Effects of silver carp and small indigenous species on pond ecology and carp polycultures in Bangladesh

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 large carps rohu (Labeo rohita), catla (Catla catla) and common carp (Cyprinus carpio) and the SIS punti (Puntius sophore) and mola (Amblypharyngodon mola), an experiment was carried out to test the effects of silver carp and of each SIS species on the growth, survival and yield of the large and small fish and on pond ecology.The ecology of the ponds was dominated by changes in time, strongly related to the development of a surface plankton scum at the beginning of the culture season and weather conditions. The surface scum increased decomposition processes and decreased algal development in the water body, promoted photosynthesis and ammonium release and reduced nitrification. Over those effects, the presence of silver carp in the ponds decreased algal biomass through grazing and promoted nitrification providing and resuspending particles in the water column. These effects were also produced by mola, but were evident only in the absence of silver carp. Punti stirring on the pond bottom increased nutrient flow from the sediments into the water column and promoted nitrification, but were also evident only in the absence of silver carp.The addition of 10 silver carp over the 99 large carps stocked in the 100 m² fishponds did not affect punti and mola reproduction in the ponds, negatively affected rohu and catla growth and yield by about 20-25% but not their survival, did not affect common carp performance, reduced punti harvested biomass by 10%, reduced mola performance by about 50%, and silver carp's own biomass increased total yield and total income in about 20% each. The addition of 250 mola or punti to the large carp polycultures did not affect the performance of any of the large carps. The decreased income from selling the more expensive large carps was more than compensated by that obtained from silver carp, which increased total income by 13-24% as compared to the corresponding treatments without silver carp. This allows the option to the farmer of selling part of the silver carp to complete the cash income that would have been obtained from large carps only if silver carp would not be stocked, and consume the rest with the family.     

Effects of adding different proportions of the small fish punti (Puntius sophore) and mola (Amblypharyngodon mola) to a polyculture of large carp

This experiment was carried out in the framework of a project to develop a viable fish polyculture technology under Bangladeshi conditions that allows simultaneous fish production of small indigenous species for the farmers' family consumption and of large carp species as a cash crop. The objectives of this experiment were to assess the effects of adding punti and mola in different proportions on the large carp and on the environment, and to assess the effects of punti on mola and mola on punti. The polyculture included the large carp rohu, catla and common carp (as cash crop fish), and the small indigenous fish punti and mola (as food for the small‐scale farmer family). The total large carp stocking density was 10 000 fish ha^?1, at a species ratio of 1:1:1. The total small fish stocking densities were 0 in the control and 30 000 punti and mola ha^?1 in the treatments, these at rates 2:1, 1:1 and 1:2. Stocking punti and mola at the density and all ratios tested were viable solutions to obtain simultaneously large carp cash crops and small fish to feed the farmer's family. Statistically marginal differences in large carp production were obtained in stocking small fish in the different proportions. These marginal differences could be explained by food competition between punti and common carp and between mola and rohu, which had different outcomes depending on the proportions of the small fish stocked. Stocking punti and mola at a 1:1 ratio would result in more small fish for the farmer's family, while the individual size of rohu, the most expensive large carp, would be somewhat smaller, but not necessarily small enough to decrease its selling price. Stocking one of the small fish in higher proportion than the other (2:1 or 1:2) would result in less mola for the family consumption, while harvesting of common carp would be somewhat lower and of smaller fish. Since common carp is the cheapest of the large carps, this small reduction would not necessarily affect the family income in an important way. With these results, farmers would now be able to reorganize their stocking practices with large carps and small fish and decide the appropriate small fish stocking ratios to meet their needs.     

Effects of adding the small fish punti (Puntius sophore Hamilton) and/or mola (Amblypharyngodon mola Hamilton) to a polyculture of large carp

