Design and development of a portable and streamlined nutrient film technique (NFT) aquaponic system

One pilot-scale portable Nutrient Film Technique (NFT) aquaponic system has been designed, developed, and tested at ICAR-CIFA, Bhubaneswar for a period of 90 days (October to December 2018) to study the efficiency of the new design. The experimental setup has three separate units, each consisting of four major components, such as Fibreglass Reinforced Plastic (FRP) round fish culture tank (ø2.15 × 0.9 m) with operational capacity 2800 L, biofilter unit made up of Polypropylene (PP) of 100 L capacity, FRP rectangular hydroponics tank (4 × 0.9 × 0.35 m) having 2.64 m² plantation area and High-density Polyethylene (HDPE) sump (ø0.6 × 0.7 m) of 200 L capacity. Implementation of custom designed and calibrated automatic water recirculation system gives an average flow rate of 94.7 L/h for continuous flow of nutrients from fish culture tank to hydroponics tank. The designed system harnesses gravity flow in 75 % of the cycle. For performance assessment, the system was initially stocked with 54 numbers of fish fry/m³ (153.7 g/m³) of pangas (Pangasius hypophthalmus) in culture tank and 27 marigold (Tagetes erecta) plants/m² in hydroponics tank. Length and weight gain of fish were by 77.04 % and 397.2 % from initial, respectively, and marigold plant harvested 107 number of flowers/m². The Total Ammoniacal Nitrogen (TAN) reduction in biofilter was found to be 61.97 %.     

Evaluation of a Low‐head Recirculating Aquaculture System Used for Rearing Florida Pompano to Market Size

A low‐head recirculating aquaculture system (RAS) for the production of Florida pompano, Trachinotus carolinus, from juvenile to market size was evaluated. The 32.4‐m³ RAS consisted of three dual‐drain, 3‐m diameter culture tanks of 7.8‐m³ volume each, two 0.71‐m³ moving bed bioreactors filled with media (67% fill with K1 Kaldness media) for biofiltration, two degassing towers for CO₂ removal and aeration, a drum filter with a 40‐µm screen for solids removal, and a 1‐hp low‐head propeller pump for water circulation. Supplemental oxygenation was provided in each tank by ultrafine ceramic diffusers and system salinity was maintained at 7.0 g/L. Juvenile pompano (0.043 kg mean weight) were stocked into each of the three tanks at an initial density of 1.7 kg/m³ (300 fish/tank). After 306 d of culture, the mean weight of the fish harvested from each tank ranged from 0.589 to 0.655 kg with survival ranging from 57.7 to 81.7%. During the culture period, the average water use per kilogram of fish was 3.26 or 1.82 m³ per fish harvested. Energy consumption per kilogram of fish was 47.2 or 22.4 kwh per fish harvested. The mean volumetric total ammonia nitrogen (TAN) removal rate of the bioreactors was 127.6 ± 58.3 g TAN removed/m³ media‐d with an average of 33.0% removal per pass. Results of this evaluation suggest that system modifications are warranted to enhance production to commercial levels (>60 kg/m³).     

Effects of photoperiod and stocking density on survival,growth and physiological responses of narrow clawed crayfish (Astacus leptodactylus)

Photoperiod and stocking density are critical factors influencing the performance of decapod crustaceans in culture, however, their influence on growth; survival and biochemical physiology of crayfish broodstock have rarely been considered. Analysis of biochemical physiology in crayfish broodstock during the non‐breeding season provides information on the energy storage requirements of broodstock for increased survival and reproductive output. Growth rate, moulting frequency, survival and biochemical physiology were measured in Astacus leptodactylus broodstock that were cultured at three different photoperiods (18L:6D; 12L:12D; and 6L:18D) and three stocking densities (10, 20 and 40 individuals m^?2) during non‐breeding season. Survival of crayfish was highest at 18L:6D photophase and 10 m^?2 (100%) than other treatments. Survival in the high stocking density was high when combined with 18L:6D photophase, but weight gain (WG) and specific growth rate (SGR) were higher at shorter photophase and lower stocking density. Longer photophase (18L:6D) increased stress responses, characterized by increased haemolymph lactate and glucose levels. Stocking density did not affect proximate composition of crayfish; however, individuals cultured at 18L:6D photophase had higher lipid content than other photoperiod treatments. The study demonstrated that culturing A. leptodactylus at 18L:6D photophase and 10 m^?2 is critical for increased survival of broodstock.     

