Effect of different broodstock densities on the reproductive performance of Nile tilapia, Oreochromis niloticus (L.), in a recycling system

The present study was conducted to evaluate the effect of different stocking densities on the seed production of Nile tilapia, Oreochromis niloticus (L.), under intensive recycling hatchery system conditions. Males and females with mean body weights of 163.2 and 105.0 g, respectively, were stocked at three broodstock densities (4, 8 and 12 fish m^–2) at a male:female ratio of 1:3 in 1 × 1 × 0.43 m (W × L × H) fibreglass tanks. The tanks were illuminated at 2500 lux for 18 h day^–1 and the water temperature was maintained at 29 ± 1 °C. Effluent from spawning tanks was recycled through a biological filter with 10–15% replacement of new water per day. The experiment lasted for 126 days. The results showed that breeders stocked at 4 fish m^–2 had significantly higher (P < 0.05) mean values for total seed production, seed kg^–1 female day^–1, seed female^–1 day^–1, seed m^–2 day^–1 and spawning synchrony than at 8 and 12 fish m^–2 broodstock densities. The mean percentage of seeds in the yolk-sac and swim-up fry stages was highest at 4 fish m^–2 broodstock density. However, the recovery rate was not affected by broodstock density. It is recommended that further research should be conducted to determine whether weight m^–2 , number m^–2 or age of broodstock should be the basis for stocking broodstock.     

Effects of controlling temperature and light duration on seed production in tilapia, Oreochromis spilurus (Günther)

An experiment was conducted to compare the seed production of tilapia, Oreochromis spilurus (Günther), under ambient water temperature and photoperiod, and under controlled water temperature (29.0 ± 2.0 °C), and photoperiods of 13 and 14 h day^?1. Male and female breeders with average weights of 155.4 and 78.7 g, respectively, were stocked in nine 1.04 X 1.04 X 0.40 m (L X W X H) fibreglass tanks. Each tank was stocked with two males and six females. Seed collection was carried out biweekly for 300 days. The results showed that the seed kg^?1 female day^?1, seed m^?2day^?1, total seed production per tank and spawning rates of females were highest during the 14 h day^?1 photoperiod (112.4, 77.4, 24 724 and 42.1%, respectively), followed by ambient spawning conditions (104.0, 57.9, 18 356 and 36.6%, respectively), and lowest during the 13 h day^?1 photoperiod (49.5, 36.5, 12 021 and 25.7%, respectively). Under ambient spawning conditions, peak spawning occurred in May. Monthly seed production was affected by changes in temperature and light duration. In both controlled photoperiods, the peak of seed production was observed during the first month after stocking. In all treatments, spawning declined gradually after the peak, which can be attributed to exhaustion of the breeders. The findings of this study indicate that seed production of O. spilurus in Kuwait can be extended beyond the restricted spawning season of 6-7 months by maintaining water temperature at 29.0 ± 2.0 °C and photoperiod at 14 h day^?1.     

Effect of different schedules for broodstock exchange on the seed production of Nile tilapia Oreochromis niloticus (L.) in freshwater

This study compares the effect of three broodstock exchange treatments (exchange every month, two months and three months) versus no-exchange control treatment on year-round seed production and spawning synchrony of Nile tilapia Oreochromis niloticus stocked in 0.4 m³ fiberglass tanks within a recirculating water system. Adult males and females with mean body weights of 161.7 g and 127.6 g, respectively, were stocked at a rate of 4 fish/tank with a male to female sex ratio of 1:3. Water temperature at 29 ± 1.0 °C and photoperiod of 18 h/day were maintained throughout the experiment. The results showed that broodstock in the exchange treatments had higher seed production and spawning synchrony compared with those in the no-exchange treatment. The exchange treatments resulted in 70.5 to 97% increase in seeds kg female^–1 day^–1. The different spawning parameters of broodstock in the two- and three-month exchange treatments were higher than the one-month exchange treatment. Results suggest that broodstock exchange strategy has a long-term improvement effect on seed production and spawning synchrony, and exchanging all breeders every three months is more profitable in terms of better utilization of breeders and in reducing both the holding/conditioning facilities for broodstock and the labor cost.     