Abstract This experiment was carried out in the framework of a project to develop a viable fish polyculture technology under Bangladeshi conditions that allows simultaneous fish production of small indigenous species for farm families' consumption and of large carp species as a cash crop. The polyculture included the large carp rohu, catla and mirror carp as cash crop fish, and the small indigenous fish punti and mola as food for the small‐scale household. Total large carp stocking density was 10 000 fish ha^?1, at a species ratio 1:1:1. Total small fish stocking densities were 0, 25 000 or 50 000 fish ha^?1. The objectives were to assess the effects of adding 25 000 punti and/or mola ha^?1 on the large carp and environment, and to assess the effects of punti on mola and mola on punti. It was found that catla was not affected by the addition of small fish in any of the combinations tested; rohu was not affected by punti, and mola reduced rohu performance by 30–40% only when punti was not present; mirror carp was not affected by punti, and mola increased mirror carp growth rate and harvesting weight by 25–30% whether punti was present or not; small fish did not significantly affect total yield and food conversion ratio; punti performance was not affected by mola; mola harvesting weight was not affected by punti, while mola harvesting numbers and biomass were reduced by 55–65% by punti. Factor analysis of water quality data identified photosynthesis–respiration and algal biomass– temperature as the main processes governing water quality. Effects of treatment on those water quality factors are analysed, and the fish–water quality relationships discussed. In a parallel polyculture experiment in 25 farm ponds, the performance of large carp species was found to be unaffected by the addition of punti and/or mola. The results indicate that, at the densities tested, punti and mola addition to the large carp polyculture is viable as they do not reduce cash crop production and might be a good food source for the farmer's family.     

Effects of increasing the stocking density of large carps by 20% on 'cash' carp–small fish polyculture of Bangladesh

This experiment was carried out in the framework of a project to develop a viable fish polyculture technology under Bangladeshi conditions that allows simultaneous fish production of small indigenous species for the farmers' family consumption and of large carp species as a cash crop. The objectives of this experiment were to assess the effects on fish performance and on the environment of adding 20% large fish to the basic ‘cash crop’ carp–small fish polyculture consisting of 10 000 fish ha^?1 of the large carp rohu Labeo rohita (Hamilton), catla Catla catla (Hamilton) and common carp Cyprinus carpio (L.) at a species ratio 1:1:1, and 15 000 fish ha^?1 of each small indigenous fish punti Puntius sophore (Hamilton) and mola Amblypharyngodon mola (Hamilton) (control). The treatment ponds were stocked with additional 2000 large fish ha^?1, either all rohu, or all catla, or all common carp, or half rohu and half common carp, or half catla and half common carp. The results obtained and the analysis of interactions through the food web that affect food resource availability of the different fish species and account for the trends and differences observed among treatments confirm the positive effect of common carp on rohu reported in previous experiments and show that a 20% increase in large carps stocking neither affect the survival of the large carps nor reduce harvesting biomass of the small fish for the farmer's family consumption. Increased stocking densities of each large carp species did not significantly reduce its own harvesting weight and mean growth rate, while significantly increased rohu and catla (but not common carp) harvesting biomass and yield. The complex relations between species led to inter‐ and intraspecific competition, which in some treatments increased growth or yield of one species and in other treatments of other species, so that the gains on one species and the losses on the other led to no significant total harvested biomass differences between treatments. Yet, the results herein reported may help farmers to select their species stocking ratios. Thus, if the main target of the farmer is rohu, then a stocking density increase of 10% common carp and 10% rohu would improve rohu growth rate (due to common carp) and result in 50% higher rohu harvesting biomass and yield. If the main target of the farmer is catla, then a 20% increase in catla stocking density would lead to 20% higher catla harvesting biomass.     

The effects of partially substituting Indian carps or adding silver carp on polycultures including small indigenous fish species (SIS)

A sustainable semi-intensive pond aquaculture technology including major carp species (Indian, Chinese and common carp) as cash-crop and small indigenous fish species (SIS) as food for the farmers' families is being optimized in Bangladesh. Silver carp inclusion in the polyculture is now being considered, because this very efficient filter feeder has a strong impact on pond ecology and also on the farmers' family nutrition because it is a cheap fish that the family can afford to eat instead of selling. The present paper is centered on the reduction of silver carp negative effects on other species while keeping the advantages of increased total yield and income due to silver carp stocking. It presents the results of two experiments, one on-station and one on-farm, in which 3–5 silver carp/100 m² were added or partially substituted major carp filter feeders. The basic stocking density was 100 carps (rohu, catla and a bottom feeder, either mrigal or common carp, at a 1:1:1 ratio) and 250 SIS (punti and mola) per 100 m². In the on-station experiment silver carp density was 3 and 5 fish/100 m² and the large carp bottom feeder was common carp. In the on-farm experiment silver carp density was 5 fish/100 m² and the bottom feeder was either common carp or mrigal.Most of the water quality and fish performance parameters tested were not affected by the polyculture composition. Adding 3–5% silver carp or substituting 3–5% of the herbivorous fish species by this highly efficient filter feeder increased grazing pressure on the phytoplankton, which led to a 25–40% reduction of the chlorophyll concentration in the water column. The increased grazing pressure was not enough to affect other water quality parameters and fewer effects on the availability of food for the other fish species occurred than when the silver carp addition was 10% of the polyculture, as reported in a previous work. The strong negative effects of silver carp on the other species of the polyculture and the higher total yields and income recorded in previous experiments with the addition of 10 silver carp/100 m² were much weaker and their expression depended on other pond conditions when 3 or 5 silver carp/100 m² were added or substituted the same number of rohu or catla, either when the bottom feeder was mrigal or common carp. It was concluded that stocking 3 silver carp/100 m² over the usual 100 large carp and 250 SIS /100 m² can be considered a ‘no effect’ stocking density in relation to the control without silver carp, while stocking 10 silver carp/100 m² should be preferred by farmers to keep the option of selling or consuming the silver carp.     

