The potential of mixed culture of genetically improved farmed tilapia (Oreochromis niloticus) and freshwater giant prawn (Macrobrachium rosenbergii) in periphyton-based systems

The production performance of genetically improved farmed tilapia (GIFT, Oreochromis niloticus) and freshwater prawn (Macrobrachium rosenbergii) in periphyton‐based systems were studied in farmers' ponds at Mymensingh, Bangladesh. Fifteen ponds (200–300 m² area and 1.0–1.5 m in depth) were used to compare five stocking ratios in triplicate: 100% GIFT, 75% GIFT plus 25% prawn, 50% GIFT plus 50% prawn, 25% GIFT plus 75% prawn and 100% prawn. Ponds were stocked at a total density of 20 000 GIFT and/or prawn ha^?1. Bamboo poles (mean diameter 6.2 cm and 5.5 pole m^?2) were posted in pond bottoms vertically as periphyton substrate. Periphyton biomass in terms of dry matter (DM), ash‐free DM and chlorophyll a were significantly higher in ponds stocked with prawn alone than in ponds with different combinations of GIFT and prawn. Survival of GIFT was significantly lower in ponds stocked with 100% GIFT (monoculture) whereas, that of prawn was significantly higher in its monoculture ponds indicating detrimental effects of GIFT on prawn's survival. Individual weight gains for both species were significantly higher in polyculture than in monoculture. The highest total fish and prawn yield (1623 kg GIFT and 30 kg prawn ha^?1) over 125–140 days culture period was recorded in ponds with 75% GIFT and 25% prawn followed by 100% GIFT alone (1549 kg ha^?1), 50% GIFT plus 50% prawn (1114 kg GIFT and 68 kg prawn ha^?1), 25% GIFT plus 75% prawn (574 kg GIFT and 129 kg prawn ha^?1) and 100% prawn alone (157 kg ha^?1). This combination also gave the highest economic return. Therefore, a stocking ratio of 75% GIFT plus 25% prawn at a total density of 20 000 ha^?1 appeared to be the best stocking ratio in terms of fish production as well as economics for a periphyton‐based polyculture system.     

Effects of stocking density of freshwater prawn Macrobrachium rosenbergii and addition of different levels of tilapia Oreochromis niloticus on production in C/N controlled periphyton based system

An on-station trial was conducted to evaluate the effect of stocking density of freshwater prawn and addition of different levels of tilapia on production in carbon/nitrogen (C/N) controlled periphyton based system. The experiment had a 2 × 3 factorial design, in which two levels of prawn stocking density (2 and 3 juveniles m^? 2) were investigated in 40 m² earthen ponds with three levels of tilapia density (0, 0.5 and 1 juveniles m^? 2). A locally formulated and prepared feed containing 30% crude protein with C/N ratio close to 10 was applied considering the body weight of prawn only. Additionally, tapioca starch was applied to the water column in all ponds to increase C/N ratio from 10 (as in feed) to 20. Increasing stocking density of tilapia decreased the chlorophyll a concentration in water and total nitrogen in sediment, and increased the bottom dissolved oxygen. The concentrations of inorganic nitrogenous species (NH₃–N, NO₂–N and NO₃–N) were low due to maintaining a high C/N ratio (20) in all treatment ponds. Increasing prawn density decreased periphyton biomass (dry matter, ash free dry matter, chlorophyll a) by 3–6% whereas tilapia produced a much stronger effect. Increasing stocking density of freshwater prawn increased the total heterotrophic bacterial (THB) load of water and sediment whereas tilapia addition decreased the THB load of periphyton. Both increasing densities of prawn and tilapia increased the value of FCR. Increasing prawn density increased gross and net prawn production (independent of tilapia density). Adding 0.5 tilapia m^? 2 on average reduced prawn production by 12–13%, and tilapia addition at 1 individual m^? 2 produced a further 5% reduction (independent of prawn density). The net yield of tilapia was similar between 0.5 and 1 tilapia m^? 2 treatments and increased by 8.5% with increasing stocking density of prawn. The combined net yield increased significantly with increasing stocking density of prawn and tilapia addition. The significantly highest benefit cost ratio (BCR) was observed in 0.5 tilapia m^? 2 treatment but freshwater prawn density had no effect on it. Therefore, both stocking densities (2 and 3 juveniles m^? 2) of prawn with the addition of 0.5 tilapia m^? 2 resulted in higher fish production, good environmental condition and economic return and hence, polyculture of prawn and tilapia in C/N controlled periphyton based system is a promising options for ecological and sustainable aquaculture.     

