Apparent absorption efficiencies of amino acids in rainbow trout, Oncorhynchus mykiss (Walbaum), fed diets containing bacterial single-cell protein

The aim of the present study was to compare the apparent absorption efficiency of individual indispensable and dispensable amino acids in diets containing varying proportions of bacterial single cell protein (BSCP) and fishmeal when fed to rainbow trout, Oncorhynchus mykiss (Walbaum). Four diets were formulated to contain a total of 45.8% crude protein, of which 0% was from BSCP in diet 1 (BSCP-0), 25% in diet 2 (BSCP-25), 62.5% in diet 3 (BSCP-62.5) and 100% in diet 4 (BSCP-100); the remainder of the protein was from fishmeal. The four diets were isonitrogenous and had similar amino acid compositions. Duplicate groups of 25 fish were fed one of the four experimental diets at the rate of 20 g kg^?1 body weight per day for 132 days. Apparent absorption efficiency was measured over a 45-day period during the growth trial. Results from two methods of faeces collection and two indicators of absorption efficiency were compared. The amino acid absorption efficiency of BSCP-100 was significantly lower (P < 0.05) when compared to the other three diets and absorption of some of indispensable amino acids was below the requirement level. The variation seen in individual amino acids absorption efficiencies between diets suggests that this parameter may be important in fish nutritional investigations.     

Growth performance and carcass composition of rainbow trout, Oncorhynchus mykiss (Walbaum), fed the (β-agonist ractopamine

Ractopamine is a synthetic catecholamine analogue which is known to improve feed efficiency, reduce carcass fat and increase lean muscle growth in terrestrial livestock (e.g. sheep, pigs, cattle and chickens), and therefore, may be useful in aquaculture production if similar effects can be elicited in fish. In the present study, rainbow trout, Oncorhynchus mykiss (Walbaum), (mean initial weight of 196 g) were fed the β-agonist ractopamine at 0, 5, 10, 20 or 40 parts per million (p.p.m.) of the diet for 12 weeks in a complete, randomized experimental design. Growth and feed intake were monitored throughout the experiment. Carcasses were analysed for protein, fat, energy and ash prior to the initiation of the trial, and subsequently, every 4 weeks. The growth rate, expressed as thermal-unit growth coefficient (TGC) was higher (P ≤ 0.05) in the 10 p.p.m. groups during weeks 1-4 and when averaged over all treatment weeks. Feed efficiency was significantly higher (P ≤ 0.05) in fish fed 10 and 5 p.p.m. ractopamine compared with controls during weeks 1-4 and 4-8, respectively, and was significantly higher (P ≤ 0.05) in all treated groups when averaged over the 12-week treatment period. Compared with controls, carcass protein was elevated in fish fed 5 p.p.m. ractopamine for 4 weeks, and in those offered 10 and 20 p.p.m. for 8 weeks. Carcass fat was significantly lower (P ≤ 0.05) in fish fed 10 p.p.m. ractopamine for 8 weeks, although the 40 p.p.m. group exhibited elevated fat levels in comparison with control fish. The data suggest that rapidly growing rainbow trout demonstrate age-related growth and carcass compositional trends similar to those of mammals, in that feed efficiency and carcass protein decrease while carcass fat increases with age. Oral administration of ractopamine to rainbow trout only modestly altered growth performance, and the trends were not as dramatic nor as consistent as those observed in homeothermic vertebrates fed ractopamine. Out of the concentrations tested, feed incorporation of ractopamine at 10 p.p.m. elicited the most pronounced effect on growth performance.     

The effect of glycyrrhizin on rainbow trout, Oncorhynchus mykiss (Walbaum), leucocyte responses

Abstract. Treatment of rainbow trout macrophages with glycyrrhizin (GL), an aqueous extract of licorice, Glycyrrhiza glabra , enhanced their respiratory burst activity. Maximal effects were seen using concentrations of 10–100 μml⁻¹. GL also modulated trout lymphocytes, increasing proliferation responses to the mitogen phytohaemagglutinin some two-fold over a range of GL concentrations. In addition, GL elicited the release of a macrophage activating factor (MAF) from head kidney leucocytes, as assessed by the ability of generated supernatants to increase respiratory burst activity of target macrophages. MAF activity was most apparent using 100 μg ml⁻¹ GL to induce MAF release and a 48-h incubation period with the target macrophages. Finally, GL was shown to enhance the release of MAF in response to the mitogen concanavalin A. The possible use of GL as a stimulant of fish innate defences is discussed.     

