Effects of replacing fish meal with soya protein concentrate on growth,feed efficiency and digestibility in diets for snakehead,Channa striata

Soya bean meal‐based formulated feeds have recently become available for snakehead culture in Vietnam. This study was conducted to determine the appropriate replacement of fish meal (FM) protein by another soya product, soya protein concentrate (SPC), in snakehead (Channa striata) diets. Five iso‐nitrogenous (45% crude protein) and iso‐caloric (19 KJ g^?1) practical diets were formulated to replace 0% (control), 40%, 60%, 80% and 100% of protein FM by protein SPC (100% FM, 40% SPC, 60% SPC, 80% SPC and 100% SPC respectively). A digestibility experiment was also conducted with the same formulated diets with addition of 1% chromic oxide. Fish fed 100% FM and 40% SPC diets had significantly better growth and survival compared with other treatments. Feed intake, feed conversion ratio, protein efficiency ratio and net protein utilization, trypsin and chymotrypsin activities of experimental fish fed 100% FM and 40% SPC diets were significantly higher than those fed other diets. The apparent digestibility coefficient (ADC) of the diet and diet components, ADC_diet, ADC_protein and ADC_lipid, of fish fed diet 40% SPC and 100% FM treatment were significantly higher than those of other treatments. The cost/kg fish produced in diets 100% FM and 40% SPC was much lower compared with other treatments. Dietary inclusion levels of SPC in diet above 40% significantly affected fish survival, growth, digestibility and trypsin and chymotrypsin activities, although fish chemical composition was not greatly affected.     

Addition of the red macroalgae Turkish Towel Chondracanthus exasperates and taurine improves the performance of alternative plant‐based feeds for juvenile sablefish Anoplopoma fimbria

Turkish Towel Chondracanthus exasperates and taurine were added to alternative plant‐based feeds for juvenile sablefish Anoplopoma fimbria to evaluate the effect of these ingredients, alone or combined, on fish growth, feed intake and efficiency, whole body nutrient composition, and liver histomorphology. Fish growth was significantly increased with the addition of taurine, and to a lesser extent, Turkish Towel to the experimental feeds. Feed efficiency and protein retention were significantly improved with the addition of taurine, but were unaffected by Turkish Towel addition. As expected, whole body taurine content was significantly affected by taurine addition. Increases in whole body lipid were observed with both taurine and Turkish Towel addition, with a significant interaction between the two factors. Liver histomorphology was generally normal; however, histopathologic changes were observed in some fish at the end of the experiment. The occurrence of hepatocellular nuclear pleomorphism and clear cell foci was less among fish that had received the Turkish Towel feeds. The addition of taurine had no effect on the number of fish with a histopathologic change. Overall, results from this study reaffirm taurine supplementation is beneficial to sablefish receiving plant‐based feeds and indicate Turkish Towel may be a promising feed ingredient for this species.     

Soy protein concentrate as an alternative in replacement of fish meal in the feeds of hybrid grouper,brown‐marbled grouper (Epinephelus fuscoguttatus) × giant grouper (E. lanceolatus) juvenile

Hybrid grouper juveniles (body weight, 6.1 ± 0.7 g) (brown‐marbled grouper, Epinephelus fuscoguttatus × giant grouper, E. lanceolatus) were fed with six isoproteic (50% crude protein) and isolipidic (12% crude lipid) feeds containing different levels of soy protein concentrate (SPC) in replacement of fish meal (SPC at 20%, 30%, 40%, 50% and 60% protein) and control feed (SPC0) for 6 weeks. Hybrid grouper juveniles were cultured in 100‐L fibreglass tank equipped with flow‐through water system and fed twice a day to apparent satiation level. The highest and lowest growth was recorded in fish fed SPC20 and SPC60 respectively. However, growth of SPC20 was not significantly higher than those fed SPC0, SPC30, SPC40 and SPC50 (p > .05). A decreasing growth trend was observed with the increasing level of SPC from feed SPC40 to SPC60. A noticeable better feed utilization was also observed in fish fed SPC0, SPC20, SPC30, SPC40 and SPC50 compared to fish fed SPC60 (p < .05). The fish condition factor, hepatosomatic index, viscerosomatic index and whole body proximate content of the fish were not affected by the graded levels of SPC. However, the body lipid content was significantly lower in fish fed SPC40 to SPC60 (p < .05). The apparent digestibility coefficient (ADC) of protein and lipid was significantly higher in fish fed SPC0 and SPC20 compared to other dietary treatments (p < .05). Based on the regression analysis on specific growth rate, the study suggests that the hybrid grouper grow best at 21.4% and can utilize up to 50% inclusion level of SPC in protein without significantly affect their growth and its body condition.     