Stocking density for the intensive rearing of juvenile crayfish, <Emphasis Type="Italic">Pacifastacus leniusculus</Emphasis> (Astacidae), using <Emphasis Type="Italic">Artemia</Emphasis> nauplii to supplement a dry diet from the onset of exogenous feeding

Recent advances in intensive rearing of astacid juvenile crayfish have greatly improved the results. This challenges the current application possibilities of the studies performed previously, and new research on density is required. A 100-day experiment was carried out under controlled conditions to evaluate density effects on survival and growth rates of juvenile crayfish in optimal conditions of feeding. Juvenile stage 2 Pacifastacus leniusculus were stocked in fibreglass tanks (1 m², 200 l water) at 20 ± 1°C and fed a dry diet for salmonids supplemented with restricted amounts of Artemia nauplii. Stocking densities were 100, 300, 600 and 1,000 crayfish m⁻². Mean survival rate was reduced significantly with increased stocking density, ranging from 86.33% (100 m⁻²) to 39.13% (1,000 m⁻²). All checks showed that at the lowest initial density (100 m⁻²) animals grew significantly faster those at higher densities, recording a final carapace length of 15.28 mm and weight of 1.08 g. Among the treatments of 300, 600 and 1,000 m⁻² no differences were found either in carapace length or in weight throughout the experimental period, with a final mean growth of 14 mm carapace length and 0.72 g weight. The final proportion of animals with chelae autotomy rose significantly with increasing stocking density, ranging from 14.44% (100 m⁻²) to 41.45% (1,000 m⁻²). This study shows that diet is a decisive factor for stocking successfully high densities under controlled conditions and provides useful information to set adequate densities in accordance with the production objectives.     

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.     

Evaluation of freshwater prawn‐small fish culture concurrently with rice in Bangladesh

An integrated aquaculture of freshwater prawn (Macrobrachium rosenbergii) and self‐recruiting small fish mola (Amblypharyngodon mola) was conducted in farmers' rice fields at Mymensingh, Bangladesh. There were four treatments with three replications. Four stocking densities, 10 000, 15 000, 20 000 and 25 000 ha^?1, of freshwater prawn were applied. The stocking density of mola was the same (20 000 ha^?1) in all treatments. During land preparation, triple super phosphate (TSP) and murate of potash (MP) were applied at the rate of 150 and 75 kg ha^?1 respectively. Urea was applied at the rate of 200 kg ha^?1 in equally distributed three installments after 16, 45 and 65 days of rice plantation. Prawns were fed with commercial pelleted feed at 3–8% body weight. All water quality parameters were found to be within the suitable range for freshwater prawn culture. There was a homogenous abundance of plankton communities in all treatments. The results of a 4‐month culture period showed that the average production of prawn ranged from 222 to 388 kg ha^?1, mola 51 to 68 kg ha^?1 and rice 2 880 to 3 710 kg ha^?1. Significantly higher production of both prawn and mola was recorded in the plots where the freshwater prawn stocking density was 15 000 ha^?1. This treatment resulted in a net profit of USD 1100 ha^?1.     