Effect of feeding on the tissue free amino acid concentrations in rainbow trout (Oncorhynchus mykiss Walbaum)

This study investigates whether tissue free amino acid (FAA) pools in rainbow trout,Oncorhynchus mykiss (Walbaum), are altered following feeding and the relationships between the amount of food consumed and the FAA pool size. Trout were starved for 7 days to provide baseline data and then refed on day 8. Individual food intake was measured by radiography and the consumption of amino acids (AA) calculated from dietary protein consumption. Total FAA concentrations in the stomach, liver and white muscle were little changed at various times after the meal and this pattern was repeated for the majority of individual FAA. Overall, the most notable change was a reduction in essential FAA concentrations (principally in valine, leucine and isoleucine) in the white muscle following feeding. However, in the caeca total FAA, total essential FAA and a number of individual FAA were significantly elevated at 4, 9 and 15h following feeding. There were few significant correlations between dietary amino acid consumption and total tissue FAA and essential FAA concentration in the stomach, caecum and white muscle; correlations were stronger in the liver. In order to explain the relative constancy of total FAA concentrations in the tissues following food intake (which represents over 100% of the total FAA pool) a model is presented that quantifies the AA flux through the free pools and considers the role of protein turnover in regulating FAA pool size.     

Identification and pathogenicity to rainbow trout, Oncorhynchus mykiss (Walbaum), of some aeromonads

Twelve strains of fish pathogenic aeromonads were identified by 16S rRNA sequencing as Aeromonas bestiarum , A. hydrophila , A. hydrophila subsp. dhakensis , A. salmonicida subsp. salmonicida , A. sobria biovar sobria and A. veronii biovar sobria. Following intramuscular injection, A. hydrophila subsp. dhakensis caused dark liquefying, raised furuncle-like lesions in rainbow trout within 48 h. Extracellular products of all cultures contained gelatinase and lecithinase, and most revealed lipase. Congo red absorption and siderophore production was recorded, but not so the suicide phenomenon or slime production. Sodium dodecyl sulphate polyacrylamide gel electrophoresis profile of the outer membrane proteins (OMP) revealed 10–25 bands, of which major bands were seen in the region of 32.5–47.5 and 62–83 kDa. Marked heterogenicity of the OMP and whole cell protein (WCP) profiles within and among the species was observed. Polypeptides of 83–173 kDa were detected in the WCP profile of the cultures, but they were not expressed in OMP fractions.     

Nutrient digestibilities, weight gain and plasma and liver levels of carbohydrate in rainbow trout, Oncorhynchus mykiss (Walbaum), fed diets with oats

Five oat products, whole oats, oat groats, rolled oats, oat meal and oat bran were tested at five inclusion levels. A growth and a digestibility trial were carried out with rainbow trout, Oncorhynchus mykiss (Walbaum), of 100 g initial mean weight. Protein, lipid and ash digestibilities were only marginally affected by dietary oat levels. Dry matter content of faeces was not affected by any of the oat products. Starch digestibility was high and showed no reduction with increased dietary content. Moderate levels of oats increased weight gain (60-170 g kg^?1), whereas the highest level tested (400 g kg^?1), appeared to cause a decrease in gain. Fat content in fish showed a slight increase as dietary incorporations of whole oats, oat bran and oat meal increased. Dressed percentage, however, was only marginally decreased. In fish fed whole oat or oat groat, plasma glucose levels increased as oat inclusion increased. Liver glycogen increased with increasing oat level. There were no clear associations between concentrations of pancreatic hormones and level of oat incorporation.     