Performance and immunological responses of rainbow trout (Oncorhynchus mykiss) fed bioprocessed plant‐based proteins

Marine‐derived fish meal (FM) is a traditional component of commercial aquaculture feeds for many farmed fish species. Modern bioprocessing technologies have been developed to produce high‐protein plant‐based ingredients for aquafeeds to further reduce FM and other conventional animal protein sources. A 90‐day feeding trial using juvenile rainbow trout (Oncorhynchus mykiss) (27.9 ± 1.4 g; mean ± SE) was completed to evaluate growth, feed efficiency, general health and immunological responses to diets containing experimental plant‐based protein sources and reduced FM. Trout were fed one of four dietary treatments: experimental bioprocessed soy protein concentrate (BSPC), commercially available enzymatically hydrolysed soy protein concentrate (CSPC), experimental bioprocessed barley protein concentrate (BBPC) and a FM control (FMC). At trial termination, there were no significant differences in relative growth (RG) or specific growth rate (SGR) between fish fed either FMC or BSPC (p > .05). Fish fed either CSPC or BBPC resulted in significantly lower RG and SGR than the FMC (p < .05). Significant respiratory burst analysis differences were observed among treatments at day 90 (p < .01), for fish fed either FMC or CSPC diets. This assessment of bioprocessed plant‐based protein ingredients facilitates the characterization and incorporation of bioprocessed soy and barley protein as the industry continues to evaluate FM replacements in rainbow trout feeds.     

Fishmeal can be completely replaced by soy protein concentrate by increasing feeding frequency in Nile tilapia (Oreochromis niloticus GIFT strain) less than 2 g

An 8‐week growth trial was conducted to determine the effects of complete replacement of fishmeal protein by soy protein concentrate (SPC) on growth performance of Nile tilapia (Oreochromis niloticus GIFT strain) fry (initial body weight 1.6 ± 0.0 g). In control diet, 135 g kg^‐1 fishmeal was used, and in the other two diets, 100% of fishmeal was replaced by SPC supplemented with or without methionine hydroxy analogue (MHA) according to the content in FM diet. Fish of FM group were fed twice daily. Fish of SPC6 group were fed SPC diet six times daily. Fish of SPCM group were fed twice (SPCM2) or six times (SPCM6) daily. The results showed that complete replacement of fishmeal with SPC did not affect survival, condition factor, visceralsomatic index of Nile tilapia. Feed intake (FI) and feed conversion ratio (FCR) of fish in SPCM2 and SPC6 groups were higher than those in FM and SPCM6 groups. Specific growth rate (SGR) of fish in SPCM6 group was highest among four treatments. Productive protein value (PPV) of SPCM2 and SPC6 groups were significantly lower than that of FM group. Fishmeal could be completely replaced by SPC without negative effect on growth by MHA supplementation and increasing feeding frequency.     