Effects of stocking density on production and economics of Nile tilapia (Oreochromis niloticus) and freshwater prawn (Macrobrachium rosenbergii) polyculture in periphyton-based systems

The present research investigated the effect of stocking density on pond (75 m², depth 1.2 m) production of Nile tilapia (Oreochromis niloticus) and freshwater prawn (Macrobrachium rosenbergii) stocked at a fixed 3:1 tilapia:prawn ratio. Three stocking densities were tried in triplicate: 20 000 ha⁻¹ (treatment TP‐20), 30 000 ha⁻¹ (TP‐30) and 40 000 ha⁻¹ (TP‐40). The ponds were provided with bamboo as substrate for periphyton development. Bamboo poles (mean diameter 5.5 cm and 5.0 poles m⁻²) were posted vertically into pond bottoms, resulting in 60% additional substrate area in each pond. On average, 43 genera of algae and 17 genera of zooplankton were identified from pond water, whereas 42 genera of algae and six genera of microfauna were attached to bamboo substrates. No differences were observed between treatments in the ash‐free dry matter (AFDM), chlorophyll a and phaeophytin a content of periphyton (P>0.05). Survival of tilapia and prawn and individual weight gain of tilapia were lower (P<0.05) in treatment TP‐40. The net yields were higher (P<0.05) in treatments TP‐30 (2209 and 163 kg ha⁻¹ 105 day⁻¹ of tilapia and prawn respectively) and TP‐40 (2162 and 141 kg ha⁻¹ of tilapia and prawn respectively) than in treatment TP‐20 (1505 and 136 kg ha⁻¹ of tilapia and prawn respectively). The net tilapia yields were quadratic correlated (R²=0.92) with fish stocking density. The cost–benefit analysis shows that the net profit margin was highest in treatment TP‐30 (69%), followed by TP‐20 (50%) and TP‐40 (44%).     

The effect of stocking density on yield,growth, and survival of Asian river catfish (<Emphasis Type="Italic">Pangasius</Emphasis><Emphasis Type="Italic">bocourti</Emphasis> Sauvage, 1880) cultured in cages

Asian river catfish (Pangasius bocourti Sauvage, 1880) were cultured at five different stocking densities in cages (submerged volume 1 m³) suspended in a dugout pond from August to November 2009. Pangasius bocourti fingerlings (mean weight 27.09 ± 0.54 g) were stocked at densities of 12, 25, 50, 100, and 200 fish m⁻³. At the end of 3 months, the harvest weights (gross yields) were, respectively, 2.05 ± 0.30, 5.20 ± 0.31, 10.60 ± 0.42, 19.98 ± 0.78, and 42.37 ± 0.41 kg m⁻³. The mean fish weights among the stocking densities of 25, 50, 100, and 200 fish m⁻³ were not significantly different, but were significantly higher than that of the 12 fish m⁻³ density. The specific growth rates among high stocking densities of 50, 100, and 200 fish m⁻³ were not significantly different; however, they were significantly higher than those of the low stocking densities of 12 and 25 fish m⁻³. Asian river catfish performed poorly at the lowest density. The results indicate an initial lower stocking threshold for Asian river catfish of above 5.20 kg m⁻³. The Asian river catfish cultured in small cages placed in a pond reached the desirable market size (>200 g) within a 90-day grow-out period. The results show that the maximum yield for Asian river catfish during a 3-month production cycle was not reached.     

Development of larval culture techniques for the shore crab, <Emphasis Type="Italic">Carcinus maenas</Emphasis> (L)

The shore crab Carcinus maenas is a commercially important species, utilised as sea angling bait as well as supporting a European-wide fishery. Hatchery production could provide an alternative source of bait crabs, alleviating potential competition between these sectors and environmental concerns regarding bait collection practices. A series of experiments were carried to investigate the potential for hatchery production, focusing on effects of dietary regimes and stocking densities through the zoeal stages and the influence of tank substrates and stocking density during the megalopa stage. Inclusion of the rotifer Brachionus plicatilis as live food for early larval stages conveyed no advantage in terms of survival or rate of development compared to a diet of Artemia nauplii. Increasing zoea stocking densities (from 94 to 557 l⁻¹) had a significantly negative effect upon survival to the megalopa stage (from 75% down to 47%), although this was off-set by a significant increase in production, with 260 megalopae⁻¹ produced from an initial density of 557 zoeae l⁻¹. The inclusion of substrates for megalopa stages had no impact on production or development rate, compared to tanks with no substrate. The completely benthic behaviour of megalopae indicates that tank floor area will be a limiting factor for crab production. Increasing stocking density of megalopae was found to significantly and negatively influence survival, although above 10,000 megalopae m⁻² the rate of decline in survival stabilised and maximum production (3,114 juveniles m⁻²) of juvenile crabs could be achieved at the highest stocking densities tested (40,000 m⁻²).     