Total fish meal replacement with rapeseed protein concentrate in diets fed to rainbow trout (Oncorhynchus mykiss Walbaum)

The potential of rapeseed protein concentrate as fish meal alternative in diets for rainbow trout (initial average weight 37.8?±?1.4?g) was evaluated. Nine experimental tanks of a freshwater flow-through system were stocked with 12 fish each. Triplicate groups of fish received isonitrogenous (47.9?±?0.5% CP) and isoenergetic (22.4?±?0.2?kJ?g^?1) experimental diets with 0, 66 and 100% of fish meal substituted with rapeseed protein concentrate (71.2% CP), thereby providing 0, 29 and 43% of dietary protein. As the amino acid profile of rapeseed protein concentrate was comparable to fish meal, there was no need to supplement experimental diets with synthetic amino acids. At the end of the 84?days of feeding period, fish growth performance, feed intake and feed efficiencies were not compromised, when 100% of fish meal in the control diet was replaced with rapeseed protein concentrate, revealing a SGR of 1.19 or 1.10, a FCR of 1.09 or 1.18 and a feed intake of 78.5 or 74.7?g in fish fed on the control diet or fed the diet devoid of fish meal, respectively. Intestinal morphology did not reveal any histological abnormalities in all dietary groups. Blood parameters including haematocrit and haemoglobin as well as glucose, triglycerides and total protein in the plasma were not different between treatment groups. Thus, the rapeseed protein concentrate tested here has great potential as an alternative to fish meal in rainbow trout diets.     

Dietary microbial phytase supplementation and the utilization of phosphorus, trace minerals and protein by rainbow trout [Oncorhynchus mykiss (Walbaum)] fed soybean meal-based diets

Effects of thermal and enzymatic treatments of soybean meal on apparent absorption of total phosphorus, phytate phosphorus, nitrogen (protein), ash, calcium, magnesium, copper, iron, manganese, strontium and zinc were examined using rainbow trout, Oncorhynchus mykiss (Walbaum), as the test species. Absorption of the test nutrients was estimated using yttrium as an inert non-absorbable indicator. Thermal treatments (microwaving, dry roasting, steam heating, cooking) had no measurable effect on the apparent absorption of phosphorus and other minerals. Phytase supplementation increased the apparent absorption of phosphorus, nitrogen (protein), ash, calcium, magnesium, copper, iron, strontium and zinc in low-ash diets containing soybean meal, but had little effect in high-ash diets containing both soybean and fish meal. In low-ash diets, the apparent absorption of phosphorus increased in accord with the level of phytase added to the diet, from 27% (no phytase added) up to 90% (phytase added, 4000 units kg⁻¹ diet) or 93% (predigested with phytase, 200 units kg⁻¹ soybean meal). In high-ash diets, dietary acidification with citric acid decreased the effect of phytase, whereas in low-ash diets, acidification markedly increased the effect of the enzyme. Excretion of phosphorus in the faeces of fish fed a low-ash diet containing phytase-treated soybean meal was 0.32 g per kg diet consumed, a 95%−98% reduction compared with phosphorus excretion by fish consuming commercial trout feeds.     

Quantification of Flavobacterium psychrophilum in rainbow trout, Oncorhynchus mykiss (Walbaum), tissues by qPCR

A qPCR assay was developed for rapid and sensitive detection of Flavobacterium psychrophilum, the aetiological agent of bacterial cold-water disease and rainbow trout fry syndrome in salmonid fish worldwide. A set of F. psychrophilum-specific primers based on 16S rRNA gene sequences was designed and validated for specific detection and quantification of DNA isolated from representative strains of F. psychrophilum. The qPCR assay exhibited a high specificity for the 16S rRNA gene of F. psychrophilum (from 4 × 10(8) down to 11 copies per reaction) but not for other Flavobacterium species or other bacteria including fish pathogens. This qPCR-based method proved to be useful in the quantification of the F. psychrophilum titre present within organs dissected out from diseased fish. As the F. psychrophilum genome contains six copies of the 16S rRNA gene, we could infer a limit of detection corresponding to two bacteria per reaction, corresponding to 800 bacteria per fish tissue sample, and therefore 20 F. psychrophilum cells mg(-1) of tissue (for sample weighing 40 mg). The qPCR assay reported here could be a useful tool for veterinary diagnostic laboratories to monitor the F. psychrophilum infection level in fish farms.     