Fisheries By-Catch and By-Product Meals as Protein Sources For Rainbow Trout Oncorhynchus mykiss

Abstract— Fisheries by-catch and by-product meals are portrayed as ingredients having a great potential as ingredients in aquaculture feeds. The present study was designed to evaluate the nutritional value of shrimp by-catch meal, shrimp processing waste meal, and two fish meals made from Pacific whiting (meal with and without solubles) for rainbow trout by determining apparent digestibilityof these ingredients and conducting a 12-wk feeding trial with juvenile fish (average initial weight 20 g/fish). Apparent digestibility coefficients (ADCs) for protein in diets containing by-catch and processing by-products were 76% for shrimp by-catch meal, 79% for shrimp processing waste meal, 88% for Pacific whiting meal without solubles, and 92% for Pacific whiting meal with solubles. ADCs for lipid were higher than 94% for all the diets. ADCs for energy were 57% for shrimp by-catch meal, 73% for shrimp processing waste meal, 70% for Pacific whiting meal without solubles, and 73% for Pacific whiting meal with solubles. Growth performance was significantly affected by dietary protein source. Fish fed the shrimp by-catch meal diet had weight gain and feed conversion ratios similar to that of fish fed the control diet with anchovy fish meal. Fish fed diets containing shrimp processing waste and Pacific whiting meal with solubles had significantly lower weight gain and higher feed conversion ratios than the control diet. Growth was significantly lower in fish fed the Pacific whiting meal diet compared to fish fed the anchovy fish meal. The lower growth of fish fed diets containing Pacific whiting meal appeared to be a result of lower feed intake, indicating perhaps a lower palatability of this ingredient. Additional research addressing processing methods, nutritional manipulations, and palatability enhancement is needed to improve potential of some fisheries byproduct meals as ingredients in the diets of rainbow trout.     

Adiposity and growth of post-smolt Atlantic salmon Salmo salar L.

The long‐term adipose homeostasis seen in mammals gives rise to a ‘lipostatic’ model in which signals produced in proportion to fat stores serve to regulate energy intake. An extension of this predicts an impact of these signals on growth; downregulation of feeding in animals with increased adiposity should result in reduced growth. This was tested by monitoring fat deposition and growth in Atlantic salmon Salmo salar L. provided with feeds that differed in fat content. Salmon parr (mass c. 20 g) were fed either high‐ (H: 34%) or low‐fat (L: 22%) feeds, based on either fish (F) or vegetable (V) oils for 6 months to create groups of fish that differed in adiposity (10–12% and 5–7% body fat) at parr–smolt transformation (mass c. 130 g). Fish fed the high‐fat feeds deposited more body fat, and this was confirmed by measurement of fat concentrations in the fillet, viscera and remaining carcass. The fish were then grown‐on in sea water (c. 35 g L^?1, 8 °C, 24L:0D) for 14 weeks while being fed either high‐ or low‐fat feed formulated with fish oil to give the following treatments: HF→ H, HF→ L, LF→ L, LF→ H, HV→ H, HV→ L, LV→ L, LV→ H. Although fish exposed to the various feed treatments did not differ markedly in growth rate (SGR range 1–1.14% day^?1) over the 14 weeks of rearing in sea water, the results were in general agreement with predictions from the ‘lipostatic’ model, i.e. fish with the greatest fat reserves after the parr–smolt transformation grew more slowly than fish that were ‘leaner’ at this time. This suggests that adiposity, or ‘fatness’, may exert a negative feedback on feeding in salmon, thereby having an influence upon growth.     

Studies on the utilization by rainbow trout (Salmo gairdneri Rich.) of feed mixtures containing soya bean meal and an addition of amino acids

This paper deals with the growth of fish and utilization of feed mixtures in which fish meal is partly substituted by powdered soya beans. In order to improve the quality of protein in the feeds, they were enriched with synthetic amino acids. The food coefficients of the feeds were calculated, as were the protein efficiency ratio and apparent net protein utilization, by chemical analysis.

The results of the experiment showed that an addition of amino acids improved the utilization of feeds. It was found that rainbow trout can utilize soya bean protein enriched with cystine (1%) and tryptophan (0.5%) almost as well as protein from the fish meal.

