Sex-Reversal of Oreochromis aureus by Immersion in Mibolerone,a Synthetic Steroid1

Mibolerone (MI), a synthetic steroid, was used to sex-reverse undifferentiated Oreochromis aureus fry. Fry were exposed to 0.0, 0.3, 0.6, or 1.0 ppm MI in static-water solutions for five weeks (immersion treatments) or were fed a diet containing 1.0 ppm MI for four weeks in a flow-through system. Following hormone treatment, the fish were grown to over 60 mm total length prior to sex determination using gonadal squash examination. Fish immersed in 1.0, 0.6, or 0.3 ppm MI had average tissue MI concentrations of 14.4, 5.6, and 3.3 ppm, respectively. Immersion in either 1.0 or 0.6 ppm MI for five weeks resulted in an average of 82% males and 18% ovo-testicular fish (inter-sex fish) with no gonadal females being produced. Exposure to 0.3 ppm MI resulted in 78.7% males, 20.7% ovo-testicular fish, and 0.7% gonadal females. Feeding a diet containing 1.0 ppm MI resulted in 85% males, 11% ovo-testicular fish, and 4% gonadal females. Fry growth and survival were negatively correlated with the MI concentration of the immersion treatments. Exposing tilapia fry to static-water solutions of 0.6 ppm MI for five weeks appears to be a feasible method of eliminating the production of functional females. Immersion solutions should be changed at least weekly to maintain an effective hormone concentration.     

Sex reversal and growth of Oreochromis spilurus (Günther) in brackish and sea water by feeding 17α-methyltestosterone

Abstract. An investigation was undertaken to study the effect of 17α-methyiteslosterone (17-MT) on the sex reversal and growth of tilapia, Oreochromis spilurus (Günther), in brackish water (3–5 ppt) and sea water (40 ppt). Seven experiments were performed; four in brackish water and three in sea water. Yolk-sac fry (1 or 3 day age) were immersed (7 or 24 h/day) in water containing hormone at a concentration of 0–10mg/l for 4–25 days. The swim-up fry were subsequently fed either normal or hormone-impregnated diets (30–70 mg/kg) for another 35 days starting from first feeding. The growth and SGR of fry were better in groups which received hormone both in water and food as compared with water or food alone. Immersion In 5–10 ppm (24 h/day) brackish water solution for 5 days did not bring any change in growth or sex ratio. However, immersion in sea water solution for 25 days (7 h/day) at a concentration of 1 ppm induced positive growth but no change in sex ratio. Higher doses either did not affect or reduced growth and SGR but induced higher percentages (77–83%) of males. Complete masculinization (100%) was observed only in brackish water when fish were immersed in 2·5 ppm solution for 4 days (24 h/day) and later fed 50 mg/kg in diets for 35 days. These results were further confirmed when the immersion schedule was modified to 10 days (7 h/day) and feeding 50–70 mg/kg hormone. It can be concluded from these studies that in brackish water complete masculinization of O. spilurus is possible through immersion in 5·0 ppm for a minimum of 75 h followed by feeding with 50 mg/kg food for a minimum of 40 days starting from first feeding.     

Effects of hormone treatments and temperature on sex-reversal of Nigorobuna Carassius carassius grandoculis

ABSTRACT: Functional sex reversal of all-female nigorobuna Carassius carassius grandoculis to phenotypic males was examined by immersion exposure of fry to 17-methyltestosterone (17-MT) and controlled water temperature during early development. Fry were reared in water containing different concentrations of 17-MT at 24 and 30°C for 80 days starting 20 days after hatching. Although the fish exposed to 0.1 and 1.0 μg/L 17-MT at 24°C were all male, treatment with 10.0 μg/L 17-MT resulted in 43% females. Twenty-two percent males appeared in the control treatment at 30°C but the control at 24°C was entirely female. The proportion of males in treatments exposed to 0.01– 1.0 μg/L 17-MT at 30°C was slightly lower than in the respective treatments at 24°C. These results indicate that the phenotypic expression of sex in nigorobuna is thermolabile and that sex determination is under the control of genetic factors and temperature. Also, control of temperature during early development has been shown to be important for the production of all-female offspring for use as breeding stock for pond culture of fish suitable for preparation as 'funazushi'.     

