecology, Catholic University of Leuven, Ch. de Be´riotstraat 32, B-3000 Louvain, Belgium b Mekong Delta Farming Systems Research and Development Institute, University of Can Tho, Duong 3/2, Can Tho, Viet Nam Received 27 February 2001; received in revised form 26 July 2001; accepted 10 December 2001

Abstract Barbodes gonionotus (Bleeker) is a popular fish species in rice – fish culture. In Vietnam’s Mekong Delta, the species is often grown in polyculture with Oreochromis niloticus (L.) and Cyprinus carpio L. In this study, we used the data of eight experiments into such polycultures at the Co Do rice – fish experimental station to look whether the rice stage or fish biomass had any effect on the specific growth rate (SGR) of B. gonionotus. The B. gonionotus net production figures varied from 1.7 to 547.7 kg/ha, while the SGR varied from 0.27% to 3.58% body weight/day. We found that SGR values did not depend on the developmental stage of the rice crop. However, the SGR values significantly decreased with increasing standing biomass (SB) of B. gonionotus, O. niloticus or wild fish. We concluded that intraspecific and interspecific competition is an important issue in B. gonionotus culture in rice fields. Besides this, SGR values were higher in the wet season and in plots with inorganic fertilizer or pig manure application. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Rice – fish culture; Barbodes gonionotus; Oreochromis niloticus; Cyprinus carpio; Polyculture; Competition

1. Introduction Barbodes gonionotus (Bleeker) is often stocked in polyculture in rice fields in South (Bangladesh and India) and Southeast (Indonesia, Malaysia, Thailand and Vietnam) Asia. In the Mekong Delta, Vietnam, the species is often stocked in polyculture with Cyprinus

*

Corresponding author. Tel.: +32-1632-3966; fax: +32-1632-4575. E-mail address: [email protected] (N. Vromant).

0044-8486/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 ( 0 2 ) 0 0 0 0 5 - 4

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carpio (L.) and to a lesser extent with Oreochromis niloticus (L.). Rice – fish systems in the Mekong Delta are characterized by intensive double rice cropping and extensive to semi-intensive fish culture. In these systems, C. carpio and O. niloticus mainly feed on detritus (Chapman and Fernando, 1994; Rothuis et al., 1998a), whereas B. gonionotus mainly feeds on the rice plant (Rothuis et al., 1998a), probably the most prevalent food item in the rice –fish system. Haroon (1998) stated that B. gonionotus escapes intraspecific competition in rice – fish systems. However, Rothuis et al. (1998b) found decreasing daily growth rates with stocking densities increasing from 4000 to 16 000 fish/ha and attributed this to intraspecific competition. As B. gonionotus has higher survival rates and growth rates and fetches higher market prices than O. niloticus and C. carpio, the species is a favorite among rice – fish farmers (Nhan et al., unpublished results). However, as rice field-based B. gonionotus research is scarce and seemingly contradictory (compare Haroon, 1998; Rothuis et al., 1998b), extension discourse on B. gonionotus culture in rice fields is often confounding, illogic and based on hearsay information obtained from a few successful farmers. Some of the pertinent questions are: (1) whether B. gonionotus growth is favored or restricted during any of the rice phases, (2) whether intraspecific competition limits the growth of B. gonionotus in macrophyte-abundant rice fields, and (3) whether there is any interspecific competition between B. gonionotus and any of the other species in the polyculture. In this paper, we brought together the data of eight rice – fish experiments on the polyculture of B. gonionotus, O. niloticus and C. carpio to investigate whether rice phases or the standing biomass of any of the three fish species affects the specific growth rate of B. gonionotus.

