Low levels of Aflatoxin B1 could cause mortalities in juvenile hybrid sturgeon,Acipenser ruthenus ♂×A. baeri♀

Toxicity of aflatoxin B₁ (AFB₁) was investigated in juvenile hybrid sturgeon Acipenser ruthenus ♂ × A. baeri♀, an important coldwater finfish farmed in China and other countries. Seven experimental diets (Diet A–G) containing different levels of AFB₁ (0, 1, 5, 10, 20, 40 and 80 μg kg^?1 diet) were fed to juvenile sturgeon weighing 10.53 ± 0.17 g kg^?1 to determine its effect on survival, growth, feed consumption, hematocrit, liver histology as well as muscular and hepatic toxin accumulation. The experiment lasted for 35 days and was conducted in two periods of 25 and 10 days each. No external changes or unusual behaviour was observed in the fish fed diets with AFB₁. Mortality was observed in fish fed with highest levels of AFB₁ (80 μg kg^?1– Diet G) from day 12 onwards. After 25 days, fish fed the diet of 80 μg AFB₁ kg^?1 showed significant lower survival (50 ± 5.77%) followed by those fed 40 μg AFB₁ kg^?1 diet (80 ± 5.77%) and 20 μg AFB₁ kg^?1 diet (86.66 ± 3.33%). No significant difference was observed in specific growth rate (SGR) or hepatosomatic index (HSI) between groups. Hematocrit was significantly higher in the fish fed the diet of highest AFB₁. The fish were weighed at day 25 in some treatments (Diets F and G) because of high mortality. However, feeding was continued for another 10 days to observe mortality or behavioural changes if any in the other groups. After 35 days, survival in the fish fed Diet F (40 μg AFB₁ kg^?1) was 40% and those fed Diet E (20 μg AFB₁ kg^?1) was 36.2%. Significant histopathological changes including nuclear hypertrophy, hyperchromasia, extensive biliary hyperplasia, focal hepatocyte necrosis and presence of inflammatory cells were observed in the liver of fish fed high levels of aflatoxin (40 and 80 μg kg^?1). AFB₁ accumulation in fish muscle and liver increased with increased dietary AFB₁ levels. It could be confirmed that 10 μg AFB₁ kg^?1 diet was the maximum allowable level in hybrid sturgeon diet.     

Effect of feeding time on dietary protein utilization and growth of juvenile Senegalese sole (Solea senegalensis)

The present study aimed to determine the effect of feeding time on growth and nitrogen excretion in juvenile sole. An 84‐day growth trial was conducted, in which food was supplied to three triplicate groups of juvenile Senegalese sole (3 g wet weight) at different schedules – diurnal, nocturnal and mixed. At the end of the growth trial, ammonia and urea excretion was assessed during a 24 h cycle. Improved growth (1.3% vs. 0.9% day^?1, specific growth rate), higher nitrogen retention (0.35 vs. 0.27 g N kg^?1 day^?1), lower ammonia excretion (209 vs. 272 mg N‐NH₄ kg^?1 day^?1) and lower total nitrogen excretion (278 vs. 352 mg N kg^?1 day^?1) were found in daytime‐fed fish compared with night‐fed fish. Fish in the mixed feeding regime showed intermediate values of ammonia and total nitrogen excretion, but did not differ from day‐fed fish regarding the other parameters. Results indicate that juvenile sole at a period of their life cycle appear to use more efficiently dietary protein for somatic growth under a diurnal than under a nocturnal feeding regime. This suggest that at least during a time‐window in the juvenile rearing a diurnal feeding regime might be more effective in the production of this species.     

