Production of Walleye,Stizostedion vitreum,in Earthen Ponds Fertilized with Organic and Inorganic Fertilizers and Stocked at Three Rates

Walleye, Stizostedion vitreum, fry were raised at densities of 20,30 and 40/m³ in ponds initially fertilized with alfalfa and yeast and supplementally fertilized with liquid inorganic fertilizers. Liquid fertilizers were added weekly to maintain N and P concentrations of 600 and 30 ug/L, respectively. Fish growth was not affected by stocking rate, but survival was significantly (P < 0.05) reduced in ponds stocked at the two higher rates. The fertilization procedure we used did not provide a sufficient forage base to support more that 20 walleye/m³.     

Pond Production of Fingerling Walleye,Stizostedion vitreum,in the Northern Great Plains

The cultural practices used to produce fingerling walleye, Stizostedion vitreum, in drainable earthen ponds are described for a state fish hatchery in Nebraska and two federal hatcheries in North Dakota. The ponds were filled 1 to 7 days before D2-D4 (Dl=the day of hatch) walleye fry were stocked. At one hatchery, ponds were sometimes double-cropped, first for production of northern pike, Esoxlucius. The two federal hatcheries fertilized ponds with ground alfalfa hay or pellets, while the standard practice at the Nebraska hatchery was not to fertilize walleye ponds, because of concern that fertilization would result in weed problems and oxygen depletion. One hatchery seeded the ponds with rye grass in the fall. Two of the hatcheries regularly used herbicides to prevent the stranding of fingerlings during harvest and their mortality caused by entangment with net algae, Hydrodicton. When used, herbicide treatment was applied before ponds were filled (Aquazine^TM) or as needed during the culture interval (Aquazine^TM) or copper sulfate). Harvesting was done after 24 to 58 days; the extreme range represented variation among hatcheries; the variation among ponds at a given hatchery ranged from 4 to 10 days. Harvest occurred when fingerlings were 25 to 50 mm total length and weighed 1,500-5,440 fish/kg. Harvests ranged from 11,933 to 308,537 fingerlings/ha. Survival ranged from 3 to 104% of the estimated number of fry stocked.     

Passive immunization of tilapia, Oreochromis niloticus (L.), with anti-Streptococcus iniae whole sera

Passive immunization of tilapia, Oreochromis niloticus, was conducted to determine whether anti- Streptococcus iniae whole sera (ASI), heat inactivated anti- S . iniae whole sera (HIASI) and normal whole sera (NWS) were protective when intraperitoneally (i.p.) injected into tilapia. The ASI was produced in tilapia actively immunized (challenged) with virulent S. iniae by i.p. injection. An antibody response against S. iniae was demonstrated by enzyme linked immunosorbent assay (ELISA) and 18% of the immunized fish died because of the S. iniae infection. The actively immunized tilapia demonstrated a secondary antibody response and immunity to S. iniae after challenge with S. iniae by i.p. injection. Survival was 100% in the actively immunized fish. The NWS was obtained from tilapia free of ASI antibody and susceptible to S. iniae infection (40% mortality). In two separate experiments, significantly higher mortality was noted in tilapia passively immunized with NWS (33 and 53%) and phosphate buffered saline (PBS) (30 and 60%), in comparison with mortalities of 0 and 10% or 3.3 and 6.7% in the fish passively immunized with ASI or HIASI 14 days after S . iniae infection by i.p. injection ( P  = 0.0003 and 0.0023). Results suggest that immunity provided by ASI and HIASI was because of antibody against S. iniae . Inactivation of complement in the HIASI treatment further suggests that ASI antibody plays a primary role in immunity against S. iniae infection.     

Impact of Sump Tank Size on Growth,Survival, and Production of Marron,Cherax tenuimanus,in an Intensive System

Juvenile marron, Cherax tenuimanus were cultured in an intensive crayfish culture system for a period of six months to study the effects of sump tank size on crayfish growth, survival, and production. Water volume in the large sump system was 2750 L and in the small sump tank system was 500 L. Crayfish were held in individual compartments (surface area 320 cm², depth 10 cm) and fed with commercial pellets (30% protein; 3-5% body weight per week). Specific growth rate (SGR) was 0.75%/day in the large sump system and 0.68%/day in the small sump system; survival was 83% in the large sump system and 71% in the small sump system; and biomass roduction (B) was 239 g/m² in the large sump system and 158 g/m² in the small sump system. These values were all higher (P < 0.05) in the large sump system. The results suggest that crayfish production could be improved by using large sump tanks in recirculating intensive crayfish culture systems.     

