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Effects of the filter feeder silver carp and the bottom feeders mrigal and common carp on small indigenous fish species (SIS) and pond ecology
Aquaculture 258 (2006) 439 – 451 www.elsevier.com/locate/aqua-online
Effects of the filter feeder silver carp and the bottom feeders mrigal and common carp on small indigenous fish species (SIS) and pond ecology A. Milstein a,⁎, A.F. Ahmed b , O.A. Masud b , A. Kadir b , M.A. Wahab b b
a Fish and Aquaculture Research Station Dor, M.P. Hof HaCarmel, 30820 Israel Department of Fisheries Management, Bangladesh Agricultural University, Mymensingh-2202, Bangladesh
Received 18 January 2006; received in revised form 11 April 2006; accepted 17 April 2006
Abstract A sustainable semi-intensive pond aquaculture technology including major carp species as cash-crop and small indigenous fish species (SIS) as food for the farmers' families is being optimized in Bangladesh. The inclusion of silver carp (Hypophthalmichthys molitrix), a cheap large species affordable by poor farmers, is now being considered. As part of a study on the effects of this filter feeder on polycultures including the SIS punti (Puntius sophore) and mola (Amblypharyngodon mola), an experiment was carried out in the ponds of the Bangladesh Agricultural University, Mymensingh, to test this fish effects in the presence of the bottom feeders either common carp (Cyprinus carpio) or mrigal (Cirrhinus cirrhosus) on production/reproduction of SIS, on the other fish species and on pond ecology. The data were analyzed using univariate and multivariate statistical techniques. Reproduction of both SIS species occurred in all ponds, their fry numbers, weight and biomass at harvest not being affected either by silver carp or by the bottom feeder species. The addition of silver carp in mrigal ponds had a negative effect on both adult SIS, while its addition to carp ponds had a weaker negative effect on mola and a positive effect on punti. Common carp favoured mola growth and reduced punti survival. Silver carp performance was not affected by the species of bottom feeder present. Common carp performance was not affected by silver carp. Mrigal harvesting biomass and survival were not affected by silver carp, but its harvesting weight, growth rate and yield decreased respectively by 29%, 42% and 39% in its presence. Large carp and total harvested biomass and yields were over 50% higher when silver carp was also present. In the presence of silver carp, large carp and total yields were 20% higher in common carp ponds, while in its absence they were somewhat higher in mrigal ponds. The FCR calculated considering only the large fish were 10% higher in mrigal ponds. FCR calculated including all species were somewhat higher in common carp ponds without silver carp, and 35% higher in mrigal ponds with silver carp. The observed results are explained and discussed considering the feeding habits of each species, the natural food web, and the ecological processes developing in the ponds. The addition of silver carp did not reduce the income obtained from the cash-crop species and could contribute to the nutrition and/or extra income of the farmer's family. From the production and ecological point of views, addition of silver carp to common carp ponds is a better proposition than to add it to mrigal ponds. © 2006 Elsevier B.V. All rights reserved. Keywords: Common carp; Food web; Mola; Mrigal; Polyculture; Punti; Silver carp; SIS small indigenous species
⁎ Corresponding author. Tel.: +972 4 6390651x23; fax: +972 4 6390652. E-mail address: [email protected] (A. Milstein). 0044-8486/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2006.04.045
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1. Introduction
2. Materials and methods
Pond aquaculture of freshwater fin fishes in Bangladesh is developing at a fast rate to compensate the loss of capture fisheries for which the country once was considered a blessed nation. Polyculture of 6–7 species of Indian and Chinese carps is widely practiced on an ad hoc basis (Wahab et al., 1997) targeting a higher fish production. In this context, as the first step of pond preparation all small fishes that naturally inhabit the ponds are removed using piscicides like rotenone or phostoxin, to eliminate competition with the target cultured species. While the benefits of these practices were not clearly established, their negative impacts on the farming households family nutrition are becoming apparent. Farmers grow carps to sell them as ‘cash-crop’ without considering the nutritional requirements of the family, since those small fishes are the principal protein source of the rural families (Thilsted et al., 1997). Facing this problem, a sustainable semi-intensive pond aquaculture technology including only a few major carp species as cash-crop and small indigenous fish species as food for the farmers' families was developed (e.g.: Mazumder and Lorenzen, 1999; Roos et al., 2002; Wahab et al. (eds.), 2003) and is being implemented by local extension agencies for its wider dissemination. The first steps to optimize the cash-SIS technology were centered on the study of fish-environment relationships in several polyculture combinations, focusing on interference on the pond bottom by the bottom feeding fish (Wahab et al., 2001, 2002; Milstein et al., 2002; Wahab et al., 2003; Alim et al., 2004, 2005). At present the research concentrates on the intervention in the water column, through the addition of a fish with ecological and socio-economic potential advantages: silver carp (Hypophthalmichthys molitrix) is expected to have a strong impact on the pond ecology, because it is a very efficient filter feeder (Milstein et al., 1985a,b; Milstein, 1992), and also on the farmers' family nutrition, because it is a cheap fish that the family can afford to eat instead of selling. It is also easily accessible to the poorer section of the population because of its low market price. In the framework of the study of the effects of silver carp on polycultures including the SIS species punti (Puntius sophore) and mola (Amblypharyngodon mola), the present article presents the results of an experiment designed to test the effects of silver carp on production/ reproduction of SIS in the presence of either common carp (Cyprinus carpio) or mrigal (Cirrhinus cirrhosus) as bottom feeders. The relationships among the fish species and their environment were also explored.
The experiment was conducted in winter–spring (Dec 2004 to April 2005) in 16 earthen fish ponds of 100 m2 area and 1.5 m depth in the Field Laboratory of the Faculty of Fisheries, Bangladesh Agricultural University (BAU), Mymensingh. Before starting the experiment ponds were drained to eradicate all predatory fishes, embankments and slopes were repaired, and agricultural lime (CaCO3) at 250 kg/ha (= 2.5 kg/pond) was applied on 22-Nov-04. Ponds were filled up with pumped water and fertilized with urea and triple phosphate (TSP), each at 100 kg/ha (= 1 kg/pond) to promote algal growth. The experiment had 4 treatments with 4 replications per treatment in a 2 × 2 factorial design (with/without silver carp; with common carp or with mrigal). In all ponds 10,000 punti/ha, 10,000 mola/ha, and 8000 bottom feeders/ha were stocked. These were either common carp or mrigal. Some ponds were also stocked with 2000 silver carp/ha. The resulting treatments were SC = silver carp + common carp, _C = only common carp, SM = silver carp + mrigal, and _M = only mrigal (Table 1). Fingerlings of the major carps were gathered from the local retailer, who collected them from rural nurseries that obtained the fertilized eggs from the nearby Government hatchery. Fingerlings of the small fish were collected from perennial ponds of the farmers, where farmers keep them together with major carps and the small fish naturally breed. Ponds were stocked on 8Dec-04 and harvested on 17-Apr-05. Fish stocking weight was calculated from the bulk weight divided by the number of individuals. No significant differences in stocking weight and biomass of each species between replications were revealed. Fertilizers and manure were applied at 10 days intervals, always at the same hour (10:00). Fertilizers were urea and TSP (0.5 kg/100 m2 Table 1 Stocking weight and number of fish in 100 m2 ponds in each treatment Fish species:
Stocking Treatment weight (g)
Silver carp 35.61 Common carp 26.59 Mrigal 36.00 Punti ⁎ 3.66 Mola ⁎ 1.18
SC _C SM _M (#/pond) (#/pond) (#/pond) (#/pond) 20 80 – 100 100
– 80 – 100 100
20 – 80 100 100
– – 80 100 100
Treatments: SC = silver carp + common carp, _C = only common carp, SM = silver carp + mrigal, and _M = only mrigal. ⁎ Small indigenous fish species (SIS).