Fry Production of Shire River Tilapia Oreochromis shiranus at Two Broodstock Sex Ratios1

Abstract.— Shire River tilapia Oreochromis shiranus broodfish were stocked at a density of 1.25 fish/m² and sex ratios of either 1:1 or 1:3 (males: females). Two weeks after stocking, fry collection began. Fry were then harvested weekly for 9 wk with a fine-meshed dip net operated from the pond bank for 1.5 h each morning and afternoon. Ten weeks after fry collection began, all ponds were drained and completely harvested. Over the entire 84 d, fry production averaged 0.78 fry/m² per d and did not differ (P < 0.05) between treatments. Fry production per female was significantly higher (P < 0.02) in ponds stocked at a 1:1 sex ratio (111 fry/female) compared to a 1:3 sex ratio (66 fry/female). To avoid inbreeding without sacrificing production, a broodstock sex ratio of 1:1 can safely be used when producing O. shiranus fry at a density of 1.25 fsh/m² in earthen ponds. Competition among females is a possible cause of reduced fry production in densely-stocked brood ponds.     

Effects of Natural EstradioI-17β and Synthetic 17α- Ethynylestradiol on Direct Feminization of European Sea Bass Dicentrarchus labrax

To produce a monosex female population of European sea bass Dicentrarchus labrax, fry were fed dry diets containing dosages of 12.5, 25, and 50 mg/kg food of either the natural estrogen estradiol-170β(E₂) or the synthetic estrogen 17α-ethynylestradiol (EE₂) for 60 d starting at 88 d post-hatch (dph). A complete feminization (100%) was achieved in all E₂-treated groups at the age of 11 mo (330 dph). All affected fish had ovaries similar in size and histological structure to those of control females. In the E₂-treated groups, feminized fish were heavier and longer than untreated controls (males and females combined). In control groups females exhibited significantly higher body weight and total length than males. Untreated females from control groups and females from the group treated with E₂ at 12.5 mg/kg food had similar body weight, suggesting that in sea bass growth is related to phenotypic sex. In the Entreated groups, survival rates were similar to those of the control fish. A relatively high percentage of females was obtained in the EE₂-treated groups (from 38.6 to 96.5%). However, the gonadal development in these fish was significantly suppressed and a dose-dependent reduction of gonadal sizes was evident. Treatments with the EE₂ (12-5, 25, and 50 mg/kg food) resulted in many fish having abnormal (2.9-5.4-39.8%, respectively) and sterile (0.6-6.0-21.6%, respectively) gonads. Effects also included significantly lower weight and shorter length when compared with controls. Furthermore, fish fed with EE₂ at the dosage of 50 mg/kg food had high mortality rate. A simple protocol was developed for the complete feminization in sea bass in which the fry (80-100 dph) were fed to satiation two times daily with a diet containing 12.5 me of E₂/ks food for a period of 60 d.     

Effects of dietary taurine levels on epidermal thickness and scale loss in red sea bream,Pagrus major

Taurine is often added to artificial fish diets to compensate for a reduction in fish meal (FM). However, the taurine content of FM‐based diets is typically lower than in diets consisting of raw fish, even in diets where FM is the only protein source. We evaluated the effects of dietary taurine in FM‐based diets on epidermal thickness and scale detachability in red sea bream Pagrus major. We compared the effect of diets containing 0% (control), 0.3% (Tau‐0.3%), 0.6% (Tau‐0.6%) and 1.0% (Tau‐1.0%) taurine. Red sea bream (average body weight, 39 g) were fed these diets for 7 weeks. Taurine supplementation had no effect on growth, feed intake, feeding efficiency, or survival. However, the epidermal thickness was higher in fish in the Tau‐0.6% and Tau‐1.0% groups than in the control and Tau‐0.3% groups. Similarly, scale loss was significantly higher in the control group than in the Tau‐0.6% and Tau‐1.0% groups. Our results suggest that supplementation with >0.6% taurine (1.0% in diet) improves skin condition.     

Production of a high percentage of male offspring in growth-enhanced transgenic tilapia using Oreochromis aureus ZZ selected pseudofemales