Effects of Fertilization and of Fry Stocking Density on Pond Production of Fingerling Walleye,Stizostedion vitreum

The influence of fertilization and of fry stocking density on production of fingering walleye, Stizostedion vitreum, was evaluated in earthen ponds at North Platte State Fish Hatchery, North Platte, Nebraska. In 1990, five 0.4-ha ponds were fertilized with alfalfa pellets, and five were fertilized with soybean meal; four unfertilized ponds served as controls. All ponds were stocked with D2 (Dl = the day at hatch) walleye fry at 250.000ha. Differences in yield, number of fingerlings harvested, mean length, and mean weight amone treatments were not statistically significant (P> 0.05). In 691, two fertilization schedules (no fertilizer and fertilization with alfalfa pellets) and two fry stocking rates (250.000 and 375,000 fry/ha) were evaluated. Four ponds were used for each treatment. Statistically significant treatment differences were found in yield, number of fingerlings harvested/ha, average length, and average weight. Yield was higher in fertilized ponds compared with yield from unfertilized ponds at both stocking densities, but yield did not differ significantly between stocking density treatments given the same fertilizer treatment. Survival did not differ between density treatments, but total number of fish harvested was significantly greater from ponds stocked at the higher density. Fingerlings with the largest average weight were raised in fertilized ponds that were stocked at 250,00O/ha, while the smallest fingerlings were from unfertilized ponds that were stocked at 375,000ka. Days in culture interval, which varied among ponds by 9 days in 1990 and 10 days in 1991, was significantly correlated with most production variables in 1990 and with all production variables in 1991. Means of water quality variables were not significantly different between fertilized and unfertilized ponds in either year, but significant differences were found in means of three water quality variables between 1990 and 1991. Yield in both fertilized and unfertilized ponds in 1991 was less than in 1990.     

吉富品系尼罗罗非鱼选育系F6、F7和F8当年鱼生长对比研究

罗非鱼是世界性养殖鱼类之一,对罗非鱼进行遗传改良研究具有重要意义。在流水池饲养环境里,对选育系F6、F7和F8 3代的当年尼罗罗非鱼Oreochromis niloticus生长性能进行同池比较。经过114 d的饲养,F6、F7和F8世代鱼体重的平均生长速度分别为1.07、1.12和1.47 gd-1,呈逐代增大的趋势。偏差校正后,F6、F7和F8 3代鱼增重率分别为1.13、1.18和1.36 gd-1。F8的体重变异系数比F7降低了6.29%,比F6降低了7.95%。F8代的生长方程是y=40.313x－61.670（R2=0.8783,R=0.9513）。     

Effect of potential probiotic bacteria on growth and survival of tilapia Oreochromis niloticus L., cultured in the laboratory under high density and suboptimum temperature

This study examined the effect of potential probiotic bacteria on growth and survival of the tilapia Oreochromis niloticus , under high density and suboptimum temperature. Presumptive Bacillus and lactic acid bacteria (LAB) were isolated from tilapia and from their culture system and were characterized for haemolytic and enzymatic activity, and antagonism against Vibrio . Selected strains were included in the diet of juvenile tilapia and evaluated during a 134-day assay. The experiment was conducted with four treatments: (1) fish fed with commercial feed plus Dry Oil^®; (2) fish fed with commercial feed plus LAB; (3) fish with bacilli in water; (4) fish with a mixture of treatments 2 and 3. Tilapias in all treatments, including bacteria, grew significantly better than fish fed with commercial feed plus Dry Oil^® (control group). Survival was similar in all treatments. The physicochemical parameters of the culture system were maintained within the optimal ranges for the species, with the exception of temperature (19.9–24.82 °C). Animals fed diet supplemented with bacilli and LAB had good survival and the best growth performance, suggesting that bacteria are appropriate growth-stimulating additives in tilapia cultivation.     