The effects of oligofructose on growth performance,survival and autochthonous intestinal microbiota of beluga (Huso huso) juveniles

Preliminary experiments were undertaken to investigate the effects of oligofructose on beluga sturgeon (Huso huso) growth performance, survival and culturable autochthonous intestinal microbiota. Juveniles (20 g) were fed diets containing varying levels of oligofructose (10, 20 and 30 g kg^?1) at 2–3% body weight per day for 7 weeks. Compared to the control group, no significant (P > 0.05) effect on growth performance was observed in fish fed diets supplemented with oligofructose at 10 and 20 g kg^?1. However, compared to the 20 g kg^?1 group, feeding oligofructose at 30 g kg^?1 resulted in adverse effects on growth performance. Dietary supplementation of oligofructose at 20 g kg^?1 significantly increased survival rate. Microbiological assessment indicated that the viable culturable autochthonous levels were not affected by dietary oligofructose. Although lactic acid bacteria (LAB) were not a dominant component of the endogenous autochthonous microbiota, LAB levels were significantly elevated in fish fed 20 g kg^?1 dietary oligofructose. This elevated LAB population was able to persist for at least 1 week after reverting the prebiotic group back to a control diet. This study encourages further research on different aspects of oligofructose in sturgeon culture with clear emphasis on optimizing dosage levels.     

Intensive carp Cyprinus carpio (L.) rearing in a farm pond in southern Germany and its effects on water quality

Experiments over three years, in a pond of approximately 1500 m², to intensify production at high stocking densities with controlled feeding resulted in a substantial increase in yield (weight gain per hectare and year: 2·6 tonne for C_3–4, 9·1 tonne for C_2–3, 15·4 tonne for C_1–2). At the same time water requirement could be reduced to 4 m³ kg^?1 fish produced. However, the stocking densities achieved resulted in a continuously increasing eutrophication effect which expressed itself in mass development of phytoplankton. This led to considerable diurnal fluctuations in oxygen concentration (0·5–20·0 mg O₂ litre^?1) and total ammonia (0–11·0 mg NH₃NH₄⁺ litre^?1; maximum of 1·5 mg NH₃ litre^?1). The high stocking density contributed indirectly to the diurnal fluctuations (fertilization through faeces and lost feeds). However, its contribution to the overall oxygen depletion amounted to less than 25% at night. The amplitude of the fluctuation was mainly determined by the photosynthetic and respiratory activity of the phytoplankton and by microbiological degradation processes. With increasing water temperature during summer, increasing feeding rate, continuous fish biomass increment, and decreasing light period per day and danger of total fish stock mortality was greatest in August. Only at maximum photosynthetic activity during daylight was the total ammonia production in the pond adequately counteracted and sufficient oxygen produced. Even heavy aeration could not always successfully provide for oxygen during night. In order to avoid a breakdown of the system, algae and nutrients had to be diluted from time to time by excessive water replacement. Despite the temporary overloading of the pond system, losses in numbers had been kept below 6·5% (normally: 10–20%).Pellets were offered via self-feeding devices. Food uptake was continuously recorded. No diurnal feeding rhythmic could be observed, but feeding activity was entirely dependent on oxygen levels in pond water. Feed conversion efficiencies reached values between 1·96 and 2·48.     

Size‐related oxygen consumption and ammonia excretion of Eurasian perch (Perca fluviatilis L.) reared in a recirculating system

Oxygen consumption (OC) and ammonia excretion rates (AE) of perch were measured under commercial‐like conditions (temperature 23.3 °C) in both fed (F) and feed‐deprived groups (D). Measurements were taken in triplicate in six sized batches of perch ranging from 44.8 to 336.2 g. The mean daily OC was 288.3–180.6 mg O₂ kg^?1 h^?1 for group F fish ranging in size from 44.8 to 279.4 g body weight. The mean daily AE expressed as total ammonia nitrogen (TAN) was 13.8–5.2 mg TAN kg^?1 h^?1 in the same groups. Daily peaks of OC in group F perch were observed 6 h after the onset of feeding for each size group with relatively stable values up to the end of feeding. Peaks of daily AE in group F perch were observed 10 h after the onset of feeding in each size group, with a rapid decrease up to 16 h after onset. In group D, OC was 181.1–110.5 mg O₂ kg^?1 h^?1 in the weight range 57.9–336.2 g. The daily mean AE was 1.7–0.5 TAN kg^?1 h^?1 in this group. No dramatic peaks of OC and AE were observed in group D perch.     

