The androgenic gland and monosex culture of freshwater prawn Macrobrachium rosenbergii (De Man): a biotechnological perspective

Males of the freshwater prawn Macrobrachium rosenbergii (De Man) grow faster and reach a larger size at harvest than females of the species. It is thus obvious that culture of monosex all‐male populations would be economically advantageous. Sexual differentiation in crustaceans is regulated by the androgenic gland (AG), which plays a pivotal role in the regulation of male differentiation and in the inhibition of female differentiation. In M. rosenbergii, AG removal from immature males resulted in sex reversal, with complete female differentiation. Similarly, AG implantations into immature females lead to the development of the male reproductive system. Sex‐reversed M. rosenbergii animals were capable of mating with normal specimens to produce offspring. Early attempts in Israel and more recently, attempts in other countries to establish all‐male populations through manual segregation showed that for the production of monosex prawn populations to be economically feasible, intervention via the AG is probably required. However, a suitable biotechnology is still to be developed, and an androgenic hormone has yet to be identified in decapods. Three lines of aquacultural and biotechnological research and development are proposed for the future: (1) Establishment of monosex cultures through manual segregation, together with the application of selective harvesting and claw ablation, as well as examination of different monosex culture strategies under a variety of economic conditions. (2) Microsurgical intervention in the AG, leading to the development of functional neo‐females, which would subsequently be mated with normal males to produce all‐male progeny. (3) Elucidation of AG bioactive products to enable biochemical or molecular manipulation of sex differentiation.     

Over-wintering growth of Macrobrachium rosenbergii (de Man) with carp polyculture in Bangladesh fed formulated diets

An experiment was conducted from December 2003 to April 2004 to observe the over‐wintering growth of freshwater prawn, Macrobrachium rosenbergii, with catla, Catla catla and rohu, Labeo rohita in polyculture using formulated diets. The study was conducted in eight experimental ponds, each 80 m². Three experimental diets containing 30% protein were prepared using fish meal, meat and bone meal, mustard oilcake, rice bran, wheat bran and molasses (binder), and assigned to treatments T₁, T₂ and T₃ respectively. A commercial diet from Saudi‐Bangla Fish Feed was assigned to T₄ (reference diet). Each treatment had two replicate ponds. Juvenile prawns and catla and rohu fingerlings (initial weight 1.60±0.10, 30.0±1.2 and 25.0±1.1 g respectively) were stocked at a ratio of 2:1:1 (prawn:catla:rohu). A total of 160 prawn and fish (20 000 ha^?1) were stocked in each pond. Fish were fed twice daily at 3% body weight (b.w.) for the first 3 months and 5% b.w. for the last 2 months. Prawns in T₁ fed diet 1 had significantly higher (P<0.05) weight gain compared with that of T₃. The reference group and T₂ had intermediate values not significantly different from either. Weight gains of catla and rohu were significantly higher in T₁. The feed conversion ratio values of different diets ranged between 1.89 and 2.13. Survival (%) ranged from 90.0% to 95.0% for catla, 87.5% to 92.5% for rohu and 70.0% to 76.3% for M. rosenbergii, and there were no significant differences (P>0.05) among different treatments. Total production ranged between 2196 and 2679 kg ha^?1, with T₁ showing significantly higher production and net profit (taka 56 531.9 ha^?1). The results of the study demonstrated that it is possible to culture M. rosenbergii with carp in polyculture during the winter utilizing the late‐produced PLs. Further study is needed to determine the optimum stocking density of M. rosenbergii in carp polyculture.     