Intensive rearing of juvenile crayfish (Pacifastacus leniusculus,Astacidae) during the first 6 months: effects of size grading

Recent advances in crayfish feeding have enabled the development of size grading studies from the start of first‐feeding. A 180‐day experiment aimed at intensive rearing of Pacifastacus leniusculus was carried out under controlled conditions, evaluating the effects of size grading at two different periods from the onset of exogenous feeding. Stage 2 juveniles were stocked in fibreglass tanks at a density of 100 m², and fed a dry diet for salmonids combined with restricted amounts of Artemia cysts. Five groups were tested: no grading, grading at 60 days (large and small size) and grading at 100 days (large and small size). After 6 months, no significant differences were found in the survival among groups (mean: 73.06%). The highest final growth (pooled results from upper and lower classes: 17.39 mm carapace length, 1.43 g weight) was achieved by the crayfish sorted at 60 days, showing significant differences from the ungraded group. Smaller crayfish graded at 60 days grew significantly faster than smaller crayfish graded at 100 days. The food conversion ratio was lower in the graded groups (mean: 2.64), showing significant differences from the ungraded group (3.23). This study shows that size grading allows a better performance and an improved feeding efficiency.     

DEVELOPMENT OF AN INTENSIVE SHRIMP CULTURE SYSTEM IN KUWAIT1

Development of an intensive culture system is presently being conducted in Kuwait as a means of farming penaeid shrimp in arid lands. Efficiency of commercial-scale hatchery production of Penaeus semisulcatus and P. japonicus showed significant improvement in 1981 over previous years. Three one-month hatchery rearing cycles in three 15 m³ concrete tanks yielded a total of 9.1 million post-larval shrimp with an average density of 160 PL/liter being obtained for one of these trials. Four other trials were discontinued due to low spawning rates and disease. Installation of a heat exchange system made it possible to begin larviculture in February, two months earlier than in previous years. Research in nursery and grow-out phases of intensive shrimp culture is being directed towards raceways, although Shigueno culture systems are also being investigated. Experimental work with six 18 m × 1.5 m raceways demonstrated that low water exchange rates (1 tank volume/day), combined with high aeration (9.5 liter/min) yielded the highest shrimp biomass. Unheated greenhouse structures constructed over the raceways showed that a single layer of 0.25 mm clear plastic sheeting could maintain water temperatures up to 2.7°C above those in unenclosed tanks. Rearing trials in two Shigueno tank systems demonstrated the importance of high water flow rates (3 tank volumes/day minimum) and adequate aeration. Due to sub-optimal conditions, survival was reduced to 7.5%, however, growth rates up to 6 g/month were obtained for P. semisulcatus.     

Stocking Density for Nursery Production of Redclaw Crayfish,Cherax quadricarinatus,in a Recirculating System

Redclaw crayfish, Cherax quadricarinatus, early juveniles were reared at different stocking densities in a closed recirculation system using 12-L plastic containers as rearing tanks. Initial stocking densities were 1.0, 1.5, 2.0, 2.5, and 3.0 per liter (66, 89, 111, 133, and 156 crayfish/m², respectively). Rearing period was 42 days. Each density was tested with five replicates. Shelter (0.112 m²) was added to double the surface area of rearing tanks. Animals were fed ad libitum twice a day with a commercial diet containing 35% crude protein. There were no significant differences (P < 0.05) in length and specific growth rate (SGR) among stocking densities. Final weight and daily weight gain, however, were significantly higher at the density of 66 per m² (1.0 per liter). Total biomass at harvest increased with density. Survival was affected by stocking density from day 28 onward, decreasing with density from 62.7 ± 7.6% obtained at 66 crayfish/m² to 44.85 ± 8.18% at 156 crayfish/m².     