The effect of feeding on oxygen consumption and ammonia excretion of juvenile tench Tinca tinca (L.) reared in a water recirculating system

The effect of feeding frequency (one, three, and continuous feeding), feed ration (0.2, 0.5, 0.8% of total fish biomass), and feeding per se on the oxygen consumption (OC, mg O₂ kg⁻¹ h⁻¹) and ammonia excretion (AE, mg TAN kg⁻¹ h⁻¹) of juvenile tench (body weight 15–19 g) and variations in these parameters in daily cycles were examined. Fish metabolism was studied in a recirculating system (rearing tanks of 0.2 m³, water temperature 23 °C). It was found that oxygen consumption and ammonia excretion depended significantly on feed ration. An increase of feed ration from 0.2 to 0.8% of fish biomass caused an increase of OC and AE from 126.80 mg O₂ kg⁻¹ h⁻¹ and 1.95 mg TAN kg⁻¹ h⁻¹ to 187.35 mg O₂ kg⁻¹ h⁻¹ and 8.80 mg TAN kg⁻¹ h⁻¹ (p<0.05). There was no dependence between feeding frequency and the mean rate of oxygen consumption. However, the relationship between feeding frequency and ammonia excretion by juvenile tench was statistically significant (p<0.05). Feeding frequency significantly affected daily fluctuations of AE and OC. It was found that diurnal variations in metabolic rates were strictly related to tench feeding, and the daily variations of AE were significantly higher than OC.     

The optimum dietary protein level for the Mexican cichlid Cichlasoma urophthalmus (Günther): a comparison of estimates derived from experiments using fixed-rate feeding and satiation feeding

Optimum dietary protein levels of young Cichlasoma urophthalmus (Günther) of 0.3 g mean weight were determined at 28°C using two methods, a fixed feeding rate of 6% body weight per day and satiation feeding. In the fixed rate trials nine isoenergetic diets were formulated with protein levels ranging from 347 to 561 g kg^?1 using brown fish meal (anchovy) as the only protein source. In the satiation feeding trials, ten diets were formulated based on brown fish meal (mackerel) ranging from 0 to 450g kg^?1 protein. When fish were fed a fixed rate diet, the best absolute growth was obtained with diets between 435 and 560 g kg^?1 protein. Broken-line analysis showed that, in terms of weight gain (%), the optimum protein level was about 453 g kg^?1. When fish were fed to satiation, the best absolute growth was obtained with the 383 g kg^?1 protein diet, and broken-line analysis suggested an optimum of about 325 g kg^?1. The difference between the two results, using the two techniques, is very marked. The probable mechanism underlying these data is discussed.     

Effects of feeding rates on growth performances of white sturgeon (Acipenser transmontanus) fries

Four 1‐week growth trials were conducted to determine the effects of feeding rates on the growth performances of white sturgeon (Acipenser transmontanus) fries 6–9 weeks after initiation of feeding. Six feeding rates with four replications were used in each of the four trials, and the feeding rates were 3.0–8.0, 2.0–7.0, 1.0–6.0 and 1.0–6.0% body weight (BW) per day in 1% increment, respectively. Number of fries per replicate and their initial BW (means ± SEM) were 60, 45, 30 and 30 and 2.8 ± 0.1, 4.5 ± 0.4, 8.5 ± 0.7 and 10.0 ± 0.7 g, respectively. The fries were kept at 18–19 °C and fed a commercial salmonid feed (488 g kg^?1 protein and 123 g kg^?1 fat). Mortality was low and unrelated to feeding rates. Final body weights, body weight increases, specific growth rates and feed efficiency were significantly (P < 0.05) affected by the feeding rates. Body moisture and lipid contents were significantly affected by feeding rates except body moisture content in trial II. Body protein contents were not affected by feeding rates except in trial III. Broken‐line analysis on specific growth rates indicated that the optimum feeding rates were 6.5 ± 0.4, 4.8 ± 0.2, 4.2 ± 0.1 and 3.8 ± 0.2% body weight per day, respectively, for white sturgeon fries 6–9 weeks after initiation of feeding.     