Grow‐out performance of Kuria labeo,Labeo gonius (Hamilton), with major carps in carp polyculture system

A year‐long grow‐out carp polyculture trial was conducted in nine earthen ponds to study the growth performance of Kuria labeo (Labeo gonius) with the different major carps such as catla (Catla catla), silver carp (Hypophthalmichthys molitrix), rohu (Labeo rohita) and mrigal (Cirrhinus mrigala). Suitable water quality parameters were maintained in ponds through intermittent liming, manuring and fertilization. Three different species combinations of carps were evaluated using silver carp and catla as the common species and varying other carp components as rohu–Kuria labeo, mrigal–Kuria labeo and rohu–mrigal in the three treatments. The ponds were stocked at a combined density of 7500 fingerlings ha^?1. Silver carp and catla showed similar growth performances in all the three combinations, suggesting that other carps in the combination do not have any differential influence on their growth. Kuria labeo was compatible with rohu, while competition was observed with mrigal. Although growth performance of Kuria labeo was inferior to that of mrigal, better compatibility of Kuria labeo with rohu helped this combination to yield a biomass equivalent to the mrigal–rohu combination, suggesting feasibility to use Kuria labeo as an alternative species to mrigal in the major carp polyculture system without compromising the total biomass yield.     

Compatibility of silver barb Puntius gonionotus (Bleeker) with Indian major carps in a grow-out polyculture

The compatibility and growth performance of silver barb Puntius gonionotus (Barbonymous gonionotus) with the three Indian major carps, i.e., Catla catla, Labeo rohita and Cirrhinus mrigala, were assessed in a 10‐month carp polyculture trial. Treatments T‐1, T‐2, T‐3 and T‐4 were stocked with three of the above four carp species, with an absence of silver barb, mrigal, rohu and catla, respectively, while all four species were stocked in treatment T‐5. The treatments were stocked at 6000 fingerlings ha⁻¹, with an equal species ratio maintained in each treatment. Incorporation of silver barb into the polyculture system neither affected the survival of any carp irrespective of species combination nor yielded significant changes in biomass production among treatments, except for the one without catla, where it was significantly low. The study revealed a higher extent of competition between silver barb and rohu, perceptible from the lower growth of one in the presence of the other. Although a certain level of competition of silver barb with mrigal was evident, competition with catla was not perceptible. Irrespective of species combination with silver barb as a component species, similar total biomass production in treatments revealed the feasibility of its incorporation into the Indian major carp‐based polyculture practice without affecting the total yield.     

The effect of the introduction of Nile tilapia (Oreochromis niloticus,L.) on small indigenous fish species (mola,Amblypharyngodon mola,Hamilton; chela,Chela cachius,Hamilton; punti,Puntius sophore,Hamilton)