Effects of Stocking Density on Nursery Production and Economics of the Freshwater Prawn,Macrobrachium rosenbergii

ABSTRACT

In temperate regions, post-larvae freshwater prawn, Macrobrachium rosenbergii, are grown to more advanced sizes in tanks prior to pond stocking. This intermediate stage of culture is referred to as the nursery period. Little research has been conducted on different management practices on juvenile prawn growth and survival during this 30-60 day period. Survival during the nursery stage has been highly variable and may be related to the cannibalistic behavior of juvenile freshwater prawn when cultured at high densities in the nursery. The objective of this study was to evaluate the effect of stocking density, relative to the provision of artificial substrate (number of prawns/m² of substrate), on growth, survival, and economic variables for freshwater prawn juveniles during nursery production. Post-larvae (0.01%0.00 g, n = 300) were stocked into nine 1900 L tanks, each provided with 20.5 m² of artificial substrate in the form of horizontal layers of black plastic mesh (10 mm) spaced 5 cm apart. Tanks were randomly assigned one of three prawn densities (215, 430, or 860 post-larvae/m² of substrate), which equated to 2.3,4.6 and 9.2 prawn/L, respectively. Juvenile prawn were fed a commercial trout diet (42% protein) at a percentage of body weight according to a feed rate table. Water quality was maintained using a flow rate of 8 L/min in each tank from a reservoir pond. Temperature was maintained at approximately 28°C using heat pumps. After 56 days there was no significant difference (P >0.05) in average weight of juvenile prawn stocked at the three densities (0 = 0.58%0.12 g, n = 9). Survival was significantly lower (P <0.05) for prawn stocked at 860 m² (62%) than in those stocked at 430/m² (78%) and 215/m² (94%), which were not statistically different (P >0.05). Even with reduced survival, the highest stocking density produced the greatest number of nursed juveniles based on both tank volume (5.5/l) and surface area (530/m²), at the lowest average cost.     

Effect of density on brood size in noble crayfish,Astacus astacus L., subjected to indoor rearing conditions*

Abstract. The effects of density, cover, and individual versus communal holding on brood size and female survival during the breeding period were examined in a large-scale experiment, involving 3591 mature noble crayfish, Astacus astacus L., females. Crayfish density during both mating/spawning and winter incubation had major effects on egg survival. In the high density experiment (67 and 44 individuals per m² during mating/spawning and winter incubation respectively) mean pleopod egg number in April was 18-20. Lowering the density to 50 during mating/spawning (still 44 individuals per m² during winter incubation) gave mean pleopod egg numbers of 31-38. In the low density experiment (50 and 33 individuals per m² during mating/spawning and winter incubation) mean pleopod egg numbers were 44-46. Egg-bearing females held in individual compartments with perforated bottom had a mean pleopod egg number of 25. Female survival during winter incubation was high, ranging from 91.8 to 98.3%. Covering the basins had no distinct effect on egg and female survival. In Astacus astacus culture we recommend that crayfish density should not exceed about 50 individuals per m² during mating/spawning and about 33 individuals per m² during winter incubation.     

Evaluation of Stocking Density during Second‐Year Growth of Largemouth Bass,Micropterus salmoides,Raised Indoors in a Recirculating Aquaculture System