Ammonia and nitrite toxicity to false clownfish <Emphasis Type="Italic">Amphiprion ocellaris</Emphasis>

False clownfish, Amphiprion ocellaris, is one of the most commercialized fish species in the world, highly produced to supply the aquarium market. The high stocking densities used to maximize fish production can increase ammonia and nitrite to toxic levels. In this study, A. ocellaris juveniles (1.20 ± 0.34 g) were exposed to six concentrations of ammonia ranged from 0.23 to 1.63 mg/L NH₃-N and eight concentrations of nitrite (26.3–202.2 mg/L NO₂ ^?-N). The LC₅₀- 24, LC₅₀-48, LC₅₀-72 and LC₅₀-96 h were estimated to be 1.06, 0.83, 0.75 and 0.75 mg/L for NH₃-N and 188.3, 151.01, 124.1 and 108.8 mg/L for NO₂ ^?-N. Analysis of gill lesions caused by sublethal concentrations of these nitrogenous compounds showed that both nitrogenous compounds induced tissue lesions such as hyperplasia of epithelium cells, hypertrophy of chloride cells and lamellar lifting to all concentrations tested. However, histopathological alterations were more conspicuous accordingly the increase of ammonia or nitrite in fish exposed to 0.57 mg/L NH₃-N or 100 mg/L NO₂ ^?-N. Based on our results, we recommend to avoid concentrations higher than 0.57 mg/L of NH₃-N and 25 mg/L of NO₂-N in water.     

Nitrogen and phosphorus digestibility and excretion of different-sized groups of milkfish (Chanos chanos Forsskal) fed formulated and natural food-based diets

This study determined the digestibility of nitrogen and phosphorus, and the excretion rate of different‐sized groups of milkfish fed a commercial diet, a SEAFDEC formulated diet or lab‐lab (natural food‐based diet). Fish (31.2–263.0 g) were stocked in 12 units of 300‐L fibreglass tanks filled with aerated seawater. The postprandial total ammonia‐nitrogen (TAN) and phosphate (PO₄‐P) excretion of fish were estimated from changes in TAN and PO₄‐P concentrations in water for 24 h. Digestibility was determined from the nitrogen, phosphorus and Cr₂O₃ content of the diets, and pooled faeces after the fish had been fed diets marked with chromic oxide. TAN excretion rate (mg TAN kg^?1 fish day^?1) was significantly lowest (P < 0.05) in medium to very big fish fed the lab‐lab diet (60.8–124.4) and highest in small and medium fish fed the SEAFDEC diet (333.3–331.6) and small fish fed the commercial diet (280.1). Regardless of size, fish fed lab‐lab excreted (mg PO₄‐P kg^?1 fish day^?1) significantly lower PO₄‐P (36.2) but did not differ with fish fed the commercial diet (64.8). Excretion rates decreased exponentially as fish weight increased but positively increased with feed ration. Excretion pattern of milkfish revealed two peaks: the first peak occurred 6 h after feeding and the second peak at 18 h for TAN and 21 h for PO₄‐P, coinciding with the start of the daylight hours. TAN and PO₄‐P excretion accounted for 20.5–34.6% of total N consumed and 18.7–42.6% of P consumed respectively. Approximately 27.9–42.5% of N consumed and 47.2–58.5% of P consumed were lost as faeces. Total nutrient losses were lower using the lab‐lab diet (0.31 g N and 0.14 g P kg^?1 fish) compared with the formulated diets (0.47–0.48 g N and 0.17–0.19 g P kg^?1 fish); the losses decreased per kg of fish as fish size increased. Results suggest that the diet and size of fish influence wastage of N and P to the environment with greater losses in small fish and when artificial diets are used. Such measurements will provide valuable information for the preparation of N and P budgets for milkfish in grow‐out systems.     

Influence of digestible protein levels on growth and feed utilization of juvenile meagre Argyrosomus regius

This study investigates the effect of digestible protein levels in experimental diets for meagre (Argyrosomus regius). A group of 253 fish, 52 g of mean weight, was distributed in 12 tanks, three replicates per treatment. Four isolipidic diets (170 g kg^?1 crude lipid) with different digestible protein levels (350 g kg^?1, 430 g kg^?1, 490 g kg^?1 and 530 g kg^?1) were formulated using commercial ingredients. The trial lasted 62 days. Meagre fed diets 430, 490 and 530 g kg^?1 obtained higher TGC (2.47, 2.57, 2.69 × 10^?3, respectively) than fish fed diet 350 g kg^?1 (2.14 × 10^?3). Group of fish fed diet with 350 g kg^?1 DP showed the lowest ammonia excretion level. According to the in vitro digestibility trial diets with 350 and 430 g kg^?1 DP released less amino acids in comparison with diet with 49% DP, although in vivo digestibility test did not show significant differences among diets 430, 490 and 530 g kg^?1 DP. Using the quadratic regression, optimal digestible protein intake according to the ECR for rearing juvenile meagre was recorded in 0.8 g DP/100 g fish and day.     