This is the first controlled experiment to quantify the effect of introduced tilapia on indigenous species. This experiment was conducted in small earthen ponds (100 m²) to assess the impact of mixed‐sex or all‐male Nile tilapia (Oreochromis niloticus) on small indigenous species (SIS) commonly found in south Asia, mola (Amblypharyngodon mola), chela (Chela cachius) and punti (Puntius sophore). Ponds were fertilized, then stocked with 0.56 fish m^?2 of water surface area in the mixed‐sex and all‐male tilapia treatments and 0.42 fish m^?2 in the treatment without tilapia. No additional nutritional inputs were applied after stocking. Treatments were: mixed‐sex tilapia with SIS, mono‐sex male tilapia with SIS and SIS without tilapia (control). All treatments were stocked with 14 fish per species. All species reproduced during the 21‐month culture duration. The number of recruits varied by species, Tilapia reproduced in greater numbers than SIS. Tilapia numbers at harvest were the highest (451 ± 25/100 m²) in the mixed‐sex treatment compared with mola (221 ± 22/100 m²), chela (94 ± 8/100 m²) and punti (100 ± 7/100 m²). The number of mola was higher (399 ± 33/100 m²) in the all‐male tilapia treatment. There was reduction in the number of mola and chela in the treatment containing mixed‐sex tilapia. Gut content analysis combined with water sampling revealed that all fish species fed selectively. Significant interspecies dietary overlap was found between Nile tilapia and SIS and among SIS. Thus, there is potential for tilapia to compete with indigenous fish species when space and other resources are limiting, but a longer duration study with varying level of management is needed to determine how successfully tilapia competes with locally adapted SIS.     

Effects of varied levels of incorporation of Labeo gonius in carp polyculture system in India

Growth performance of kuria labeo, Labeo gonius as a component species in the major carp polyculture system was evaluated at two incorporation levels against a control without the species through a year‐round grow‐out study in nine earthen ponds (0.08 ha). Three species ratio of catla, silver carp, rohu, mrigal and kuria labeo at 15:15:40:30:0 (T‐1: control), 15:15:40:20:10 (T‐2) and 15:15:40:10:20 (T‐3) were evaluated as three treatments. The carps were stocked at a combined density of 7500 fingerlings ha^?1. Silver carp demonstrated the highest survival (75–81%) followed by rohu (70–76%), catla (69–76%), kuria labeo (69–71%) and mrigal (67–69%). Species‐wise yield attributes such as survival, harvest weight, SGR and biomass yield of silver carp, catla and rohu were similar in their respective treatments. Kuria labeo at 10% inclusion demonstrated 12% higher harvest weight than its 20% inclusion. However, such higher weight gain could not affect the total biomass yields of carps which remained similar among the treatments. Furthermore, harvest weight of kuria labeo at 10% inclusion was comparable to that of mrigal when the latter incorporated at 20–30% level. Therefore, the study suggested 10% to be a suitable incorporation level for kuria labeo in the commercial grow‐out carp polyculture system.     

The effect of common carp, Cyprinus carpio (L.) and mrigal, Cirrhinus mrigala (Hamilton) as bottom feeders in major Indian carp polycultures

A polyculture experiment with the large carp rohu, Labeo rohita (Hamilton), catla, Catla catla (Hamilton) and either mrigal, Cirrhinus mrigala (Hamilton) or common carp, Cyprinus carpio (L.) (as cash crop fish), and the small indigenous fish punti, Puntius sophore (Hamilton) (as food for the small‐scale farmer family) was carried out at the Field Laboratory of the Faculty of Fisheries, Bangladesh Agricultural University, Mymensingh. The main objective was to compare polycultures of large carp in which the bottom feeder is either the native mrigal or the exotic common carp. Secondary objectives were to assess the effects of adding the small indigenous species punti to polycultures of large carp, and to compare the effects of mrigal and common carp on punti production and reproduction. It was found that (i) common carp damaged embankments, had no effect on catla, improved rohu performance by 50% and total fish production by 20%; (ii) punti addition did not affect rohu, catla and total yield, improved mrigal performance by 50%, and decreased common carp performance by 20%; and (iii) punti was not affected either by common carp or by mrigal. However, its performance was not satisfactory, probably owing to frequent netting, which might have hindered growth and breeding. In spite of the embankment damage caused by common carp, this bottom feeder seems to be more promising than mrigal, because it leads to higher fish production. The addition of punti to the large carp polyculture is a viable proposition, as it does not reduce cash crop production, and might be a good food source for a small‐scale farmer's family.     