Spawning performance and seed production from hybridization between Oreochromis spilurus and the GIFT strain of the Nile tilapia Oreochromis niloticus

This study compared the spawning performance of four crosses between the salt‐tolerant tilapia Oreochromis spilurus and the fast‐growing genetically improved farmed tilapia (GIFT) strain of the Nile tilapia Oreochromis niloticus: (1) O. spilurus♀×O. spilurus♂ (SS); (2) Nile ♀× Nile ♂ (NN); (3) Nile ♀×O. spilurus♂ (NS) and (4) O. spilurus♀× Nile ♂ (SN). In each cross, males and females having mean weights of 157 and 115 g, respectively were stocked in 0.43 m³ tanks at a male to female ratio of 1:3. Seeds (unhatched eggs, yolk‐sac fry and free swimming fry) were harvested biweekly for 127 days. Viable spawns resulted from all crosses. The NN cross produced the highest total seeds tank^?1 (2890), seeds (kg female)^?1 day^?1 (48.3), seeds m^?2 day^?1 (23.3) and seeds female^?1 day^?1 (7.9), followed by the SS and SN crosses. The NS cross had the lowest fecundity. Lower spawning parameters in the NS and SN crosses may be attributed to the lower social compatibility between the males and females compared with that of the pure parents. The survival and growth of the offspring from the four crosses will be compared in seawater (37–40 g L^?1) to assess whether hybrid vigor was produced.     

Observations on the reproductive performance of three mouth-brooding tilapia species in low-salinity underground water

The reproductive performance and seed production of three mouth‐brooding tilapia species, Oreochromis spilurus (Günther), Oreochromis aureus (Steindachner) and Taiwanese red tilapia hybrid, were compared in low‐salinity underground water (3–5 g L^?1) to select species with the highest reproductive rate. Two‐year‐old males and females having average body weights of 454 and 259 g, respectively, were stocked in 2‐m³ fibreglass breeding tanks (two tanks per species) at 1:3 male to female sex ratio and at average density of three fish m^?2. Seeds (unhatched eggs; yolk‐sac fry and the swim‐up fry) were collected weekly for 112 days. The results showed O. spilurus to have significantly higher (P<0.05) mean spawning parameters expressed as total seed tank^?1 (84 154), seed kg female^?1 day^?1 (284), seed m^?2 day^?1 (190.3) and seed female^?1 day^?1 (76.3), than O. aureus and red tilapia. Monthly seed production in O. spilurus and red tilapia gradually increased from May, peaked in July and declined in August, while in O. aureus, it continued to decrease from May to August. In this study, the observed production levels of seed in O. spilurus were among the highest levels reported for tilapia; therefore, O. spilurus could be selected as a potential candidate for mass seed production in commercial tilapia hatcheries.     

Effects of incubation water hardness and salinity on egg hatch and fry survival of Nile tilapia Oreochromis niloticus (Linnaeus)

This study examined the effects of water hardness and salinity on yolk sac larvae and swim‐up fry survival of Nile tilapia, Oreochromis niloticus (Chitralada strain), eggs during artificial incubation. Four experiments were conducted to evaluate the effects of hardness, salinity and the sources of saline incubation water. High water hardness treatments (500–4200 mg L^?1 as CaCO₃) resulted in higher yolk sac larvae and swim‐up fry survival than low water hardness treatments (50.0 and 132 mg L^?1 as CaCO₃); although yolk sac larvae and swim‐up fry survival did not differ among the high or low hardness treatments. Salinity of 4.0 g L^?1 using seawater, and 4.0 and 8.0 g L^?1 using unprocessed common salt resulted in the higher survival rate of yolk sac larvae and swim‐up fry than other salinity treatments. Yolk sac larvae and swim‐up fry survival was found to decrease with the increase in salinity and increase with the increase in water hardness. The present study demonstrated the positive effects of increased water hardness level (>132 mg L^?1) on yolk sac larvae and swim‐up fry survival. The study also showed that seawater salinity of 4 g L^?1 was the most appropriate salinity level for incubating Nile tilapia eggs.     