Stocking density effects on growth and production of the threatened silurid catfish,Mystus cavasius (Hamilton) fingerlings in nursery ponds

The fingerling‐rearing experiment of the threatened catfish, Mystus cavasius was carried out at different stocking densities in earthen nursery ponds. Twelve‐day‐old fry were stocked at 200 000 ha^?1 in treatment‐1 (T₁), 250 000 ha^?1 in treatment‐2 (T₂) and 300 000 ha^?1 in treatment‐3 (T₃) respectively. The mean length and weight of fry at stocking was 1.24 ± 0.25 cm and 0.11 ± 0.04 g respectively. Fry in all the experimental ponds were supplemented with SABINCO nursery feed for the first 14 days and starter‐I feed for days 15–56. The physico‐chemical parameters and plankton population of pond water were within the suitable level for fish culture. Growth in terms of final weight, final length, weight gain, length gain and specific growth rate and survival of fingerlings were significantly higher in T₁ than those in T₂ and T₃. Feed conversion rate was significantly lower in T₁ followed by T₂ and T₃ in that order. Significantly higher number of fingerlings was produced in T₃ than that in T₂ and T₁. Even then, consistently higher net benefits were obtained from T₁ than those from T₃ and T₂. Among the treatments evaluated, 200 000 fry ha^?1 was the best stocking density considering the highest growth, production and net benefits of fingerlings of M. cavasius in nursery ponds.     

Resistance to vibriosis,production performance and tolerance to stress of the selected giant freshwater prawn (Macrobrachium rosenbergii)

The purpose of this study was to evaluate the resistance to vibriosis, growth, survival and tolerance to stress of the selected prawn, second generation, compared to a non‐selected control. The first generation of selected giant freshwater prawn, which has 10.4% higher of resistance, was used to attain disease resistant freshwater prawn (Macrobrachium rosenbergii) generation through challenge test‐based selection. Resistance test was conducted by infecting the prawn (mean body weight of 10.29 ± 1.40 g) with pathogenic Vibrio harveyi (5 × 10⁵ cfu prawn^?1). The growth and survival of the prawn were evaluated by rearing the two populations of prawn in both nursery and grow‐out phases. Stress tolerance test was done by evaluating the viability of postlarvae exposed to environmental stressors, i.e. temperature, salinity, NH₃ and formaldehide. Post‐challenge survival of the selected prawn (55.0 ± 5.0%) was about 46% higher than that of the control (37.5 ± 7.5%). The survival of the selected prawn in nursery culture (77.16 ± 0.841%) was significantly higher (p < .05) than that of the control (51.31 ± 2.938%), while the survival in grow‐out culture was similar (p > .05). The growth of selected prawn (4.99 ± 0.03% day^?1) was significantly higher than that of the control (4.81 ± 0.05% day^?1). There was no difference between treatments on the tolerance level against the tested environmental stressor. Overall data suggested that the selected prawn showed better performance in growth and resistance against vibriosis.     

Effects of Two Densities of Caged Monosex Nile Tilapia, Oreochromis niloticus, on Water Quality, Phytoplankton Populations, and Production When Polycultured with Macrobrachium rosenbergii in Temperate Ponds

The effects of different densities of caged Nile tilapia, Oreochromis niloticus, on water quality, phytoplankton populations, prawn, and total pond production were evaluated in freshwater prawn, Macrobrachium rosenbergii, production ponds. The experiment consisted of three treatments with three 0.04‐ha replicates each. All ponds were stocked with graded, nursed juvenile prawn (0.9 ± 0.6 g) at 69,000/ha. Control (CTL) ponds contained only prawns. Low‐density polyculture (LDP) ponds also contained two cages (1 m³; 100 fish/cage) of monosex male tilapia (115.6 ± 22 g), and high‐density polyculture (HDP) ponds had four cages. Total culture period was 106 d for tilapia and 114 d for prawn. Overall mean afternoon pH level was significantly lower (P ≤ 0.05) in polyculture ponds than in CTL ponds but did not differ (P > 0.05) between LDP and HDP. Phytoplankton biovolume was reduced in polyculture treatments. Tilapia in the LDP treatment had significantly higher (P ≤ 0.05) harvest weights than in the HDP treatment. Prawn weights were higher (P ≤ 0.05) in polyculture than prawn monoculture. These data indicate that a caged tilapia/freshwater prawn polyculture system may provide pH control while maximizing pond resources in temperate areas.     