A preliminary study on tailoring of fillet iodine concentrations in adult Atlantic salmon (Salmo salar L.) through dietary supplementation

The present study was performed to assess to what degree supplemented dietary iodine (I) was retained in selected tissues, including the fillet of adult Atlantic salmon (Salmo salar) reared in sea water. Atlantic salmon weighing approximately 1.5 kg were randomly assigned to three net pens per treatment and fed moist pellets (based on minced saithe and herring) supplemented with 0, 40 or 80 mg iodine (as KI) kg^?1 on dry weight basis for 150 days. The iodine concentrations in the experimental feeds were analysed to be 10, 54 and 86 mg kg^?1 dry weight, respectively. Growth, mortality and blood haemoglobin concentration (Hb) were recorded. Iodine concentrations were measured in muscle, liver and kidney after 90 and 150 days of feeding by inductively coupled plasma‐mass spectrometry. In addition, plasma thyroxine (T₄) and triiodo‐thyronine (T₃) were determined. The weight gain during the period was approximately 1 kg for all treatments. There were no mortalities, and blood Hb levels were within normal ranges. The iodine concentration in muscle, liver and kidney were all affected by the dietary iodine level, despite wide intratreatment variation. After 150 days, fillets of fish fed 10, 54 and 86 mg I kg^?1 showed mean concentrations of 0.4, 0.5 and 0.9 mg I kg^?1 wet weight, respectively, whereas the iodine concentration in the liver and the kidney increased approximately three times in the dietary groups. Similarly, plasma T₄ and T₃ showed great variation within the treatments. No significant correlations were found between individual tissue iodine concentration and thyroid hormone concentration in any of the groups at any sampling time. This preliminary feeding experiment showed that fillet iodine in adult Atlantic salmon can be increased up to 1.4 mg I kg^?1 wet weight by dietary iodine 80 times the minimum requirement for salmonids, without impacting health, performance or plasma thyroid hormone status.     

Evaluation of dietary yellow loess as an antibiotic replacer on growth,immune responses,serological characteristics and disease resistance in rainbow trout,Oncorhynchus mykiss

A 12 weeks of feeding trial was conducted to evaluate the effects of different levels of dietary yellow loess as an antibiotic (oxytetracycline) replacer in rainbow trout, Oncorhynchus mykiss. Five experimental diets were formulated to contain no antibiotics or yellow loess (control/CON), three graded levels of yellow loess 5 (YL₅), 10 (YL₁₀) and 20 g YL kg^?1 diet (YL₂₀) and oxytetracycline at 5 g OTC kg^?1 diet. Forty‐five fish averaging 39.4 ± 1.6 g (mean ± SD) were randomly distributed in to 15 aquaria. Triplicate groups of fish were fed one of the experimental diets at 1.5 ~ 1.9% of wet body weight per day. At the end of the feeding trial, average weight gain (WG) and specific growth rate (SGR) from fish fed CON diet were significantly lower than those from fish fed YL₁₀, YL₂₀ or OTC diets (P < 0.05). Lysozyme activity from fish fed YL₂₀ was detected to be significantly higher than that from fish fed CON diet (P < 0.05). While, superoxide dismutase (SOD) activity from fish fed YL₁₀ and YL₂₀ was recorded to be significantly higher than that from fish fed CON diet (P < 0.05). Fourteen days of challenge test with bacteria A. salmonicida showed significantly lower survival rate for CON than those of fish fed other experimental diets. Therefore, these results indicated that dietary yellow loess at 10–20 g kg^?1 could be a promising alternative of oxytetracycline in rainbow trout.     

