The use of Kluyveromyces marxianus in the diet of Red‐Stirling tilapia (Oreochromis niloticus,Linnaeus) exposed to natural climatic variation: effects on growth performance,fatty acids,and protein deposition

The purpose of this study was to evaluate the use of single cell proteins (Kluyveromyces marxianus) as fishmeal substitutes for Oreochromis niloticus during long‐term feeding under natural climatic conditions. Juveniles (150) were divided into two groups: the control group, fed with conventional fish diet (T1); and the experimental group, fed with the alternative diet (T2). T1 animals had a higher final weight and per cent weight gain than T2 animals, which we mainly observed in winter and autumn. The lipid content ranged from 1.9% to 3.8% in liver and from 0.2% to 1% in white muscle for both groups. T2 animals had a higher protein concentration in the fillet and the lipid content in this tissue was not altered by diet. Liver and plasma protein content in both groups and the muscle content in T1 decreased during growth. Specific fatty acids (FAs) were selectively retained in fillet phospholipids (PLs), primarily as C22:6n3, which was found at high levels in the PLs of both groups during all seasons. Despite the differences observed, the percentage of polyunsaturated FAs in the fillet was similar throughout the experiment for both diets, reaching approximately 50% of the total FA in the PLs.     

Comparison of growth, fillet yield and proximate composition between Stirling Nile tilapia (wild type) (Oreochromis niloticus, Linnaeus) and red hybrid tilapia (Florida red tilapia×Stirling red O. niloticus) males

A comparison was made of some productive traits of Stirling Nile tilapia (wild type) (Oreochromis niloticus, Linnaeus) and red hybrid tilapia (Florida red tilapia× Stirling red O. niloticus) males during a 98‐day grow‐out period. Twenty‐two males from each genetic group with initial weights of 139.0 g for O. niloticus and 207.3 g for the red hybrid were placed in triplicate tanks. The fish were fed with a feed containing 36.8% crude protein. Survival was 97.0% for the red hybrid and 83.3% for O. niloticus. Daily individual weight gains were 2.95 and 2.50 g and final body weights were 473.0 and 348.8 g for the red hybrid and O. niloticus respectively. Fillet yield was similar for both the species, with 33.4% for the red hybrid and 32.0% for O. niloticus. Fresh fillet lipid content was perceptibly less in the red hybrid (0.33%) than in O. niloticus (2.07%). Some benefits of a red low‐fat tilapia genotype are discussed.     

Evaluation of growth and fatty acid profile of Nile tilapia (Oreochromis niloticus) fed with Schizochytrium sp.

The objective of this study was to evaluate growth, body chemical composition and lipid profile of Nile tilapia juvenile fed with Schizochytrium sp. Two hundred and forty Nile tilapia (Oreochromis niloticus) juvenile (1.33 ± 0.11 g) were distributed in 20 aquariums, at the density of 12 fish per aquarium. The juvenile were fed with five levels of Schizochytrium sp.: Control (0 g/kg of Schizochytrium sp. in feed); AS10 (10 g/kg of Schizochytrium sp. in feed); AS20 (20 g/kg of Schizochytrium sp. in feed); AS30 (30 g/kg of Schizochytrium sp. in feed) and AS40 (40 g/kg of Schizochytrium sp. in feed). The inclusion of Schizochytrium sp. increase the body weight, weight gain, final biomass and biomass gain of tilapia juvenile. The body crude protein of tilapia was increased after addition of microalgae. Juvenile lipid profile also was influenced when fed with 40 g of Schizochytrium sp. per kg and have it levels of omega‐3 and docosahexaenoic acid (DHA) increased, and omega 6:3 ratio decreased. Tilapia juvenile fed with Schizochytrium sp. per kg have better growth, omega‐3 and DHA levels rising in the body, being a considerable source of fatty acids for human nutrition. Levels above 20 g of Schizochytrium sp. per kg in the feed favour the increase of protein in Nile tilapia body.     