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.     

Effect of time-restricted feeding and refeeding regimes on compensatory growth,body composition,and feed utilization in prawn (Macrobrachium rosenbergii) culture system

ABSTRACT

A 14-week feeding experiment was conducted to determine the effect of a time-restricted feeding and refeeding regime on compensatory growth, body composition, and feed utilization in improved traditional prawn (Macrobrachium rosenbergii) culture systems. Three groups of prawn were used in which the control group (C) received feed regularly at ad libitum and the other two groups, Treatment 1 (T₁) and Treatment 2 (T₂), consumed feed 4 and 5 days per week respectively throughout the first 9 weeks. After feeding restriction, satiation of feed was given in the last 5 weeks. At the end of the experiment, the final weight of prawn in C, T₁, and T₂ were found to be 17.23 ± 6.56g, 16.04 ± 6.18g, and 15.64 ± 6.34 g respectively, which was insignificant. FCR decreased with increased restriction of feeding, while PER and NPU increased. Moisture content increased significantly, while protein and lipid content decreased significantly with increasing restriction time of feeding. After the satiation period, only the T2 group was able to replenish the lipid content. Water quality parameters didn’t vary significantly among the experimental groups and remained within optimum level. Therefore, time restriction could be applied in Macrobrachium rosenbergii culture without detrimental effects on growth, feed efficiency, and nutrient utilization.     

Live transportation of Macrobrachium rosenbergii (De Man) in chilled sawdust

Three cooling rates of 1.26±0.09°C h^?1 within 8 h (slow, T₁), 2.52±0.18°C h^?1 within 4 h (moderate, T₂) and 5.04±0.36°C h^?1 within 2 h (fast, T₃) were tested to cold‐anaesthetize farm raised Macrobrachium rosenbergii (De Man) (45–52 g) in each case from 25°C down to 15±1°C in a refrigerated chilling tank, provided with aeration. The cold‐anaesthetized prawns subjected to each chilling rate were packed in an insulated cardboard box (triplicate) between two layers of moist and chilled (2–3°C) sawdust, and kept inside a chilled storage cabinet at 15±1°C, for set durations of 6, 9, 12, 15 and 18 h. Survival was determined by revitalizing the prawns in aerated water with an initial temperature of 20°C, which was raised to 29±1°C within 3 h. The experiment was repeated using berried females acclimated to brackishwater of 12 g L^?1 salinity and the percentage survival recorded after live storage for durations ranging from 6 to 24 h at intervals of 3 h. Statistically valid safe durations for obtaining 100% survival of the cold anaesthetized and live stored prawns were determined using probit analysis at the three chilling rates tested, and were found to be 7.39, 6.98 and 4.54 h in the case of adult prawns, and 7.87, 8.17 and 6.43 h for berried females for T₁, T₂ and T₃ respectively. For practical purposes, the durations that yielded 95% survival rates were computed to be 16.47, 12.14 and 8.35 h in the case of adult prawns and 18.49, 19.02 and 11.11 h for berried females for T₁, T₂, and T₃ respectively. The berried prawns revitalized after live storage were incubated in tanks and the zoea larvae reared up to postlarvae (PL‐5), and compared against a control. No significant difference was found in larval hatch fecundity, survival rate and the production of PL L^?1 between the treatment and control, indicating that the method of cold anaesthetization and live storage of berried prawns could be used for successful transportation of broodstock.     