Gene transfer has offered a new tool for the development of improved fish strains for aquaculture. Monosex fish populations could minimize the impact of genetically modified organisms in the environment. In Oreochromis aureus, the use of pseudofemale spawners (sex-reversed male with a female phenotype) is an alternative technique for producing genetically male tilapia offspring. O. aureus fry were treated with 17β-estradiol at 100 mg/kg of food for 45 days. We obtained 77.1% females and 45.9% in the control group. Females randomly taken from the treated group were crossed with normal males. Fry from pseudofemales producing more than 90% male progenies were submitted to 17 β-estradiol treatment to obtain F₂ pseudofemales. The results of the sex-reversal were low and variable ranging between 66.0 and 84.3% females. F₂ pseudofemales were crossed with transgenic males from the F70 line (O. aureus × O. urolepis hornorum) and non-transgenic (O. aureus) males. The sex ratio of progeny of F₂ pseudofemale deviated significantly (P < 0.01) in favor of males in the crosses with transgenic (90.2%) and non-transgenic (89.3%) males compared to the results observed with normal females (51.0 and 52.3%, respectively). The mean fry production with pseudofemales (per m²/day) was similar to the normal females in the crosses with transgenic and non-transgenic males. To our knowledge this is the first report on the production of a near monosex population in genetically modified fish.     

Effects of Dietary Protein Levels on Gonad Maturation, Size and Age at First Maturity, Fecundity and Growth of Nile Tilapia

Six-week-old Nile tilapia (Oreochromis niloticus) fry with an average weight (SD) of 0.51(0.2) g were reared for 140 days on five formulated, isocaloric diets of different protein levels (25, 30, 40 and 45% by dry weight). Fish fed diets of higher protein levels (40 and 45%) showed better growth and feed conversion ratio than those on lower protein levels. Fast-growing fish matured earlier. Maturation rate was affected by the dietary protein levels. Males matured earlier than females: the first mature males were recorded when they were 14 weeks old, whereas the females matured after 18 weeks. In both sexes, mean percentage of mature fish rose with increasing dietary protein level, the percentage of mature males being higher than that of the females. Similarly, the percentage of mature fish rose with the increasing age of fish, with more than 50% males and females mature at the age of 22 and 24 weeks, respectively, the exception being the 25% protein diet fed fish, where the percentage of mature fish was below 50%. In all treatments, spawning was initiated when the fish were 22 weeks old. Smallest size at spawning of males and females was 9.2 cm (13.1 g) and 8.1 cm (8.9 g), respectively, and dietary protein levels influenced the size of fish at first maturity. For both sexes, no difference was found in the gonado-somatic index (GSI) among different treatments. Fecundity increased with increasing dietary protein levels, but significant differences were found only between 40–45% and 25–35% dietary protein levels. The relative fecundity (eggs g^–1 female) was higher at the lower dietary protein levels (25–35%) than at the higher dietary protein levels (40–45%). The dietary protein levels did not have any significant influence on the size and weight of mature eggs. The chemical composition of fish and mature ovaries was significantly influenced by the dietary protein level.     

Effects of dietary sunflower oil on growth parameters,fatty acid profiles and expression of genes regulating growth and metabolism in the pejerrey (Odontesthes bonariensis) fry

Aquaculture fish diets usually contain an addition of fish oil to improve their nutritional value. The effect of the replacement of dietary fish oil (FO) by sunflower oil (SfO) on growth, fatty acid composition and expression of genes implicated in somatic growth, feed intake and fatty acid metabolism was studied in pejerrey fry. Fry were fed per 45 days with diets containing FO/SfO ratios of 100% FO; 50% FO:50% SfO; 20% FO:80% SfO; and 100% SfO. No differences were detected in growth and in the total per cent of saturated and monounsaturated fatty acids. Gh, ghr‐I and ghr‐II showed a higher mRNA expression in head and trunk of fry fed with 100% SfO diet. Expression of igf‐II was higher in trunk of fry fed with 100% SfO diet compared with 100% FO diet. The Δ6‐desaturase gene expression was upregulated in head and trunk of fry fed with 100% SfO diet. The nucb2/nesfatin‐1 gene expression decreased in the trunk of fry with increasing dietary SfO. We conclude that the replacement of fish oil by sunflower oil in pejerrey fry feed does not affect growth and is a viable strategy to reduce production costs of this fish.     