Dietary ascorbic acid requirement of juvenile ayu (Plecoglossus altivelis)

To investigate dietary ascorbic acid (AA) requirement of juvenile ayu (Plecoglossus altivelis) weighing 1.27 ± 0.02 g, eight diets were formulated with graded levels (0, 20, 40, 80, 160, 320, 640 and 1280 mg AA kg^?1) of AA supplied as ascorbyl polyphosphate. Each experimental diet was fed to four‐replicate groups to apparent satiation three times a day for 8 weeks. At the end of the feeding trial, fish fed AA‐deficient diet showed visible AA deficiency signs and low survival. Based on the four‐parameter saturation kinetics model, the calculated AA requirement levels for each dose‐dependent response [weight gain, hepatic AA concentration, hydroxyproline (HyPro) concentration in skin and HyPro concentration in backbone] were 116, 226, 47 and 35 mg kg^?1, respectively. Based on the maximal growth performance, a level of 116 mg AA kg^?1 was recommended for commercial diet of juvenile ayu. To maintain tissue HyPro saturation and avoid AA deficiency symptoms, the minimum required dietary AA level was 47 mg kg^?1. Hepatic AA saturation was considered as the most stringent criterion for determination of AA requirement.     

Effect of high dietary intakes of vitamin E and n-3 HUFA on immune responses and resistance to Edwardsiella tarda challenge in Japanese flounder (Paralichthys olivaceus, Temminck and Schlegel)

A feeding experiment was conducted to investigate the effects of high dietary intake of vitamin E (supplied as dl ‐α‐tocopheryl acetate) and n‐3 highly unsaturated fatty acid (n‐3 HUFA) on the non‐specific immune response and disease resistance in Japanese flounder Paralichthys olivaceus. Nine practical diets were formulated to contain one of three levels of vitamin E namely, 0, 80 or 200 mg kg^?1 (the total α‐tocopherol contents in the diets were 21, 97 and 213 mg kg^?1 based on analysis), and at each vitamin E level with one of three n‐3 HUFA levels i.e. 0.5%, 1.5% or 2.0%. Each diet was randomly assigned to triplicate groups of Japanese flounder (initial body weight: 40.5±1.0 g, mean±SD) in a re‐circulation rearing system. Fish were fed twice daily to apparent satiation at 07:00 and 18:00 hours for 12 weeks. During the experimental period, water temperature was maintained at 18±1°C, salinity 31–35 g L^?1, and pH 7.8–8.2. Dissolved oxygen was not less than 6 mg L^?1, and there were negligible levels of free ammonia and nitrite. The results showed that the increase in dietary n‐3 HUFA from 0.5% to 1.0% significantly decreased muscle α‐tocopherol contents in fish‐fed diets with 21 and 97 mg α‐tocopherol kg^?1 diet (P<0.05). In 1.0% HUFA groups, alternative complement pathway activity (ACH₅₀) of fish fed the diet containing the 213 mg α‐tocopherol kg^?1 diet was significantly higher than noted for fish fed the diet containing 97 mg α‐tocopherol kg^?1 diet (P<0.05). Fish fed the diet with 213 mg α‐tocopherol kg^?1 and 2.0% n‐3 HUFA had the highest lysozyme activity (131.7 U mL^?1) among all the dietary treatments. Fish fed the diets containing 97 and 213 mg α‐tocopherol kg^?1 with 1.0% n‐3 HUFA had significantly higher respiratory burst activity than those fed the diets containing 21 mg α‐tocopherol kg^?1 with 0.5 and 1.0% n‐3 HUFA (P<0.05). In the disease resistance experiment, high intake of dietary vitamin E with 213 mg α‐tocopherol kg^?1 significantly decreased cumulative mortality and delayed the days to first mortality after a 7‐day Edwardsiella tarda challenge (P<0.05). In addition, under the experimental conditions, dietary vitamin E and n‐3 HUFA had a synergistic effect on the non‐specific immune responses and disease resistance in Japanese flounder (P<0.05).     

Estimation of dietary biotin requirement of Japanese seabass,Lateolabrax japonicus C.