Largemouth bass (LMB), Micropterus salmoides, are a highly desirable food fish especially among Asian populations in large cities throughout North America. The primary production method for food‐size LMB (>500 g) has been outdoor ponds that require two growing seasons (18 mo). Indoor, controlled‐environment production using recirculating aquaculture system (RAS) technologies could potentially reduce the growout period by maintaining ideal temperatures year‐round. Researchers conducted a 26‐wk study to evaluate optimal stocking densities for growout of second‐year LMB to food‐fish size in an indoor RAS. LMB fingerlings (112.0 ± 38.0 g) were randomly stocked into nine 900‐L tanks to achieve densities of 30, 60, or 120 fish/m³ with three replicate tanks per density. The RAS consisted of a 3000‐L sump, ¼ hp pump, bead filter for solids removal, mixed‐moving‐bed biofilter for nitrification, and a 400‐watt ultraviolet light for sterilization. Fish were fed a commercially available floating diet (45% protein and 16% lipid) once daily to apparent satiation. At harvest, all fish were counted, individually weighed, and measured. Total biomass densities significantly increased (P ≤ 0.05) with stocking rate achieving 6.2, 13.2, and 22.9 kg/m³ for fish stocked at 20, 60, and 120 fish/m³, respectively. The stocking densities evaluated had no significant impact (P > 0.05) on survival, average harvest weight, or feed conversion ratio which averaged 92.9 ± 5.8%, 294.5 ± 21.1 g, and 1.8 ± 0.3, respectively. After approximately 6 mo of culture, LMB did not attain target weights of >500 g. Observed competition among fish likely resulted in large size variability and overall poor growth compared to second‐year growth in ponds. Additional research is needed to better assess the suitability of LMB for culture in RAS.     

Survey of large circular and octagonal tanks operated at Norwegian commercial smolt and post-smolt sites

A survey was conducted to determine the geometry, operating parameters, and other key features of large circular or octagonal culture tanks used to produce Atlantic salmon smolt and post-smolt at six major Norwegian Atlantic salmon production companies. A total of 55 large tanks were reported at seven land-based hatchery locations, i.e., averaging 7.9 (range of 4–12) large tanks per land-based site. In addition, one 21,000 m³ floating fiberglass tank in sea was reported. Culture volume ranged from 500 to 1300 m³ for each land-based tank. Most tanks were circular, but one site used octagonal tanks. Land-based tank diameters ranged from 14.5 to 20 m diameter, whereas the floating tank was 40 m diameter. Maximum tank depths ranged from 3.5 to 4.5 m at land-based facilities, which produced diameter-to-average-depth ratios of 3.6:1 to 5.5:1 m:m. The floating tank was much deeper at 20 m, with a diameter-to-average-depth ratio of only 2.4:1 m:m. All land-based tanks had floors sloping at 4.0–6.5% toward the tank center and various pipe configurations that penetrated the culture tank water volume at tank center. These pipes and sloping floors were used to reduce labor when removing dead fish and harvesting fish.Maximum flow ranged from 3 to 19 m³/min per land-based tank, with 400 m³/min at the floating tank, but tank flow was adjustable at most facilities. Land-based tanks were flushed at a mean hydraulic retention time (HRT) of 35–170 min. Maximum feed load on each land-based tank ranged from 525 to 850 kg/day, but the floating tank reached 3700 kg/day. Almost half of the large tanks reported in this survey were installed or renovated since 2013, including the three tank systems with the highest flow rate per tank (greater than 17.6 m³/min). These more recent tanks were operated at more rapid tank HRT’s, i.e., from 34.8 to 52.5 min, than the 67–170 min HRT typical of the large tanks built before 2013. In addition, flow per unit of feed load in land-based tanks that began operating before 2010 were lower (19–30 m³ flow/kg feed) than in tanks that began operating later (33–40 m³ flow/kg feed). In comparison, the floating tank operates at a maximum daily tank flow to feed load of 160 m³ flow/kg feed, which is the least intensive of all tanks surveyed. Survey results suggest that the recently built tanks have been designed to operate at a reduced metabolic loading per unit of flow, a tendency that would improve water quality throughout the culture tank, all else equal. This trend is possible due to the ever increasing application of water recirculating systems.     

Preliminary Pond Production of the Red Claw Crayfish,Cherax Quadricarinatus,in the Central United States

The Australian red claw crayfish, Cherax quadricarinatus, has recently attracted attention as a potential candidate for aquaculture in the United States. This paper reports the first experimental yield trials of the red claw crayfish in the central United States. Two earthen ponds (0.30 and 0.31 ha) were stocked with juvenile red claw crayfish at either 3.5 or 7.0 crayfish/m². Crayfish were fed a diet of pelletized shrimp ration supplemented with corn silage. When stocked at 3.5/m₂, red claw crayfish grew from an average weight of 3.0 g to 27.0 g in 97 days; average survival was 79.6%; net yield was 653 kg/ha; and feed conversion was 1.4. When stocked at 7.0/m² red claw crayfish grew from an average weight of 4.1 g to 19.7 g in 99 days; average survival was 70.0%; net yield was 678.9 kg/ha; and feed conversion was 1.8. During the yield trial, temperatures were optimal for growth (25 to 33°C) 60% of the time. These findings are sufficiently encouraging to warrant further trials in locations with a longer growing season, in order to evaluate more fully the potential of the red claw crayfish as a candidate for aquaculture in the United States.     