Effect of density on survival and growth of cyprinid fish fry

One of the bottlenecks in Israel's cold water ornamental fish industry is the large loss of fish during the post-larval stage. As a first step towards increasing survival rates, the optimal stocking density in earthen ponds for fry of each species should be determined. The results of five consecutive experiments with fry of goldfish (Carassius auratus), common carp and koi (respectively, edible and ornamental morphotypes of Cyprinus carpio) are presented here. The experiments were run in 18 cages of 1 m³ and 0.8 mm mesh size placed in a 0.02 ha earthen pond, stocked at 50,100 and 200 fry per cage. The variables analyzed were growth rate, harvesting weight, biomass and survival. Equations to estimate growth rates under different conditions are given. The three fish types present different growth rates and survival patterns in relation to stocking density and season, which lead to different management implications. Koi can be stocked up to 2 million fry per ha without showing negative density effects. Goldfish should be stocked at low density (500,000–1 million fry per ha) in spring and at higher rate (2 million fry per ha) in summer. Common carp stocking density should be adjusted to obtain the required fish size at harvest.     

POLYCULTURE OF THE FRESHWATER PRAWN (Macrobrachium rosenbergii) WITH TWO COMBINATIONS OF CARPS IN MANURED PONDS

A study was conducted to 1) evaluate the compatibility of prawns (Macrobrachiurn rosenbergii) with carps, especially the effect of the common carp (Cyprinus carpio); and 2) measure the ability of prawns to utilize the foods available in manured systems. Carps used were the silver (Hypophthalmichthys molitrix), bighead (Aristichthys nobilis), grass (Ctenopharyngodon idella) and common carp (Cyprinus carpio). Four ponds (0.09 to 0.17 ha) were stocked with silver, bighead and grass carps while common carp were stocked in only two ponds. Total fish densities were 8,600/ha where four carps were stocked, but only 6,200/ha when common carp were omitted. Post-larval prawns (wt 0.116 g) were stocked at densities of 17/m². Swine (approximately 60/ha of pond surface area) were housed on pond dikes so that fresh manure continuously entered each pond. Two lots of swine were fattened during the experimental period and loadings of swine manure were calculated based on hog size and feed consumption. Growth periods were 163 days for fish and 107 and 121 days for separate stockings of prawns. Gains in biomass by prawns averaged 714 kg/ha in ponds without common carp and 364 kg/ha in ponds containing common carp. Gains in biomass for both fish and prawns were 3,619 kg/ha (19.8 kg/ha/day) in ponds with common carp and 2,924 kg/ha (18.0 kg/ha/day) in those without common carp. Prawn survival was higher (X = 84.5% vs X = 72.5%), but final average weight of prawns was lower (2.64 vs 5.86 g) in the presence of common carp due to competition for food between common carp and prawns. Competition from carp is believed to have exerted less influence on prawn survival than predation on small prawns by larger, faster-growing prawns.     

Digestive proteases of three carps Catla catla, Labeo rohita and Hypophthalmichthys molitrix: partial characterization and protein hydrolysis efficiency

Characteristics and functional efficacy of digestive proteases of Catla catla, catla, Labeo rohita, rohu and Hypophthalmichthys molitrix, silver carp were studied. Total protease activity was significantly (P < 0.05) higher in rohu (1.219 ± 0.059 U mg protein⁻¹ min⁻¹) followed by silver carp (1.084 ± 0.061 U mg  protein⁻¹ min⁻¹), and catla (0.193 ± 0.006 U mg  protein⁻¹ min⁻¹). Trypsin activity of silver carp and rohu was 89–91% higher than catla. Chymotrypsin activity was significantly (P < 0.05) higher in silver carp compared with rohu and catla. The protease activity of rohu and silver carp displayed bell‐shaped curves with maximum activity at pH 9; whereas in catla, maximum activity was found between pH 8 and 11. Inhibition of protease activity with soybean trypsin inhibitor (SBTI), phenylmethylsulfonyl fluoride (PMSF) revealed the presence of serine proteases and inhibition of activity with N‐α‐p‐tosyl‐L‐lysine‐chloromethyl ketone (TLCK) and N‐tosyl‐L‐phenylalanychloromethane (TPCK) indicated the presence of trypsin‐like and chymotrypsin‐like enzymes in all these three carps. SDS‐PAGE showed the presence of several protein bands ranging from 15.3 to 121.9 kDa in enzyme extracts of catla, rohu and silver carp. The substrate SDS‐PAGE evidenced the presence of various protease activity bands ranging from 21.6–93.7, 21.6–63.8 and 26.7–98.5 kDa for catla, rohu and silver carp respectively. In pH‐stat hydrolysis of Chilean fishmeal showed significantly (P < 0.05) higher degree of hydrolysis compared with soybean meal, silver cup (a commercial fish feed of Mexico) and wheat flour, with enzyme preparations of three fishes. The rate of hydrolysis was significantly (P < 0.05) higher in silver carp compared with others.     