Induction of meiotic gynogenesis in bagrid catfish (Pseudobagrus ussuriensis) with homologous sperm and its confirmation for female homogamety

The bagrid catfish, Pseudobagrus ussuriensis, exhibits significant sexual dimorphism in growth rate and body size with males growing faster than females. Therefore, an all‐male culture can dramatically increase the output and profitability of fishery products. According to the monosex breeding route, super‐male individuals’ acquirement by XY male sex reversal and artificial gynogenesis is the key step. An effective protocol to induce meiotic gynogenesis using homologous sperms has been developed in this study. The most effective UV irradiation for sperm genetic inactivation was found to be at a distance of 20 cm with 66 μW/cm² light intensity for 25 min. Optimal treatment for cold shock was 5 min post‐fertilization at 0‐4°C for 30 min, which gave the best survival rate of 13.65 ± 2.87%. The sex ratio in the meiotic gynogens showed a significant female‐biased deviation (p < .05); thirty meiogynogens and their parents were further analysed using a male‐specific AFLP marker, of which only the male parent produced a male‐specific DNA band of 412 bp. These results indicated the female homogametic XX/XY sex determination system in P. ussuriensis. The optimization of a protocol for the successful induction of meiogynogenesis in the bagrid catfish lays the basis for all‐male production and is useful in ascertaining the genetic sex determination system in this promising aquaculture species.     

Dynamics of glucose in the haemolymph of female giant freshwater prawn,Macrobrachium rosenbergii,influences reproductive and non‐reproductive moulting cycles

Glucose, when measured in haemolymph, has been found to reflect a useful predictor of energetic investment. This study evaluated the pattern of glucose in the haemolymph, with an attempt to gain a better insight into the role of glucose as nutritional source of ovarian development and energy reserves during reproductive and non‐reproductive moulting cycles. The haemolymph of female giant freshwater prawn, Macrobrachium rosenbergii, was obtained at eight different moulting stages, and levels of glucose were determined using an enzymatic colorimetric glucose‐oxidase method in parallel with a histological examination of ovarian development. Glucose levels were relatively low (0.15 ± 0.02 mg mL^?1) at D₀ stage, an abrupt increase (0.52 ± 0.13 mg mL^?1) during premoult D₁ stage and declined (0.32 ± 0.10 and 0.31 ± 0.09 mg mL^?1) during premoult D₂ and D₃ stages, respectively; thereafter, a slight increase (0.43 ± 0.09 mg mL^?1) occurred at post‐moult A stage. The progression of ovarian growth, marked by an increasing gonadosomatic index (GSI) pattern during the reproductive moulting cycle (C₀–D₃ stages), was directly proportionate to fluctuations in glucose levels. GSI was significantly positively correlated with glucose (R = 0.40; P < 0.05). In contrast, glucose was notably higher at post‐moult A and premoult D₂ stages during non‐reproductive moulting cycle, the period during which glucose is crucial for exoskeletal chitin synthesis. At this particular stage, a negative correlation between body weight and glucose (R = ?0.36; P < 0.05) was observed. The dynamics of glucose in the haemolymph of female M. rosenbergii correlated with ovarian growth, which signify that glucose as nutritional source for vitellogenesis, and affects the body weight of this species.     

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 water exchange protocols on water quality,sedimentation rate and production performance of Penaeus monodon in earthen ponds

This study was carried out in farmers' fields to quantify the total water and consumptive water use in grow‐out culture of Penaeus monodon under recommended package of practice with two different water management protocols: T₁, with no water exchange and T₂, with regulated water exchange. Treatment‐wise estimated total water use, was 2.09 and 2.43 ha‐m 122 day^?1, while the computed consumptive water use index (m³ kg^?1 biomass) was 5.35 and 6.02 in T₁ and T₂ respectively. Lower rates of water exchange (T₂) showed significantly improved (P < 0.05) crop performance in terms of performance index (19.75 ± 0.75), production‐size index (74.1 ± 3.4), survival rate (80.13 ± 1.7%) and productivity (2.44 ± 0.08 t) over the zero water exchange. The shrimp pond water quality suitability index (WQSI) infers that regulated water exchange (T₂) improved the overall suitability of water quality for shrimp culture. WQSI up to 90 days of culture ranged between 7.5–9.0 in T₂, needs little management while in the last month of rearing, it was good with moderate management requirements. Treatment‐wise sediment load ranged between 50.4–56.3 m³ t^?1 shrimp biomass. High intensity of water exchange and low apparent feed conversion ratio influenced in lowering the sedimentation rate. Regulated water exchange protocol (T₂) performed well (higher net total water productivity and net consumptive water productivity) against no water exchange (T₁). A higher OV:CC ratio (ratio of the output value to the cost of cultivation) indicated that T₂ had a distinct edge over the T₁ protocol.     