Changes in commercially exploited populations of tench, Tinca tinca (L.), in littoral zones of lakes of northeastern Poland

Commercially exploited tench, Tinca tinca, (L.) populations in 480 lakes in the northeastern Poland were analyzed on the basis of the fishery data from 1949 to 1994. The long-term changes in tench catch were studied in four categories of lakes with respective proportions of littoral zones (<30, 31–60, 61–90 and <90% of total lake area). The analyses were performed with using relative catch (kg ha⁻¹ month⁻¹) of tench. Mean relative catch of tench for 1949–1994 ranged from 0.36 kg ha⁻¹ month⁻¹ in lakes with smallest littoral zone (>30%) to 0.93 kg ha⁻¹ month⁻¹ in lakes with the largest littoral (<90%). Mean relative catch increased with increasing proportions of littoral zones in lakes. However, long-term trends in mean relative catch of tench showed decreasing tendencies in all lakes. The beginning of these tendencies was found about ten years earlier in lakes with smaller littoral (<60%) in comparison to lakes with larger littoral zones (>60%). Mean stocking rates of tench (number of 1-year-old fish per ha of littoral) for 1949–1994 were not significantly different in lakes of studied categories. However, lakes with proportionally less littoral zone had reduced stocking efficiency estimated as number of stocked fish per ha of littoral to require 1 kg ha⁻¹ month⁻¹ of tench.     

Potential of mixed culture of freshwater prawn (Macrobrachium rosenbergii) and self-recruiting small species mola (Amblypharyngodon mola) in rotational rice–fish/prawn culture systems in Bangladesh

The effects of different stocking densities of freshwater prawn (Macrobrachium rosenbergii) on its growth and production in relation to the presence of small self‐recruiting species mola (Amblypharyngodon mola) were investigated in modified rice fields after rice harvest at Mymensingh, Bangladesh. Keeping the stocking density of mola fixed at 20 000 ha^?1 in each treatment, four densities of freshwater prawn (treatments) were maintained: 10 000, 15 000, 20 000 and 25 000 ha^?1 respectively. The rice plots were limed (CaCO₃) and fertilized with urea, triple super phosphate and cowdung regularly. The prawns were fed daily with commercial pellets. Water quality parameters such as temperature, dissolved oxygen, transparency, pH, total alkalinity, inorganic nitrogen (nitrate, nitrite and total ammonia), chlorophyll a and orthophosphate were determined fortnightly. Numerical analysis of plankton communities was performed monthly. All water quality parameters were found to be within the suitable range for freshwater prawn culture, except high temperature, in the peak summer months. There was a homogenous abundance of plankton communities in all treatments throughout the experimental period. Mola started to breed in the second month of the original stocking with partial harvesting after the second month and continued until the final harvest. The total production of mola ranged between 124 and 152 kg ha^?1 during the 4‐month culture period. The average prawn survival ranged from 49% to 57% without any significant difference among treatments. Freshwater prawn production ranged from 294 to 596 kg ha^?1 with significantly higher production in the treatment where 20 000 ha^?1 freshwater prawn were stocked. This treatment also resulted in a higher net profit margin (74%), indicating that stocking at a combination of 20 000 ha^?1 freshwater prawn and 20 000 ha^?1 mola could be the optimum proposition for prawn–mola culture in modified rain‐fed rice fields after rice cultivation.     