2. Materials and methodology 2.1. Description of the Co Do experiments Results from eight rice – fish experiments carried out at the rice – fish research station at the Co Do state farm, Can Tho province, Vietnam, between November 1995 and October 1999 were used to analyze the specific growth rate of B. gonionotus in intensively cultured rice fields. The Co Do area is characterized by about 2500 hours of sunshine a year and an annual rainfall of approximately 1600 mm. There are two main seasons: a dry season from December to mid-May and a wet season from mid-May to November. The region is also characterized by yearly flooding (0.5 – 1-m depth) from August to November during which rice cropping is impossible and fish culture is hazardous. Farming in the Co Do area is characterized by an intensive direct-seeded double rice cropping system: a dry season crop (December– March) followed by a wet season crop (May – August). All plots used in the experiments measure about 650 m2 and have a rice field area (550 2 m ) and a 1-m-deep L-shaped trench (100 m2) that serves as fish refuge. All plots are surrounded by a dike and have their own water inlet and outlet. A detailed description of the study area, experimental station, rice – fish field layout, soil and water resources and

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experimental results can be found in Rothuis et al. (1998a,b, 1999) and Vromant et al. (1998). Half of the experiments used for the study took place in the wet season; the other half took place in the dry season (Table 1). Experiments in the wet season included a rice crop followed by a rice ratoon crop. (Ratooning is the ability of rice plants to regenerate new tillers after harvest (Chauhan et al., 1985).) The experiments in the dry season included a rice crop—followed in only one experiment (experiment 7) by a period during which the fish remained in the trench without any access to the field. In the first four experiments, we used the rice variety IR56279, and in the other, we used IR62032. Both varieties are short duration varieties; they can be harvested at about 100 days after seeding (DAS). Whereas we applied inorganic fertilizer to the rice at normal rates, we did not apply organic manure. All fields were hand-weeded, and when necessary, insecticides or fungicides were applied. The water level in the rice field was kept between 10 and 15 cm, except in the ratoon crop when the water levels were between 30 and 40 cm. O. niloticus (L.), C. carpio L. and B. gonionotus (Bleeker) were stocked in all eight experiments. Trichogaster pectoralis Regan was stocked in three experiments, whereas Osphronemus goramy Lacepoˆde, Hypophthalmichthys molitrix (Valenciennes), Labeo rohita (Hamilton) and Clarias sp. were stocked in one experiment. All fish were stocked at about 20 DAS. A few days before rice seeding, trenches of all plots were treated with 15 ppm Derris root (5% rotenone) to eradicate wild fish. A 5-mm screen at the field water inlets and a 10-mm screen at the main water gate of the station prevented other fish from entering the fields. 2.2. B. gonionotus B. gonionotus is a native of the Mekong and Chao Phraya basins, the Malay Peninsula, Sumatra and Java (Kottelat, 1998). The species is known to feed on Ceratophyllum sp., Najas sp. (Schuster, 1952), filamentous algae and terrestrial plants (Hora and Pillay, 1962; Haroon and Pittman, 1997a). In intensively cultured rice fields, B. gonionotus mainly feeds on aquatic macrophytes (Haroon and Pittman, 1997a,b; Rothuis et al., 1998a), most likely rice leaves (Rothuis et al., 1998a). Vromant et al. (in press) found that B. gonionotus, when stocked in rice fields, reduce the number of rice tillers. 2.3. Data collection and processing Since the variable specific growth rate (SGR) allows for the comparison of the individual growth of fish with different starting weights, we used SGR B. gonionotus in a multiple regression using the data of the eight experiments (Table 1). The SGR in percent (%) was calculated according to Brett and Groves (1979): SGR ¼ ðlnW2  lnW1 Þ100=ðt2  t1 Þ where t1 and t2 (days) are the start and end of the period under consideration, and W1 and W2 (g/fish) are the mean start and end weights for that period. Details on all experiments can be found in Table 1. This resulted in a data set comprising 107 plot entries. These were used to calculate the production data for B. gonionotus (Table 2).

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Table 1 Overview of the experiments used Season

Rice variety

Experimental design

Repl.