10种中草药及4种组方对罗非鱼致病性海豚链球菌体外抑菌试验

采用琼脂扩散法(打孔法)研究了黄连等10种中草药及4种组方对尼罗罗非鱼(Oreochromis niloticus)致病性海豚链球菌(Streptococcus iniae)的抑制作用,并通过二倍稀释法测定抑菌作用较强的中草药对海豚链球菌的抑菌和杀菌浓度.结果显示:10种中草药对海豚链球菌均有不同程度的抑制作用,其中黄...     

Fisheries by-catch and processing waste meals as ingredients in diets for Nile tilapia, Oreochromis niloticus

Digestibility, feeding and growth studies were conducted with Nile tilapia using diets containing fishery by‐catch and processing waste meals. Three meals manufactured from sorted fisheries by‐catch (MBM, from mixed benthic species, SPM, from small pelagic species, CAM, from mixed catfish species) one from tuna cannery waste (TCW) and one commercial anchovy meal (COM) were tested. By‐catch and processing waste meals had lower protein, lower lipid and higher ash contents than anchovy meal. The meals were all highly digestible and no significant differences (P≥0.05) were observed between apparent protein digestibility measurements. Five feeds, containing fish meal as the major protein ingredient, were formulated and fed to triplicate groups of 30 juvenile tilapias for 9 weeks. Survival, weight gain, specific growth rate (SGR), feed conversion ratio, protein efficiency ratio, phosphorus retention and whole body proximate composition were compared. Weight gain and SGRs were similar for each treatment group and compared favourably with the results obtained from juvenile tilapia elsewhere. Growth was the highest for CAM (P≤0.05), which contained both the highest essential amino acid levels and the highest ash content. Phosphorus retention was significantly lower in fish fed with high ash meals, MBM, CAM and TCW (P≤0.05) than in fish fed with the lower ash meals COM and SPM. Overall, the fisheries by‐catch and processing waste meals evaluated in this study are suitable protein ingredients for juvenile tilapia feeds.     

Effect of probiotics, Enteroccus faecium, on tilapia (Oreochromis niloticus) growth performance and immune response

The aim of this study was to analyze the effect of a probiotic bacterium, Enterococcus faecium ZJ4 on growth performances and immune responses of tilapia (Oreochromis niloticus). The tilapias were treated with E. faecium ZJ4 at a final concentration of 1 × 10⁷ cfu ml^− 1 in aquaria water every 4 days. Six aquaria with three replicates for treated and controls were used. After 40 days, the tilapias supplemented with the probiotic showed significantly better final weight and daily weight gain (DWG) than those fed the basal diet (Control) (P < 0.05). There was no remarkable difference (P > 0.05) in the total serum protein, albumin content, globulin concentration and A/G ratio between the treated and control tilapias. The result of lysozyme activity assay was similar to these biochemical indexes. However, the complement component 3 content, myeloperoxidase (MPO) activity and the respiratory burst activity of blood phagocytes were higher (P < 0.05) in E. faecium treated tilapias (trial 1) than the controls.     

Streptococcus agalactiae Vaccination and Infection Stress in Nile Tilapia,Oreochromis niloticus

Abstract

The stress response following intraperitoneal (IP) injection of a non-adjuvant Streptococcus agalactiae vaccine in cultured warmwater Nile tilapia, Oreochromis niloticus, has not been investigated. Further, little or no information is available on stress following S. agalactiae infection and what effect, if any, vaccination has on susceptibility to infection. The objective of this study was to develop preliminary information on the associations between vaccination, stress, and infection. Blood glucose levels were used to evaluate stress in the fish at different time intervals following vaccination and challenge with S. agalactiae. Blood glucose levels were measured in vaccinates and controls at 0, 2, 6, 24 hours, and 28 days post-immunization (0 hours pre-challenge), and at 2,6, 24,48,72, and 312 hours following challenge with 1.5 × 10⁴ colony forming units (CFU) of S. agalactiae/fish. Significant increases in blood glucose levels were observed only in association with the injection of the vaccine and at 2 hours after injection. After S. agalactiae challenge, both controls and vaccinates had significantly (P < 0.05) higher blood glucose values at 2, 24,48, and 72 hours than at 0 hours. However, blood glucose levels in vaccinates were significantly (P < 0.05) lower than the controls at 24, 48, and 312 hours. Blood glucose levels and mortality of the infected controls were significantly correlated (r² = 0.9236, P = 0.0134). The cumulative mortality of the vaccinates and controls was 10% and 60% after 13 days post-challenge, respectively. The relative percent survival (RPS) was 83.4. Our results indicate that the vaccine was efficacious against S. agalactiae and induced short-term stress in tilapia. These preliminary results also suggested, for the first time, that vaccination may significantly reduce the infection stress associated with S. agalactiae infection in tilapia.     