Chemical composition of the experimental fish is discussed, as well as the amino acid composition of fish muscles and selected physiological parameters: haematocrit, haemoglobin content in the fish blood, the hepasomato index, and the content of protein, amino-transferases and cholesterol in the fish blood serum.     

Effects of natural feeding stimulants and glutamic acid supplementation on the feed intake,growth performance and digestive enzyme activities of red sea bream (Pagrus major) fed fish meal‐free soy protein concentrate (SPC)‐based diet

Two experiments were conducted for red sea bream (Pagrus major). In experiment 1, the optimum level of glutamic acid and natural feeding stimulants to enhance feed intake were determined and found that glutamic acid level of 0.5% and fish meat hydrolysate (FMH) were effective. In experiment 2, fish were fed with soy protein concentrate (SPC)‐based diet with synthetic feeding stimulants (Basal diet), the Basal diet with FMH (FMH diet), the FMH diet with glutamic acid (FMHG diet) and with fish meal diet (FM diet) as a control until satiation for 8 weeks. Feed intake of FMHG‐fed fish was significantly higher than others (p < 0.05). Specific growth rate and the feed conversion ratio of FMHG were comparable to those of FM‐fed fish (p > 0.05). Relative visceral fat ratio and crude lipid content of any SPC‐based diet‐fed fish tended to be lower than those of FM diet‐fed fish. There were no significant differences in trypsin and lipase activities hepatopancreas among treatments. SPC can be utilized as a sole protein source in a diet for red sea bream. The lower growth performance in SPC‐based diet‐ fed fish was not due to poor digestive enzyme secretion but could be associated with lipid utilization disorder.     

Is there lipostatic regulation of feed intake in Atlantic salmon Salmo salar L.?

High‐energy feeds, with increased fat concentrations, are often used in salmon farming to improve feed:gain ratio. However, fish are thought to regulate ingestion to meet their energy and nutrient intake requirements. Further, feeds with excessive fat content will lead to increased adiposity, which is thought to exert a negative feedback on feed intake via lipostatic regulation mechanisms. A test of the lipostatic model of feed intake regulation was carried out on juvenile Atlantic salmon Salmo salar L. (c. 165 g) in which body fat content (5.6% and 9.4% body fat) had been manipulated by feeding feeds with different fat concentrations. Thereafter, the fish were offered the high‐ and low‐fat feeds (15.6% and 26.5% dietary fat content) simultaneously, and our hypothesis was that feed preference and intake would reflect the state of fat storage. Thus, we predicted that, when given a choice of feeds, the lean fish would eat more, and that the body fat status of the groups would converge over time. The results indicated a general preference for the leaner feed irrespective of adiposity level, but leaner fish consumed more feed, grew faster and deposited more body fat than their fatter counterparts. Over time, body compositions converged among treatments, and differences in feed intake ablated. These findings seem to provide supportive evidence for a lipostatic regulation of feed intake in fish.     

Surveillance of selected nutrients,additives and undesirables in commercial Norwegian fish feeds in the years 2000–2010

This article presents data on commercial, Norwegian fish feeds from 2000 to 2010, including elements, additives, some selected vitamins and a range of environmental contaminants. Iodine, selenium and vitamin D concentrations, all declined during the time period; simultaneously, a reduction in marine ingredients in feeds has occurred. Still, the feeds fulfilled known fish nutrient requirements. Arsenic (As) in the feed was fitted with a polynomial regression with peak concentrations in 2004, correlating with fishery landings of blue whiting (Micromesistus poutassou), a reduction species with high content of As. A polynomial regression was also significant for mercury, which peaked in 2005 and was correlated to fish meal (FM) inclusion in the feeds. Residues of the pesticide DDT and its metabolites, chlordane and toxaphene, as well as polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers and sum dioxins and dioxin‐like PCBs, all decreased during this time period. These positive changes in undesirable substances can likely be attributed to the increasing replacement of marine ingredients with plant ingredients, as well as the increased use of South American fish oils rather than North Atlantic ones. On the other hand, cadmium concentrations were twofold higher in South American FMs, and increased in feeds from 2000 to 2010.     