Treatment

STD (fish/m2)

Fish species (%)

1

Dry 1995 – 1996

IR56279

CRBD

3

2

Rainy 1996

IR56279

CRD

3

3

Dry 1996 – 1997

IR56279

CRD

3

1 2 2 2 2 1 0.45

SB SB SB SB SB SB SB

4

Rainy 1997

IR56279

2  2 factorial

3

A: 1

A: SB (60), CC (20), NT (20)

B: 1

B: SB (60), SC (20), MR (20)

0.5

SB (48), CC (22), NT (30)

0.5, 1.5 &

SB (25), CC (25), NT (25), SG (25)

0.5

5

Dry 1997 – 1998

IR62032

CRD

4

6

Rainy 1998

IR62032

2  3 factorial

3

7

Dry 1998 – 1999

IR62032

CRD

4

T1 – T4: fish nursing treatments T5: higher fish stocking T1: fish polyculture T2: fish polyculture T3: fish polyculture T4: fish polyculture T1: low (100 kg/ha) rice seeding rate T2: medium (200 kg/ha) rice seeding rate T3: high (300 kg/ha) rice seeding rate F1: low (100 kg/ha) vs. high (300 kg/ha) rice seeding rate F2: rice – fish culture A and rice – fish culture B T1: low ( F 5 cm) water level T2: high ( F 15 cm) water level F1: extra feeding for fish vs. no extra feeding for fish F2: fish stocking density: fish/m2 T1 – T4 different inputs to fish culture

8

Rainy 1999

IR62032

2  2 factorial

4

F1: row seeding vs. hand seeding

2.5 0.75

(50), (50), (20), (50), (80), (45), (82),

CC CC CC CC CC NT CC

(30), NT (20) (30), NT (20) (53), NT (27) (33), NT (17) (13), NT (7) (38), CA (17) (7), NT (11)

SB (30), CC (30), NT (30), SG (10) SB (54), CC (16), NT (20), SG (9), GG (1)

F2: after rice harvest: no-ratoon fertilization vs. ratoon fertilization STD: stocking density of fish; CRBD: complete randomized block design; CRD: complete randomized design; T1 – T5: treatment 1 to treatment 5; F1 and F2: experimental factors 1 and 2; SB: silver barb; NT: Nile tilapia; CC: common carp; SC: silver carp; MR: mrigal; CA: catfish; SG: snakeskin gourami; GG: giant gourami.

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Experiment

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Table 2 Descriptive statistics for B. gonionotus culture in intensively cultured rice fields (N = 107) Variable

Unit

Mean

Standard deviation

Min.

Max.

Culture period Stocking biomass Stocking density Stocking rate Gross production Net production Survival rate SGR Mean daily gain Harvesting weight

days kg/ha fish/m2 % kg/ha kg/ha % %body weight/day g/fish/day g/fish

120.1 38.8 0.44 47.0 181.8 142.9 78.6 1.74 0.39 65.9

31.0 40.6 0.27 17.1 137.3 117.6 18.7 0.92 0.31 56.8

66.0 3.5 0.12 20 18.0 1.7 27.5 0.27 0.04 8.5

156.0 146.5 1.60 80.0 604.1 547.7 100.0 3.58 1.58 253.9

When possible, fish were sampled during the rice crop. As such, we got SGRs for different developments of the rice crop: vegetative stage (0– 40 days after rice seeding (DAS)), reproductive stage (41 –70 DAS), ripening stage (71 – 100 DAS), fallow stage (before rice seeding and after rice harvest) and ratoon stage (after wet season rice harvest). However, very often, not enough fish could be sampled as a large number of fish forage in the rice field where they cannot be caught. Nevertheless, we were able to increase the data set to 260 entries.