Production of heterozygous and homozygous clones in Nile tilapia

Production of heterozygous and homozygous clones of Nile tilapia (Oreochromis niloticus L.) was successfully carried out. An outbred clonal line was produced by cross-breeding between a viable mitotic gynogenetic female and male (recessive mutation in a sex determining gene). At the same time, an inbred clonal line was produced by gynogenetic reproduction (retention of 2nd polar body) using optimal pressure/heat shock treatments. The heterozygous and homozygous nature of the two types of clonal lines were checked and identified at the adenosine deaminase ADA* marker locus. A model for the large scale production of similar clones is presented. The possible implications of such clonal lines in genetic research and in the culture of Oreochromis spp. are discussed. © Rapid Science Ltd. 1998     

Physiological and Biochemical Determination of Rainbow Trout,Oncorhynchus mykiss,Semen Quality for Cryopreservation

In rainbow trout, Oncorhynchus mykiss, parameters to determine semen fitness for cryopreservation and quality control of cryopreserved semen were investigated. The following parameters can be used to evaluate semen fitness for cryopreservation as they are statistically significant (P < 0.01) correlated to the post-thaw fertilization rate: motility rate of fresh semen (y = 4.996x - 0.0958x² + 0.0006x³ - 5 1.7363); sperm velocity of fresh semen (y = 6.741x - 0.036x² - 268.37); seminal plasma osmolality (y = 0.539x - 125.59); seminal plasma pH (y = -82.768x + 728.133); seminal plasma triglyceride levels (y = 0.069x + 29.863); seminal plasma ß-D-glucuronidase activity (y = -1.112x + 0.0058x² + 82.229); seminal [lasma lactate dehydrogenase activity (y = -0.096x + 0.00006x² + 583.80); spermatozoan acid phosphatase activity (y = -132.51x + 126.38x² + 66.48); spermatozoan adenylate kinase activity (y = 3.474x + 4.925). Quality of deep-frozen semen can be evaluated by motility parameters (P < 0.01): frozen/thawed semen motility rate and post-thaw fertilization rate: y = 1.943x + 28.002; sperm velocity and post-thaw fertilization rate: y = 0.8812x - 0.0059x² + 24.9686.     

Evaluation and Nutrient Quality of Detoxified Jackbean Seeds,Cooked in Distilled Water or Trona Solution,as a Substitute for Soybean Meal in Nile Tilapia,Oreochromis niloticus,Diets

ABSTRACT

Jackbean, Canavalia ensiformis, seeds were detoxified using eight different wet thermal detoxification methods in either distilled water (5% w/v) or trona (NaCO₂.NaHCO₃.2H₂O) solution. The effects of the detoxification methods on hemagglutinating, antitryptic and urease activities, and on the digestibility coefficient of crude protein and gross energy in the jackbean seeds were investigated. Cracked jackbean seeds cooked (100°C) in trona solution for one hour proved to be more effective as a method of inactivating the antinutritional factors in jackbean seeds. Dried detoxified jackbean seed meals were incorporated in dry practical diets providing 20% or 30% of total protein and fed to Nile tilapia, Oreochromis niloticus, fingerlings (mean weight, 6.7 g) twice daily to apparent satiation for 70 days. Mortality was low and no abnormal fish behavior was noted even when the detoxified jackbean meals provided 30% of total protein in diet. Growth and feed utilization indices, were similar (P > 0.05) between the diet in which cracked jackbean seeds cooked in water provided 20% of total protein and in diets in which cracked jackbean seeds cooked in trona solution provided 20% or 30% of total protein. Carcass composition and hepatosomatic index showed no definite trend among O. niloticus fingerlings fed the experimental diets.     