Olive barb, Puntius sarana (Hamilton) is a potential candidate species for introduction into the grow-out carp polyculture system

The compatibility of olive barb, Puntius sarana (Hamilton) with major carps was studied in grow-out carp polyculture system for one year in a set of nine earthen ponds of 0.08 ha each. Three different species combinations evaluated were Control: catla (Catla catla Ham.), silver carp (Hypophthalmichthys molitrix Valenciennes), rohu (Labeo rohita Hamilton) and mrigal (Cirrhinus mrigala Hamilton) at 0.5:0.5:1:1; T1: catla, silver carp, rohu and olive barb at 0.5:0.5:1:1 and T2: catla, silver carp, mrigal and olive barb at 0.5:0.5:1:1 at combined density of 7500 fingerlings/ha. While survival levels of the carps did not differ significantly in treatments (P > 0.05), silver carp recorded highest survival levels (94–96%) followed by olive barb (87–90%), mrigal (72–74%), rohu (72–73%) and catla (67–69%). The specific growth rate (SGR) and average harvested body weight (ABW) of catla and silver carp did not differ significantly among the treatments revealing their competition with mrigal or olive barb to be minimum. In absence of rohu in T2, both mrigal and olive barb showed higher SGR and ABW revealing minimal competition between these two species, while their lower performance in presence of rohu in Control and T1 indicated inter-specific competition with the latter. Such olive barb–rohu inter-specific competition, however, failed to yield significant effect on growth of rohu as revealed from its non-significant SGR difference in presence and absence of olive barb. The lower FCR (2.54 ± 0.06) and higher treatment biomass production (3418.4 ± 95.0 kg ha^− 1 year^− 1) in T1 with rohu–olive barb combination compared to T2 with mrigal–olive barb (2.84 ± 0.11; 3155.1 ± 104.7 kg ha^− 1 year^− 1) indicated feasibility and advantage of culturing rohu with olive barb rather than mrigal in carp polyculture. Further, similar biomass production in Control and T1 also indicated feasibility of replacing mrigal with olive barb in the grow-out carp polyculture system.     

Nursing duration and pond fertilization level affect polycultures of Indian major carp (rohu Labeo rohita and mrigal Cirrhina mrigala) with monosex Nile tilapia Oreochromis niloticus

This study investigated the effects of nursing duration on the subsequent performance of rohu (R) Labeo rohita and mrigal (M) Cirrhina mrigala in polyculture with monosex male Nile tilapia (T) Oreochromis niloticus at four levels of pond fertilization. Nile tilapia, rohu and mrigal were stocked at a ratio of 4:1:1 in a 90‐day trial based on 40 20‐m² pens fixed in four 400‐m² earthen ponds. Growth of carp fingerlings during prolonged nursing (5 or 12 months) was stunted compared with fish nursed over a conventional duration of 3 months (3) but showed superior growth subsequently. Mean daily weight gain of stunted rohu (12) ranged from 2.2 to 2.8 g per fish day^?1 compared with 1.1–1.6 g per fish day^?1 for younger fish (3). The comparable ranges for mrigal were 1.9–2.8 and 1.4–2.1 g per fish day^?1. Growth of Nile tilapia was inversely related to duration of carp nursing at the four levels of fertilization. Nile tilapia showed more response to increasing levels of fertilizer input (Y=?1.421+1.716X, where Y is the daily weight gain of Nile tilapia and X is the fertilizer level, r²=0.98, P<0.01, n=12). At a high level of fertilization (3.0 kg N:1.5 kg P ha^?1 day^?1), performance of stunted fingerlings (5 and 12) of both rohu and mrigal was similar (range 2.3–2.8 g per fish day^?1, P>0.05), but younger mrigal (M3) grew faster than rohu (2.1 g per fish day^?1 and 1.6 g per fish day^?1 respectively). Older rohu (12) appeared to perform particularly well, and Nile tilapia poorly at the lowest level of fertilization (1.5 N:0.75 kg P ha^?1 day^?1), suggesting the impact of age of seed on competition within polycultures. The net fish yield (NFY) of tilapia was not affected significantly (P>0.05) by differential stocking age of carps; therefore, combined NFY of the three experimental fish species was not affected by the age of carp, as tilapia was the dominant species in polyculture. The highest combined NFY of all species in the most intensively fertilized pond (3.0 N:1.5 P kg ha^?1 day^?1) was calculated at 4.06±0.08 g·m^?2 day^?1, which was significantly higher (P<0.001) than the yield (1.82±0.12 g·m^?2 day^?1) from the pond with the lowest fertilization. At the highest fertilizer level, tilapia, rohu and mrigal contributed 72%, 14% and 14%, respectively, to the NFY, whereas the ratio was 60%, 20% and 20% at the lowest fertilization level. The study indicated that yields from tilapia in polyculture with the two carp species in more eutrophic water can be optimized if advanced nursing of carps is practised. Moreover, higher inputs of inorganic fertilizer and advanced nursing of carp are economically attractive under Bangladeshi conditions. Advanced nursing of rohu also improves its performance in more extensive systems when tilapia densities are high.     