A 9‐week feeding experiment was conducted to determine the dietary biotin requirement of Japanese seabass, Lateolabrax japonicus C. Six isonitrogenous and isoenergetic purified diets (Diets 1–6) containing 0, 0.01, 0.049, 0.247, 1.238 and 6.222 mg biotin kg^?1 diet were fed twice daily to triplicate groups (30 fish per group) of fish (initial average weight 2.26 ± 0.03 g) in 18 fibreglass tanks (300 L) filled with 250 L of water in a flow‐through system. Water flow rate through each tank was 2 L min^?1. Water temperature ranged from 25.0 to 28.0 °C, salinity from 28.0 to 29.5 g L^?1, pH from 8.0 to 8.1 and dissolved oxygen content was approximately 7 mg L^?1 during the experiment. After the feeding experiment, fish fed Diet 1 developed severe biotin deficiency syndromes characterized by anorexia, poor growth, dark skin colour, atrophy and high mortality. Significant lower survival (73.3%) was observed in the treatment of deficient biotin. The final weight and weight gain of fish significantly increased with increasing dietary biotin up to 0.049 mg kg^?1 diet (P < 0.05), and then slightly decreased. Both feed efficiency ratio and protein efficiency ratio showed a very similar change pattern to that of weight gain. Dietary treatments did not significantly affect carcass crude protein, crude lipid, moisture and ash content. However, liver biotin concentration (0–6.1 μg g^?1) significantly increased with the supplementation of dietary biotin (P < 0.05), and no tissue saturation was found within the supplementation scope of biotin. Broken‐line regression analysis of weight gain showed that juvenile Japanese seabass require a minimum of 0.046 mg kg^?1 biotin for maximal growth.     

Effect of supplemental dietary zinc sources on the growth and carbohydrate utilization of tilapia Smith 1840, Oreochromis niloticus × Oreochromis aureus

An 8‐week feeding trial was conducted to investigate the effect of supplemental dietary zinc sources on the growth performance and carbohydrate utilization of juvenile tilapia Smith 1840, Oreochromis niloticus × O. aureus. The goal was to compare the bioavailability of two zinc sources, zinc sulphate (ZnSO₄) or zinc methionine (ZnMet), by using two practical basal diets with 350 g kg^?1 (C350) or 400 g kg^?1 (C400) carbohydrates based on wheat as the carbohydrate source. The results showed that fish fed with a diet supplemented with 60 mg kg^?1 Zn from either ZnSO₄ or ZnMet had a significantly (P < 0.05) greater specific growth rate and protein efficiency ratio than those fed with the diets of ≤30 mg kg^?1 Zn. The composition of tilapia carcass was also found to be influenced by various levels of dietary zinc from the two zinc sources. The G6P‐DH in fish fed with the 20 mg kg^?1 ZnMet diet and the PK levels in fish fed with 20 mg kg^?1 ZnSO₄ and 30 mg kg^?1 ZnMet diet were significantly (P < 0.05) higher than those in fish fed with the C400 diet. The data suggest that supplemental dietary zinc from either ZnMet or ZnSO₄ significantly affects the growth performance and carbohydrate utilization of tilapia.     

Dietary levamisole modulates the immune response and disease resistance of Asian catfish Clarias batrachus (Linnaeus)

In order to determine the immunomodulatory effect of dietary levamisole in Asian catfish (Clarias batrachus), fish were fed four different diets for 10 days: a formulated diet as control and the same diet supplemented with 50, 150 or 450 mg levamisole kg^?1 feed. The serum bacterial agglutination titre against Aeromonas hydrophila as a measure of specific immunity, serum haemagglutination titre, natural haemolytic complement activity (ACH₅₀), myeloperoxidase and lysozyme activities, total protein level and oxidative radical production by neutrophils as a measure of non‐specific immunity as well as disease resistance against A. hydrophila challenge to separate vaccinated and non‐vaccinated groups were evaluated at 0, 1, 2 and 3 weeks after last administration of levamisole. Levamisole supplement at the lowest level (50 mg kg^?1) significantly enhanced oxidative radical production and serum myeloperoxidase (MPO) content immediately after 10 days of feeding, which reached peak values after 3 and 2 weeks of feeding respectively. Haemolytic complement and haemagglutination titre were significantly enhanced after 3 and 1 weeks respectively. Haemolytic complement activity and MPO activities were significantly raised to 150 mg kg^?1 after 3 and 2 weeks, respectively. At the highest level of levamisole feeding (450 mg kg^?1) significant decreases in superoxide production and complement activity were measured immediately after levamisole feeding, which returned to the normal level after 1 week post‐ feeding. Fish were challenged with a virulent strain of A. hydrophila at 0, 1, 2 and 3 weeks after levamisole feeding, and the cumulative per cent survival was recorded over 10 days. Feeding levamisole at 50, 150 or 450 mg kg^?1 increased per cent survival in vaccinated fish immediately after levamisole feeding, and survival was significantly higher at 450 mg kg^?1. There was no difference in mortality patterns in non‐vaccinated fish. The results support the use of levamisole at 50 mg kg^?1 feed for 10 days as an immunostimulant in Asian catfish farming.     