Growth response and acquired resistance of Nile tilapia, Oreochromis niloticus (L.) that survived Streptococcus iniae infection

This study determined the growth performance and acquired resistance of Nile tilapia, Oreochromis niloticus (L.) that survived Streptococcus iniae infection. Tilapia were challenged with three doses of S. iniae (8.8 × 10³, 8.8 × 10⁴ and 8.8 × 10⁵ CFU fish^?1 for low, medium and high challenges respectively). Groups of non‐injected and tryptic soy broth‐injected fish were maintained as controls. Significantly (P<0.05) higher mortality (45.0%) occurred in the high challenge treatment than in the low challenge treatment group (29.6%). The medium challenge group had mortality (36.3%) that did not differ significantly from the high or low treatment. Few fish died in the non‐injected and broth‐injected treatments (3.4% and 0.8% respectively). The tilapia that survived S. iniae infection used to assess growth performance were selected from survivors without gross clinical signs of disease. These fish were randomly stocked at a rate of 30 fish into each 57 L aquarium in triplicate and fed to apparent satiation for 8 weeks. No significant differences were detected in weight gain, feed intake, feed efficiency ratio or survival between S. iniae‐survived tilapia and the control treatments following the 8‐week growth performance trial. Following the 8‐week feeding study, tilapia were challenged with 1 × 10⁶ CFU fish^?1 of S. iniae to assess acquired immunity. Mean cumulative mortality was significantly higher (P<0.05) in the control treatments (41.7% for the non‐injected and 43.3% for the broth‐injected fish) than in the low, medium and high challenge treatments (7.4%, 3.3% and 8.3% respectively). Serum protein was significantly (P<0.05) elevated in the S. iniae‐survived tilapia that were subsequently challenged when compared with controls challenged for the first time. Agglutinating antibody titre was significantly higher in the fish in the medium and high challenge treatments, compared with the control fish challenged for the first time. The results suggest tilapia that survive S. iniae challenge without showing overt disease signs performed as well as non‐infected tilapia. Further, the S. iniae‐survived tilapia challenged following the 8‐week growth performance trial gained acquired resistance to homologous S. iniae challenge.     

Digestibility, faeces recovery, and related carbon, nitrogen and phosphorus balances of five feed ingredients evaluated as fishmeal alternatives in Nile tilapia, Oreochromis niloticus L.

This study shows that alternatives for fishmeal in a fish diet affect not only fish growth but also faeces stability and nitrogen (N) and phosphorus (P) waste production. Wheat gluten diet (WGD), soybean meal extract diet (SBE), soybean meal diet (SBM), duckweed diet (DWD) and single‐cell protein diet (SCP) were evaluated as a fishmeal replacement on a 15% weight weight^?1 basis in tilapia diets. Fishmeal replacement affected dry matter (dm), protein, ash and P digestibility significantly. Faeces recovery (6.8–11.2%) was not significantly affected, although the amount of non‐recovered faeces and total faeces showed significant differences. Duckweed diet and SCP resulted in the largest amounts of non‐recovered and total faeces (199–210, 224–225 g dm kg^?1 feed dm). Compared with fishmeal diet (FMD), the WGD and SBE resulted in similar growth, but higher non‐faecal N losses (471–495 vs. 416 g N kg^?1 N). Soybean meal diet, DWD and SCP resulted in lower growth but less non‐faecal loss (409–450 g N kg^?1 N). The DWD and FMD had the highest N retention (480 g N kg^?1 N) compared with the other diets (431–451 g N kg^?1 N). Carbon retention, faecal and non‐faecal losses and P retention were similar for all diets (302–358, 142–176 and 489–523g C kg^?1 C, 606–704 g P kg^?1 P). Phosphorus faecal loss was lower for all diets (329–381 g P kg^?1 P) than for the FMD (401 g P kg^?1 P).     

Influence of Dietary Lipid Sources on the Growth Performance,Immune Response and Resistance of Nile Tilapia,Oreochromis niloticus,to Streptococcus iniae Challenge