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pond each). Manure (6.5 kg/100 m2 pond) was applied wet in the four corners of each pond. Due to symptoms of epizootic ulcerative syndrome (EUS), liming was repeated on 14 and 26-Jan-05 also applied at 2.5 kg/ 100 m2 pond; when the symptoms disappeared liming was suspended. Supplementary feed consisted of rice bran and soaked oil cake (2:1, both weighed dry, before oil cake was soaked in water), given 6 times a week at a daily rate of 1.5% of the large carp (silver carp, common carp and mrigal) body weight until February (cold period) and at 3% from March onward. During the winter the fish were not sampled to avoid disturbing them. When the temperature rose they were weighed monthly to adjust feeding. Environmental sampling was carried out at 10 day intervals, one or two days after fertilization, always at the same hour (09:00–10:00). The parameters measured were: temperature, transparency (Secchi disk), pH (EC 10 Portable pH meter), dissolved oxygen (digital Jenway portable DO-meter Model-58), total alkalinity, phosphorus and nitrogen compounds (P–PO4, N–NH4, N–NO2, N–NO3, HACH Kit DR/2010) and chlorophyll-a (standard procedures, APHA, 1992). Ecological processes that account for the main variability of the measured variables were identified through factor analysis (Kim and Mueller, 1978; Milstein, 1993), run from the correlation matrix among water quality variables. The purpose of factor analysis is to explain the relationships among a set of variables in terms of a limited number of new variables, which are assumed to be responsible for the covariation among the observed variables. The first factor extracted from that matrix is the linear combination of the original variables that accounts for as much of the variation contained in the samples as possible. The second factor is the second linear function of the original variables that accounts for most of the remaining variability, and so on. The factors are independent of one another, have no units and are standardized variables (normal distribution, mean = 0, variance = 1). The coefficients of the linear functions defining the factors are used to interpret their meaning, using the sign and relative size of the coefficients as an indication of the weight to be placed upon each variable. The effect of silver carp presence or absence and bottom feeder species on the performance of each fish species, on water quality variables and on the factors resulting from the factor analysis were tested with ANOVA. Differences between treatment levels were tested with the Duncan multicomparison test of means. A significance level of P < 0.05 was used. Survival and specific growth rate (percentage) data were normalized
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using the arcsine of the square root transformation. Feed conversion ratios were transformed to ranks (nonparametric technique appropriate for ratios) before performing further analyses. The analyses were run using the SAS statistical package. 3. Results 3.1. Fish performance Results of fish performance are presented in Tables 2–6 and Figs. 1–4. Common carp and punti grew at the same rate in winter and spring, while spring growth rate of the other species at least doubled that of the winter. However, if specific growth rates are considered, the large carp did not present seasonal differences while punti SGR was higher in winter and that of mola was higher in spring (Table 2). Significant differences by treatments within one season occurred only for mrigal in spring, which grew 0.4 g/day in the presence of silver carp and 1.1 g/day in its absence. Growth rates of all species were low, since most of the experiment was during the cold period, but the results can be used for comparison purposes. Survival of the large carps was at least 80%, of punti about 70% and of mola about 45%. Reproduction of both SIS species occurred in all ponds, independently of the polyculture combination present, starting in early spring when temperature increased. Table 3 presents the one-way-ANOVA results at harvest of the large fish analyses. Except for survival, there was a large variability in the silver carp variables; the ANOVA models were almost significant (P > 0.06) and in spite of the apparent differences between ponds with common carp and with mrigal, the corresponding mean multicomparisons did not disclose significant differences at the P > 0.05 level. Thus, silver carp
Table 2 Average growth rate and specific growth rate (SGR) of each fish species in winter (8-Dec-04 to 17-Feb-05, 71 days) and spring (17Feb-05 to 17-Apr-05, 59 days) Fish species
Silver carp Common carp Mrigal Punti Mola
Growth rate(g/day)
SGR ⁎ (%/day)
Winter
Winter
Spring
1.69 0.94 0.70 1.49a 0.82b
1.37 0.81 0.90 0.93b 1.06a
b
1.19 0.38 0.33b 0.11 0.02b
Spring a
2.56 0.52 0.73a 0.13 0.04a
Duncan mean multicomparisons significant at P < 0.05 indicated with different superscript letters in the same line. ⁎ Statistical tests based on transformed data. Values of means given untransformed.
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Table 3 ANOVA and Duncan mean multicomparisons of each large carp species harvesting parameters Variable unit
Weight Biomass (g) (kg/pond)
Survival ⁎ (%)
Silver carp ANOVA Signif. ns ns r2 0.47 0.47 Mean multicomparisons by bottom feeder C.carp 314 6.3 100 Mrigal 227 4.5 100 Common carp ANOVA Signif. ns ns ns r2 0.01 0.07 0.11 Mean multicomparisons by silver carp presence Present 83 6.5 91 Absent 86 6.0 96 Mrigal ANOVA Signif. ⁎⁎ ns ns r2 0.71 0.49 0.16 Mean multicomparisons by silver carp presence Present 85 _b 5.8 86 Absent 120 a_ 7.7 80
Growth (g/day)
Yield (kg/pond)
ns 0.49
ns 0.49
2.6 1.5
5.6 3.8
ns 0.06
ns 0.13
0.42 0.47
3.9 4.6
⁎ 0.69
⁎ 0.59
0.38 _b 3.3 _b 0.65 a_ 5.4 a_
r2 = coefficient of determination. %SS = % of the total sum of squares. Sign = Significance levels: ⁎ = 0.05, ⁎⁎ = 0.01, ⁎⁎⁎ = 0.001, ns = not significant. Mean multicomparisons: different letters in each column indicate significant differences at the 0.05 level. Pond area = 100 m2. ⁎ Statistical tests based on transformed data. Values of means given untransformed.
performance (number, weight and biomass at harvest, survival, growth rate and yield) was not affected by the species of bottom feeder present in the polyculture (either common carp or mrigal). Common carp performance was not affected by the presence or absence of silver carp. Mrigal harvesting biomass and survival were not affected by silver carp, but its harvesting weight, growth rate and yield decreased respectively by 29%, 42% and 39% in the presence of silver carp. Table 4 presents the two-way-ANOVA results at harvest of punti analyses. Punti (adults, fish bigger than 9 g) survival was significantly affected by the silver carp⁎bottom feeder combination (which accounted for 74% of the explained variability of punti survival) and also by the species of bottom feeder present in the polyculture (which accounted for the remaining 26% of the explained variability). Average punti survival was 15% lower in the presence of common carp than in the presence of mrigal. The silver carp⁎bottom feeder combination indicates that in the presence of silver
carp, punti survival was somewhat higher in carp ponds, while in the absence of silver carp punti survival was about half in common carp than in mrigal ponds (Fig. 1). Punti reproduced in the ponds, the numbers, weight and biomass of their fry at harvest not being affected either by silver carp or by the bottom feeder species in the polyculture. Table 5 presents the two-way-ANOVA results at harvest of mola analyses. As opposed to punti, mola (adults, fish bigger than 4 g) survival was not affected by silver carp or bottom feeder species, but mola harvesting weight, growth rate and yield were. The presence of silver carp reduced mola harvesting weight by 18% and growth rate by 33%. In common carp ponds mola weight and growth rate were respectively 20% and 50% higher than in mrigal ponds. The silver carp⁎bottom feeder combination indicates that in the absence of silver carp, mola yield was somewhat higher in mrigal ponds, while in the presence of silver carp mola yield was about half in mrigal than in common carp ponds (Fig. 2). Mola reproduced in the ponds, the numbers, weight and biomass of their fry at harvest not being affected either by silver carp or by the bottom feeder species in the polyculture. Table 6 presents several total harvesting biomass, yields and feed conversion ratios (FCR) results. The significant biomass and yield models accounted for about 70% and the FCR models for 50% of their variability. Large carp and total harvested biomass and yields were over 50% higher when silver carp was also present, as expected. Besides, the significant silver carp⁎bottom feeder combination indicates that in the presence of silver carp, large carp and total yields were 20% higher in common carp ponds, while in the absence of silver carp large carp and total yields were somewhat higher in mrigal ponds (Fig. 3). The FCR calculated considering only the large fish were 10% higher in mrigal ponds. FCR calculated including all species were somewhat higher in common carp ponds in the absence of silver carp, and 35% higher in mrigal ponds in the presence of silver carp (Fig. 4). 3.2. Water quality results Table 7 presents the ANOVA results of each water quality parameter. The model applied was highly significant for all variables, and accounted for all the variability of temperature, 60–80% of the variability of DO, pH and nitrogen ions, 53% of alkalinity and Secchi, and only 30–40% of phosphate and chlorophyll-a variability. Time was the most important source of variability of all variables, responsible for
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Table 4 ANOVA and Duncan mean multicomparisons of punti harvesting parameters Punti (adults, fish bigger than 9 g) ANOVA unit
Weight (g)
Biomass (kg/pond)
Survival ⁎ (%)
Growth (g/day)
Yield (kg/pond)
Signif. r2 Source Silver carp Bottom feeder Silver*bottom
ns 0.25 Sign ns ns ns
ns 0.41 Sign ns ⁎ ns
⁎⁎ 0.66 Sign ns ⁎ ⁎⁎
ns 0.25 Sign ns ns ns
ns 0.37 Sign ns ⁎ ns
Mean multicomparisons by silver carp Present 18 Absent 19
1.2 1.4
68 69
0.11 0.12
1.0 1.1
Mean multicomparisons by bottom feeder C.carp 17 Mrigal 20
1.0 _b 1.6 a_
62 _b 76 a_
0.10 0.13
0.8 _b 1.3 a_
%SS 0 26 74
Punti fry (fish smaller than 9 g) ANOVA unit
Number (#/pond)
Weight (g)
Biomass (kg/pond)
Total punti biomass (kg/pond)
Signif. r2 Source Silver carp Bottom feeder Silver⁎bottom
ns 0.30 Sign ns ⁎ ns
ns 0.35 Sign ns ns ns
ns 0.1 Sign ns ns ns
ns 0.39 Sign ns ns ns
Mean multicomparisons by silver carp Present 22 Absent 27
2.9 3.9
0.11 0.15
1.36 1.52
Mean multicomparisons by bottom feeder C.carp 41 a_ Mrigal 8 _b
4.9 2.0
0.21 0.06
1.25 1.63
r2 = coefficient of determination. %SS = percentage of Sum of Squares. Sign = Significance levels: ⁎ = 0.05, ⁎⁎ = 0.01, ⁎⁎⁎ = 0.001, ns = not significant. Mean multicomparisons: different letters in each column indicate significant differences at the 0.05 level. Pond area = 100 m2. ⁎ Statistical tests based on transformed data. Values of means given untransformed.