Food selection, electivity and dietary overlap among planktivorous Chinese and Indian major carp fry and fingerlings grown in extensively managed, rain-fed ponds in Bangladesh

Abstract. The fry of four species of planktivorous carp, silver carp, Hypophthalmichthys molitrix (Valenciennes), bighead carp, Aristichthys nobilis (Richardson), rohu, Labeo rohita (Hamilton), and catla, Catla catla (Hamilton), were grown in monoculture in ponds in Mymensingh, Bangladesh in order to determine electivity and the extent of dietary overlap. Other than an avoidance for Cyanobacteria, electivity was found to be low in most instances, although the low productivity of the ponds may have had some influence. Dietary overlap was greatest between catla and silver carp (0–52) and between catla and bighead carp (0–48).     

Cage‐pond integration of African catfish (Clarias gariepinus) and Nile tilapia (Oreochromis niloticus) with carps

Cage‐pond integration system is a new model for enhancing productivity of pond aquaculture system. A field trial was conducted using African catfish (Clarias gariepinus) and Nile tilapia (Oreochromis niloticus) in cages and carps in earthen ponds. There were four treatments replicated five times: (1) carps in ponds without cage, (2) tilapia at 30 fish m^?3 in cage and carps in open pond, (3) catfish at 100 fish m^?3 in cage and carps in open pond, (4) tilapia and catfish at 30 and 100 fish m^?3, respectively, in separate cages and carps in open pond. The carps were stocked at 1 fish m^?2. The cage occupied about 3% of the pond area. The caged tilapia and catfish were fed and the control ponds were fertilized. Results showed that the combined extrapolated net yield was significantly higher (P < 0.05) in the catfish, tilapia and carps integration system (9.4 ± 1.6 t ha^?1 year^?1) than in the carp polyculture (3.3 ± 0.7 t ha^?1 year^?1). The net return from the tilapia and carps (6860 US$ ha^?1 year^?1) and catfish, tilapia and carps integration systems (6668 US$ ha^?1 year^?1) was significantly higher than in the carp polyculture (1709 US$ ha^?1 year^?1) (P < 0.05). This experiment demonstrated that the cage‐pond integration of African catfish and Nile tilapia with carps is the best technology to increase production; whereas integration of tilapia and carp for profitability.     

Environmental effects of common carp Cyprinus carpio (L.) and mrigal Cirrhinus mrigala (Hamilton) as bottom feeders in major Indian carp polycultures

A polyculture experiment with the large carp rohu, catla and either mrigal or common carp (as cash crop fish), and the small indigenous fish punti (as food for the farmer's family) was carried out at Bangladesh Agricultural University, Mymensingh. The main objectives were to compare polycultures of large carp in which the bottom feeder is either the native mrigal or the exotic common carp, and to assess the effects of adding the small indigenous species punti to those polycultures. The results of fish–fish interactions and overall fish production have already been reported. The present paper presents the effects on the water quality, and discusses fish–environment interactions. The main conclusions are: time changes in the pond environment were stronger than fish composition effects. The main practice affecting water quality was liming, that incresed alkalinity, pH and water transparency and decreased ammonia. Rain affected photosynthesis and the match‐mismatch of the two steps of nitrification. The more that bottom feeding fish species disrupt the mud bottom, the stronger their effects on pond environment. Common carp produce the strongest disruption of the mud bottom, followed by punti and then by mrigal. Mud disruption produced by common carp leads to a stronger liming effect, nutrient release into the water, and provides more particles that rain‐floods wash out, facilitating the mismatch of the two steps of nitrification, and increased phosphorus adsorption into the mud bottom. Mud disruption by punti is only enough to improve the liming effect. Mud disruption by mrigal is the least, hence less particles are resuspended, nitrification is not affected during floods and relatively more phosphate remains in the water available for photosynthesis. The bottom feeder common carp can be seen not only as a target‐cultured fish but also as a management tool. Farmers can get double benefit in introducing common carp in the ponds as it enhances the effectiveness of lime application and increases the availability of nutrients to phytoplankton. Through the manipulation of species in the polyculture alone, farmers can maintain the environment better and also reduce input costs.     