Effects of different fertilization and feeding regimes on the production of integrated farming of rice and prawn Macrobrachium rosenbergii (De Man)

An experiment was conducted in rice field plots each of 30 m² to determine the appropriate combination of feeding and fertilization regimes for the giant freshwater prawn Macrobrachium rosenbergii (De Man, 1879) cultured along with rice in rice fields. There were four treatments: rice culture only with regular fertilization (A, control); rice–prawn integrated culture with regular fertilization (B); rice–prawn integrated culture with basal fertilization and commercial feed (C) and rice–prawn integrated culture with regular fertilization and commercial feed (D). Juvenile prawns of 1.5‐g size were stocked on the third day after rice transplantation at a density of 2 prawns m^?2 and fed twice daily at 5% reducing to 2% of body weight during the experiment. The rice yield in treatment D (0.42 kg m^?2) was significantly higher than that in other treatments (0.34, 0.36, 0.34 kg m^?2 in treatments A, B, C respectively). Prawns grew significantly faster (P<0.05) in treatments C and D (23.8±0.9 and 22.0±1.7 g prawn^?1 respectively) than in treatment B (14.7±1.6 g prawn^?1). Prawn production in treatment C (347±13 kg ha^?1 crop^?1) was significantly (P<0.05) higher than that (234±30 kg ha^?1 crop^?1) in treatment B, while in treatment D (296±53 kg ha^?1 crop^?1) it was not significantly different from that in treatments B and C (P>0.05). Treatment C gave the highest economic returns among all treatments, followed by treatments D and B, indicating that the combination of basal fertilization and commercial feed is the most appropriate nutrient input regime for the rice–prawn integrated culture system.     

Effect of stocking density on survival and growth performance of pikeperch, <Emphasis Type="Italic">Sander lucioperca</Emphasis> (L.), larvae under controlled conditions

The effect of stocking density on the survival and growth of pikeperch, Sander lucioperca (L.), larvae was examined in two consecutive experiments. In experiment I, 4-day-old larvae [body wet weight (BW): 0.5 mg; total body length (TL): 5.6 mm] were reared in 200-l cylindro-conical tanks in a closed, recirculating system (20 ± 0.5°C) at three stocking densities (25, 50 and 100 larvae l⁻¹) and fed a mixed feed (Artemia nauplii and Lansy A2 artificial feed) for 14 consecutive days. At densities of 25 and 100 larvae l⁻¹, growth rate and survival ranged from 2.7 to 1.9 mg day⁻¹ and from 79.2 to 72.3%, and fish biomass gain ranged from 0.6 to 2.0 g l⁻¹, respectively. There were two periods of increased larval mortality: the first was at beginning of exogenous feeding and the second during swim bladder inflation. In experiment II, 18-day-old larvae (BW: 35 mg; TL: 15.6 mm) obtained from experiment I were reared under culture conditions similar to those of experiment I, but at lower stocking densities (6, 10 and 15 larvae l⁻¹). The fish were fed exclusively with artificial feed (trout starter) for 21 consecutive days. At densities of 6 and 15 larvae l⁻¹, the growth rate and fish biomass gain ranged from 28.8 to 23.1 mg day⁻¹ and from 2.0 to 3.3 g l⁻¹, respectively. The highest survival (56.5%) was achieved at a density of 6 larvae l⁻¹. Mortality at all densities was mainly caused by cannibalism II type behaviour (27–35% of total). In both experiments, growth and survival were negatively correlated and fish biomass gain positively correlated with stocking densities. The present study suggests that the initial stocking density of pikeperch larvae reared in a recirculating system can be 100 individuals l⁻¹ for the 4- to 18-day period post-hatch and 15 individuals l⁻¹ for the post-19-day period.     

Determining the effects of stocking density and temperature on growth and food consumption in the pharaoh cuttlefish, <Emphasis Type="Italic">Sepia pharaonis</Emphasis>, Ehrenberg 1890

The purpose of this experiment was to observe the impact of stocking density on growth and food consumption of juvenile Sepia pharaonis reared at 23 and 28°C. Two groups of 32 cuttlefish each were reared in closed recirculating seawater systems with water temperatures of 23°C (group A) and 28°C (group B). Each group was divided into three treatments with two replicates per treatment: low-density (equivalent to 20 cuttlefish m⁻²), medium-density (equivalent to 100 cuttlefish m⁻²), and high-density (equivalent to 200 cuttlefish m⁻²). Measured amounts of live food were added three times a day and the wet body weight of each cuttlefish was measured once a week during the 42-day study. Cuttlefish in group B had higher growth rates and food consumption than cuttlefish in group A. The different stocking densities in group B affected the size of the cuttlefish whereas the stocking densities of the cuttlefish in group A treatments did not lead to different sizes between densities. Overall, the gross growth efficiency of the high-density treatments was lower than that of the low-density treatments, as was the weight of the cuttlefish in the high-density treatment. Although the wet weights of group A treatments were not significantly different (P > 0.05), the wet weights of the cuttlefish in the high-density, group B, treatment were lower than those in the low and medium density treatments. This decrease in individual size suggests that stocking densities of 100 to 200 cuttlefish m⁻² may interfere with growth.     