Table 3 B. gonionotus: mean, standard deviation, minimum and maximum of the variables used in the SGR multiple regression (N = 187)

Dependent variable B. gonionotus specific growth rate Independent variables Extra food Pig manure Inorganic fertilizer Rice vegetative phase Rice reproductive phase Rice ripening phase Ratoon Trench Season Initial weight of B. gonionotus Log10 wild fish weight Log10 B. gonionotus standing biomass Log10 C. carpio standing biomass Log10 O. niloticus standing biomass

Data

Unit

Metric

%body weight/day

Dummy Metric Dummy Dummy Dummy Dummy Dummy Dummy Dummy Metric Metric Metric

0 = no; 1 = yes pigs/plot 0 = no; 1 = yes 0 = no; 1 = yes 0 = no; 1 = yes 0 = no; 1 = yes 0 = no; 1 = yes 0 = no; 1 = yes 0 = dry; 1 = wet g/fish kg/ha g/m2

Metric Metric

g/m2 g/m2

Mean

Standard deviation

Min.

Max.

1.97

1.82

 1.07

8.72

0.48 0.047 0.19 0.57 0.42 0.45 0.18 0.14 0.52 29.9 1.13 0.73

0.50 0.17 0.39 0.49 0.49 0.49 0.39 0.35 0.50 35.4 0.49 0.45

0 0 0 0 0 0 0 0 0 0.7 0 0.54

1 1 1 1 1 1 1 1 1 150.0 2.15 1.59

0.45 0.60

0.39 0.32

0.02 0.017

1.38 1.22

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We set up a multiple regression to determine the variables affecting the SGR of B. gonionotus. An overview of the dependent and all the independent variables can be found in Table 3. The variable ‘‘Extra food for fish’’ stands for a daily application (at a rate of 5% of the total fish biomass) of a mixture of fine rice bran (40%), rough rice bran (30%) and fresh water spinach (Ipomoea aquatica Forsk.) (30%) to the trench. The variable ‘‘Pig manure’’ stands for the number of pigs that provided manure per plot. The variable ‘‘Inorganic fertilizer’’ stands for the application of inorganic fertilizer to the trench at a rate of 7 kg N/ ha and 10 kg/P2O5/ha once every 2 weeks. The variables rice vegetative phase, rice reproductive phase, rice-ripening phase, trench and ratoon indicate to which period of the rice – fish cycle each data entry belongs. Some entries span for more than one period. We used the B. gonionotus weight as recorded at the beginning of each period (Costa-Pierce et al., 1993). The wild fish weight is the weight of the non-stocked fish at fish harvest. To estimate the fish biomass for each data entry, we calculated the standing biomass (SB): SB ¼ Wi  Sh  SD where Wi is the individual fish weight at the beginning of each period, Sh is the survival rate at fish harvest and SD is the initial stocking density for a given plot. We assumed that fish mortality took place immediately after stocking. The multiple regression analysis aimed at finding causal relationships. We used the default analysis as presented in the multiple regression module of STATISTICA for Windows, version 5.1. The procedure followed was based on Iles (1996): assumption testing (evenness of data; xs are not random variables and are measured without errors; errors have a mean of zero, are independent, have the same variance and are normally

Fig. 1. The specific growth rate of B. gonionotus decreases with increasing B. gonionotus standing biomass.

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Fig. 2. The specific growth rate (%body weight/day) of B. gonionotus decreases with increasing O. niloticus standing biomass.

Fig. 3. The specific growth rate of B. gonionotus (%body weight/day) does not change with increasing C. carpio standing biomass.

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distributed), transformations, creation of the model, checking of the autocorrelation, checking of the influential data, checking of the residuals using Cook’s D and multicollinearity testing through the use of the principal component analysis. In Fig. 1, we used a logarithmic trendline. For Figs. 2 and 3, we used the bicubic spline smoothing procedure to fit the surfaces.

3. Results B. gonionotus culture in rice fields resulted in extremely different production results (Table 2): the net production varied between 1.7 and 547.7 kg/ha, the SGR between 0.27% and 3.58% body weight/day, and the harvesting weight between 8.5 and 253.9 g/fish. The multiple regression solution includes six significant variables which explain 73.5% of the total variation (Table 4). In the wet season, B. gonionotus had a high SGR. Adding inorganic fertilizer or pig manure increased the SGR. In the presence of a high biomass of O. niloticus or wild fish, the SGR dropped, suggesting interspecific competition. The lower SGR with increasing B. gonionotus standing biomass (Fig. 1) suggests intraspecific competition. The variables used to indicate the different rice stages had no effect on SGR.