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.     

Low-cost diet for monoculture of giant freshwater prawn (Macrobrachium rosenbergii de Man) in Bangladesh

An experiment was conducted for 3 months in 12 experimental ponds, each of 30 m², with a view to develop a low‐cost diet for monoculture of Macrobrachium rosenbergii in ponds. Three experimental diets (30% protein) were formulated using fish meal, meat and bone meal, mustard oilcake, sesame meal and rice bran in different combinations partially replacing fish meal by meat and bone meal and sesame meal and assigned to treatments T₁, T₂ and T₃ respectively. A commercial golda feed (Starter‐III) was assigned to T₄ (reference diet). Each treatment had three replicates. Juveniles of M. rosenbergii (2.90±0.21 g) were stocked at the rate of 40 000 ha⁻¹. Prawns were fed three times daily at the rate of 10% and 5% of their body weight at the beginning and for the last 2 months respectively. The ponds were provided with aeration during the night using air pumps. The ranges of water quality parameters recorded in different ponds were: temperature 28.9–32.5°C, dissolved oxygen 5.1–8.1 mg L⁻¹ and pH 6.4–7.7. The results showed that the weight gain of prawns fed diet 1 was significantly higher (P<0.05) than those fed diets 2 and 3, but was not significantly different from those fed diet 4 (reference diet). The feed conversion ratio (FCR) values of diets ranged between 2.21 and 2.96 with diets 1 and 4 showing significantly lower (P<0.05) FCR values. The survivals (%) ranged between 68% and 78% with prawns fed diets 1 and 4 showing significantly higher survival. The production of prawn ranged between 921 and 1428 kg ha⁻¹ and diet 1 resulted in a significantly high (P<0.05) production. A simple economic analysis showed that diet 1 generated the maximum net profit of Tk 159 178 ha⁻¹. The results of the study showed that a diet containing 20% fish meal, 10% meat and bone meal, 15% mustard oilcake, 15% sesame meal, 35% rice bran, 4% molasses and 1% vitamin–mineral premixes may be recommended to the farmers for monoculture of M. rosenbergii in ponds.     

Effect of ammonia-N on growth and feeding of juvenile Macrobrachium rosenbergii (De-Man)

Survival rate, growth and feed intake were determined for late juveniles (4.31 ± 0.18 g) of river prawn Macrobrachium rosenbergii in freshwater with total ammonia‐N (NH₃‐N+NH₄‐N) concentrations of 0.015 (control), 0.5, 1.0 and 1.5 mg L⁻¹ for 60 days at pH 7.53 ± 0.04 and temperature 24.0 ± 2.5°C. Survival rate was significantly (P<0.05) lower (54 ± 4.2–70 ± 5.4%) for total ammonia concentrations from 0.5 to 1.5 mg L⁻¹ [0.0139–0.0419 mg L⁻¹ of unionized ammonia (NH₃)]. Growth (0.026–0.030 g day⁻¹ range) of the prawns did not differ for the different NH₃ levels but were significantly (P<0.05) lower compared with control (0.056 g day⁻¹). Feed intake rates also diminished significantly (P<0.05) from 77.60 ± 2.45% at control (0.015 mg L⁻¹ NH₃‐N) to 48.69 ± 2.13% at 1.5 mg L⁻¹ NH₃‐N (0.0419 mg L⁻¹ of unionized NH₃).     