Increased spawning activity of female Nile tilapia (Oreochromis niloticus) (L.) after stocking density and photoperiod manipulation

This study was conducted to evaluate the effects of stocking density and photoperiod in increasing the reproductive performance of Oreochromis niloticus. In experiment 1, a change in stocking density (from 47.7 to 6.8 kg m^?3) was performed, with groups of 48 females moved to single compartments. In experiment 2, 36 females experienced a 6L:18D photoperiod for 21 or 28 days (stocking density: 31.3 kg m^?3) before being placed in individual compartments (stocking density of 6.9 kg m^?3, 12L:12D photoperiod). The spawning rates in experiment 1 (32.2%) and in experiment 2 (21 days: 65.2%, 28 days: 36.1%) were significantly higher than those in the control (17.7%). On the other hand, hatching and swim‐up fry rates were significantly lower in experiment 2 – 21 days (41.3%, 40.4%) but not in experiment 1 (64.0%, 56.3%) compared with the control (67.0%, 62.3%). The spawning rate in experiment 2 – 21‐day treatment group was the highest, while the number of eggs collected per female was significantly lower than that in the control. In experiments 1 (20.8%) and 2 (21 days: 44.4%, 28 days: 19.4%), the daily spawning rates were the highest 3 days after maintaining females in single compartments. The current experiment demonstrates how specific stocking density changes can be used to induce higher spawning rates in Nile tilapia.     

Effects of photoperiod on growth and spawning efficiency of Nile tilapia (Oreochromis niloticus L.) broodstock in a recycling system

This study was conducted to evaluate the effects of photoperiod on the growth, feed conversion ratio (FCR) and spawning performance of Nile tilapia (Oreochromis niloticus) broodstock. Duplicate groups of two males (66±3.04 g) and six females (50.5±1.58 g) were stocked in 0.4 m³ fibreglass tanks in a recirculating water system at a male:female ratio of 1:3. The fish were subjected to four photoperiod treatments: 24:0 light:dark (L:D), 18L:6D, 12L:12D and 6L:18D. Light intensity was kept constant at about 2500 lx throughout the study. The fish were fed a commercial tilapia diet (35% crude protein and 16.6 MJ GE kg⁻¹) at a daily rate of 2–3% of tank biomass, twice a day, for 130 days. Males grew significantly faster than females under all photoperiod regimes. The best growth rate and FCR were attained at 18L:6D, followed by 24L:0D, 12L:12D and 6L:18D respectively. The number of eggs per female, number of eggs per spawn and number of spawnings per female were all significantly higher in the 12L:12D treatment than in all other photoperiod cycles. Interspawning intervals and days elapsed per spawn were also shorter in the 12L:12D treatment. The time to first spawning was slightly longer in the 24L:0D and 6L:18D than in 12L:12D and 18L:6D light phases. The 18L:6D and 6L:18D photoperiods produced the lowest spawning performance. It is concluded that a 12L:12D photoperiod regime should be adopted for maximum fecundity, seed production and spawning frequencies of Nile tilapia broodstock reared in intensive, recirculating systems. If maximum reproduction is desired, a near‐natural day length photoperiod should be used.     

Nursing duration and pond fertilization level affect polycultures of Indian major carp (rohu Labeo rohita and mrigal Cirrhina mrigala) with monosex Nile tilapia Oreochromis niloticus