over half of the explained variability of Secchi and over 90% for the other variables. The letters in the ‘mean multicomparison by date’ section of Table 7 that indicate significant differences between dates are placed showing the time patterns of each variable. Water temperature was cool until mid January (around 20 °C) and warm in March–April (28–30 °C). Most of the water quality parameters show significant differences in the cool and warm periods. Silver carp accounted for a small (3–6%) but significant part of the explained variability of turbidity (Secchi), chlorophyll-a and nitrite. Secchi disk readings decreased and chlorophyll-a and nitrite increased in the presence of this filter feeding fish. Bottom feeder had an important (39%) contribution to turbidity, which was higher
(lower Secchi) in the presence of common carp than of mrigal. No other parameters showed significant bottom feeder effect. Phosphate was the only parameter that showed a significant silver carp⁎bottom feeder cross effect (Fig. 5), with lower PO4–P in the water in ponds with silver carp and common carp than in the other treatments. Table 8 presents the results of the factor analysis performed on the water quality parameters, and the ANOVA of the extracted factors. Two factors accounted for 53% of the overall data variability. The first factor (FACTOR1) shows two groups of variables (one with positive and one with negative high coefficients) that are positively correlated within the group and negatively correlated with the other group. The factor reflects the
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Table 5 ANOVA and Duncan mean multicomparisons of mola harvesting parameters Mola (adults, fish bigger than 4 g) ANOVA unit
Number (#/pond)
Weight (g)
Biomass (kg/pond)
Survival ⁎ (%)
Growth (g/day)
Yield (kg/pond)
Signif. r2 Source Silver carp Bottom feeder Silver*bottom
ns 0.36 Sign ns ns ⁎
⁎⁎ 0.65 Sign ⁎⁎ ⁎⁎ ns
ns 0.44 Sign ns ns ⁎
ns 0.35 Sign ns ns ⁎
⁎⁎ 0.65 Sign ⁎⁎ ⁎⁎ ns
⁎ 0.50 Sign ns ns ⁎
4.6 _b 5.6 a_
0.21 0.23
47 42
0.02 _b 0.03 a_
0.14 0.17
Mean multicomparisons by bottom feeder C.carp 45 5.6 a_ Mrigal 44 4.6 _b
0.24 0.21
45 44
0.03 a_ 0.02 _b
0.17 0.14
Mean multicomparisons by silver carp Present 47 Absent 42
%SS 57 43 0
%SS 57 43 0
%SS 11 22 67
Mola fry (fish smaller than 4 g) ANOVA unit
Number (#/pond)
Weight (g)
Biomass (kg/pond)
Total mola biomass (kg/pond)
Signif. r2 Source Silver carp Bottom feeder Silver⁎bottom
ns 0.22 Sign ns ns ns
ns 0.40 Sign ns ⁎ ns
ns 0.24 Sign ns ns ns
ns 0.42 Sign ns ns ns
Mean multicomparisons by silver carp Present 14 Absent 12
1.3 1.7
0.03 0.03
0.25 0.27
Mean multicomparisons by bottom feeder C.carp 19 Mrigal 7
2.3 a_ 0.7 _b
0.05 0.02
0.28 0.22
r2 = coefficient of determination. %SS = percentage of Sum of Squares. Sign = Significance levels: ⁎ = 0.05, ⁎⁎ = 0.01, ⁎⁎⁎ = 0.001, ns = not significant. Mean multicomparisons: different letters in each column indicate significant differences at the 0.05 level. Pond area = 100 m2. ⁎ Statistical tests based on transformed data. Values of means given untransformed.
main differences between winter and spring. In winter temperature and pond metabolism were low. When temperature is low the saturation point of oxygen is high so that the water can retain more oxygen, respiration of pond organisms is low so that they do not consume much DO and do not reduce pH. Phytoplankton biomass (chlorophyll) was also low, hence water transparency was high (high Secchi disk visibility), and nitrogenous nutrients were not much consumed so that they remained in the water. With increased temperature the oxygen saturation point decreases so that water retain less DO, pond metabolism increases and respiration reduces pH and oxygen, phytoplankton biomass increases reducing transparency and consuming nutri-
ents. The ANOVA model accounted for 92% of the variability of this factor, most of it due to sampling date. Over the date effect there was a significant bottom feeder effect, with lower FACTOR1 values in ponds with common carp than in ponds with mrigal. The second factor (FACTOR2) shows lower phosphate and ammonia levels when chlorophyll-a and pH were high and water transparency low, which reflects nutrient absorption by phytoplankton. The ANOVA model explained 62% of this factor variability. Most of the explained variability was due to time, with a seasonal pattern as shown in the ‘mean multicomparison by date’ section of Table 8. Over the date effect there were significant silver carp and bottom feeder effects,
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Table 6 ANOVA and Duncan mean multicomparisons of total fish harvesting biomass and yield and feed conversion ratio (FCR) ANOVA
Large carp biomass (kg/pond)
Large carp yield (kg/pond)
Total SIS ⁎ biomass (kg/pond)
Total biomass (kg/pond)
Total yield (kg/pond)
Large carp FCR ⁎⁎
All fish FCR ⁎⁎
Significance r2 Source Silver carp Bottom feeder Silver*bottom
⁎⁎⁎ 0.75 Sign ⁎⁎⁎ ns ns
⁎⁎ 0.70 Sign ⁎⁎⁎ ns ⁎
ns 0.37 Sign ns ns ns
⁎⁎ 0.70 Sign ⁎⁎⁎ ns ⁎
⁎⁎ 0.63 Sign ⁎⁎ ns ⁎
⁎ 0.52 Sign ns ⁎ ns
⁎ 0.49 Sign ns ns ⁎
Mean multicomparisons by silver carp Present 11.3 a_ 8.3 a_ Absent 7.1 _b 5.0 _b
1.6 1.8
12.8 a_ 8.9 _b
9.6 a_ 6.5 _b
1.9 2.1
1.6 1.6
Mean multicomparisons by bottom feeder C.carp 9.4 7.1 Mrigal 9.0 6.3
1.5 1.8
10.9 10.9
8.3 7.7
1.9 _b 2.1 a_
1.6 1.7
%SS 0 12
%SS 78 4 18
%SS 79 0 \21
%SS 71 2 27
%SS 11 57 32
%SS 0 34 66
r2 = coefficient of determination. %SS = % of total the sum of squares. Sign = Significance level: ⁎ = 0.05, ⁎⁎ = 0.01, ⁎⁎⁎ = 0.001, ns = not significant. Mean multicomparisons: different letters in each column indicate significant differences at the 0.05 level. Pond area = 100 m2. ⁎ Small indigenous fish species (SIS), punti + mola. ⁎⁎ Statistical tests based on rank-transformed FCR data. Values of means given untransformed.
As expected in an experiment carried out during winter and spring, there were strong differences between both seasons. Most water quality parameters had clearly different values in the cool and warm periods, while FACTOR1 indicated that pond metabolism was low in winter and high in spring. Most of the fish also grew slower in winter than in spring, and SIS reproduction only occurred in spring with the temperature increase.
Over the winter–spring differences, there were effects due to the polyculture composition that affected ecological processes developing in the ponds. Fig. 6 presents a conceptual representation of the pond ecosystem functioning under each combination of fish species, based on the significant interactions observed in this experiment. Left–right comparisons characterize differences due to the presence of silver carp, and top– bottom comparisons denote differences due to the bottom feeder species. Size of organisms and arrows represent importance of effects. Thus, the relationships among the different elements in the ponds can be described as follows: While searching for food common carp produces a stronger stirring of the mud bottom than mrigal
Fig. 1. Average punti survival (%) by silver carp and bottom feeder presence in the polyculture.
Fig. 2. Average mola yield (kg/100 m2 pond) by silver carp and bottom feeder presence in the polyculture.
with higher factor values when silver carp was present and in common carp ponds. 4. Discussion 4.1. Ecological considerations
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Fig. 3. Average total yield in each treatment including each species in the polyculture, by silver carp and bottom feeder presence in the polyculture (treatment). Yield scale in a per ha basis.
(Milstein et al., 2002), increasing water turbidity and facilitating nutrient flow through the autotrophic food web. Thus, in common carp ponds there were more phytoplankton that absorbed more nutrients (FACTOR2) and the overall respiration in the pond (FACTOR1) was higher than in mrigal ponds (Fig. 6, top vs. bottom graphs: resuspended particles, nutrients arrow, Secchi disk and phytoplankton). Together with this, grazing by silver carp on phytoplankton maintained the algae populations continually reproducing, thus absorbing more nutrients and increasing biomass as compared with populations in ponds without this fish (Fig. 6, left vs. right graphs). Mrigal is a bottom feeder that feeds on detritus, plants and zooplankton. It is mainly benthic but also migrates throughout the water column to feed (Chakrabarty and Shing, 1963). The decrease of mrigal harvesting weight, growth rate and yield in the presence of silver carp was accounted for by silver carp removal of phytoplankton, which then is less available in the water column for other fish species and does not precipitate onto the bottom where mrigal mainly feeds from (Fig. 6, left vs. right upper graphs). This effect was not felt by common carp, since this species does not feed on phytoplankton in the water column and can feed in the bottom browsing in deeper layers than mrigal (Fig. 6, left vs. right lower graphs). Punti survival seems to be related to interactions with the large fish. As the large bottom feeders, punti feed on the bottom and on detritus (Kohinoor, 2000). The turbidity produced by common carp activity near the bottom may have negatively affected punti survival, possibly through gill clogging (Fig. 6, top
vs. bottom left graphs). On the other hand, silver carp is a very efficient filter feeder (Milstein, 1992; Milstein et al., 1985a,b), and in its presence less particles sediment onto the pond bottom preventing detritus enrichment. Thus, in carp ponds silver carp reduced the excess of particles resuspended by carp allowing increased punti survival, while in mrigal ponds silver carp grazing decreased food availability for the bottom feeders, reducing punti survival (Fig. 6, left vs. right graphs). The negative effect of silver carp on mola may be due to competition for food resources, since both species graze on phytoplankton (Kohinoor, 2000; Miah and Siddique, 1992; Milstein, 1992). The positive effect of common carp on mola may be due to increased food resources through its activity on the pond bottom. Thus, in the absence of silver carp mola grew better in common carp than in mrigal ponds (Fig. 6, top vs. bottom left graphs). In mrigal ponds silver carp strongly reduced food resources for mola, reducing mola growth (Fig. 6, left vs. right upper graphs). In common carp ponds, the strong bottom disturbance produced by this bottom feeder increased phytoplankton availability, hence reduced competition between silver carp and mola (Fig. 6, left vs. right lower graphs). Summarising the results obtained, silver carp performed equally well in the presence of either bottom feeder and did not disturb punti and mola reproduction; its addition increased total biomass and yield to mrigal and common carp ponds; its addition into mrigal ponds had a negative effect on mrigal and on both SIS, while its addition into common carp ponds had a weaker negative effect on mola, a positive effect on punti, and did not affect common carp. In view of these results, it seems more convenient to add silver carp to common carp ponds than to mrigal ponds.