Heritability for growth traits in giant freshwater prawn,Macrobrachium rosenbergii (de Mann 1879) based on best linear unbiased prediction methodology

In this study, heritability was estimated for growth‐related traits of giant freshwater prawn (Macrobrachium rosenbergii) before and after morphological sexual differentiation. Estimation was made on data from 16 full‐sib and eight half‐sib families. The variance estimation was performed using a univariate mixed linear animal model and variance components were analysed following an animal model using restricted maximum likelihood procedure using average information algorithm. Heritability estimates (h²) varied considerably with ages. At 2 months old, h²for carapace length (CL; 0.35±0.15) and body weight (BW; 0.26±0.13) were higher than those estimated at 5 months old, based on mixed sex data. However, when data were sorted by sex, h² calculated from data of females were higher than those of males for CL (0.26±0.16 vs. 0.10±0.06), BW (0.28±0.17 vs. 0.12±0.08), body length (0.40±0.17 vs. 0.11±0.07), total length (0.47±0.18 vs. 0.11±0.07) and claw length (0.29±0.16 vs. 0.03±0.04). The same trend was observed for traits at 6 months old in both bulk and individual rearing.     

Effects of salinity on standard metabolic rate and critical oxygen tension in the giant freshwater prawn (Macrobrachium rosenbergii)

The extensively farmed giant freshwater shrimp, Macrobrachium rosenbergii, can survive salinities up to 26 g L^?1, but the commercially important grow‐out occurs exclusively in freshwater areas. Recent studies suggest the shrimp equally capable of growing in brackish as fresh water and a better understanding of how this species responds to changing salinity could significantly impact freshwater prawn farming in deltas and coastal areas. Here, the effect of salinity (0 and 15 g L^?1) on standard metabolic rate (SMR) and critical oxygen tension (P_crit) was measured in adult M. rosenbergii using intermittent closed respirometry. SMR was 79.8 ± 3.1 and 72.7 ± 2.9 μmol kg^?1 min^?1 in fresh and brackish water, respectively, with no significant difference between the two salinities (P = 0.122). During hypoxia M. rosenbergii maintained oxygen uptake down to a P_crit of 26.3 ± 1.4 mmHg in fresh and 27.2 ± 2.0 mmHg in brackish water (P = 0.682), showing that salinity had no overall effect on oxygen conductance in the animals. These findings are in agreement with recent growth studies and provide further evidence that grow‐out phase could be accomplished in brackish water areas. Thus, the predicted intrusions of brackish water in tropical deltas as a consequence of future global warming may not impact this important production.     

Growth and production performance of threatened snakehead fish,Channa striatus (Bloch), at different stocking densities in earthen ponds

Production trials of threatened snakehead fish (Channa striatus) were carried out under different stocking densities in earthen ponds of Bangladesh. The average weight and length of the fingerlings during stocking was 17.63 ± 1.23 g and 13.21 ± 0.52 cm. Fingerlings were stocked at 5000 ha^?1 in treatment‐1 (T₁), 6250 ha^?1 in treatment‐2 (T₂) and 7500 ha^?1 in treatment‐3 (T₃) respectively. Fish in all the experimental ponds were fed with supplementary feed comprising of fish meal (30%) and mustard oilcake (70%) at the rate of 3–6% of estimated body weight two times per day. In addition, trash fish were supplied at the rate of 2–3% of the estimated biomass on each alternate day. In situ water quality parameters of the pond were within the suitable range for fish culture. The growth and survival of fingerlings were significantly higher in T₁ than in T₂ and T₃. The food conversion ratio was significantly lower (P<0.05) in T₁ than in T₂ and T₃. The estimated gross and net production of fish was higher in T₁, followed by T₂ and T₃. Overall, the highest growth, survival and production were obtained from T₁. Therefore, it could be concluded that of 5000 fingerlings ha^?1 is the most suitable stocking density for culturing C. striatus under a monoculture system in the earthen ponds for better production.     