Preliminary study on the use of synthetic substrate for juvenile stage production of the yabby, Cherax destructor (Clark) (Decapoda: Parastacidae)

Juvenile Cherax destructor (commonly called the yabby) (mean weight 48.3 mg) were cultured intensively (stocking density 360/m²) under controlled conditions for 48 days. The animals were provided with a combination of food (high protein pellets and/or natural feed organisms attached to a conditioned synthetic substrate) and refuge. Fastest growth and highest yield was recorded when both pellets and the conditioned synthetic material were provided. Although the yabbies sheltered in the synthetic substrate, it did not increase survival. Juvenile yabbies (< 200 mg) were able to graze on small organisms attached to the synthetic material but this ability appeared to decline as the yabbies grew to a larger size. The use of artificial substrates in the intensive nursery phase production of juvenile freshwater crayfish is discussed.     

Performance of Pacific White Shrimp,Litopenaeus vannamei,Reared in Zero‐Exchange Tank Systems Exposed to Different Light Sources and Intensities

The development of biofloc production technology has generated significant commercial and research interest directed toward the inland culture of Pacific white shrimp, Litopenaeus vannamei. Most work to date has been conducted in greenhouses, where photoautotrophic organisms are significant contributors to system functionality. In more temperate locations, operations in insulated buildings would reduce heating costs. This experiment was designed to evaluate the effect of light on shrimp cultured in intensive biofloc systems. A 92‐d experiment was conducted in 3.8‐m³ tanks. There were five light treatments: (1) natural sunlight (SUN) as a control (midday: 718 lx); (2) one metal halide light (MHL) (1074 lx); (3) one fluorescent light (1FL) (214 lx); (4) two fluorescent lights (2FL) (428 lx); and (5) three fluorescent lights (3FL) (642 lx). Artificial light treatments operated on a 12:12 daily cycle. There were three replicate tanks per treatment and each was separated by black plastic to prevent light transmission between replicates. Each tank was stocked at 465 shrimp/m² of tank bottom (initial mean weight = 0.4 g). Light treatment had a significant (P≤ 0.05) impact on average individual weight, survival, harvest yield (kg/m²), and feed conversion ratio (FCR). Harvest yield and survival among shrimp in the SUN, MHL, and 1FL treatments were not significantly different. However, there was an inverse linear relationship (P≤ 0.05; R² = 0.76) between the number of fluorescent fixtures and survival, which was related to greater concentrations of filamentous bacteria as the intensity of fluorescent light increased, causing gill fouling. Natural light and MHL did not result in high concentrations of filamentous bacteria. These results indicate that natural light, metal halide lighting, and/or relatively low levels of fluorescent lighting are suitable for indoor production of Pacific white shrimp in biofloc systems. Light spectrum and intensity can affect bacterial community structure, which has a profound effect on shrimp survival and production.     

Water velocity in commercial RAS culture tanks for Atlantic salmon smolt production