Growth of control and androgen-treated Nile tilapia, Oreochromis niloticus (L.), during treatment, nursery and grow-out phases in tropical fish ponds

Abstract. Masculinization of sexually undifferentiated tilapia fry is achieved by oral administration of the androgen 17-α methyltestosterone (MT). An anabolic response to androgen treatment of tilapia has been reported. Growth of control and MT-treated tilapia was evaluated during consecutive treatment, nursery, and grow-out phases under conditions approximating commercial, semi-intensive tilapia farms in Central America. Oreochromis niloticus (L.) fry were fed a 0 or 60 mg/kg MT diet for 28 days. Growth curves for control and MT-treated fish did not have significantly different slopes. Mean harvest fry weights were similar, averaging 0·1 g/fry for both treatments. Fry were subsequently stocked into 0·2-ha nursery ponds for 94 days growth. Slopes of control and MT-treated fish growth curves were not significantly different. Mean final individual weights did not differ significantly between treatments. Control fish did not deviate significantly from the1:1 male: female ratio, but MT-treated fish were 97% males. Control male and MT-treated male fingerlings were stocked for grow-out into 0·1-ha organically fertilized earthen ponds. No significant difference in growth was observed between control and MT-treated fish. Mean gross yields after 150 days and mean final individual weights were similar for both treatments.     

苯甲酸雌二醇诱导新吉富罗非鱼雌性化的研究

2009年7月12日至10月21日和2010年5月1日至11月6日,水温23～31℃范围下,在3口2.72m×1.55m×0.72m、水深0.62m的水泥育苗池和3口1.2m×1.0 m×1.0 m、网目20目/英寸的小网箱中,分别放养100尾体长0.7～1.9㎝的新吉富尼罗罗非鱼(Oreochromis niloticus)苗,投喂含不同剂量的苯甲酸雌二醇(EB)激素的饲料,以获得用于生产YY超雄鱼的XY转化雌鱼。结果显示,用100 mg.kg-1EB饲料投喂26d、36d、46d、59d和饲养102d后,实验鱼的雌性率分别为80.0%、93.4%、93.9%和94.0%,分别比对照鱼(54.8%)高25.2%、38.6%、39.1%和39.2%;用200 mg.kg-1EB饲料投喂24d和饲养188d后,实验鱼的雌性率为83.1%,比对照鱼(46.9%)高出36.2%。经Χ2检验,两次试验的雌性转化率差异均极显著(P<0.001)。实验鱼和对照鱼的成活率差异不显著,未见畸形现象。实验结束时两批实验鱼的体质量和全长均显著小于对照鱼(P<0.01和P<0.05),未转化的实验雄鱼与对照雄鱼的全长、体质量差异不显著(P>0.50)。综合分析认为,用EB诱导新吉富罗非鱼苗雌性化的适宜剂量为200 mg.kg-1饲料,投喂35d左右较为适宜。     

Effect of oestradiol-17β immersion treatments on sex reversal of mud loach, Misgurnus mizolepis (Günther)

Mud loach, Misgurnus mizolepis (Günther), were sex reversed using the immersion technique. Fry were raised in water containing 50, 100 or 200 μg oestradiol-17β I^?1 for 1, 2 or 3 weeks. Survival rates decreased as dose and duration of treatment increased: in most instances they were significantly (P < 0.05) lower than those of the controls. Growth rates were unaffected except for fish in the 200 μg I^?1 treatments. The 200 μg T^?1 treatments for 2 and 3 weeks produced monosex female populations. The 100 μg I^?1 treatment for 3 weeks produced 99% females and 1% intersex individuals. Histologically, ovaries of sex-reversed fish were similar to those of normal females. Morphologically, however, they were different in that the ovaries of sex-reversed fish were not connected to the genital pore. The hormone treatments affected the size and morphology of the pectoral fins, which exhibit sexual dimorphism in mud loach.     

The influence of triiodothyronine (T<Subscript>3</Subscript>) on the early development of piracanjuba (<Emphasis Type="Italic">Brycon orbignyanus</Emphasis>)

This paper reports the triiodothyronine’s (T₃) effects on the early growth and survival of piracanjuba (Brycon orbignyanus) produced from fertilized eggs hormone exposed. The study was carried out in two phases. In the first phase, eggs divided in 6 batches were immersed in T₃ solutions: 0.01; 0.05; 0.1; 0.5 ppm; 1 ppm and control (no T₃). After a 15-min immersion, eggs were transferred to incubators where larvae were kept up to 72 h after hatching. Larval weight, length and yolk sac volume were determined every 12 h. Sixty and 72 h after hatching, larvae exposed to 0.5 ppm T₃ were significantly heavier than the others, and those exposed to 1 ppm T₃ showed the lowest weight. The yolk sac absorption was not affected. In the second experimental phase, the resulting fry from the first phase were stocked into 3 boxes per treatment (5 larvae L⁻¹) and fed with plankton, fish larvae and feed prepared in the hatchery (48% CP) in the first 3 days, plankton and feed from the 4th to the 10th day and only feed in the next (last) 5 days. Fry weight, length and specific growth rate were determined at 1, 5, 10 and 15 days. Survival was calculated in the last day. In the 15th day, fry length did not differ among treatments but the weight of the control group was higher. Higher survival in the T₃-treated groups suggested lower predation among fry. The results allowed us to conclude that there was no expressive effect of T₃ on the growth, but it improved the survival of the piracanjuba progeny.     