Fig. 4. Average FCR calculated including all fish species, by silver carp and bottom feeder presence in the polyculture.
Table 7 ANOVA and Duncan mean multicomparisons of water quality parameters
A. Milstein et al. / Aquaculture 258 (2006) 439–451 r2 = coefficient of determination. Significance levels: ⁎ = 0.05, ⁎⁎ = 0.01, ⁎⁎⁎ = 0.001, ns = not significant. %SS = percentage of total sums of squares. Mean multicomparisons: different letters in each column indicate significant differences at the 0.05 level. a>b>…. Number of observations = 240. 447
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Table 8 Results of factor analysis, ANOVA and Duncan mean multicomparisons of water quality parameters
Fig. 5. Average phosphate (mg/l) in the water by silver carp and bottom feeder presence in the polyculture.
4.2. Nutritional and economic considerations The amounts of fish harvested might look unimportant to western readers. However, under Bangladesh rural conditions they are considerable. In rural areas SIS and vegetables with rice or bread are the principal diet, and 250 g SIS would be sufficient for a meal of a 6 member rural family because they use SIS to give taste to their vegetable curry. This amount of SIS can easily be retrieved at short intervals throughout the culture season, from the moment SIS start to reproduce in the ponds. A meal richer in protein is prepared with silver carp. For a 6 member family 3 fish of 200 g would suffice for one meal, while a 1 kg large fish would suffice for two meals. From the two SIS tested, punti was found to perform well in all situations. Punti is considered the most consumed fish in Bangladesh (Roos, 2001), and resilient in harsh conditions. In contrast, mola production was low in all fish species combinations, most probably due to their culture during the cold season and to its reproduction that started only towards the end of the experiment, so that the fry biomass produced was also relatively low. In other experiments performed entirely during the warm season, although in different polyculture combinations, mola fry production was higher, and for the same mola stocking density 20% more harvested biomass was obtained (Alim et al., 2004). Other polycultures carried out also in the warm season with somewhat higher mola stocking densities (1.25 to 1.5 mola/m2) recorded 200 to 400% higher harvested biomass than in the present cool season experiment (Wahab et al., 2003; Alim et al., 2005). The higher warm season production would allow farm families to harvest periodically (say weekly) an amount required for the family nutrition. Mola is highly rich in vitamin A
Factor coefficients in bold were used for interpretation. r2 = coefficient of determination. Significance levels: ⁎ = .05, ⁎⁎ = 0.01, ⁎⁎⁎ = 0.001, ns = not significant. %SS = percentage of total sums of squares. Mean multicomparisons: different letters in each column indicate significant differences at the 0.05 level. a>b>…. Number of observations = 240.
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Fig. 6. Conceptual representation of the pond ecosystem functioning under each treatment. Upper graphs with mrigal, lower graphs with common carp. Left graphs without silver carp, right graphs with silver carp. Size of organisms and arrows represent importance of effects. Number of SIS related to their survival. Secchi disk represented to the right of the phytoplankton icons.
(Thilsted et al., 1997), therefore its importance for the family is higher than its economic return. In the present experiment the culture period not only covered the winter months, but also was rather short. As a result, the large carps were harvested when they were still small, total yields were 25–50% lower than those obtained during the warm season in the same ponds (Fig. 3 here 500–1000 kg/ha/4 months culture, compared to 1900–2600 kg/ha/5 months culture in Wahab et al., 2002, 2003; Alim et al., 2004, 2005), and the SIS reproduced but still did not attain large biomass. The price of all the involved species at these sizes are rather similar (bottom feeders 30–40 Tk./kg, silver carp 25– 35 Tk./kg, SIS 35–40 Tk./kg, 1 U$ = 65 Tk.), and the yield and harvested biomass were similar in all treatments, except for the presence of silver carp. Thus, at this harvesting stage there were no significant differences in income from each species among the treatments, showing that the addition of silver carp did
not reduce the income obtained from the cash-crop species and could contribute to the nutrition and/or extra income of the farmer's family. This is very important for poor farmers that own seasonal small ponds (up to 1000 m2 in area) in which they can culture fish only for about 4 months during the warm rainy season. In that short culture period the silver carp cannot attain large size, and for mid-size fish (250–500 g) silver carp is 25– 40% cheaper than the cash-crop fish (35–40 Tk./kg vs. 45–60 Tk./kg). In ponds with a water source that allows longer culture cycles, the cash-crop fish can reach large size while the SIS continue reproducing. Since the ecological relationships established in the different fish combinations tested affect fish performance from the very beginning of the culture, a stronger effect on fish production is expected in these ponds. Then, since for fish larger than 1 kg silver carp is 25–50% cheaper than the other large carps (60–70 Tk./kg vs. 80–150 Tk./kg) while the price of punti and mola remains unchanged,
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differences in income between the treatments may be expected and consuming the SIS and silver carp instead of selling them should be more attractive for the farmers. 5. Conclusions • Both SIS reproduced in all ponds, independently of the large fish combination present. • Silver carp performed equally well in the presence of either bottom feeder. • Addition of silver carp increased yields in both, mrigal and common carp ponds. • Addition of silver carp in mrigal ponds had a negative effect on mrigal and on both SIS. • Addition of silver carp in common carp ponds had a weaker negative effect on mola, a positive effect on punti, and did not affect common carp. • Common carp favoured mola growth and reduced punti survival. • The observed effects are the result of ecological interactions among the fish species through the natural food web and the pond environment. • The addition of silver carp did not reduce the income obtained from the cash-crop species and could contribute to the nutrition and/or extra income of the farmer's family. • It is more recommended to add silver carp to common carp ponds than to mrigal ponds. Acknowledgements This research was supported under Grant No. TAMOU-03-C23-022 U.S.–Israel Cooperative Development Research Program, Economic Growth, U.S. Agency for International Development. Prof. R.I. Sarker of Bangladesh Agricultural University (BAU), Mymensingh, is acknowledged for his cooperation during this study. This work could not have been done without the assistance of the staff of the Fisheries Field Laboratory and Water Quality and Pond Dynamics Laboratory, BAU, Mymensingh Bangladesh.
References APHA, 1992. Standard Methods for the examination of Water and Waste Water, 18th ed. American Public Health Association, Washington DC. 1268 pp. Alim, M.A., Wahab, M.A., Milstein, A., 2004. Effects of adding different proportions of the small fish punti (Puntius sophore) and mola (Amblypharyngodon mola) to a polyculture of large carp. Aquac. Res. 35, 124–133.