Sex reversal in Nile tilapia (Oreochromis niloticus Linnaeus) by androgen immersion

Two experiments were conducted to investigate the efficacy of three androgens applied through immersion treatments on the sex ratio of Nile tilapia (Oreochromis niloticus L.) fry. In experiment 1, 14 days post‐hatching (DPH) larvae were exposed to a single immersion treatment in 17α‐methyltestosterone (MT), 17α‐methyldihydrotestosterone (MDHT) or 17α‐ethynyltestosterone (ET) at 200, 600 and 1800 μg L^?1 over 4 h (130 larvae per treatment). In experiment 2, Nile tilapia larvae were exposed to the higher androgen concentration (1800 μg L^?1) applied as either a single immersion (14 DPH) or double immersion (10 and 14 DPH) over 4 h (125 larvae per treatment). Change in sex proportion within each experiment as well as between experiments was analysed by the chi‐square test. In experiment 1, MT, MDHT and ET were equally effective in significantly increasing the proportion of males when applied at 1800 μg L^?1 (86.0%, 90.0% and 86.7% respectively). At 200 μg L^?1 none of the androgens altered sex ratio. At 600 μg L^?1, only MDHT slightly, but significantly skewed the sex ratio towards males (73.0%). In experiment 2, a single immersion treatment at 14 DPH (1800 μg L^?1) significantly increased the proportion of males, but at this time the response was significantly hormone dependent (MDHT, 100.0%; MT, 91.6%; ET, 76.9%). When compared with a single immersion, two‐immersion treatments significantly increased the proportion of males in the MT‐treated group (from 91.6% to 98.3%), decreased the proportion of males in the MDHT group (from 100.0% to 93.4%) and had no significant effect the ET‐treated group (change from 76.9% to 82.5%). The overall comparison of the sex ratio among same treatments from different experiments (a single immersion in 1800 μg L^?1) was not significantly different.     

Optimization of flaxseed oil feeding time length in adult Nile tilapia (Oreochromis niloticus) as a function of muscle omega‐3 fatty acids composition

This study evaluated the omega‐3 (n‐3) fatty acids and the proximate composition of muscle tissue of adult Nile tilapias to select the best feeding time length with a diet containing 70 (g kg^?1 wt) flaxseed oil (FO). The results showed that dietary complementation with FO for 45 days is suitable for obtaining high levels of protein (164 g kg^?1), total lipids (94 g kg^?1), and ash (18 g kg^?1). Furthermore, there was a significant difference (P < 0.05) in the reduction of n‐6 and an increase in the concentration of n‐3. With 45 days’ time of FO feeding, fish weight was 532 g and it was improved by the incorporation of total n‐3 (9.8%), consisting of alpha‐linolenic acid (LNA; 6.3%), and n‐3 very long‐chain polyunsaturated fatty acid (n‐3 VLC‐PUFA; 3.5%), and including docosahexaenoic acid (DHA; 1.2%). This gave a better n‐6/n‐3 ratio (1.1) of muscle tissue, a more desirable ratio than the present ratio sometimes as high as 1 : 20 in human diets. The concentrations of n‐3 VLC‐PUFA were higher than those of native Brazilian freshwater fish. Thus, 45 days is the shortest time period required for the inclusion of FO oil in tilapia feed to raise the nutritional value of adult Nile tilapia.     

Nutrient composition and sensory evaluation of fillets from wild-type Nile tilapia (Oreochromis niloticus, Linnaeus) and a red hybrid (Florida red tilapia × red O. niloticus)

A comparison of composition and sensory evaluation were performed on fillets of Oreochromis niloticus (wild type) and red hybrid tilapia (RHT) (Florida red tilapia ♂× red O. niloticus♀). Fifty male tilapia, 25 O. niloticus (initial weight 159.3 g) and 25 red hybrid (initial weight 132.4 g), were placed in each of three 2.0 m³ volume tanks. The fish were fed a commercial feed containing 35.9% crude protein during the 120‐day experimental period and then killed in cold water (3°C). All the fish were filleted. Twelve fillets from each treatment were used for proximate analyses, five fillets from each treatment were used for fatty acids analyses and the remaining fillets were used for sensory evaluation. The compositional analyses showed similar moisture, true protein and ash content in both genetic groups, but a lower crude fat content was measured in the red hybrid fillets (0.70%) compared with that of O. niloticus fillets (0.97%). No differences between O. niloticus and the red hybrid were observed in their fillet profile of fatty acids. In the sensory evaluation test, a difference in fillet flavour between both genetic groups was perceived. Of the 112 consumers in the preference test, 81.2% perceived a difference in fillet flavour between the two tilapias, with a general preference for the red hybrid over the wild‐type O. niloticus. The benefits of cultivating a RHT with a low lipid content are discussed.     