Feed discrimination and selection in self-fed European sea bass Dicentrarchus labrax L.

Abstract This study was conducted to evaluate feed discrimination and preference of European sea bass Dicentrarchus labrax L., taking into account the effects of feed location, previous feeding experience and light conditions. Fish (63 g) were held in tanks and exposed to a 12L:12D photoperiod and ambient temperature. The following feeding practices were applied in triplicate: three‐choice feeding (self‐ feeding with feeds formulated for carnivorous, herbivorous and omnivorous fish rotated on a weekly basis) and monofeeding (self‐feeding with one of these feeds). After 9 weeks (phase I), fish previously held in monofeeding, were subjected to three‐choice feeding for 2 weeks (phase II). In phase I, fish discriminated between feeds after their rotational displacements and preferred the feed formulated for carnivorous species. This also supported the best growth. Fish used in phase II preferred the same feed. There was no preference for any location within tanks. Feeding was generally diurnal, although in some tests with the feeds for herbivores and omnivores, diurnal and nocturnal feed demands were similar and fewer in number than with the feed formulated for carnivores.     

Compensatory Growth of Pond-reared Hybrid Striped Bass, Morone chrysops × Morone saxatilis, Fingerlings

Compensatory growth (CG) or “catch‐up growth” is a period of super‐accelerated growth following a period of suboptimal conditions (i.e., lack of prey availability or overwintering). Little is known about the CG response in pond‐raised fish and whether hybrid striped bass (HSB), Morone chrysops × Morone saxatilis, might exhibit the rapid growth states or improvements in other production characteristics that may accompany the response. To evaluate the potential for CG in HSB culture, a 16‐wk growth trial in twelve 0.1‐ha earthen ponds was conducted. Approximately 2850 fish (mean weight ± SD = 3.2 g ± 1.1) were stocked into ponds and subjected to one of four cyclic feeding regimens. Treatment regimens included a control (0 wk, fed twice daily to apparent satiation) and cycles of 1, 2, or 4 wk of feed deprivation, followed by 1, 2, or 4 wk of feeding to apparent satiation. Fish in the 4‐wk feeding regimen were offered feed twice during the feed‐deprivation period (once every other week). Growth, specific growth rate (SGR), hepatosomatic index (HSI), intraperitoneal fat ratio, and condition factor (CF) were measured every other week, while overall growth, feed efficiency (FE) (FE = [weight gain/feed fed] × 100), and survival were calculated at the trial termination. The effect of these feeding regimens on water quality was examined by monitoring pH, turbidity, total ammonia nitrogen, nitrite–nitrogen, nitrate–nitrogen, soluble reactive phosphorus, and chlorophyll a weekly; total nitrogen and phosphorous biweekly; and dissolved oxygen and temperature twice daily. Cyclic feeding elicited CG; fish subjected to the 2‐wk regimen had a significantly higher SGR than 0‐wk controls during all but the final refeeding period. FE was higher for all fish in the cyclic regimens, although only FE for fish in the 2‐wk regimen was statistically greater (40%) than the controls. HSI was the most responsive measure and significantly decreased in the 2‐ and 4‐wk treatments during feed‐deprivation period and overcompensated during the refeeding period. CF also varied with feeding cycle and proved to be an effective nonlethal measure of predicting a CG response. No statistical differences in water quality parameters were observed. These data suggest that CG can be effectively induced in pond‐raised HSB and that the increase in FE warrants further research for practical application. Future pond studies with fingerling HSB fish should be conducted with emphasis on feed‐deprivation periods of 2 wk and refeeding periods of at least twice that of the feed‐deprivation period.     