Growth and fry production of Nile tilapia, Oreochromis niloticus (L.), on different feeding schedules

The effect of scheduled use of high‐protein and low‐protein diets on body weight and fry production of Nile tilapia, Oreochromis niloticus (L.), was determined. A preliminary feeding trial was first conducted on fingerlings. These were fed a high‐protein diet (H, 25% protein) or a low‐protein diet (L, 18% protein) daily, or diet H for 1–3 days followed by diet L for 1–4 days. Final body weight was significantly higher (P < 0.05) in fish fed diet H daily and in fish fed diet H for 2–3 days followed by diet L for 1 day (2H?1L and 3H?1L). Fingerlings on 1H?1L and 3H?2L had slightly lower growth. Based on the response of the fingerlings, five feeding schedules were tested with the broodstock. A high‐protein diet (HP, 40%) and a low‐protein diet (LP, 25%; same as H for fingerlings) were used. Feeding schedules significantly influenced body weight of female but not the male fish. Fry production was not significantly affected by the feeding schedule for broodstock. When growth, fry production and saving in feed cost were all considered, the broodstock on 1HP?1LP and 3HP?2LP feeding schedules both gave the highest overall performance. These findings give fish farmers an option in the management of feeding of tilapia broodstock.     

双斑东方鲀全人工繁殖试验

对野生双斑东方鲀(Takifugu bimaculatus)人工繁殖的子一代进行人工培育,雌鱼须经三年性腺成熟,成熟系数为17.55%,雄鱼性腺成熟为两年。产卵前一个月,雌亲鱼经强化培育、催熟后,成熟系数可达到25.00%。对7组亲鱼注射LRH-A_2激素,共获鱼苗1.2048×10~6尾,其中全人工繁殖鱼苗9.592×10~5尾,对照组获鱼苗2.456×10~5尾。并就双斑东方鲀性成熟年龄等进行讨论。     

Vaccination of channel catfish, Ictalurus punctatus (Rafinesque), by immersion and oral booster against Edwardsiella ictaluri

Abstract. A commercially prepared vaccine against Edwardsiella ictaluri was used to vaccinate 12-day-old channel catfish fry by immersion, or by immersion plus an oral booster 2 months later. One month after the fish were fed the booster vaccine, they were challenged by waterborne exposure to 2·1 × 10⁶ cells ml⁻¹ of E. ictaluri. Immersion only vaccinated fish suffered 6·7% mortality and immersion plus oral-boosted fish had a 3·3% mortality. Mortality among non-vaccinated controls was 96·7% and was significantly ( P < 0·01) above the vaccinated mortality. The relative per cent survival for the immersion-only fish was 93·1, while it was 96·6 for the immersion plus oral-boosted fish. Agglutinating antibody titres of the vaccinated fish were significantly ( P < 0·05) higher than the control fish. When the ponds were drained 6 months after stocking, 42·7% of non-vaccinated, 56·3% of immersion-only and 70·8% of immersion plus oral-boosted fish were harvested. Survival of immersion plus orally-boosted fish was significantly ( P < 0·05) higher than the controls of immersion-only fish. Duplicate populations of immersion plus oral-booster-vaccinated fish grew 34% and 56% faster, respectively, on an average daily gain than the control fish, while immersion-only fish in one pond grew 20% less per day and fish in the second pond grew 48% faster.     

Androgen-induced sex-inversion of Oreochromis (Trewavas) hybrid fry stocked into cages standing in an earthen pond

Gonadally undifferentiated Oreochromis niloticus × O. aureus hybrid fry (F₂ generation), were fed a single level of androgen-treated diet (60 mg/kg 17-α-ethynyltestosterone), in earthen pond cages, an indoor tank and outdoor concrete tanks. All fry fed androgen-treated diets had populations consisting of ≥ 96% males, whilst those populations fed control diets had sex ratios which were not significantly different from an expected 1:1. No all-male broods were obtained, but populations composed of 96% and 99% males were produced in earthen pond cages following 25- and 28-day treatment periods, respectively, at a mean temperature of 30°C. Fry in outdoor pond cage treatments appeared to grow faster than those inside. Additional, naturally occurring food in the ponds is considered partly responsible for this. Survival of fry during androgen treatments depended upon size and density at stocking. Larger and more vital fry survived better than newly hatched fry.     