Dietary valine requirement of juvenile Nile tilapia,Oreochromis niloticus

An 8‐week feeding trial was conducted to quantify the dietary valine requirement of cultured juvenile Nile tilapia, Oreochromis niloticus. Six isonitrogenous (280 g/kg crude protein) and isoenergetic (16.06 MJ/kg gross energy) diets with graded levels of valine (amounting to 4.1, 7.2, 9.9, 12.7, 15.6 and 18.8 g/kg of dry diet) were formulated. Each diet was randomly assigned to triplicate groups of 20 fish (6.48 ± 0.06 g). Results showed that the weight gain, specific growth rate, protein efficiency ratio and protein retention efficiency all increased with an increasing level of dietary valine up to 12.7 g/kg, but remained relatively constant for fish fed higher levels of dietary valine. In addition, the total protein concentration and aspirate aminotransferase activity in plasma, hepatic lysozyme and catalase activities were all significantly (p < .05) improved by dietary valine supplementation. Based on the broken‐line regression analysis of weight gain and protein retention efficiency, the optimal dietary valine requirement for juvenile Nile tilapia occurred between a level of 11.5 g/kg of diet (equivalent to 41.1 g/kg of dietary protein) and 12.7 g/kg of diet (equivalent to 45.3 g/kg of dietary protein).     

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.     

Growth performance and feed utilization of Nile tilapia Oreochromis niloticus (Linnaeus, 1758) and tilapia galilae Sarotherodon galilaeus (Linnaeus, 1758) fingerlings fed plant protein-based diets

This study was designed to determine the effect of complete substitution of fish meal (FM) by three plant protein sources including extruded soybean meal (SBM), extruded full‐fat soybean (FFSB) and corn gluten meal (CGM) on growth and feed utilization of Nile tilapia Oreochromis niloticus and tilapia galilae Sarothrodon galilaeus. Four isonitrogenous of crude protein (ca. 28.0%) and isocaloric (ca. 19 MJ kg⁻¹) experimental diets were formulated. The control diet (diet 1) was prepared with FM as the main protein sources. Diets 2–4, each FM control diet, were completely substituted with SBM (diet 2), FFSB (diet 3) and CGM (diet 4). l ‐lysine and dl ‐methionine were added to plant protein diets to cover the nutritional requirements of tilapia. Each treatment was allocated to three net pens and fed for 17 weeks. Nile tilapia fed the control diet showed significantly higher (P≤0.05) values for final body weight (FBW), feed intake (FI), weight gain (WG) and specific growth rate (SGR), whereas fish fed the diet with CGM achieved the lowest values. Tilapia galilae fed SBM diet recorded the highest (P≤0.05) values for growth performance. Better feed conversion ratio (FCR) for both Oreochromis niloticus and Sarothrodon galilaeus was observed when fish were fed SBM diet, whereas the worse FCR was recorded for FFSB diet. Feed utilization parameters including protein productive value (PPV), fat retention (FR) and energy retention (ER) showed significant differences (P≤0.05) for both the species fed different dietary protein sources. The present results suggest that, for Nile tilapia, both SBM and FFSB supplemented with dl ‐methionine and l ‐lysine can completely replace dietary FM. Meanwhile, S. galilaeus fed SBM diet exhibited comparable growth and feed utilization with those fish fed a fish‐meal‐based diet.     