Supplementation of Protein Hydrolysates to a Low‐fishmeal Diet Improves Growth and Health Status of Juvenile Olive Flounder,Paralichthys olivaceus

An 11‐wk feeding trial was conducted to evaluate three different protein hydrolysates as feed ingredients in high‐plant‐protein diets for juvenile olive flounder. Five experimental diets were fed to juvenile olive flounder to examine the effect of three different protein hydrolysates on growth performance, innate immunity, and disease resistance against bacterial infection. A basal fishmeal (FM)‐based diet was regarded as a high‐FM diet (HFM) and a diet containing soy protein concentrate (SPC) as a substitute for 50% FM protein was considered as a low‐FM diet (LFM). Three other diets contained three different sources of protein hydrolysates, including shrimp, tilapia, and krill hydrolysates (designated as SH, TH, and KH), replacing 12% of FM protein. All diets were formulated to be isonitrogenous and isocaloric. Triplicate groups of fish (15.1 ± 0.1 g) were handfed one of the diets to apparent satiation twice daily for 11 wk and subsequently challenged against Edwardsiella tarda. Growth performance and feed utilization of fish fed hydrolysate‐supplemented diets were significantly improved compared to those of fish fed the LFM diet. Dietary inclusion of the protein hydrolysates significantly enhanced apparent digestibility of dry matter and protein of the diets. In the proximal intestine, histological alterations were observed in the fish fed the LFM diet. The fish fed the hydrolysate diets showed significantly longer mucosal fold and enterocytes and greater number of goblet cells compared to fish fed the LFM diet. Respiratory burst activity was significantly higher in fish fed the TH and KH diets than fish fed the LFM diet. Significantly higher immunoglobulin levels were found in fish fed SH and KH diets compared to those of fish fed the LFM diet. Dietary inclusion of the protein hydrolysates in SPC‐based diets exhibited the highest lysozyme activity. Significantly higher superoxide activity was observed in groups of fish fed the KH diet. Fish offered the protein hydrolysates were more resistant to bacterial infection caused by E. tarda. The results of this study suggest that the tested protein hydrolysates can be used as potential dietary supplements to improve growth performance and health status of juvenile olive flounder when they were fed a LFM diet.     

Replacement of fishmeal by soya protein concentrate in the diets of Totoaba macdonaldi (Gilbert, 1890) juveniles: effect on the growth performance,in vitro digestibility,digestive enzymes and the haematological and biochemistry parameters

The potential of soya protein concentrate (SPC) as an alternative protein source in diets for Totoaba macdonaldi juveniles was evaluated. Seven isonitrogenous and isolipidic diets were formulated containing 15–100% SPC (SPC15, SPC30, SPC45, SPC60, SPC75, SPC90 and SPC100) to replace fishmeal (FM‐protein), and a FM‐protein‐based diet without SPC was used as a reference diet (RD). Each diet was randomly assigned to triplicate groups of 20 totoaba (50 ± 1.0 g) and was fed twice daily to apparent satiation. After 60 days of experiment, effects on totoaba were evaluated. Growth performance in fish fed RD to SPC45 was similar. The maximum FM‐protein replacement for weight gain (g kg ABW^?1 day^?1) was estimated to be 34.17% using a broken‐line model. In vitro digestibility of fish fed RD, SPC15 and SPC30 was similar. Trypsin activity was higher in fish fed the RD (9.38 ± 0.52 mU × 10^?3 mg protein^?1. Chymotrypsin activity was similar in fish fed RD, SPC15 and SPC30. Activity of alkaline protease and phosphatase was similar in fish fed RD, SPC15 and SPC30. Red blood cells (RBC) were the highest in fish fed RD to SPC60. Fish fed the RD, SPC15 and SPC30 present the highest haematocrit (HT) and haemoglobin (HB) content. Diets SPC90 and SPC100 presented similar MCV and MCHC. Total protein (TP) profile overall decreased in fish fed SPC90 and SPC100, pointing to nutritional hypoproteinemia due to deficient digestion and absorption. These results indicate that SPC‐based diets could be used efficiently by totoaba with FM‐protein replacement of less than 45%.     