Approaches to optimizing dietary protein to energy ratio for African catfish Clarias gariepinus (Burchell, 1822)

An 8‐week feeding trial was conducted in a recycling water system at 28 ± 1 °C to investigate protein to energy ratio (P/E ratio) in African catfish Clarias gariepinus (10.9 ± 0.04 g). Six fishmeal‐based diets of two protein levels (330 and 430 g kg^?1), each with three lipid levels (40, 80 and 120 g kg^?1) resulted in P/E ratios ranging from 15.5 to 21.3 mg protein kJ^?1 gross energy (GE) were fed to 20 fish (per 30‐L tank) in triplicate. Fish were fed 50 g kg^?1 of their body weight per day adjusted fortnightly. Significantly higher (P < 0.05) growth rates and feed conversion efficiency were evident in fish fed with higher protein diet. The highest growth rate was found by fish fed 430 g kg^?1 protein, 21.2 kJ^?1 GE with a P/E ratio of 20.5 mg protein kJ^?1 GE. Significantly indifferent (P > 0.05) values of protein utilization were found in‐between the both (higher and lower) protein diets. Higher lipid deposition (P < 0.05) in whole body and liver was observed with increasing dietary lipid level at each protein diet and as higher (P < 0.05) for the lower protein diets. Liver glycogen tended to decrease with increasing gross energy at each protein diet and higher protein diet showed comparatively lower values (P > 0.05). Digestive enzyme activities (protease and lipase) and histological examination of intestine and liver of fish fed varying P/E diets found no significant differences in response to experimental diets. The study reveals that African catfish C. gariepinus performed best the diet containing 430 g kg^?1, 21.2 kJ g^?1 and 20.5 mg protein kJ g^?1 GE protein, gross energy and P/E ratio, respectively.     

Effect of different dietary levels of commercial wood charcoal on growth,body composition and environmental loading of red tilapia hybrid

A 60‐day feeding study was performed to evaluate the role of dietary commercial wood charcoal (CWC) in fish growth performance, body composition and water quality of fingerlings red tilapias (Oreochromis mossambicus × Oreochromis niloticus). Four levels of charcoal (10, 20, 30 and 40 g kg^?1) were added to the control diet (0 g kg^?1 CWC) and fed to red tilapias (initial weight of 1.20 ± 0.04 g). At the end of the feeding trial, the fish weight gain, specific growth rate, feed conversion ratio, protein efficiency ratio and energy retention of the fish groups fed 30 and 40 g kg^?1 (CWC) in diet were significantly (P < 0.05) higher comparing with all other tested fish groups. Moreover, some proximate composition such as crude protein and nitrogen retention efficiency g kg^?1 of the fish groups fed on 30 and 40 g kg^?1 CWC diets showed higher values (P < 0.05) than those of other tested fish groups (0, 10 and 20 g kg^?1 CWC). Apparent digestibility coefficients of protein and dry matter were improved (P < 0.05) in CWC‐tested fish groups compared to the control fish group. These data were powered by the data of the water quality that showed significant (P < 0.05) enhancement in both dissolved oxygen and ammonia concentrations by the increment in dietary charcoal levels. The above‐mentioned parameters' data suggested that 30 g kg^?1 dietary CWC can be considered as a suitable level to maintain normal growth of red tilapia juveniles as well as to enhance water quality of the rearing area.