4. Discussion 4.1. Ratoon Extension work on rice –fish culture in the Mekong Delta, Vietnam, stresses the importance of the rice ratoon crop as a food source for B. gonionotus. This recommendation is mainly based on the observation that the main daily gains of B. gonionotus are noticeably higher in the ratoon crop than in the main rice crop. The main rice crop is not the most favorable environment to raise fish. With increasing rice biomass, the primary production by phytoplankton drops severely, resulting in low

Table 4 Multiple regression solution for SGR B. gonionotus (R2 = 0.735, F(6,180) = 83.384, p < 0.0001, S.E. estimate = 0.951)

Intercept Log10 O. niloticus standing biomass Inorganic fertilizer Season Log10 B. gonionotus standing biomass Log10 wild fish weight Pig manure

b

S.E. of b

B

S.E. of B

t(180)

p-level

 0.34

0.077

3.89  1.89

0.24 0.43

16.22  4.38

0.000001 0.000021

0.37 0.56  0.39

0.046 0.060 0.081

1.71 2.03  1.59

0.21 0.22 0.33

8.05 9.35  4.87

0.000001 0.000001 0.000002

 0.26 0.11

0.051 0.040

 0.95 1.16

0.19 0.42

 5.06 2.79

0.000001 0.005924

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dissolved oxygen concentrations (often below 2 mg l  1) (Rothuis et al., 1999; Vromant et al., 2001). Besides this, Halwart et al. (1996) pointed out that high tillering rice plants may cease fish from entering the dense vegetation. Vromant et al. (1998) found that the fish could still enter the rice field at high rice tiller densities but suggested that movements might be restrained. The ratoon crop is characterized by a lower biomass (Rothuis et al., 1998a) and higher water levels (30 – 45 cm), which make fish access to the field easier. Besides this, the ratoon crop often has a lot of weeds, and the higher water levels should make it possible for B. gonionotus to feed on leaves high up in the rice plant. Our analysis (Table 4) does not show any positive effect of the ratoon crop on the SGR of B. gonionotus. Indeed, none of the rice stages is withheld in the regression, indicating that B. gonionotus growth does not depend on the rice stage. Since the ratoon crop always follows the main rice crop, higher mean daily gains in the ratoon are due to the higher individual fish weight in the ratoon and are not due to better environmental or feeding conditions. 4.2. Intraspecific competition As the B. gonionotus standing biomass increased, the SGR dropped significantly (Table 4 and Fig. 1), indicating intraspecific competition. Also, Rothuis et al. (1998b) found clear indications of intraspecific competition for B. gonionotus in rice fields. Rice, supposedly the main food item of B. gonionotus in rice –fish fields (Rothuis et al., 1998a), is omnipresent. Rothuis et al. (1998a) found that in a polyculture of B. gonionotus, O. niloticus and C. carpio in rice –fish fields, B. gonionotus is the only species that mainly feeds on macrophytes. Macrophytes were good for 82.2 –100% of the food found in the stomach of B. gonionotus, 4.7– 10.8% in O. niloticus and 2.7– 5% in C. carpio (Rothuis et al., 1998a). A study by Haroon and Pittman (1997a) on the feeding of B. gonionotus in rice fields also showed a high positive selectivity index of this species for macrophytes. Haroon (1998) postulated that the species escapes intraspecific food competition in rice – fish systems. However, our data suggest that food competition does limit the growth of B. gonionotus in the rice field. While it cannot be denied that the rice biomass increases throughout the rice crop, it is also important to note that rice plants grow taller while the field floodwater remains at a level of approximately 15 cm. The fact that field water levels of 25 cm are a common practice in Bangladesh (Haroon and Pittman, 1997a) and the fact that the Bangladeshi experiments only lasted for 2– 3 days (Haroon, 1998) might explain why Haroon (1998) did not find intraspecific competition in his experiments. Farmers in Vietnam prefer water levels of 10 –15 cm because rice yields decrease 0.06 ton/ha with every 1 cm increase in the water level (Vromant et al., in press). As such, the increasing B. gonionotus standing biomass goes together with an increasing rice biomass at an increasing height, out of reach of the species. Besides that, rice becomes less palatable as the plant matures. Low dissolved oxygen levels (as mentioned above) with simultaneously increasing rice and fish biomass might also partly explain the observed competition. From a practical point of view, it is important that farmers are aware of this potential intraspecific competition, and consequently, consider lowering the B. gonionotus stocking densities.