Alim, M.A., Wahab, M.A., Milstein, A., 2005. Effects of increasing by 20% the stocking density of large carps of the basic “cash” carp — small fish polyculture of Bangladesh. Aquac. Res. 36, 317–325. Chakrabarty, R.D., Shing, S.B., 1963. Observations on some aspects of the fishery and biology of the mrigal Cirrhinus mrigala (Hamilton) from Allahabad. Indian J. Fish. 10A (1), 209–232. Kim, J.O., Mueller, C.W., 1978. Factor Analysis. Statistical Methods and Practical Issues. Quantitative applications in the social sciences, vol. 14. Sage University. 8 pp. Kohinoor, A.H.M., 2000. Development of culture technology of three small indigenous fish mola (Amblypharyngodon mola), punti (Puntius sophore) and chela (Chela cachius) with notes on some aspects of their biology. Ph.D. Thesis, Department of Fisheries Management, Bangladesh Agricultural University, Mymensingh, 263 pp. Mazumder, D., Lorenzen, K., 1999. Developing aquaculture of small native species (SNS) in Bangladesh: village level agroecological change and the availability of SNS. NAGA — The ICLARM Q. 22 (3), 20–23. Miah, M.J.U., Siddique, W.H., 1992. Studies on the food and feeding habits of mola, Amblypharyngodon mola. Bangladesh J. Agric. Sci. 19 (2), 165–170. Milstein, A., 1992. Ecological aspects of fish species interactions in polyculture ponds. Hydrobiologia 231, 177–186. Milstein, A., 1993. Factor and canonical correlation analyses: basic concepts, data requirements and recommended procedures. In: Prein, M., Hulata, G., Pauly, D. (Eds.), Multivariate Methods in Aquaculture Research: Case Studies of Tilapias in Experimental and Commercial Systems. ICLARM Studies and Reviews, vol. 20, pp. 24–31. Milstein, A., Hepher, B., Teltsch, B., 1985a. Interactions between fish species and the ecological conditions in mono- and polyculture pond system. I. Phytoplankton. Aquac. Fish. Manage. 16, 305–317. Milstein, A., Hepher, B., Teltsch, B., 1985b. Interactions between fish species and the ecological conditions in mono- and polyculture pond system II. Zooplankton. Aquac. Fish. Manage. 16, 319–330. Milstein, A., Wahab, M.A., Rahman, M.M, 2002. Environmental effects of common carp Cyprinus carpio (L.) and mrigal Cirrhinus mrigala (Hamilton) as bottom feeders in major Indian carp polycultures. Aquac. Res. 33, 1103–1117. Roos, N., 2001. Fish consumption and aquaculture in rural Bangladesh: Nutritional contribution and production potential of culturing small indigenous fish species (SIS) in pond polyculture with commonly cultured carps. Ph.D. Thesis, The Royal Veterinary and Agricultural University, Frederiksberg, Denmark. 121 pp. Roos, N., Thilsted, S.H., Wahab, M.A., 2002. Culture of small indigenous fish species in seasonal ponds in Bangladesh: the potential for production and impact on food and nutrition security. In: Edwards, P., Little, D.C., Demaine, H. (Eds.), Rural aquaculture. CABI Publishing, Wallingford, UK, pp. 245–252. Thilsted, S.H., Roos, N., Hasan, N., 1997. The role of small indigenous fish species in food and nutrition security in Bangladesh. NAGA News Letter July–December (Supplement) 20, 13. Wahab, M.A., Rahmatullah, S.M., Beveridge, M.C.M., Baired, D.J., Tollervey, A.G., 1997. Impact of Sharpunti (Puntius goninotus) in the polyculture of native carps and mitigative measures using duckweed (Lemna sp). Proc. Fourth Asian Fisheries Forum, 16–20 October 1995, Beijing, pp. 60–64. Wahab, M.A., Rahman, M.M., Milstein, A., 2001. The effect of common carp Cyprinus carpio and mrigal Cirrhinus mrigala as
A. Milstein et al. / Aquaculture 258 (2006) 439–451 bottom feeders in Indian carp polyculture. World Aquac. 32 (4), 50–52 (and 69). Wahab, M.A., Rahman, M.M., Milstein, A., 2002. The effect of common carp Cyprinus carpio (L.) and mrigal Cirrhinus mrigala (Hamilton) as bottom feeders in major Indian carp polycultures. Aquac. Res. 33, 547–557. Wahab, M.A., Alim, M.A., Milstein, A., 2003. Effects of adding the small fish punti (Puntius sophore) and/or mola
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(Amblypharyngodon mola) to a polyculture of large carp. Aquac. Res. 34 (2), 149–164. Wahab, M.A., Thilsted, S.H., Hoq, E.M. (Eds.), 2003. Small Indigenous Species of Fish in Bangladesh. Proceedings of BAU–ENRECA/DANIDA Workshop on 30–31 October 2002, BAU, Mymensingh, Bangladesh. Bangladesh Agricultural University, Mymensingh, Bangladesh. 150 pp.
Effects of the filter feeder silver carp and the bottom feeders mrigal and common carp on small indigenous fish species (SIS) and pond ecology A. Milstein a,⁎, A.F. Ahmed b , O.A. Masud b , A. Kadir b , M.A. Wahab b b
a Fish and Aquaculture Research Station Dor, M.P. Hof HaCarmel, 30820 Israel Department of Fisheries Management, Bangladesh Agricultural University, Mymensingh-2202, Bangladesh
Received 18 January 2006; received in revised form 11 April 2006; accepted 17 April 2006
Abstract A sustainable semi-intensive pond aquaculture technology including major carp species as cash-crop and small indigenous fish species (SIS) as food for the farmers' families is being optimized in Bangladesh. The inclusion of silver carp (Hypophthalmichthys molitrix), a cheap large species affordable by poor farmers, is now being considered. As part of a study on the effects of this filter feeder on polycultures including the SIS punti (Puntius sophore) and mola (Amblypharyngodon mola), an experiment was carried out in the ponds of the Bangladesh Agricultural University, Mymensingh, to test this fish effects in the presence of the bottom feeders either common carp (Cyprinus carpio) or mrigal (Cirrhinus cirrhosus) on production/reproduction of SIS, on the other fish species and on pond ecology. The data were analyzed using univariate and multivariate statistical techniques. Reproduction of both SIS species occurred in all ponds, their fry numbers, weight and biomass at harvest not being affected either by silver carp or by the bottom feeder species. The addition of silver carp in mrigal ponds had a negative effect on both adult SIS, while its addition to carp ponds had a weaker negative effect on mola and a positive effect on punti. Common carp favoured mola growth and reduced punti survival. Silver carp performance was not affected by the species of bottom feeder present. Common carp performance was not affected by silver carp. Mrigal harvesting biomass and survival were not affected by silver carp, but its harvesting weight, growth rate and yield decreased respectively by 29%, 42% and 39% in its presence. Large carp and total harvested biomass and yields were over 50% higher when silver carp was also present. In the presence of silver carp, large carp and total yields were 20% higher in common carp ponds, while in its absence they were somewhat higher in mrigal ponds. The FCR calculated considering only the large fish were 10% higher in mrigal ponds. FCR calculated including all species were somewhat higher in common carp ponds without silver carp, and 35% higher in mrigal ponds with silver carp. The observed results are explained and discussed considering the feeding habits of each species, the natural food web, and the ecological processes developing in the ponds. The addition of silver carp did not reduce the income obtained from the cash-crop species and could contribute to the nutrition and/or extra income of the farmer's family. From the production and ecological point of views, addition of silver carp to common carp ponds is a better proposition than to add it to mrigal ponds. © 2006 Elsevier B.V. All rights reserved. Keywords: Common carp; Food web; Mola; Mrigal; Polyculture; Punti; Silver carp; SIS small indigenous species
⁎ Corresponding author. Tel.: +972 4 6390651x23; fax: +972 4 6390652. E-mail address: [email protected] (A. Milstein). 0044-8486/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2006.04.045
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1. Introduction
2. Materials and methods
Pond aquaculture of freshwater fin fishes in Bangladesh is developing at a fast rate to compensate the loss of capture fisheries for which the country once was considered a blessed nation. Polyculture of 6–7 species of Indian and Chinese carps is widely practiced on an ad hoc basis (Wahab et al., 1997) targeting a higher fish production. In this context, as the first step of pond preparation all small fishes that naturally inhabit the ponds are removed using piscicides like rotenone or phostoxin, to eliminate competition with the target cultured species. While the benefits of these practices were not clearly established, their negative impacts on the farming households family nutrition are becoming apparent. Farmers grow carps to sell them as ‘cash-crop’ without considering the nutritional requirements of the family, since those small fishes are the principal protein source of the rural families (Thilsted et al., 1997). Facing this problem, a sustainable semi-intensive pond aquaculture technology including only a few major carp species as cash-crop and small indigenous fish species as food for the farmers' families was developed (e.g.: Mazumder and Lorenzen, 1999; Roos et al., 2002; Wahab et al. (eds.), 2003) and is being implemented by local extension agencies for its wider dissemination. The first steps to optimize the cash-SIS technology were centered on the study of fish-environment relationships in several polyculture combinations, focusing on interference on the pond bottom by the bottom feeding fish (Wahab et al., 2001, 2002; Milstein et al., 2002; Wahab et al., 2003; Alim et al., 2004, 2005). At present the research concentrates on the intervention in the water column, through the addition of a fish with ecological and socio-economic potential advantages: silver carp (Hypophthalmichthys molitrix) is expected to have a strong impact on the pond ecology, because it is a very efficient filter feeder (Milstein et al., 1985a,b; Milstein, 1992), and also on the farmers' family nutrition, because it is a cheap fish that the family can afford to eat instead of selling. It is also easily accessible to the poorer section of the population because of its low market price. In the framework of the study of the effects of silver carp on polycultures including the SIS species punti (Puntius sophore) and mola (Amblypharyngodon mola), the present article presents the results of an experiment designed to test the effects of silver carp on production/ reproduction of SIS in the presence of either common carp (Cyprinus carpio) or mrigal (Cirrhinus cirrhosus) as bottom feeders. The relationships among the fish species and their environment were also explored.
The experiment was conducted in winter–spring (Dec 2004 to April 2005) in 16 earthen fish ponds of 100 m2 area and 1.5 m depth in the Field Laboratory of the Faculty of Fisheries, Bangladesh Agricultural University (BAU), Mymensingh. Before starting the experiment ponds were drained to eradicate all predatory fishes, embankments and slopes were repaired, and agricultural lime (CaCO3) at 250 kg/ha (= 2.5 kg/pond) was applied on 22-Nov-04. Ponds were filled up with pumped water and fertilized with urea and triple phosphate (TSP), each at 100 kg/ha (= 1 kg/pond) to promote algal growth. The experiment had 4 treatments with 4 replications per treatment in a 2 × 2 factorial design (with/without silver carp; with common carp or with mrigal). In all ponds 10,000 punti/ha, 10,000 mola/ha, and 8000 bottom feeders/ha were stocked. These were either common carp or mrigal. Some ponds were also stocked with 2000 silver carp/ha. The resulting treatments were SC = silver carp + common carp, _C = only common carp, SM = silver carp + mrigal, and _M = only mrigal (Table 1). Fingerlings of the major carps were gathered from the local retailer, who collected them from rural nurseries that obtained the fertilized eggs from the nearby Government hatchery. Fingerlings of the small fish were collected from perennial ponds of the farmers, where farmers keep them together with major carps and the small fish naturally breed. Ponds were stocked on 8Dec-04 and harvested on 17-Apr-05. Fish stocking weight was calculated from the bulk weight divided by the number of individuals. No significant differences in stocking weight and biomass of each species between replications were revealed. Fertilizers and manure were applied at 10 days intervals, always at the same hour (10:00). Fertilizers were urea and TSP (0.5 kg/100 m2 Table 1 Stocking weight and number of fish in 100 m2 ponds in each treatment Fish species:
Stocking Treatment weight (g)
Silver carp 35.61 Common carp 26.59 Mrigal 36.00 Punti ⁎ 3.66 Mola ⁎ 1.18
SC _C SM _M (#/pond) (#/pond) (#/pond) (#/pond) 20 80 – 100 100
– 80 – 100 100
20 – 80 100 100
– – 80 100 100
Treatments: SC = silver carp + common carp, _C = only common carp, SM = silver carp + mrigal, and _M = only mrigal. ⁎ Small indigenous fish species (SIS).