This study investigated the effects of nursing duration on the subsequent performance of rohu (R) Labeo rohita and mrigal (M) Cirrhina mrigala in polyculture with monosex male Nile tilapia (T) Oreochromis niloticus at four levels of pond fertilization. Nile tilapia, rohu and mrigal were stocked at a ratio of 4:1:1 in a 90‐day trial based on 40 20‐m² pens fixed in four 400‐m² earthen ponds. Growth of carp fingerlings during prolonged nursing (5 or 12 months) was stunted compared with fish nursed over a conventional duration of 3 months (3) but showed superior growth subsequently. Mean daily weight gain of stunted rohu (12) ranged from 2.2 to 2.8 g per fish day^?1 compared with 1.1–1.6 g per fish day^?1 for younger fish (3). The comparable ranges for mrigal were 1.9–2.8 and 1.4–2.1 g per fish day^?1. Growth of Nile tilapia was inversely related to duration of carp nursing at the four levels of fertilization. Nile tilapia showed more response to increasing levels of fertilizer input (Y=?1.421+1.716X, where Y is the daily weight gain of Nile tilapia and X is the fertilizer level, r²=0.98, P<0.01, n=12). At a high level of fertilization (3.0 kg N:1.5 kg P ha^?1 day^?1), performance of stunted fingerlings (5 and 12) of both rohu and mrigal was similar (range 2.3–2.8 g per fish day^?1, P>0.05), but younger mrigal (M3) grew faster than rohu (2.1 g per fish day^?1 and 1.6 g per fish day^?1 respectively). Older rohu (12) appeared to perform particularly well, and Nile tilapia poorly at the lowest level of fertilization (1.5 N:0.75 kg P ha^?1 day^?1), suggesting the impact of age of seed on competition within polycultures. The net fish yield (NFY) of tilapia was not affected significantly (P>0.05) by differential stocking age of carps; therefore, combined NFY of the three experimental fish species was not affected by the age of carp, as tilapia was the dominant species in polyculture. The highest combined NFY of all species in the most intensively fertilized pond (3.0 N:1.5 P kg ha^?1 day^?1) was calculated at 4.06±0.08 g·m^?2 day^?1, which was significantly higher (P<0.001) than the yield (1.82±0.12 g·m^?2 day^?1) from the pond with the lowest fertilization. At the highest fertilizer level, tilapia, rohu and mrigal contributed 72%, 14% and 14%, respectively, to the NFY, whereas the ratio was 60%, 20% and 20% at the lowest fertilization level. The study indicated that yields from tilapia in polyculture with the two carp species in more eutrophic water can be optimized if advanced nursing of carps is practised. Moreover, higher inputs of inorganic fertilizer and advanced nursing of carp are economically attractive under Bangladeshi conditions. Advanced nursing of rohu also improves its performance in more extensive systems when tilapia densities are high.     

Effects of stocking density and feeding levels on growth and feed efficiency of Nile tilapia (Oreochromis niloticus L.) fry

The effects of stocking density and feeding levels on larval survival, growth rates, feed utilization efficiency and body composition of Nile tilapia (Oreochromis niloticus L.) fry were investigated in two consecutive experiments. In experiment 1, swimup fry (0.016 g average body weight) were stocked in 20‐L fibreglass tanks, in a closed, recirculating indoor system, at five stocking densities (3, 5, 10, 15 and 20 fry L^?1) and fed a larval test diet (40% crude protein) to apparent satiation, three times a day, for 40 days. Fish survival, percentage weight gain and specific growth rate (% SGR) were negatively correlated with stocking density. The best performance was achieved at 3 fry L^?1. However, no significant differences in growth parameters were found between 3 and 5 fry L^?1. Body composition was not significantly affected by stocking density. In experiment 2, fry (0.016 g average body weight) were stocked at 5 fry L^?1, and fed a larval test diet (40% crude protein) at six feeding levels (10%, 20%, 25%, 30% and 35% BW day^?1) and to satiation, three times a day for 40 days. Fish growth rates and survival were extremely poor at 10% feeding level, and improved significantly with increasing feeding levels up to 30%, and levelled off with further increase in feeding levels. On the contrary, feed conversion ratio (FCR) and protein production value (PPV) were negatively correlated with feeding level. The lowest feeding level (10%) produced significantly lower body lipid and higher protein and ash than other feeding levels. The present results suggest that the optimum stocking density and feeding level of Nile tilapia fry are 5 fry L^?1 and 30% per day respectively.     

Characterization and comparison of variations in reproductive performance of Chitralada strain Nile tilapia, Oreochromis niloticus (L.)