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4.3. Interspecific competition In a balanced polyculture system, synergistic fish – fish and fish – environment relationships are maximized and antagonistic relationships are minimized (Milstein, 1992). However, if a pond is overloaded at any trophic level, interspecific and/or intraspecific competition will occur and productivity will decline (Bardach et al., 1972). Competition can be due to food shortage or worsening environmental conditions (Milstein, 1992). Increasing the O. niloticus standing biomass reduced the SGR (Fig. 2) of B. gonionotus, whereas increasing the C. carpio standing biomass had no negative effect on the SGR of B. gonionotus (Fig. 3), indicating that there is an interspecific competition with O. niloticus but not with C. carpio. This is in agreement with the findings of Haroon et al. (1992) who found that the daily growth rate of B. gonionotus lowered when raised in polyculture with O. niloticus. Though macrophyte feeders have no direct benefit from fertilization (Bardach et al., 1972; Saha et al., 1990), we found that fertilization of the trench with inorganic fertilizer or pig manure had a positive effect on B. gonionotus growth (Table 4), most probably through increasing algal biomass. Haroon and Pittman (1997a) found that besides feeding on macrophytes, B. gonionotus also feed substantially on Microcystis (22.9% of the gut content), Brachionus (5.6%) and Cyclops (4.4%) in ponds and on Spirogyra (12.6%), Oedogonium (12.9%) and Cyclops (23.6%) in rice fields. Since most of these are also consumed by O. niloticus (Chapman and Fernando, 1994) and since O. niloticus is known to partly feed on macrophytes (Rothuis et al., 1998a), food competition between the two fish is indeed very likely in environments with a high degree of food limitation such as rice – fish fields (Vromant et al., 2001). Adding fertilizers and manure reduces the chance of food competition. In our experiments, the predator Channa striata (Bloch) was the main wild fish species (both in number and weight). Since the anti-predator behavior is known to be costly both in terms of energy required and because it prevents fish from pursuing foraging (Abrahams and Kattenfeld, 1997), any stress by a predator further decreases the SGR.

5. Conclusion Though rice –fish systems have a large macrophyte biomass (mainly rice), the SGR of B. gonionotus remains low throughout the different rice stages. Our data suggest that this is due to a shortage of food, and as a result, intraspecific and interspecific competition.

Acknowledgements This study was part of a cooperative research project ‘‘Impact analysis and improvement of rice – fish farming systems in the semi-deep freshwater area of the Mekong Delta, Vietnam’’ of the University of Can Tho (Mekong Delta Farming Systems R&D Institute) and the Catholic University of Leuven (Laboratory of Ecology and Aquaculture and the Laboratory of Soil Fertility and Soil Biology) with support from the

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Flemish Inter-university Council (VLIR) and the Belgian Administration for Development Cooperation (BADC). The first author is seconded to the Mekong Delta Farming Systems R&D Institute by the Flemish Office for Development Cooperation and Technical Assistance (VVOB). The authors wish to thank H.C. Linh, N.T.H. Chau, V.T.M. Huong, V.T.T. Huong and N.V. Nhat for their assistance during the sampling.

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