Effects of all‐male,mixed‐sex and all‐female freshwater prawn in polyculture with major carps and molas in the fallow rice fields

This study was conducted to evaluate the effects of all‐male, mixed‐sex and all‐female freshwater prawn (Macrobrachium rosenbergii) in a polyculture with major carps (Catla catla and Labeo rohita) and self‐recruiting small fish molas (Amblypharyngodon mola) in the fallow rice fields of Bangladesh. There were three treatments with three replicates. All ponds were stocked with carps and molas plus either all‐male prawns (treatment MP), mixed‐sex prawns (MFP) or all‐female prawns (FP). Prawn, mola, catla and rohu were stocked 20 000, 20 000, 1750 and 750 ha^?1, respectively, in all treatments. The prawns were fed twice daily, starting at 8% body weight and gradually reduced to 3% body weight. The fish were fed in the morning with mustard oil cake and rice bran (1:2 ratios) at 3% body weight. Significantly higher production of prawns (697 kg ha^?1) was obtained in treatment MP, which yielded 34.7% and 56.2% more production than MFP and FP respectively. Significantly higher total production of 1620 kg ha^?1 and a higher benefit–cost ratio of 2.10:1 were also obtained in treatment MP. It can be concluded that an all‐male prawn culture is economically more viable than all‐female and mixed‐sex prawn cultures, along with other fish like major carps and the nutrient‐dense molas in the polyculture.     

Optimization of stocking density of freshwater prawn Macrobrachium rosenbergii (de Man) in carp polyculture in Bangladesh

A study was conducted to optimize stocking density of freshwater prawn, Macrobrachium rosenbergii, in carp polyculture for 3 months in 10 experimental ponds of 80 m². Five stocking densities of prawn, 2500, 5000, 7500, 10 000 and 12 500 ha^?1, were assigned to treatments T₁, T₂, T₃, T₄ and T₅ respectively. The densities of catla, Catla catla, rohu, Labeo rohita and silver carp, Hypophthalmicthys molitrix, were 2500, 5000, and 2500 ha^?1, respectively, in each treatment. Each treatment had two replicate ponds. The mean initial weights of prawn, catla, rohu and silver carp were 1.1±0.02, 8.28±0.1, 25.2±1.1 and 36.32±1.2 g respectively. A pelleted diet containing 30% protein was prepared using fish meal, meat and bone meal, mustard oilcake, rice bran, wheat bran and molasses, and was fed twice daily at a rate of 5% of fish biomass. Water quality parameters were measured fortnightly and the ranges of temperature, pH and dissolved oxygen were 27.5–1.3°C, 6.9–8.6 and 4.5–8.6 mg L^?1 respectively. Feed conversion ratios ranged from 2.05 to 2.20 among the treatments. Per cent survival (%) of prawns ranged from 72% to 78%, while it varied from 80% to 93%, 90% to 95% and 90% to 92% for catla, rohu and silver carp respectively. The results showed that there were no significant differences among the weight gains of prawn and carp in different treatments. However, the overall total production of prawn and fish together was significantly (P<0.05) higher in T₃ and T₄ compared with other treatments. The total production for 3 months ranged between 2618 and 2916 kg ha^?1. The production of prawn was significantly higher (361.3 kg ha^?1) in T₅ with a highest stocking density of 12 500 prawn ha^?1. Although there was no significant difference (P>0.05) between the total production of prawn and fish together in T₃ and T₄, the highest net profit (Tk. 69 006 ha^?1) was obtained in T₄. Therefore, from the result of the study it may be concluded that a stocking ratio of 4:1:2:1 of prawn:catla:rohu:silver carp at a total density of 20 000 ha^?1 may be recommended for prawn–carp polyculture in ponds.