Effects of stocking density on survival and growth of juvenile tench (<Emphasis Type="Italic">Tinca tinca</Emphasis> L.)

In two 120-day experiments, performed in the laboratory at 22°C, the effects of stocking density on the survival and growth of juvenile tench (Tinca tinca L.) were evaluated. Fish were kept in fibreglass tanks, supplied throughout with flow of artesian water, and fed a dry diet for salmonids, in excess, supplemented with restricted amounts of Artemia nauplii. In the first experiment four-month-old juveniles (0.31 ± 0.04 g and 32.00 ± 1.17 mm TL) were stocked at four densities—0.18, 0.88, 1.05, and 2.10 g l⁻¹. Survival was high (>89%) for all treatments. Final densities ranged between 1.10 g l⁻¹ (significantly lowest) and 10.46 g l⁻¹ (significantly highest). The density increase was significantly higher (611%) for fish stocked at the lowest initial density (0.18 g l⁻¹) than for fish stocked at 0.88, 1.05, and 2.10 g l⁻¹, for which the density increase averaged 457%. In the second experiment, 4.5-month-old juveniles (0.58 ± 0.17 g and 39.54 ± 0.83 mm TL) were stocked at three densities—1.05, 3.00, and 4.00 g l⁻¹. Survival was high (>96%) for all treatments. Final densities ranged between 4.08 and 16.53 g l⁻¹ and were significantly higher for greater initial densities. The density increase was greatest (413%) for fish stocked at the highest density (4 g l⁻¹) and was not significantly different for fish stocked at 1.05 and 3 g l⁻¹. Considering all the densities in the two experiments, for stocking at 4 g l⁻¹ the final density was 15 times higher than that reached after stocking at 0.18 g l⁻¹, without harmful effects on survival and growth. This final density (equivalent to 16.53 kg m⁻³) is in the range recommended for other fish species in this period under intensive conditions.     

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.     

Effect of salinity on carrying capacity of adult Nile tilapia Oreochromis niloticus L. in recirculating systems

Effect of salinity on carrying capacity of a recirculation system for Nile tilapia, Oreochromis niloticus L.; production was assessed. Survival, growth and feed conversion ratio of adult Nile tilapia fed 30% crude protein diet for 88 days were measured at three different salinity levels (8, 15 and 25 g L^?1) and two stocking densities (20 and 40 m^?3) in three independent recirculating systems. Highest survival (98%) and a linear growth in net biomass (P<0.01) was observed in both densities at 8 g L^?1 and in 20 m^?3 treatment at 15 g L^?1. Highest net biomass growth was observed in the 40 m^?3 stocking density treatment at 8 g L^?1 salinity level (P<0.05). Overall biomass growth was significantly affected by salinity indicating a decrease in Nile tilapia carrying capacity with increased salinity. About 11 000 kg ha^?1 crop^?1 of Nile tilapia can be obtained in recirculating systems at 8 g L^?1 salinity, significantly higher than the net production at 15 g L^?1 (5200 kg ha^?1 crop^?1) and 22 g L^?1 (4425 kg ha^?1 crop^?1).     