Replacing the substitute—Guar meal as an alternative for non‐genetically modified soybean meal in the nutrition of rainbow trout (Oncorhynchus mykiss,Walbaum, 1792)

Soybean meal (SBM) is widely used in aqua feed products. In the European Union, only non‐genetically modified (non‐GM) SBM is accepted. However, global production of non‐GM soybeans is declining and is limiting its utilization in aqua feeds. Therefore, non‐GM SBM alternatives are required. In this study, guar meal was evaluated as a substitute for non‐GM SBM in test feeds for rainbow trout (Oncorhynchus mykiss). Fish (initial weight of 57 ± 10.1 g) were stocked to 12 experimental tanks of a freshwater recirculation system with a stocking density of 6 kg/tank (500 L). Fish were organized in triplicate groups and received isonitrogenous and isocaloric experimental feeds designated as control and test feeds 1–3, where 0, 50, 100 and 150 g/kg feed of non‐GM SBM was replaced by guar meal. During the experimental test period of 56 days, fish were fed twice a day until apparent satiation. Compared to the control group, daily feed intake of test feeds 1–3 was significantly increased by guar meal inclusion. No significant differences in feed conversion ratio and specific growth rate were detected among all the treatments. Furthermore, guar meal inclusion did not affect protein efficiency ratio. However, protein productive value was significantly impaired when guar meal incorporation was above 100 g/kg feed. In accordance with the affected protein productive value, the crude protein content of the proximate whole fish body declined significantly. In contrast, condition factor, hepatosomatic index, slaughter yield and fillet yield provided no significant differences between the experimental groups. In conclusion, this study demonstrated that non‐GM SBM could be entirely replaced by guar meal without affecting the general performance of fish and suggests guar meal as an appropriate alternative for non‐GM SBM in the nutrition of rainbow trout.     

Effect of feeding level on feed losses and feed utilization of soya and fish meal diets in Colossoma macropomum (Cuvier)

Juvenile, 1-g Colossoma macropomum (Cuvier) were fed two different diets: one with fish meal and another with soya meal as the main dietary protein source. Both diets were provided at ad libitum feeding level, and at two restricted feeding levels of ～ 80% and 60% of the ad libitum level. The experiment was performed in 30 aquaria, each stocked with 12 fish. For each treatment (two diets X three feeding levels), there were five replicates. Fish were fed three times daily at 0900, 1300 and 1700 h. At sampling days (days 14, 29, 44) in each aquarium in one of these feedings, chromic-oxide-marked feed was used. Shortly after the last meal, fish were weighed and four fish were taken from each aquarium for determination of the chromic oxide content in their digestive tract. Fish fed the fish meal diet attained a higher weight but had a lower body protein content. At the ad libitum feeding level, feed intake of the fish meal diet was higher, but feed and protein utilization efficiency were lower than with the soya diet. However, statistical analysis of the data of both the ad libitum and the two restricted feeding levels revealed that the reduced voluntary feed intake of the soya diet was the cause of its better utilization with ad libitum feeding. There is no evidence that soya protein is more freely available than fish meal protein. The chromic oxide data showed that 15 min after feeding, all of the feed (99.8%) could be traced back in fish fed the lowest feeding level, while at the ad libitum feeding level the recovery was only 72%. These data confirmed the hypothesis that at high feeding levels, considerable amounts of feed remain uneaten. Feed losses were not significantly different between the two diets. The chromic oxide recovery data of the 0900 h and 1300 h feedings showed that the feed passage rate in the digestive tract was similar for the three different feeding levels. Due to the high feed losses at high feeding levels, the relation between feed ration and feed utilization is determined mainly by the percentage of feed losses at the different feed rations.     