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pond each). Manure (6.5 kg/100 m2 pond) was applied wet in the four corners of each pond. Due to symptoms of epizootic ulcerative syndrome (EUS), liming was repeated on 14 and 26-Jan-05 also applied at 2.5 kg/ 100 m2 pond; when the symptoms disappeared liming was suspended. Supplementary feed consisted of rice bran and soaked oil cake (2:1, both weighed dry, before oil cake was soaked in water), given 6 times a week at a daily rate of 1.5% of the large carp (silver carp, common carp and mrigal) body weight until February (cold period) and at 3% from March onward. During the winter the fish were not sampled to avoid disturbing them. When the temperature rose they were weighed monthly to adjust feeding. Environmental sampling was carried out at 10 day intervals, one or two days after fertilization, always at the same hour (09:00–10:00). The parameters measured were: temperature, transparency (Secchi disk), pH (EC 10 Portable pH meter), dissolved oxygen (digital Jenway portable DO-meter Model-58), total alkalinity, phosphorus and nitrogen compounds (P–PO4, N–NH4, N–NO2, N–NO3, HACH Kit DR/2010) and chlorophyll-a (standard procedures, APHA, 1992). Ecological processes that account for the main variability of the measured variables were identified through factor analysis (Kim and Mueller, 1978; Milstein, 1993), run from the correlation matrix among water quality variables. The purpose of factor analysis is to explain the relationships among a set of variables in terms of a limited number of new variables, which are assumed to be responsible for the covariation among the observed variables. The first factor extracted from that matrix is the linear combination of the original variables that accounts for as much of the variation contained in the samples as possible. The second factor is the second linear function of the original variables that accounts for most of the remaining variability, and so on. The factors are independent of one another, have no units and are standardized variables (normal distribution, mean = 0, variance = 1). The coefficients of the linear functions defining the factors are used to interpret their meaning, using the sign and relative size of the coefficients as an indication of the weight to be placed upon each variable. The effect of silver carp presence or absence and bottom feeder species on the performance of each fish species, on water quality variables and on the factors resulting from the factor analysis were tested with ANOVA. Differences between treatment levels were tested with the Duncan multicomparison test of means. A significance level of P < 0.05 was used. Survival and specific growth rate (percentage) data were normalized
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using the arcsine of the square root transformation. Feed conversion ratios were transformed to ranks (nonparametric technique appropriate for ratios) before performing further analyses. The analyses were run using the SAS statistical package. 3. Results 3.1. Fish performance Results of fish performance are presented in Tables 2–6 and Figs. 1–4. Common carp and punti grew at the same rate in winter and spring, while spring growth rate of the other species at least doubled that of the winter. However, if specific growth rates are considered, the large carp did not present seasonal differences while punti SGR was higher in winter and that of mola was higher in spring (Table 2). Significant differences by treatments within one season occurred only for mrigal in spring, which grew 0.4 g/day in the presence of silver carp and 1.1 g/day in its absence. Growth rates of all species were low, since most of the experiment was during the cold period, but the results can be used for comparison purposes. Survival of the large carps was at least 80%, of punti about 70% and of mola about 45%. Reproduction of both SIS species occurred in all ponds, independently of the polyculture combination present, starting in early spring when temperature increased. Table 3 presents the one-way-ANOVA results at harvest of the large fish analyses. Except for survival, there was a large variability in the silver carp variables; the ANOVA models were almost significant (P > 0.06) and in spite of the apparent differences between ponds with common carp and with mrigal, the corresponding mean multicomparisons did not disclose significant differences at the P > 0.05 level. Thus, silver carp
Table 2 Average growth rate and specific growth rate (SGR) of each fish species in winter (8-Dec-04 to 17-Feb-05, 71 days) and spring (17Feb-05 to 17-Apr-05, 59 days) Fish species
Silver carp Common carp Mrigal Punti Mola
Growth rate(g/day)
SGR ⁎ (%/day)
Winter
Winter
Spring
1.69 0.94 0.70 1.49a 0.82b
1.37 0.81 0.90 0.93b 1.06a
b
1.19 0.38 0.33b 0.11 0.02b
Spring a
2.56 0.52 0.73a 0.13 0.04a
Duncan mean multicomparisons significant at P < 0.05 indicated with different superscript letters in the same line. ⁎ Statistical tests based on transformed data. Values of means given untransformed.
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Table 3 ANOVA and Duncan mean multicomparisons of each large carp species harvesting parameters Variable unit
Weight Biomass (g) (kg/pond)
Survival ⁎ (%)
Silver carp ANOVA Signif. ns ns r2 0.47 0.47 Mean multicomparisons by bottom feeder C.carp 314 6.3 100 Mrigal 227 4.5 100 Common carp ANOVA Signif. ns ns ns r2 0.01 0.07 0.11 Mean multicomparisons by silver carp presence Present 83 6.5 91 Absent 86 6.0 96 Mrigal ANOVA Signif. ⁎⁎ ns ns r2 0.71 0.49 0.16 Mean multicomparisons by silver carp presence Present 85 _b 5.8 86 Absent 120 a_ 7.7 80
Growth (g/day)
Yield (kg/pond)
ns 0.49
ns 0.49
2.6 1.5
5.6 3.8
ns 0.06
ns 0.13
0.42 0.47
3.9 4.6
⁎ 0.69
⁎ 0.59
0.38 _b 3.3 _b 0.65 a_ 5.4 a_
r2 = coefficient of determination. %SS = % of the total sum of squares. Sign = Significance levels: ⁎ = 0.05, ⁎⁎ = 0.01, ⁎⁎⁎ = 0.001, ns = not significant. Mean multicomparisons: different letters in each column indicate significant differences at the 0.05 level. Pond area = 100 m2. ⁎ Statistical tests based on transformed data. Values of means given untransformed.
performance (number, weight and biomass at harvest, survival, growth rate and yield) was not affected by the species of bottom feeder present in the polyculture (either common carp or mrigal). Common carp performance was not affected by the presence or absence of silver carp. Mrigal harvesting biomass and survival were not affected by silver carp, but its harvesting weight, growth rate and yield decreased respectively by 29%, 42% and 39% in the presence of silver carp. Table 4 presents the two-way-ANOVA results at harvest of punti analyses. Punti (adults, fish bigger than 9 g) survival was significantly affected by the silver carp⁎bottom feeder combination (which accounted for 74% of the explained variability of punti survival) and also by the species of bottom feeder present in the polyculture (which accounted for the remaining 26% of the explained variability). Average punti survival was 15% lower in the presence of common carp than in the presence of mrigal. The silver carp⁎bottom feeder combination indicates that in the presence of silver
carp, punti survival was somewhat higher in carp ponds, while in the absence of silver carp punti survival was about half in common carp than in mrigal ponds (Fig. 1). Punti reproduced in the ponds, the numbers, weight and biomass of their fry at harvest not being affected either by silver carp or by the bottom feeder species in the polyculture. Table 5 presents the two-way-ANOVA results at harvest of mola analyses. As opposed to punti, mola (adults, fish bigger than 4 g) survival was not affected by silver carp or bottom feeder species, but mola harvesting weight, growth rate and yield were. The presence of silver carp reduced mola harvesting weight by 18% and growth rate by 33%. In common carp ponds mola weight and growth rate were respectively 20% and 50% higher than in mrigal ponds. The silver carp⁎bottom feeder combination indicates that in the absence of silver carp, mola yield was somewhat higher in mrigal ponds, while in the presence of silver carp mola yield was about half in mrigal than in common carp ponds (Fig. 2). Mola reproduced in the ponds, the numbers, weight and biomass of their fry at harvest not being affected either by silver carp or by the bottom feeder species in the polyculture. Table 6 presents several total harvesting biomass, yields and feed conversion ratios (FCR) results. The significant biomass and yield models accounted for about 70% and the FCR models for 50% of their variability. Large carp and total harvested biomass and yields were over 50% higher when silver carp was also present, as expected. Besides, the significant silver carp⁎bottom feeder combination indicates that in the presence of silver carp, large carp and total yields were 20% higher in common carp ponds, while in the absence of silver carp large carp and total yields were somewhat higher in mrigal ponds (Fig. 3). The FCR calculated considering only the large fish were 10% higher in mrigal ponds. FCR calculated including all species were somewhat higher in common carp ponds in the absence of silver carp, and 35% higher in mrigal ponds in the presence of silver carp (Fig. 4). 3.2. Water quality results Table 7 presents the ANOVA results of each water quality parameter. The model applied was highly significant for all variables, and accounted for all the variability of temperature, 60–80% of the variability of DO, pH and nitrogen ions, 53% of alkalinity and Secchi, and only 30–40% of phosphate and chlorophyll-a variability. Time was the most important source of variability of all variables, responsible for
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Table 4 ANOVA and Duncan mean multicomparisons of punti harvesting parameters Punti (adults, fish bigger than 9 g) ANOVA unit
Weight (g)
Biomass (kg/pond)
Survival ⁎ (%)
Growth (g/day)
Yield (kg/pond)
Signif. r2 Source Silver carp Bottom feeder Silver*bottom
ns 0.25 Sign ns ns ns
ns 0.41 Sign ns ⁎ ns
⁎⁎ 0.66 Sign ns ⁎ ⁎⁎
ns 0.25 Sign ns ns ns
ns 0.37 Sign ns ⁎ ns
Mean multicomparisons by silver carp Present 18 Absent 19
1.2 1.4
68 69
0.11 0.12
1.0 1.1
Mean multicomparisons by bottom feeder C.carp 17 Mrigal 20
1.0 _b 1.6 a_
62 _b 76 a_
0.10 0.13
0.8 _b 1.3 a_
%SS 0 26 74
Punti fry (fish smaller than 9 g) ANOVA unit
Number (#/pond)
Weight (g)
Biomass (kg/pond)
Total punti biomass (kg/pond)
Signif. r2 Source Silver carp Bottom feeder Silver⁎bottom
ns 0.30 Sign ns ⁎ ns
ns 0.35 Sign ns ns ns
ns 0.1 Sign ns ns ns
ns 0.39 Sign ns ns ns
Mean multicomparisons by silver carp Present 22 Absent 27
2.9 3.9
0.11 0.15
1.36 1.52
Mean multicomparisons by bottom feeder C.carp 41 a_ Mrigal 8 _b
4.9 2.0
0.21 0.06
1.25 1.63
r2 = coefficient of determination. %SS = percentage of Sum of Squares. Sign = Significance levels: ⁎ = 0.05, ⁎⁎ = 0.01, ⁎⁎⁎ = 0.001, ns = not significant. Mean multicomparisons: different letters in each column indicate significant differences at the 0.05 level. Pond area = 100 m2. ⁎ Statistical tests based on transformed data. Values of means given untransformed.