In an attempt to better understand and characterize the variability in the female reproduction of individual Thai‐Chitralada strains of Nile tilapia, Oreochromis niloticus (L.), tagged mature females (n=68) from a single population were monitored in a hapa‐in‐pond system over a 12‐month experimental period. Spawn female⁻¹, eggs female⁻¹ day⁻¹, eggs kg female⁻¹ day⁻¹, day spawn⁻¹ and weight (g) at first spawning of individual females were determined from the regular weekly sampling of weight (g) and eggs per spawn. For analysis, the females were grouped into two, high spawning (HSF) and low spawning frequency (LSF) classes based on their spawning frequency (SF) record. Moreover, nested under these two classes were two groups each based on growth rate, i.e., high frequency – large size (HL), high frequency – small size (HS), low frequency – large size (LL) and low frequency – small size (LS). There was no difference in eggs spawn⁻¹ among all females. The HSF group produced 68% and 361% more eggs female⁻¹ day⁻¹ than the population mean and LSF group respectively. Eggs female⁻¹ day⁻¹ and spawn female⁻¹ remained high in the HSF group and low in the LSF group throughout the 12‐month experimental period. This suggests that individual female spawning activity is consistent within a population in a common environment. The inter‐spawn interval increased with age in all four groups, and day spawn⁻¹ was shorter by 130% in HSF females compared with day spawn⁻¹ in LSF. The HSF group also spawned more successively (≥3), while the LSF group of females had fewer successive spawns (≤2). Body weight (g) had no influence on the number of eggs produced. The study indicates that separating frequently spawning females could be used as an important strategy to improve commercial seed production of Nile tilapia.     

Dietary taurine improves reproductive performance of Nile tilapia (Oreochromis niloticus) broodstock

This study was conducted to evaluate the effects of supplemental taurine on reproductive performance of Nile tilapia (Oreochromis niloticus) broodstock fed soybean meal‐based diets. Four isonitrogenous (350 g kg^?1 protein), isocaloric (18 MJ kg^?1) diets were formulated to contain 0, 5, 10 and 15 g kg^?1 taurine. The diets were fed to triplicate groups of juvenile Nile tilapia (10–15 g average body weight) at a female: male ratio of 3 : 1, to apparent satiation, three times per day for 130 days. The size at first maturation decreased with increasing dietary taurine to 10 g kg^?1 and levelled off with further taurine supplementation. The time to first spawning was also significantly shorter at 10 g kg^?1 taurine level. Spawning performances, including spawning frequencies, total number of spawnings per tank, number of spawnings per female and absolute fecundity, were all significantly improved with increasing dietary taurine up to 10 g kg^?1. However, the quadratic regression analyses indicated that the maximum spawning performance occurred at 8 g kg^?1 of supplemental taurine. Eggs produced from broodstock fed 10 g kg^?1 taurine exhibited significantly higher hatchability and required shorter time for hatching and yolk‐sac absorption and also resulted in higher larval weight than at other dietary taurine levels. The highest egg protein, total amino acids and taurine were also obtained at 10 g kg^?1 taurine. These results suggest that 8 g kg^?1 dietary taurine is required for optimum reproductive outputs of Nile tilapia broodstock.     

The Effect of Photoperiod on Sexual Maturation,Appetite and Growth in Wild Atlantic Cod (Gadus morhua L.)

An experiment was conducted to examine (a) the effects of photoperiod on timing of sexual maturation (b) the relationship between plasma steroid levels, appetite and growth in male and female Atlantic cod (Gadus morhua L.). Wild caught Norwegian coastal cod were subjected to either a 6L/18D photoperiod typical of January at 60° N-(Short day group) or a simulated natural photoperiod (Normal day group) from June 2000 until spawning started. Appetite of individual fish were measured twice weekly, while weight, length and plasma levels of the sex steroids testosterone (T), 11 keto-testosterone (11-KT) and estradiol-17β (E2) were monitored bimonthly. Cod in the Short day group matured 3 months ahead of the cod in the Normal day group and started spawning in early November. Appetite decreased in both sexes 2–3 months prior to spawning in both groups, but this reduction was stronger among males. In both sexes, length growth was reduced concurrently with the appetite loss. Overall, females had significantly higher somatic growth, put relatively less energy into length growth and had developed larger livers compared to males at the time of spawning in the Short day group. Plasma steroid levels increased in both groups throughout the experiment, reaching peak levels of ca 10 ng ml⁻¹ (T) and 15–20 ng ml⁻¹ (11-KT) in males, and 1.5–2 ng ml⁻¹ (T) and 12–18 ng ml⁻¹ (E2) in females at the onset of spawning. Steroid levels increased more rapidly among Short day cod verifying the earlier onset of maturation. These results confirm that photoperiod is a major cue to maturation in cod and imply that the high cost of spawning for females incur differences in appetite between the sexes.     