An optimal flow domain in culture tanks is vital for fish growth and welfare. This paper presents empirical data on rotational velocity and water quality in circular and octagonal tanks at two large commercial smolt production sites, with an approximate production rate of 1000 and 1300 ton smolt/yr, respectively. When fish were present, fish density in the two circular tanks under study at Site 1 were 35 and 48 kg/m³, and that in four octagonal tanks at Site 2 were 54, 74, 58 and 64 kg/m³, respectively. The objective of the study was twofold. First, the effect of biomass on the velocity distribution was examined, which was accomplished by repeating the measurements in empty tanks under same flow conditions. Second, the effect of operating conditions on the water quality was studied by collecting and analysing the water samples at the tank’s inlet and outlet. All tanks exhibited a relatively uniform water velocity field in the vertical water column at each radial location sampled. When fish were present, maximum (40 cm/s) and minimum (25–26 cm/s) water rotational velocities were quite similar in all tanks sampled, and close to optimum swimming speeds, recommended for Atlantic salmon-smolt, i.e., 1–1.5 body lengths per second. The fish were found to decrease water velocity by 25% compared to the tank operated without fish. Flow pattern was largely affected by the presence of fish, compared to the empty tanks. Inference reveals that the fish swimming in the tanks is a major source of turbulence, and nonlinearity. Facility operators and culture tank designers were able to optimize flow inlet conditions to achieve appropriate tank rotational velocities despite a wide range of culture tank sizes, HRT’s, and outlet structure locations. In addition, the dissolved oxygen profile was also collected along the diametrical plane through the octagonal tank’s centre, which exhibits a close correlation between the velocity and oxygen measurements. All tanks were operated under rather intensive conditions with an oxygen demand across the tank (inlet minus outlet) of 7.4–10.4 mg/L. Estimates of the oxygen respiration rate in the tank appears to double as the TSS concentration measured in the tank increases from 3.0 mg/L (0.3 kg O₂/kg feed) up to 10–12 mg/L (0.7 kg O₂/kg feed). Improving suspended solids control in such systems may thus dramatically reduce the oxygen consumption and CO₂ production.     

Development of a zero water discharge (ZWD)—Recirculating aquaculture system (RAS) hybrid system for super intensive white shrimp (Litopenaeus vannamei) culture under low salinity conditions and its industrial trial in commercial shrimp urban farming in Gresik,East Java,Indonesia

This study aims to develop a hybrid zero water discharge (ZWD) - recirculating aquaculture system (RAS) system to improve water quality, as well as the growth, survival, and productivity, of the super-intensive white shrimp culture under low salinity conditions at semi-mass and the industrial level. The study consisted of two parts: (1) a semi-mass trial for the optimization of shrimp production using a hybrid ZWD-RAS system with a total volume of 2.7 m³ at the different shrimp stocking densities of 500 PL/m³, 750 PL/m³, and 1,000 PL/m³ and (2) an industrial trial at a commercial shrimp urban farming facility in Gresik, East Java, with total volume of 110 m³ at the optimum shrimp stocking density from the semi-mass trial. Both the semi-mass and industrial trials were performed in five steps: (1) preparation and installation of the RAS and ZWD system components; (2) preparation of microbial components including nitrifying bacteria, the microalgae Chaetoceros muelleri, and the probiotic heterotrophic bacteria Bacillus megaterium; (3) acclimatization of white shrimp post larvae from the salinity level of 32 ppt to 5 ppt; (4) conditioning of the biofilter used in the RAS and shrimp tank (microbial loop manipulation in ZWD); and (5) shrimp grow-out rearing for 84 days and 60 days for the semi-mass trial and the industrial trial, respectively. The hybrid system combined a ZWD system and an RAS. Shrimp tanks were conditioned with the addition of microbial components for ZWD at the beginning of the culture period. The RAS was operated when NH₄⁺ and NO₂⁻-N levels in shrimp culture reached above 1 ppm until the levels decreased to 0–0.5 ppm. The culture performance in the semi-mass trial at 500 PL/m³, 750 PL/m³, and 1,000 PL/m³ stocking densities was not significantly different for final mean body weight (12.06 ± 5.72, 11.84 ± 3.58, 12.04 ± 3.71 g/ind, respectively) and productivity (4.205 ± 0.071, 4.691 ± 0.025, 4.816 ± 0.129 kg/m³, respectively). Significant differences in survival (70 ± 7%, 53 ± 3%, 40 ± 4%, respectively) and feed conversion ratios (1.54 ± 0.01, 1.82 ± 0.00, 2.16 ± 0.03, respectively) were observed between the three different stocking densities. Water quality parameters and microbial loads during the semi-mass trial were similar for all stocking densities and were within the tolerance levels for white shrimp grow-out production. The results of the semi-mass trial showed that the hybrid ZWD-RAS system can maintain water quality and a microbial load up to a 1,000 PL/m³ stocking density; however, the optimum performance based on survival, feed conversion ratio, and productivity was reached at the 500 PL/m³ stocking density. The industrial trial of the application of the hybrid ZWD-RAS system using the optimal stocking density of 500 PL/m³ resulted in a comparable shrimp survival of 78% with a total production of 298 kg shrimp biomass (equal to a productivity level of 2.7 kg/m³). The overall results of both the semi-mass and industrial trials showed that the application of a hybrid ZWD-RAS system allows optimal shrimp survival and growth at the stocking density of 500 PL/m³ and has high potential for application in commercial shrimp grow-out production at low salinity levels.     