over half of the explained variability of Secchi and over 90% for the other variables. The letters in the ‘mean multicomparison by date’ section of Table 7 that indicate significant differences between dates are placed showing the time patterns of each variable. Water temperature was cool until mid January (around 20 °C) and warm in March–April (28–30 °C). Most of the water quality parameters show significant differences in the cool and warm periods. Silver carp accounted for a small (3–6%) but significant part of the explained variability of turbidity (Secchi), chlorophyll-a and nitrite. Secchi disk readings decreased and chlorophyll-a and nitrite increased in the presence of this filter feeding fish. Bottom feeder had an important (39%) contribution to turbidity, which was higher
(lower Secchi) in the presence of common carp than of mrigal. No other parameters showed significant bottom feeder effect. Phosphate was the only parameter that showed a significant silver carp⁎bottom feeder cross effect (Fig. 5), with lower PO4–P in the water in ponds with silver carp and common carp than in the other treatments. Table 8 presents the results of the factor analysis performed on the water quality parameters, and the ANOVA of the extracted factors. Two factors accounted for 53% of the overall data variability. The first factor (FACTOR1) shows two groups of variables (one with positive and one with negative high coefficients) that are positively correlated within the group and negatively correlated with the other group. The factor reflects the
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Table 5 ANOVA and Duncan mean multicomparisons of mola harvesting parameters Mola (adults, fish bigger than 4 g) ANOVA unit
Number (#/pond)
Weight (g)
Biomass (kg/pond)
Survival ⁎ (%)
Growth (g/day)
Yield (kg/pond)
Signif. r2 Source Silver carp Bottom feeder Silver*bottom
ns 0.36 Sign ns ns ⁎
⁎⁎ 0.65 Sign ⁎⁎ ⁎⁎ ns
ns 0.44 Sign ns ns ⁎
ns 0.35 Sign ns ns ⁎
⁎⁎ 0.65 Sign ⁎⁎ ⁎⁎ ns
⁎ 0.50 Sign ns ns ⁎
4.6 _b 5.6 a_
0.21 0.23
47 42
0.02 _b 0.03 a_
0.14 0.17
Mean multicomparisons by bottom feeder C.carp 45 5.6 a_ Mrigal 44 4.6 _b
0.24 0.21
45 44
0.03 a_ 0.02 _b
0.17 0.14
Mean multicomparisons by silver carp Present 47 Absent 42
%SS 57 43 0
%SS 57 43 0
%SS 11 22 67
Mola fry (fish smaller than 4 g) ANOVA unit
Number (#/pond)
Weight (g)
Biomass (kg/pond)
Total mola biomass (kg/pond)
Signif. r2 Source Silver carp Bottom feeder Silver⁎bottom
ns 0.22 Sign ns ns ns
ns 0.40 Sign ns ⁎ ns
ns 0.24 Sign ns ns ns
ns 0.42 Sign ns ns ns
Mean multicomparisons by silver carp Present 14 Absent 12
1.3 1.7
0.03 0.03
0.25 0.27
Mean multicomparisons by bottom feeder C.carp 19 Mrigal 7
2.3 a_ 0.7 _b
0.05 0.02
0.28 0.22
r2 = coefficient of determination. %SS = percentage of Sum of Squares. Sign = Significance levels: ⁎ = 0.05, ⁎⁎ = 0.01, ⁎⁎⁎ = 0.001, ns = not significant. Mean multicomparisons: different letters in each column indicate significant differences at the 0.05 level. Pond area = 100 m2. ⁎ Statistical tests based on transformed data. Values of means given untransformed.
main differences between winter and spring. In winter temperature and pond metabolism were low. When temperature is low the saturation point of oxygen is high so that the water can retain more oxygen, respiration of pond organisms is low so that they do not consume much DO and do not reduce pH. Phytoplankton biomass (chlorophyll) was also low, hence water transparency was high (high Secchi disk visibility), and nitrogenous nutrients were not much consumed so that they remained in the water. With increased temperature the oxygen saturation point decreases so that water retain less DO, pond metabolism increases and respiration reduces pH and oxygen, phytoplankton biomass increases reducing transparency and consuming nutri-
ents. The ANOVA model accounted for 92% of the variability of this factor, most of it due to sampling date. Over the date effect there was a significant bottom feeder effect, with lower FACTOR1 values in ponds with common carp than in ponds with mrigal. The second factor (FACTOR2) shows lower phosphate and ammonia levels when chlorophyll-a and pH were high and water transparency low, which reflects nutrient absorption by phytoplankton. The ANOVA model explained 62% of this factor variability. Most of the explained variability was due to time, with a seasonal pattern as shown in the ‘mean multicomparison by date’ section of Table 8. Over the date effect there were significant silver carp and bottom feeder effects,
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Table 6 ANOVA and Duncan mean multicomparisons of total fish harvesting biomass and yield and feed conversion ratio (FCR) ANOVA
Large carp biomass (kg/pond)
Large carp yield (kg/pond)
Total SIS ⁎ biomass (kg/pond)
Total biomass (kg/pond)
Total yield (kg/pond)
Large carp FCR ⁎⁎
All fish FCR ⁎⁎
Significance r2 Source Silver carp Bottom feeder Silver*bottom
⁎⁎⁎ 0.75 Sign ⁎⁎⁎ ns ns
⁎⁎ 0.70 Sign ⁎⁎⁎ ns ⁎
ns 0.37 Sign ns ns ns
⁎⁎ 0.70 Sign ⁎⁎⁎ ns ⁎
⁎⁎ 0.63 Sign ⁎⁎ ns ⁎
⁎ 0.52 Sign ns ⁎ ns
⁎ 0.49 Sign ns ns ⁎
Mean multicomparisons by silver carp Present 11.3 a_ 8.3 a_ Absent 7.1 _b 5.0 _b
1.6 1.8
12.8 a_ 8.9 _b
9.6 a_ 6.5 _b
1.9 2.1
1.6 1.6
Mean multicomparisons by bottom feeder C.carp 9.4 7.1 Mrigal 9.0 6.3
1.5 1.8
10.9 10.9
8.3 7.7
1.9 _b 2.1 a_
1.6 1.7
%SS 0 12
%SS 78 4 18
%SS 79 0 \21
%SS 71 2 27
%SS 11 57 32
%SS 0 34 66
r2 = coefficient of determination. %SS = % of total the sum of squares. Sign = Significance level: ⁎ = 0.05, ⁎⁎ = 0.01, ⁎⁎⁎ = 0.001, ns = not significant. Mean multicomparisons: different letters in each column indicate significant differences at the 0.05 level. Pond area = 100 m2. ⁎ Small indigenous fish species (SIS), punti + mola. ⁎⁎ Statistical tests based on rank-transformed FCR data. Values of means given untransformed.
As expected in an experiment carried out during winter and spring, there were strong differences between both seasons. Most water quality parameters had clearly different values in the cool and warm periods, while FACTOR1 indicated that pond metabolism was low in winter and high in spring. Most of the fish also grew slower in winter than in spring, and SIS reproduction only occurred in spring with the temperature increase.
Over the winter–spring differences, there were effects due to the polyculture composition that affected ecological processes developing in the ponds. Fig. 6 presents a conceptual representation of the pond ecosystem functioning under each combination of fish species, based on the significant interactions observed in this experiment. Left–right comparisons characterize differences due to the presence of silver carp, and top– bottom comparisons denote differences due to the bottom feeder species. Size of organisms and arrows represent importance of effects. Thus, the relationships among the different elements in the ponds can be described as follows: While searching for food common carp produces a stronger stirring of the mud bottom than mrigal
Fig. 1. Average punti survival (%) by silver carp and bottom feeder presence in the polyculture.