Immuomodulatory effect of dietary turmeric supplementation on Nile tilapia (Oreochromis niloticus)

This study aimed to assess the immunomodulatory effect of dietary turmeric (TUM) in Oreochromis niloticus juveniles. A total of 240 Oreochromis niloticus fingerlings were divided into four equal groups (n = 60), three replicates (n = 20) for each: control and TUM‐treated groups at 2 (T1), 4 (T2) and 8 (T3) g kg^?1 diets for 8 weeks. Body weight gain, leucocytes counts, plasma levels of interleukin 2 (IL‐2), interleukin 4 (IL‐4), lymphocyte proliferation index to pokeweed mitogen (PWM), nitric oxide (NO) and lysozyme enzyme activities were measured. Histopathology of spleen was performed. TUM at 2 g kg^?1 significantly (p < .05) improved weight gain, leucocytes, NO and lysozyme activity than control and other treatment groups. IL‐2 level was significantly lower (p < .05) in T2 and T3 than in control and T1, while IL‐4 and lymphocyte proliferation index were significantly (p < .05) elevated in T3 than control, T1 and T2. Spleens of T2 showed improvement in white pulp, while spleen of T3 and T4 revealed white pulp depletion. In conclusion, TUM supplementation exerted immunomodulatory effect in Oreochromis niloticus through manipulation of lymphocyte count, IL‐2, IL‐4 and antibacterial enzymatic activity (NO and lysozyme) that resulted in higher weight gain. Increasing TUM supplementation of fish diets beyond 2 g kg^?1 was not beneficial.     

Optimum selenium requirement of juvenile Nile tilapia,Oreochromis niloticus

A feeding experiment was conducted to determine the optimum selenium requirement in juvenile Nile tilapia. Each of six purified diets with Se‐methionine levels at 0.05, 0.21, 0.41, 0.57, 0.79 and 1.00 mg/kg was assayed in triplicate with initial body weight of 3.00 ± 0.01 g for 8 weeks. The growth of fish was obviously increased when the dietary Se was less than 0.57 mg/kg diet and reached a plateau when the dietary Se was ≥0.57 mg/kg. Serum and hepatopancreatic glutathione peroxidase (GPx) activity increased markedly when the dietary Se was less than 0.57 mg/kg, but then decreased when the dietary Se was higher than 0.57 mg/kg. The malondialdehyde contents in hepatopancreas were significantly decreased when the dietary Se was higher than 0.79 mg/kg. No significant differences were observed in hepatopancreatic total antioxidant capability (T‐AOC) among the groups (p > .05). The results of this study indicated that Se addition as Se‐methionine was essential, while both the deficiency and excess levels of dietary Se would cause negative effects on growth or antioxidant capability in juvenile Nile tilapia. Based on broken‐line regression of WG and piecewise regression of liver GPx, the optimum requirement of Se for juvenile Nile tilapia is 0.57 mg/kg diet.     

Effect of replacing soybean meal with yeast‐fermented canola meal on growth and nutrient retention of Nile tilapia,Oreochromis niloticus (Linnaeus 1758)

Yeast fermentation integrated with water soaking was applied as a method to reduce the levels of some anti‐nutritional factors in canola meal. The procedure completely eliminated glucosinolates and it reduced phytic acid content by 18%. It also led to increase of 9% of crude protein and 8–32% of some minerals in the meal. A 60‐day feeding trial was conducted to evaluate replacement of dietary soybean meal protein with yeast‐fermented canola meal for Nile tilapia at ratios of 0, 25, 50, 75 and 100% respectively. Three groups of fish with an initial weight of 10 g were fed with each diet twice a day to an apparent satiation. The results showed that there were non‐significant differences (P > 0.05) in growth, survival rate, feed intake and feed conversion ratio for all fish fed with the test diets. However, replacement with 75 and 100% levels significantly reduced (P < 0.05) protein efficiency ratio and nutrient digestibility of protein, lipid, ash, Ca, Mg and P of fish compared with the lower levels. The low retention of protein, lipid, ash, Ca, Mg and P were also observed. These effects seemed to be related to an increased dietary phytic acid.     

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.     