ABSTRACT

This experiment was conducted to determine the effect of dietary lipid sources on growth performance, body proximate composition, hematology, immune response and resistance of Nile tilapia, Oreochromis niloticus, to Streptococcus iniae infection. Six isocaloric (3.2 kcal/g) and isonitrogenous (34% crude protein) semi-purified diets were supplemented with 7% of various sources of lipid, namely, corn oil (CO), beef tallow (BT), menhaden fish oil (FO), linseed oil (LO), and equal combinations of FO+CO+BT or LO+CO+BT. Diets were fed to tilapia in quadruplicate aquaria to apparent satiation, twice daily for 12 weeks. Fish fed the BT-diet exhibited significantly lowest weight gain, diet intake, feed and protein efficiency ratios, apparent protein utilization, and survival. Whole-body protein and ash were significantly (P < 0.05) lowest and highest, respectively, for fish fed the beef tallow-diet, but the values of these parameters did not differ among fish fed other diets. No significant differences (P > 0.05) were found among hematological values, except for fish fed the FO-diet which had abnormally high red and white blood cell counts. Serum protein concentration, lysozyme activity, and natural hemolytic complement activity were significantly (P < 0.05) reduced in fish fed the BT-diet. The values of these parameters did not differ among fish fed other diets. Post-challenge antibody titer was not influenced by dietary lipid sources. Cumulative mortality 15 days post-challenge with S. iniae was significantly lower (P < 0.05) for fish fed the BT diet compared with those fed FO or FO+CO+BT diets. No significant differences were observed in fish fed other dietary lipid sources.     

Re-evaluation of total sulphur amino acid requirement and determination of replacement value of cystine for methionine in semi-purified diets of juvenile Nile tilapia, Oreochromis niloticus

Two feeding experiments were conducted to re-evaluate the total sulphur amino acid (TSAA; methionine and cystine) requirement and determine the replacement value of cystine for methionine of juvenile Nile tilapia ( Oreochromis niloticus ). Semi-purified diets used in both experiments contained 3510 kcal gross energy and 280 g of protein per kilogram diet from casein, gelatin and crystalline amino acids. The basal diet of the first experiment contained 3.1 g methionine and 0.4 g cystine per kilogram. l -methionine was added to the seven remaining diets at 1.0 g kg⁻¹ increment to produce methionine levels ranging from 3.1 to 10.1 g kg⁻¹ diet. Each diet was fed to four replicate groups of juvenile Nile tilapia (1.28 g mean weight) in a recirculation system for 8 weeks. Broken-line regression analysis of weight gain data indicated that the TSAA requirement of juvenile Nile tilapia was 8.5 g kg⁻¹ of the diet or 30.4 g kg⁻¹ of dietary protein. In the second experiment, TSAA level was set at 95% of the requirement value determined in the first experiment. Seven diets were made with different ratios of l -methionine and l -cystine (20 : 80, 30 : 70, 40 : 60, 50 : 50, 60 : 40, 70 : 30 and 80 : 20, based on an equimolar sulphur basis). Each diet was also fed to four replicate groups of juvenile Nile tilapia (4.14 g mean weight) in a recirculation system for 8 weeks. Regression analysis of weight gain data using broken-line model indicated that cystine (on a molar sulphur basis) could replace up to 49% of methionine requirement in semi-purified diets for juvenile Nile tilapia.     

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.     

Performance of plastic biofilter media with different configuration in a water recirculation system for the culture of Nile tilapia (Oreochromis niloticus)

Three kinds of locally available plastic biofilter media with different configurations (plastic rolls, PVC pipes and scrub pads) were evaluated for their efficiency in organic waste removal from the effluents of an intensive recirculating tilapia culture system. A set of three types of solid-removing filters consisting of screened sedimentation; upflow sand as well as plastic bead filtration accomplished the mechanical filtration. Values of critical metabolic wastes like total ammonia nitrogen (TAN) (0.92 ppm) and nitrite-nitrogen (NO₂-N) (0.22 ppm) were found to be well within the acceptable limits, while other water quality parameters in the culture water were also maintained within the normal range by the filtration system. Removal rates of 3.46 g TAN/m³ per day and 0.77 g NO₂-N/m³ per day, as well as TAN and NO₂-N removal efficiencies of 29.37 and 27.3% respectively, were established to be the best for the plastic-roll biofilter medium as compared to PVC-pipe and scrub-pad media. Percent removal of TAN and NO₂-N per pass of the biofilter (25.49 and 26.3% respectively) and the specific TAN and NO₂-N removal rates (43 and 9.6 mg/m² per day) of plastic rolls were also found to be superior to the other two biofilter media. Pieces of PVC pipes as biofilter medium is recommended to be used in the biofilters in view of their cheaper cost.