Abundance and production of the introduced signal crayfish ina British lowland river

In 1984, 40 mature and 100 immature signal crayfish, Pacifastacus leniusculus (Dana), were introduced into the River Great Ouse, England. By 1994, the population had spread up and down stream to occupy an 11.4 km river section. The estimated density and biomass (wet weight) of the population in 1993 were highest in summer at 4.0 m^–2 and 133 g m^–2 respectively, with annual means of 2.2 m^–2 and 82 g m^–2 for crayfish of >30 mm carapace length (CL) in the pool (P2) where the crayfish were originally introduced. Density and biomass were 15 m^–2 and 78 g m^–2, with annual means of 6.1 m^–2 and 33 g m^–2 for crayfish of all sizes in a riffle 300 m downstream from P2. An annual survival rate of 14% was estimated for crayfish >30 mm CL in P2 in 1993. The relative abundance estimated for six riffles and six pools in 1994 showed that crayfish abundance decreased gradually from the original site of introduction both up and down the river. Estimated annual production of crayfish >35 mm CL in P2 for 1993 was 52.58 g m^–2 WW, with a turnover ratio (production/biomass) of 0.44.     

Effect of Density on Growth and Feeding of the Crayfish Cambarellus montezumae (Saussure, 1857)

The acocil C. montezumae is a freshwater crayfish endemic to the Central Plateau of Mexico, but, in recent years, the natural population of this species has diminished considerably. In this work was investigated growth performance and feeding of this crayfish reared at high densities. A random block experimental design with two repetitions and three treatments (77, 154 and 231 crayfish/m²) was carried out. Organisms were fed every third day with 15% of their total biomass of shrimp commercial food with 25% of crude protein, and individual food consumption (IFC) was calculated. Significant differences (ANOVA, P < 0.05) were detected in final weight, absolute increase, relative rate of increase, instantaneous rate of increase, yield and survival rate between the densities, with those reared at a density of 77 organisms/m² reaching the largest sizes. There were no-significant differences (P > 0.05) among treatments in terms of initial weight, specific growth rate and feed conversion l rate.     

Growout production of camouflage grouper, Epinephelus polyphekadion (Bleeker), in a tank culture system

Growout production of the camouflage grouper, Epinephelus polyphekadion (Bleeker), in a 10-m³-capacity fibreglass tank culture system was evaluated, using hatchery-produced fingerlings (56-59 g initial weight) at stocking densities of five, 15 and 45 fish m^?3. During the first 9 months of a 12-month growout period, the fish were fed twice a day with a moist pellet feed containing 40.9% protein. From month 10 onwards until harvest, the fish were fed moist pellets in the morning and trash fish in the evening at a 1:1 ratio. The final weight of fish at harvest was up to 900 g, with mean weights of 544.6 ± 170.72 g at five fish m^?3, 540.2 ± 150.82 g at 15 fish m-^?3 and 513.3 ± 134.52 g at 45 fish m^?3. The results showed no significant differences (P > 0.05) in growth rate and fish size between the different stocking densities tested. The average daily growth rate ranged from 0.62 to 3.38 g fish^?1 day^?1, with mean weights of 1.49 ± 0.74 g fish^?1 day^?1 at five fish m^?3 through 0.53 to 2.38 g fish^?1 day^?1, 1.32 ± 0.57 g fish^?1 day^?1 at 15 fish m^?3 to 0.48-3.32 g fish^?1 day^?1 and 1.31 g fish^?1 day^?1 at 45 fish m^?3 stocking density. Although up to 100% survival was observed at the lowest stocking density, the survival rate significantly decreased (P < 0.05) with increasing stocking density. The food conversion ratio (FCR) significantly decreased (P <0.05) with increasing stocking densities, showing efficient feed utilization with increasing stocking densities of E. polyphekadion. The FCR averaged 2.1 at a stocking density of 45 fish m^?3. The yield in terms of kg fish produced m^?3 of water used in the culture system significantly increased (P < 0.001) from five to 45 fish m^?3. The yield averaged 17.3 ±0.53 kg m^?3 at a stocking density of 45 fish m^?3. The present results show that the present tank culture system could sustain more biomass in terms of increasing fish stocking densities. The growth performance of E. polyphekadion observed during this investigation has been reviewed with other grouper species.     