Efficacy of 17 α‐methyl testosterone and letrozole on sex reversal of protogynous grouper,Epinephelus tauvina (Forskal, 1775) and spawning performance of sex‐reversed males

This study aimed to test the efficacy of 17 α‐methyl testosterone (17 α‐MT) alone and in combination with letrozole, an aromatase inhibitor, for the induction of sex reversal in protogynous greasy grouper, Epinephelus tauvina. Further, the long‐lasting effects of these treatments and spawning performance of sex‐reversed males were also investigated. Greasy grouper with oocytes in the perinucleolus stage were implanted with 5 mg 17 α‐MT kg^?1 body weight (T₁), 5 mg 17 α‐MT and 0.2 mg letrozole kg^?1 body weight (T₂) and 5 mg 17 α‐MT with 0.4 mg letrozole kg^?1 body weight (T₃) and no androgens/enzyme inhibitor implanted (C). The 17 α‐MT alone and in combination of letrozole‐induced sex reversal in greasy grouper, whereas untreated control fish (C) showed normal ovarian development. However, T₂ and T₃ group showed 100% sex reversal and completion of spermatogenesis up to functional male phase in 2 and 3 months, respectively, whereas T₁ group resulted in only 66.67% functional male with motile spermatozoa after 4 months. Sex‐reversed males successfully fertilized the eggs during induced spawning. There were significant differences on fertilization and hatching rates between T₂ group (79.00 ± 4.36%; 77.67 ± 2.87%, respectively) and T₁ group (57.67 ± 3.17%; 63.87 ± 2.91%, respectively). The result suggested that 17 α‐MT (5.0 mg kg^?1 BW) in combination with letrozole (0.2 mg kg^?1 BW) has the potential to produce 100% sex‐reversed male in short period in greasy grouper, which might greatly help in seed production of greasy grouper.     

Prawn (Macrobrachium rosenbergii)–plant (Hydrilla verticillata) co‐culture system improves water quality,prawn production and economic benefit through stocking density and feeding regime manage

The effects and commercial implications of aquatic plant addition, and variable prawn stocking density and supply of commercial aquafeed on water quality, prawn production and economic benefit, are investigated in a prawn (Macrobrachium rosenbergii) + plant (Hydrilla verticillata) co‐culture system. Our experimental design includes a control (PM, prawn monoculture without H. verticillata, with 30 prawns/m²) and four treatments with 15% plant cover of total pond area in each replicate. Dissolved oxygen, pH, N, P, total ammoniacal nitrogen, nitrite and nitrate in this co‐culture system were significantly lower than those of PM. Prawn survival (96.2 ± 14.9%), average final weight (68.5 ± 4.9 g) and yield (879.1 ± 102.4 kg/ha) in treatment 20W80%A (20 prawns/m², 80% of control aquafeed) were obviously greater than in PM and other treatments. Despite small males (SM) and immature females (IF) being predominant in prawn + plant cultures, more than 77.2% of prawns reached or exceeded 40 g on completion of trials, and by six months were of appropriate size for market. We report culture of M. rosenbergii with H. verticillata to be both feasible and profitable. The optimal treatment, a stocking density of 20 prawns/m² with these plants, enabled reduction of commercial aquafeed to 20% conventional culture levels. For this optimal treatment, we estimate total gross revenue, profit and internal rate of return to be US$ 6,593.3 ± 103.3 ha^?1, US$ 3,095.5 ± 42.6 ha^?1 and 127.5 ± 20.7% for 20W80%A respectively; we estimate U.S. Dollar (US$) invested generates 3.87 times conventional culture revenue. Co‐culture of M. rosenbergii and plants renders prawn production ecologically and economically feasible on larger farms.