Antimicrobial susceptibility and enrofloxacin resistance of streptococcal bacteria from farmed Nile tilapia,Oreochromis niloticus (Linnaeus 1758) in Thailand

This study examined the antimicrobial susceptibility and mutation(s) in quinolone resistance‐determining regions (QRDRs) in streptococcal pathogens isolated from farmed Nile tilapia Oreochromis niloticus in Thailand. Surveillance of antimicrobial susceptibility in tilapia streptococcal pathogens reveals that Streptococcus agalactiae (n = 97) and Streptococcus iniae (n = 3) from diseased tilapia were susceptible to amoxicillin, florfenicol, sulfamethoxazole/trimethoprim and sulfadimethoxine/ormetoprim, however, only 78 isolates were susceptible to enrofloxacin. Twenty‐two enrofloxacin‐resistant S. agalactiae isolates were further examined for mutations in the QRDRs of gyrA, gyrB, parC and parE genes. Twenty isolates had single base pair changed in the gyrA sequence, C‐242‐T. Point mutations in gyrB, GC‐1135, 1136‐AA and T‐1466‐G, were identified in one isolate. All resistant isolates harboured a mutation in the parC gene, C‐236‐A, while no mutations were observed in the parE gene. The study represented mutations of gyrA and parC genes as marked modification of the enrofloxacin‐resistant S. agalactiae from farmed tilapia. This study is a primary report of the QRDRs mutations associated with fluoroquinolone resistance from streptococcal pathogen in the cultured fish. The phenotypic and genotypic characterization of enrofloxacin resistance S. agalactiae evident in this study has led to an improved regulation of antimicrobial use in Thai aquaculture.     

罗非鱼不同组织脂肪酸含量的分析

分别取罗非鱼(Oreochromis niloticus)的肉、脂肪、肝、眼和皮等组织,用氯仿-甲醇(2∶1,v/v)提取,以C17∶0为内标物,利用气相色谱-质谱法(GC/MS)分析了各组织脂肪酸种类及含量。结果显示:罗非鱼各种组织中含有16种脂肪酸,富含多不饱和脂肪酸。脂肪组织中的脂肪酸含量最高,其次是眼睛、肝脏、鱼肉和鱼皮;各组织中饱和脂肪酸(SFA)含量为202~8869 mg/kg,主要为棕榈酸(C16∶0)和硬脂酸(C18∶0);单不饱和脂肪酸(MUFA)含量为293~11219 mg/kg,主要为棕榈油酸(C16∶1)和油酸(C18∶1);多不饱和脂肪酸(PUFA)含量为362~8293 mg/kg,主要是亚油酸(C18∶2)、亚麻酸(C18∶3)和二十二碳六烯酸(DHA,C22∶6);n-6/n-3为1.20~2.46。     

Relative performance of two Nile tilapia ( Oreochromis niloticus Linnaeus) strains in Egypt: The Abbassa selection line and the Kafr El Sheikh commercial strain

The Abbassa selection line (developed by selective breeding) and the Kafr El Sheikh commercial strain (widely used in Egypt), both Oreochromis niloticus, were compared at two stocking densities (two and four fish m^?2). Harvest weight, length, depth, width and head length were recorded. The Abbassa line showed a superior harvest weight (28 per cent) over the Kafr El Sheikh strain. Males were heavier than females, but the between‐sex difference was greater in the commercial than in the Abbassa line (39 and 31 per cent respectively). Females in the Abbassa line grew almost as fast as males in the commercial line. Both strains grew faster at the lower density, and the percentage reduction in harvest weight at the higher density was about the same for both strains (27 per cent). The advantage of the Abbassa line was 28 per cent at both densities. Both strains had a similar survival rate (approximately 80 per cent) during the grow‐out period. We conclude that the Abbassa line is ready for release to the tilapia industry in Egypt. Further evidence is being sought in currently underway on‐farm evaluations. Measures should be taken to ensure the long‐term viability of the Abbassa line.     