Pond farming of Nile tilapia, Oreochromis niloticus (L.) in northern Cameroon. Comparing two different strategles for feeding cottonseed cake in tilapia male monosex culture

Two cottonseed cake feeding strategies were tested in pond culture of hand-sexed male Nile tilapia, Oreochromis niloticus (L.). (W₀ > 80 g), with the objective of making better use of the algal blooms which develop when cottonseed cake is applied to a pond. In the conventional ‘progressive feeding strategy’, the daily amount of cottonseed cake was adjusted monthly for the total tilapia bioweight estimated from sampling, assuming that cottonseed cake primarily acted as a tilapia feed. In the ‘linear feeding strategy’, ponds received a fixed daily amount of cottonseed cake throughout the trial, based on the hypothesis that tilapia would cover their nutritional requirements mostly by grazing the algae. Each feeding strategy was tested at two feeding levels. Mean daily feeding rate R in the progressive feeding strategy averaged 48.6 (low feeding level) and 102.5 kg ha^?1 day^?1 (high feeding level), and 49.8 and 82.9 kg ha^?1 day^?1 in the linear feeding strategy. Significantly better growth rates and feed conversion rates were obtained in the progressive feeding strategy. Average extrapolated net pond production was highest at R_pr=6% body weight day^?1 (10.61 ha^?1, year^?1). It was concluded that cottonseed cake contributed to tilapia growth mainly directly as a tilapia feed rather than indirectly as a pond fertilizer via algal production. Finally, the appropriateness of the metabolic feeding rate R_mb(BW^0.8 day^?1) was discussed, compared with the progressive feeding rate R_pr (%BW day^?1). (BW denotes body weight). A practical, two-step linear feeding method was proposed for cottonseed cake as a ‘rule of thumb’ that is workable in the field.     

Pond farming of Nile tilapia, Oreochromis niloticus (L.), in northern Cameroon. Mixed culture of large tilapia (> 200 g) with cattle manure and cottonseed cake as pond inputs, and African catfish, Clarias gariepinus (Burchell), as police-fish

An experiment was conducted with tilapia-catfish polyculture at the Lagdo Fisheries Station in northern Cameroon. The objectives were: 1. To estimate the effect of supplementary cottonseed cake on net pond production in ponds already receiving dried cattle manure as basic treatment: and 2. To study the performance of African catfish, Clarias gariepinus (Burchell). in recruitment control of Nile tilapia, Oreochromis niloticus (L.). Recruitment control is essential in obtaining large tilapia sizes demanded in the market. Cottonseed cake, the most important agricultural by-product in the region, is expensive. Dried cattle manure may be collected free from corrals deserted by pastoral ethnic groups. Three treatments were tested in duplo in six earthen ponds of 525 m² each; treatment A. daily application of dried cattle manure only (266 kg ha^?1 day^?1); treatment B, daily manure + cottonseed at a nominal daily rate of 3% of tilapia biomass: treatment C, daily manure + cottonseed cake at 6% of tilapia biomass. Stocking densities per pond were 250 male Nile tilapia (mean W_o 222 g), 150 female tilapia (W₀= 202 g), 30 ‘large’ African catfish (W_o= 198 g); and 30 ‘small’ catfish (W₀= 52 g). Mean fish densities were 0.76 tilapia m^?2 and 0.11 catfish m^?2. Application of dried manure and cottonseed cake was 6 days per week, and the culture period was 100 days. Fish were sampled every month and feeding rates were adjusted accordingly. Dissolved oxygen content and algal turbidity (Secchi disc) were measured once a week. Extrapolated net pond productions, including recruits, were: -0.41 ha^?1 year^?1 (treatment A); 4.8 t ha^?1 year^?1 (treatment B) and 6.5 t ha^?1 year^?1 (treatment C). Differences between treatments B and C were not significant(P < 0.05). Fertilization with dried cattle manure only (zero cottonseed cake) led to a negative net pond production in treatment A (negative net tilapia production but slightly positive net catfish production). Dried manure at the given application rate did not contribute sufficient nutrients to maintain the stocked fish biomass via enhanced natural production, while pond biomass was high for such an extensive system (manure only). Best fish growth was observed in treatment C (male tilapia, 0.9 gday^?1: large catfish, 6.9 g day^?1) although differences between treatments B and C were not significant. Growth of male and female was not significantly different, but growth rates of tilapia and catfish were significantly different (P & lt; 0.05). Average yields of tilapia recruits in treatment B (1539 kg ha^?1 year^?1) and C (1829 kg ha^?1year^?1) were about four times the average yield of recruits in treatment A (468 kg ha^?1 year^?1) but differences between treatments A, B and C were not significant. It was sugcess, or the reproductive efficiency of tilapia in treatment A could have been lower as a result of that treatment. However, clouds of up-swimming fry appeared to be at least as numerous in the replicate ponds of treatment A as in the ponds of treatments B and C.     