Fig. 2. Average mola yield (kg/100 m2 pond) by silver carp and bottom feeder presence in the polyculture.
with higher factor values when silver carp was present and in common carp ponds. 4. Discussion 4.1. Ecological considerations
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Fig. 3. Average total yield in each treatment including each species in the polyculture, by silver carp and bottom feeder presence in the polyculture (treatment). Yield scale in a per ha basis.
(Milstein et al., 2002), increasing water turbidity and facilitating nutrient flow through the autotrophic food web. Thus, in common carp ponds there were more phytoplankton that absorbed more nutrients (FACTOR2) and the overall respiration in the pond (FACTOR1) was higher than in mrigal ponds (Fig. 6, top vs. bottom graphs: resuspended particles, nutrients arrow, Secchi disk and phytoplankton). Together with this, grazing by silver carp on phytoplankton maintained the algae populations continually reproducing, thus absorbing more nutrients and increasing biomass as compared with populations in ponds without this fish (Fig. 6, left vs. right graphs). Mrigal is a bottom feeder that feeds on detritus, plants and zooplankton. It is mainly benthic but also migrates throughout the water column to feed (Chakrabarty and Shing, 1963). The decrease of mrigal harvesting weight, growth rate and yield in the presence of silver carp was accounted for by silver carp removal of phytoplankton, which then is less available in the water column for other fish species and does not precipitate onto the bottom where mrigal mainly feeds from (Fig. 6, left vs. right upper graphs). This effect was not felt by common carp, since this species does not feed on phytoplankton in the water column and can feed in the bottom browsing in deeper layers than mrigal (Fig. 6, left vs. right lower graphs). Punti survival seems to be related to interactions with the large fish. As the large bottom feeders, punti feed on the bottom and on detritus (Kohinoor, 2000). The turbidity produced by common carp activity near the bottom may have negatively affected punti survival, possibly through gill clogging (Fig. 6, top
vs. bottom left graphs). On the other hand, silver carp is a very efficient filter feeder (Milstein, 1992; Milstein et al., 1985a,b), and in its presence less particles sediment onto the pond bottom preventing detritus enrichment. Thus, in carp ponds silver carp reduced the excess of particles resuspended by carp allowing increased punti survival, while in mrigal ponds silver carp grazing decreased food availability for the bottom feeders, reducing punti survival (Fig. 6, left vs. right graphs). The negative effect of silver carp on mola may be due to competition for food resources, since both species graze on phytoplankton (Kohinoor, 2000; Miah and Siddique, 1992; Milstein, 1992). The positive effect of common carp on mola may be due to increased food resources through its activity on the pond bottom. Thus, in the absence of silver carp mola grew better in common carp than in mrigal ponds (Fig. 6, top vs. bottom left graphs). In mrigal ponds silver carp strongly reduced food resources for mola, reducing mola growth (Fig. 6, left vs. right upper graphs). In common carp ponds, the strong bottom disturbance produced by this bottom feeder increased phytoplankton availability, hence reduced competition between silver carp and mola (Fig. 6, left vs. right lower graphs). Summarising the results obtained, silver carp performed equally well in the presence of either bottom feeder and did not disturb punti and mola reproduction; its addition increased total biomass and yield to mrigal and common carp ponds; its addition into mrigal ponds had a negative effect on mrigal and on both SIS, while its addition into common carp ponds had a weaker negative effect on mola, a positive effect on punti, and did not affect common carp. In view of these results, it seems more convenient to add silver carp to common carp ponds than to mrigal ponds.
Fig. 4. Average FCR calculated including all fish species, by silver carp and bottom feeder presence in the polyculture.
Table 7 ANOVA and Duncan mean multicomparisons of water quality parameters
A. Milstein et al. / Aquaculture 258 (2006) 439–451 r2 = coefficient of determination. Significance levels: ⁎ = 0.05, ⁎⁎ = 0.01, ⁎⁎⁎ = 0.001, ns = not significant. %SS = percentage of total sums of squares. Mean multicomparisons: different letters in each column indicate significant differences at the 0.05 level. a>b>…. Number of observations = 240. 447
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Table 8 Results of factor analysis, ANOVA and Duncan mean multicomparisons of water quality parameters
Fig. 5. Average phosphate (mg/l) in the water by silver carp and bottom feeder presence in the polyculture.
4.2. Nutritional and economic considerations The amounts of fish harvested might look unimportant to western readers. However, under Bangladesh rural conditions they are considerable. In rural areas SIS and vegetables with rice or bread are the principal diet, and 250 g SIS would be sufficient for a meal of a 6 member rural family because they use SIS to give taste to their vegetable curry. This amount of SIS can easily be retrieved at short intervals throughout the culture season, from the moment SIS start to reproduce in the ponds. A meal richer in protein is prepared with silver carp. For a 6 member family 3 fish of 200 g would suffice for one meal, while a 1 kg large fish would suffice for two meals. From the two SIS tested, punti was found to perform well in all situations. Punti is considered the most consumed fish in Bangladesh (Roos, 2001), and resilient in harsh conditions. In contrast, mola production was low in all fish species combinations, most probably due to their culture during the cold season and to its reproduction that started only towards the end of the experiment, so that the fry biomass produced was also relatively low. In other experiments performed entirely during the warm season, although in different polyculture combinations, mola fry production was higher, and for the same mola stocking density 20% more harvested biomass was obtained (Alim et al., 2004). Other polycultures carried out also in the warm season with somewhat higher mola stocking densities (1.25 to 1.5 mola/m2) recorded 200 to 400% higher harvested biomass than in the present cool season experiment (Wahab et al., 2003; Alim et al., 2005). The higher warm season production would allow farm families to harvest periodically (say weekly) an amount required for the family nutrition. Mola is highly rich in vitamin A
Factor coefficients in bold were used for interpretation. r2 = coefficient of determination. Significance levels: ⁎ = .05, ⁎⁎ = 0.01, ⁎⁎⁎ = 0.001, ns = not significant. %SS = percentage of total sums of squares. Mean multicomparisons: different letters in each column indicate significant differences at the 0.05 level. a>b>…. Number of observations = 240.
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Fig. 6. Conceptual representation of the pond ecosystem functioning under each treatment. Upper graphs with mrigal, lower graphs with common carp. Left graphs without silver carp, right graphs with silver carp. Size of organisms and arrows represent importance of effects. Number of SIS related to their survival. Secchi disk represented to the right of the phytoplankton icons.
(Thilsted et al., 1997), therefore its importance for the family is higher than its economic return. In the present experiment the culture period not only covered the winter months, but also was rather short. As a result, the large carps were harvested when they were still small, total yields were 25–50% lower than those obtained during the warm season in the same ponds (Fig. 3 here 500–1000 kg/ha/4 months culture, compared to 1900–2600 kg/ha/5 months culture in Wahab et al., 2002, 2003; Alim et al., 2004, 2005), and the SIS reproduced but still did not attain large biomass. The price of all the involved species at these sizes are rather similar (bottom feeders 30–40 Tk./kg, silver carp 25– 35 Tk./kg, SIS 35–40 Tk./kg, 1 U$ = 65 Tk.), and the yield and harvested biomass were similar in all treatments, except for the presence of silver carp. Thus, at this harvesting stage there were no significant differences in income from each species among the treatments, showing that the addition of silver carp did
not reduce the income obtained from the cash-crop species and could contribute to the nutrition and/or extra income of the farmer's family. This is very important for poor farmers that own seasonal small ponds (up to 1000 m2 in area) in which they can culture fish only for about 4 months during the warm rainy season. In that short culture period the silver carp cannot attain large size, and for mid-size fish (250–500 g) silver carp is 25– 40% cheaper than the cash-crop fish (35–40 Tk./kg vs. 45–60 Tk./kg). In ponds with a water source that allows longer culture cycles, the cash-crop fish can reach large size while the SIS continue reproducing. Since the ecological relationships established in the different fish combinations tested affect fish performance from the very beginning of the culture, a stronger effect on fish production is expected in these ponds. Then, since for fish larger than 1 kg silver carp is 25–50% cheaper than the other large carps (60–70 Tk./kg vs. 80–150 Tk./kg) while the price of punti and mola remains unchanged,
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differences in income between the treatments may be expected and consuming the SIS and silver carp instead of selling them should be more attractive for the farmers. 5. Conclusions • Both SIS reproduced in all ponds, independently of the large fish combination present. • Silver carp performed equally well in the presence of either bottom feeder. • Addition of silver carp increased yields in both, mrigal and common carp ponds. • Addition of silver carp in mrigal ponds had a negative effect on mrigal and on both SIS. • Addition of silver carp in common carp ponds had a weaker negative effect on mola, a positive effect on punti, and did not affect common carp. • Common carp favoured mola growth and reduced punti survival. • The observed effects are the result of ecological interactions among the fish species through the natural food web and the pond environment. • The addition of silver carp did not reduce the income obtained from the cash-crop species and could contribute to the nutrition and/or extra income of the farmer's family. • It is more recommended to add silver carp to common carp ponds than to mrigal ponds. Acknowledgements This research was supported under Grant No. TAMOU-03-C23-022 U.S.–Israel Cooperative Development Research Program, Economic Growth, U.S. Agency for International Development. Prof. R.I. Sarker of Bangladesh Agricultural University (BAU), Mymensingh, is acknowledged for his cooperation during this study. This work could not have been done without the assistance of the staff of the Fisheries Field Laboratory and Water Quality and Pond Dynamics Laboratory, BAU, Mymensingh Bangladesh.
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