The On-Farm Trials of the Rotational Prawn-Rice Farming in a Semi-Deep Water Area

ABSTRACT

The on-farm trials of rotational rice-prawn farming in a semi-deep water area in the Mekong Delta of Vietnam was carried out in six rice fields (0.7–1.0 ha) to evaluate the production and economic efficiency of using low and high cost feed on prawn culture in paddies at different densities. Two stocking densities of 4 and 5 PL/m² were investigated. Two treatments of feed types (pellets only or a combination of pellets, trash fish, and snail meat) were applied at stocking 4 PL/m². At the treatment of 5 PL/m², prawns were fed a combination of pellets, trash fish, and snail meat. Prawns were stocked in the mid-April and harvested in mid‐November before the next dry season rice crop. Rice farming was started 3 to 5 days after prawn harvesting. By cull harvesting during the culture period, final mean weights of prawn ranged from 47.2 to 57.2 g/prawn and the male:female ratio at harvesting was 2.5:1.0. The prawn yield of treatment 5 PL/m² was highest (630 ± 22 kg/ha). Net profits in treatments using the combination of pellets, trash fish, and snail meat were 861 ± 193 US$/ha to 1,019 ± 25 US$/ha for the prawn crop and 1,393 ± 71 US$/ha to 1,576 ± 180 US$/ha for the whole system (prawn crop + dry rice crop) and significantly higher than in treatment using pellet only (P < 0.05). Prawns fed on pellets or a combination of pellets and snail meat both offer similar results in terms of production and economics. The dry rice crop offered high cost benefit ratio (2.29–2.33) with low operating cost (414–434 USD/ha) and it made a better use in sustainable rice fields through the rotational rice-prawn system.     

Density effects of silver carp Hypophthalmichthys molitrix and catla Catla catla on the production system in all‐male freshwater prawn–finfish polyculture ponds

The effects of three different combinations of silver carp Hypophthalmichthys molitrix and catla Catla catla density on the production system in all‐male freshwater prawn–finfish polyculture ponds were evaluated in triplicate. The stocking density of silver carp and catla, respectively, were maintained at 2000 and 500 ha^?1 in treatment SC₂₀₀₀C₅₀₀, 1500 and 1000 ha^?1 in treatment SC₁₅₀₀C₁₀₀₀ and 1000 and 1500 ha^?1 in treatment SC₁₀₀₀C₁₅₀₀. Male freshwater prawn Macrobrachium rosenbergii and small fish mola Amblypharyngodon mola densities were fixed in all treatments at 12 000 and 20 000 ha^?1 respectively. Management practices were same for all treatments. Blue‐clawed male prawns were harvested twice during the 122‐day culture at 15‐day intervals before the final harvest. Plankton and macro‐benthos abundance and water quality parameters (except transparency and chlorophyll a) did not vary significantly (P>0.05) among treatments. Mean final weights of both silver carp and catla were decreased with the increasing of their own stocking density. The treatment SC₁₅₀₀C₁₀₀₀ resulted in 25–32% increased net production of silver carp plus catla (461 kg ha^?1) and 20–21% increased net production of all species combined (874 kg ha^?1) as compared with the other treatments, although the differences in production of prawn and mola among treatments were not significant.     

Cultivation of <Emphasis Type="Italic">Chondrus canaliculatus</Emphasis> (C. Agardh) Greville (Gigartinales,Rhodophyta) in controlled environments

Increased market demand for algal raw materials has stimulated research and development into new cultivation technologies, particularly in countries with economically important seaweed industries. Chondrus canaliculatus is a red alga endemic to the temperate Pacific coast of South America and produces a complex carrageenan. In Chile, this potential marine resource has been underutilized since the early 1990s, after it was replaced mainly by imports of Kappaphycus and Eucheum from Asia. The study of the cultivation of C. canaliculatus in outdoor tanks demonstrates the potential to produce large quantities of algal material due to the high productivity observed. The material produced may be important in Chile as feed for cultured algal herbivores such as Haliotis spp. Data were obtained on the key factors affecting the growth of this alga, including light, temperature, and nutrient concentrations. The results showed that, on a dry weight basis, productivity varied seasonally between 40 g m⁻³ wk⁻¹ in autumn and 200 g m⁻³ wk⁻¹ in spring, and the optimal stocking density in our experiments was 8 kg m⁻³. Effects of self-shading were among the most important limiting factors in the culture. Discussion is presented on the interactions between temperature, pH, salinity, enrichment with N and P, and water exchange on the maximal growth of C. canaliculatus in this culture.