Effects of supplementary feeds with different protein levels on growth and economic performances of Nile tilapia (Oreochromis niloticus) cultured in a rain-fed rice–fish ecosystem

This study investigated the effects of supplementary feeds with different crude protein (CP) levels on the growth and economic performances of sex-reversed Nile tilapia (Oreochromis niloticus) reared in a rain-fed rice–fish ecosystem for a period of 120 days. There were four treatments—namely, traditional feed (21% CP), formulated feed (26% CP), commercial feed (30% CP), and control (no feed was used)—with three replicates per treatment. Fish (30.04 g) were stocked in ditches at a rate of 0.5 fish m^?2 considering the total area and fed one of the tested feeds twice daily at the rate of 2%–5% of body weight. Water-quality parameters of the ditches were monitored fortnightly, and these parameters were found suitable for fish farming. Organic matter, total nitrogen, and available phosphorous in soil were increased significantly (P < 0.05) in the feed treatments after fish cultivation. Highly significant weight gain and SGR were observed with fish fed the commercial feed as compared with other treatments. The highest benefit-cost ratio (2.15) was found in the control, followed by formulated feed (1.90), but in the case of numerical net return, it was the opposite. However, commercial feed showed the lowest benefit-cost ratio (1.67) due to higher feed cost than other supplementary feeds. Therefore, this study suggests that fish fed with a formulated feed (26% CP) produced significantly higher net economic return in the rice–fish farming system.     

Evaluation of full-fat and defatted maggot meals in the feeding of clariid catfish Clarias gariepinus fingerlings

The nutritional implication of processed maggot meals, hydrolysed, defatted, full‐fat, sun‐dried and oven‐dried, in the diets of Clarias gariepinus fingerlings were evaluated. The nutrient density, proximate composition, gross energy and mineral constituents of the maggot meals were determined. Five isonitrogenous and isocaloric diets were formulated to provide 40% crude protein in which fish meal in the reference diet was completely replaced by any of the following processed maggot meals: oven‐dried full‐fat (ODM), sun‐dried full‐fat (SDM), defatted oven‐dried (DODM) and defatted sun‐dried (DSDM) maggot meals respectively. Fish were fed ad libitum in triplicate treatments for 56 days in glass tanks. Biological evaluations of the fish were based on growth performance and nutrient utilization indices. The result of the nutrient composition analysis showed that processing methods of defattening and drying influenced the nutrient density of maggot meal, with crude protein content ranging between 43.30% and 46.70% for full‐fat sun‐dried and hydrolysed/defatted oven‐dried maggot meals respectively. The result showed that growth performance and nutrient utilization of fish fed DODM and DSDM dietary inclusions were not significantly different and similar to fish fed the fish meal‐based diet. However, fish fed the diet containing SDM exhibited lower daily weight gain, protein efficiency ratio and specific growth. Generally, fish performed better when fed diets containing defatted maggot meals than full‐fat maggot meal, and compared favourably with fish fed the fish meal‐based diet.     

Partial replacement of soybean meal with Methylobacterium extorquens single‐cell protein in feeds for rainbow trout (Oncorhynchus mykiss Walbaum)

A feeding trial was conducted with juvenile rainbow trout (15–16 g initial weight) to assess the effects of including single‐cell protein (SCP) produced from Methylobacterium extorquens in trout feeds. Three isonitrogenous and isoenergetic diets were produced: a control diet and two experimental diets containing 5% or 10% bacterial protein meal replacing soybean meal. Triplicate tanks, each containing 35 fish, were fed each diet to apparent satiation in a constant‐temperature (15°C), flow‐through tank system for 12 weeks. No statistically significant differences in final fish weight or other fish growth parameters were observed. Similarly, feed efficiency parameters showed no significant differences among groups. Nutrient retention indices (protein, fat, energy) were relatively high and similar among fish in each dietary treatment group, as were whole body proximate compositions. Fish survival was high, with a small but statistically significant increase for the 10% SCP diet. Overall, results demonstrate that SCP from M. extorquens is a safe and effective alternative protein for rainbow trout diets at the low inclusion levels tested. Slightly lower weight gain in fish fed the 10% SCP diet was largely due to lower feed intake, suggesting that adding palatability‐enhancing ingredients to feeds may allow higher levels of M. extorquens SCP to be used without compromising fish growth.