A comparative study of growth and feed utilization efficiency of sex-reversed diploid and triploid Nile tilapia, Oreochromis niloticus L.

Growth performance, survival and feed utilization of diploid (2n) and triploid (3n) sex‐reversed male and female Nile tilapia were evaluated at maintenance feeding (1% body weight (BW) day^?1), fixed feeding (3% BW day^?1) and apparent satiation feeding levels in a freshwater recirculation system comprised of thirty‐six 1‐m³ concrete tanks at the Asian Institute of Technology, Bangkok, Thailand. Triploid Nile tilapia (3n) was produced by subjecting fertilized diploid (2n) tilapia eggs to heat shock. After hatching, fish were sex‐reversed to all‐male and all‐female populations by oral administration of 17 α‐methyltestosterone (60 mg kg^?1 feed) and ethynylestradiol (100 mg kg^?1 feed) respectively. There was significantly higher growth with increased ration levels in both male and female groups. There were no significant differences in final BW, specific growth rate, survival rate, feed conversion ratio and protein efficiency ratio between diploid and triploid fish. Triploids had lower gonad weights than diploids, and this was particularly evident at the satiation feeding level. Triploid fish had a significantly higher apparent net protein utilization and percentage of gutted weight than diploids at all feeding levels. Higher protein utilization efficiency of triploids might be an advantage for commercial tilapia culture but further research is necessary to make such a conclusion.     

Substitution of fish meal with raw or treated cowpea (Vigna unguiculata L Walp,IT86‐D719) meal in diets for Nile tilapia (Oreochromis niloticus L.) fry

Two feeding experiments were carried out to evaluate the effect of partial substitution of fish meal (FM) with raw or heat‐treated cowpea Vigna unguiculata L. Walp var. IT86‐D719 seeds on growth performance, digestibility and pancreas tissue in Nile tilapia (Oreochromis niloticus L.) fry. Experiment 1 involved six isonitrogenous and isocaloric diets: four containing different concentrations of raw, whole cowpea meal (protein basis), a positive control with FM as sole protein source and a negative with cowpea meal as sole protein source. Substitution at up to 200 g kg^?1 had no significant effect on production parameters, although growth was negatively affected in the negative control because of seed antinutritional factor content. Experiment 2 evaluated the effect of heat treatment (oven drying at 48 °C; hot air drying at 70 °C; or autoclaving at 119 °C) and/or seed dehulling using diets containing 200 g kg^?1 cowpea meal presoaked in water. Only autoclaving eliminated trypsin inhibitor and lectin contents, independent of dehulling. Histological analysis indicated no histological changes in pancreas tissue. Raw or treated cowpea meal can replace FM in tilapia fingerling diets at up to 200 g kg^?1 protein content without significantly affecting productive parameters or pancreas tissue.     