The influence of reproductive status on the stimulatory action of N-methyl-D,L-aspartate on growth hormone secretion, in vitro in rainbow trout, Oncorhynchus mykiss

The glutamate agonist, N-methyl-D,L-aspartate (NMA) stimulates the secretion of growth hormone (GH) from pituitary fragments in vitro and increases plasma GH levels in vivo in rainbow trout, Oncorhynchus mykiss (Flett et al. 1994; Holloway and Leatherland 1997a,b); however gonadal steroid hormones appear to modulate this response in experimental situations. This study examines whether steroid hormones also modulate the GH-regulatory actions of NMA during the normal reproductive cycle of rainbow trout by examining the relationship between the stage of sexual maturation and the pituitary release of GH in vitro in response to an NMA (10^-8 M) challenge. NMA had no effect on mean GH release from the pituitary glands of fish that were immature (GSI <1.0), from males during early development (GSI 1.0-3.0), or from sexually mature males (with free running milt) and females (ovulated). However, NMA significantly increased GH release from pituitary glands taken from females during the early stages of gonadal growth (GSI 1.0-9.0) and from males and females sampled during the later stages of gonadal growth (males GSI 3.01-6.0; females GSI 9.01-15.0). The GH-stimulatory action of NMA in males and females progressed to a maximum effect during the late stages of gonadal growth, and disappeared in ovulated females and free running males. Moreover, in female fish, the maximal GH release in response to the NMA challenge is positively correlated with plasma 17β-estradiol levels; no such correlation was evident for plasma testosterone levels in males. Changes in the GH response to NMA during maturation while gonadal steroid levels fluctuate provides further evidence to suggest that the effects of NMA on GH secretion are intimately linked to endogenous gonadal steroid hormone levels. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evaluation of Decapsulated Artemia Cysts in Hatchery Diets for Channel Catfish Ictalurus punctatus Fry and Effects on Subsequent Fingerling Production

Abstract.— Two 10-d hatchery growth trials were conducted to determine if differences in growth, body composition, and survival existed among catfish fry Ictalurus punctatus fed a diet of 100% catfish-starter (CS) or CS in combination with decapsulated Artemia cysts (DAC) at 25%, 50 % , or 75% of diet weight. A 120–d production trial was undertaken immediately following the first hatchery trial to evaluate the effects of diet on production characteristics (survival, length, weight, feed conversion ratio, and total yield) of fingerling catfish produced from hatchery-raised fry. Fry fed diets containing DAC weighed 61–98% more ( P < 0.05) than fry fed only CS. The size differential of DAC-fed fry was maintained through 120 d of growth in simulated nursery ponds (0.001-ha earthen-bottom pools). The increased growth of fry fed DAC might have been related to differences in dietary lipid content, amino acid composition, or digestibility of CS and DAC. Body weights of fingerlings produced under simulated production conditions from fry fed hatchery diets containing 50% and 75% DAC were 17% and 25% higher, respectively, 130 d post-hatch, than fingerlings produced from fry fed only CS. In addition, the total yield of fingerlings produced from fry fed 75% DAC was 17% greater that that of fingerlings produced from fry fed only CS. The increased performance of fingerlings produced from DAC-fed fry could have resulted from their larger size at stocking, since larger fry might be capable of capturing natural food organisms and ingesting prepared diets more effectively than smaller fish. CS-DAC diet combinations could be used to increase weight gain of hatchery-raised fry if the cost-benefit ratio of adding DAC to the standard hatchery diet warrants its use.     

Controlled hatchery production of Clarias gariepinus (Burchell 1822): growth and survival of fry at high stocking density

Abstract. The growth and survival of Clarias gariepinus (Burchell) fry was investigated at high stocking density. Significant increases in mean fry weight, and concomitant significant decreases in specific growth rate, were recorded over successive 5-day periods. Fry growth was negatively density dependent. Fry survival was in excess of 90% in all treatments. Increasing stocking density between 50 and 150 fish/l altered the pattern of mortality; non-cannibalistic deaths decreased significantly with increasing stocking density though cannibalism did not significantly increase. Periods of weaning fish onto larger feed particles were associated with temporarily increased rates of cannibalism.