Growth and survival of the freshwater clam,Galatea paradoxa (Born 1778) cultured on different substrata at the Volta estuary,Ghana

This study was conducted to assess the optimum clam size and substratum type for the culture of Galatea paradoxa. The experiment was conducted over a 90‐day period at the Volta Estuary, Ghana. Three size classes of G. paradoxa categorized as small (27.6 ± 0.4 mm), medium (36.8 ± 0.4 mm) and large (50.0 ± 0.6 mm), were used to ascertain the effect of a sandy and muddy substratum on growth performance and survival. Growth increased from the small‐sized clams to the large‐sized clams at 2.54, 3.03 and 3.43 g, respectively, over the experimental period. Growth was higher (P < 0.05) in the muddy substratum (3.58 g) compared with sandy (2.41 g). Similarly, the specific growth rate increased from 0.99% day^?1 in the small‐sized clams to 1.36% day^?1 in the large‐sized clams. Survival rates were significantly higher (99.4%) for the large‐sized clams compared with the medium (78.8%) and the small‐size clams (74.1%). There was significant interaction between the size class and the substratum type with a trend towards increasing survival rate from the small to the large size clams in both substrata. The survival rate was lower in the muddy substratum (71.4%) compared with the sandy substratum (96.7%). The yield was significantly higher in the muddy (2.01 kg m^?2 90 day^?1) compared with sandy substratum (1.87 kg m^?2 90 day^?1). The results of this study indicate that the culture of small‐sized clams is best practiced on sandy substratum due to their elevated survival rates, whereas the muddy substratum appears most suitable for the culture of larger clams (>40 mm) because of their relatively higher survivorship and better growth performance.     

Effects of blue shrimp Litopenaeus stylirostris and goldlined rabbitfish Siganus lineatus in mono‐ and polyculture on production and environmental conditions

This study was conducted to compare the effects of shrimp and rabbitfish in mono‐ and polyculture stocked at high biomass on production and environmental conditions in a mesocosm system. Shrimp (14 g) and/or rabbitfish (19 g) were stocked in four treatments with different density but with the same total biomass (236 g m^?2), including shrimp monoculture (SM) (17 shrimp m^?2), shrimp–fish polyculture (SF) (11 shrimp and 4 rabbitfish m^?2), fish–shrimp polyculture (FS) (6 shrimp and 8 rabbitfish m^?2) and fish monoculture (FM) (12 rabbitfish m^?2). After 10 weeks of experiment, shrimp survival and biomass were low in the treatments where shrimp were dominant (SM, SF), while rabbitfish survival and biomass were high in all the treatments. Shrimp mortality was assumed to be related to an excess of the system carrying capacity (CC). Results suggested that CC is linked to shrimp biomass/density rather than the system eutrophication level. The ecosystem became heterotrophic as daily feed supply was beyond 7 g m^?2 per day. This threshold corresponded to the environmental CC of the semi‐intensive shrimp culture system. Under these conditions, the combination of high fish biomass and low shrimp biomass appeared as the most valuable in terms of system performances.