Cage‐pond integration of African catfish (Clarias gariepinus) and Nile tilapia (Oreochromis niloticus) with carps

Cage‐pond integration system is a new model for enhancing productivity of pond aquaculture system. A field trial was conducted using African catfish (Clarias gariepinus) and Nile tilapia (Oreochromis niloticus) in cages and carps in earthen ponds. There were four treatments replicated five times: (1) carps in ponds without cage, (2) tilapia at 30 fish m^?3 in cage and carps in open pond, (3) catfish at 100 fish m^?3 in cage and carps in open pond, (4) tilapia and catfish at 30 and 100 fish m^?3, respectively, in separate cages and carps in open pond. The carps were stocked at 1 fish m^?2. The cage occupied about 3% of the pond area. The caged tilapia and catfish were fed and the control ponds were fertilized. Results showed that the combined extrapolated net yield was significantly higher (P < 0.05) in the catfish, tilapia and carps integration system (9.4 ± 1.6 t ha^?1 year^?1) than in the carp polyculture (3.3 ± 0.7 t ha^?1 year^?1). The net return from the tilapia and carps (6860 US$ ha^?1 year^?1) and catfish, tilapia and carps integration systems (6668 US$ ha^?1 year^?1) was significantly higher than in the carp polyculture (1709 US$ ha^?1 year^?1) (P < 0.05). This experiment demonstrated that the cage‐pond integration of African catfish and Nile tilapia with carps is the best technology to increase production; whereas integration of tilapia and carp for profitability.     

A comparative study on the apparent digestibility of selected feedstuffs in hybrid catfish (Clarias macrocephalus × Clarias gariepinus) and Nile tilapia (Oreochromis niloticus)

The apparent digestibility (AD) of dry matter (DM), organic matter (OM), crude protein (CP) and essential amino acids (EAA) in five selected feedstuffs was evaluated in hybrid catfish and Nile tilapia. The AD of DM, OM and CP in cassava leaf meal (CLM) was lower (P < 0.05) than in groundnut meal (GNM), soybean meal (SBM), sesame husk meal (SSH) and shrimp head meal (SHM). The AD of DM, OM and CP among GNM, SBM, SSH and SHM was not different (P > 0.05). The AD of most EAA was higher in SSH (P < 0.05) than in CLM, GNM, SBM and SHM. The AD of most EAA among CLM, GNM, SBM and SHM was not different (P > 0.05). The AD of DM and OM in CLM was higher (P < 0.05) in hybrid catfish than in Nile tilapia, while there was no difference in the AD of CP in CLM between fish species. The AD of DM, OM, CP and EAA among GNM, SBM, SSH and SHM did not differ between hybrid catfish and Nile tilapia. In conclusion, there were only small differences in the nutritional properties between the selected feedstuffs in both fish species. Most EAA in the selected feedstuffs were equally well utilized by hybrid catfish and Nile tilapia.     

Optimizing diets to decrease environmental impact of Nile tilapia (Oreochromis niloticus) production

Aquaculture is one of the most thriving animal production sectors, and Nile tilapia (Oreochromis niloticus) farming represents 8% of total finfish culture. However, the industry sustainability depends on the development of cost‐effective and environmental friendly feeds. This study aimed to reduce dietary protein levels in diets for juvenile tilapia and to minimize diet environmental impact while maximizing biological efficiency. A growth trial was performed using five isoenergetic plant protein‐based diets with decreasing levels of crude protein: 360, 340, 320, 300 and 280 g/kg diet (D360, D340, D320, D300 and D280, respectively). Dietary protein utilization was assessed by metabolic trials using a radiolabelled amino acid mixture. Tilapia in all treatments showed similar growth performance and feed intake. Feed conversion ratio was significantly higher in fish fed the D280 than the D360 diet, while no differences were found for other treatments. Protein retention was significantly higher in tilapia fed the D300 than the D360 diet. Amino acid catabolism increased in fish fed the D360 diet, though without significant differences in muscle amino acid retention. This study demonstrates that dietary protein levels can be reduced to 300 g/kg diet without hindering tilapia growth and feed conversion ratio, while reducing environmental nitrogen losses.