Protein requirements of Nile tilapia (Oreochromis niloticus) fry cultured at different salinities

Effect of isolipidic (62.7 ± 5.0 g kg^?1) diets with protein levels of 204.6 (T20), 302.3 (T30), 424.6 (T40) or 511.0 g kg^?1 (T50) on growth and survival in Nile tilapia (Oreochromis niloticus Linnaeus 1758) fry cultured for 70 days at one of four salinities (0, 15, 20 and 25 g L^?1) was evaluated. A bifactorial (4 × 4) design was used with 16 treatments run in triplicate and 20 fry (0.25 ± 0.04 g) per replicate under semi‐controlled conditions. Four independent, recirculating systems (one per salinity level) were used, each one with 12 circular tanks (70 L capacity), filters and constant aeration. The different salinities had no significant effect on growth. Weight gain improved significantly as dietary protein content increased, although organisms fed the T50 diet had a lower growth rate. Survival was highest (98.33%) in the T50/15 (protein/salinity levels) treatment and lowest (71.0%) in the T20/20 treatment, with no pattern caused by the variables. The T40/25, T40/20 and T50/0 treatments produced the most efficient growth and feed utilization values while the T20 treatments at all the salinities resulted with the lowest performance. With the exception of the T50 treatments, a non‐significant tendency to increased weight gain was observed as water salinity increased, suggesting that the salinity of the culture environment does not influence dietary protein requirements in Nile tilapia O. niloticus fry.     

Effects of common carp (Cyprinus carpio Linnaeus, 1758) and the African catfish (Clarias gariepinus Burchell, 1822) on growth and reproductive performance of native tilapia Oreochromis shiranus (Boulenger, 1896)

A study was conducted to assess the effects of common carp and the African catfish on growth and reproduction of the native tilapia Oreochromis shiranus in Malawi. The study was done from 1 May to 1 October 2018 at the National Aquaculture Centre (NAC), Zomba. Four triplicated treatments were used: O. shiranus + carp (T1), O. shiranus + catfish (T2), O. shiranus + carp + catfish polyculture (T3) and O. shiranus monoculture (T4). Fish were stocked at uniform density of 0.8 fish/m². Data collection was done once every month. Results showed that O. shiranus mean weight gain (%), specific growth rate (% body weight/day), average daily gain (g fish^?1 day^?1) and condition factor (g/cm³) were highest in T3 and lowest in T1 and T4 treatments. T3 had better water quality regime and higher tilapia biomass than T1 and T4 treatments. Tilapia fry production (no. fry pond^?1 day^?1) was highest in T4 but did not significantly differ (p > .05) between T2 and T3 treatments. It is concluded that the farming of common carp in aquatic ecosystems containing the African catfish may not adversely affect growth and reproduction of O. shiranus and that the polyculture of the African catfish, common carp and tilapia can be adopted to mitigate the potential adverse effects of carp on the environment and improve tilapia growth.     

Maturation and spawning performance of pond-reared Penaeus merguiensis in different combinations of temperature, light intensity and photoperiod

The combined effects of temperature (23 and 27 °C), light intensity (1100 and 2 lux or ≈ 21.5 and 0.04 μEinst m^?2 s respectively) and photoperiod (10 h light/14 h dark and 14 h light/10 h dark) on ovarian maturation and spawning performance of ablated pond‐reared Penaeus merguiensis were investigated in a 51‐day experiment. The results showed that temperature was the most influential factor, followed by light intensity, whereas the effect of photoperiod was minor. As the maturation process progressed, the effects of temperature and light intensity became stronger while that of photoperiod became less significant. Temperature significantly affected all the reproductive parameters assessed. Prawns in the 27 °C treatments outperformed those in the 23 °C treatments (P < 0.05). The effect of light intensity was found to have a significant effect (P < 0.05) only on the proportion of spawners (prawns that spawned) and spawning rate. More prawns spawned at a higher rate under dim light (2 lux) than under strong light (1100 lux). Photoperiod affected only the proportion of prawns reaching stage III of ovarian development (P < 0.05). There were interactions between temperature and light intensity affecting the proportion of prawns reaching stage III and, although not significantly, the proportion of spawners (P = 0.177), fecundity (P = 0.134) and survival (P = 0.061). Overall, it is recommended that a combination of 27 °C, 2 lux and 10 h light/14 h dark is suitable for the maturation of pond‐reared P. merguiensis. There were indications that temperature can be used to control the rate of ovarian maturation. Also, it is possible to increase light intensity up to 1100 lux in P. merguiensis hatcheries if prawns are ablated.