Apparent amino acid availability from feedstuffs in extruded diets for rainbow trout Oncorhynchus mykiss

Apparent amino acid availability coefficients of several typical and novel feed ingredients were determined in rainbow trout using extruded diets and the faecal stripping technique. The ingredients were tested included five fish meals, three terrestrial animal by‐products, five plant protein concentrates, four plant meals, and seven low‐protein plant ingredients. Amino acid availability from the fish meals was relatively high ranging from 90 to 101%. Lower coefficients overall were observed for Menhaden fish meal FAQ when compared to the other fish meals. No differences in apparent amino acid availability were detected among the animal by‐products. Within the plant concentrate group, rice protein concentrate and barley protein concentrate exhibited generally lower amino acid availabilities compared to other concentrates tested. Among the plant meals, only the availabilities of histidine, valine, isoleucine and lysine in flaxseed meal were lower than those of soybean meal. Apparent amino acid availabilities among the low‐protein plant products were variable and significantly different.     

Influence of pre-stocking feeding levels on catchability of rainbow trout, Oncorhynchus mykiss (Walbaum), in a small fishery

Abstract. The effects of different pre-stocking feeding levels on the subsequent catchability of rainbow trout, Oncorhynchus mykiss (Walbaum), were investigated in a 12-ha put-and-take fishery. Two sibling batches of Winthrop strain were fed high or low rations of a pelleted diet to produce similar sized, but recently fast- and slow-growing fish. Each batch was given a distinctive dye mark. Anglers'catch records showed that although the 'slow-growing'fish were significantly the more easy to catch during the first week after stocking, over the remaining 5 months of the fishing season there was little difference in catchability between batches. This suggests that previous observations of sustained between-strain differences were not the result of selection of fast- and slow-growing fish per se, but were probably genetically determined.     

Bacterial pathogens in rainbow trout, Oncorhynchus mykiss (Walbaum), reared at Danish freshwater farms

During a 2-year period, bacterial fish pathogens were monitored on five rainbow trout, Oncorhynchus mykiss (Walbaum), freshwater farms in Denmark. A total of 1206 fish were examined and 361 bacterial isolates were identified phenotypically. Enteric redmouth disease, furunculosis and rainbow trout fry syndrome/coldwater disease were recorded. Infections caused by Flavobacterium psychrophilum occurred most frequently, but only one outbreak of enteric redmouth disease caused by Yersinia ruckeri serotype O1 and one of furunculosis caused by Aeromonas salmonicida were recorded during the monitoring period. Flavobacterium psychrophilum was isolated on all farms, both during disease outbreaks and from fish without any signs of disease. Serological investigations of F. psychrophilum showed that serotype Th was the dominant serotype found. The serotypes Th and Fd were involved in disease outbreaks of fry and larger fish. All isolates of F. psychrophilum showed proteolytic activities; however, a few isolates, belonging to serotype FpT did not degrade elastin and were not associated with mortality. Increasing resistance problems to oxytetracycline were demonstrated. More than half of the F. psychrophilum isolates showed resistance to oxolinic acid and oxytetracycline. No antibiotic resistant isolates were found among Y. ruckeri and A. salmonicida .     

Gut histology,immunology and the intestinal microbiota of rainbow trout,Oncorhynchus mykiss (Walbaum), fed process variants of soybean meal

Processed soybean meal ingredients have become an emerging plant‐based protein used in aquafeed formulations and have been shown to influence gastrointestinal health and microbiota in cultured species. Two 60‐day feeding trials with juvenile rainbow trout (Oncorhynchus mykiss, Walbaum) were conducted to compare the effects of fishmeal (FMC), defatted soybean meal (SBM), bioprocessed soybean meal (BSBM) and commercial soy protein concentrate (CSPC) ingredients on intestinal histology, innate immunity and microbiota profiles. Results indicated no significant differences in intestinal immunoglobulin concentrations (p = .41) or gut leukocyte phagocytosis at day 15 samplings (p = .41). Intestinal lysozyme content was found to be highest at day 60 in the BSBM treatment group (p < .01). No significant differences across treatments were observed in histological profiles at day 0 (p = .75), day 15 (p = .08) and day 60 (p = .22). ANOSIM analysis for microbiota beta diversity showed differences among all treatment groups, with the exception of the high and low levels of BSBM (p = .40). There were also differences observed in the alpha diversity ratios, with significant differences in gut communities among dietary groups as demonstrated by Chao‐1 (p < .01) and Shannon indices (p = .02). The incorporation of processed soy‐based proteins alters the microbial community composition within the distal intestine, and modulates lysozyme concentrations within the distal intestinal tissues.     

Partial substitution of fish meal and full-fat soya bean meal with wheat gluten and influence of lysine supplementation in diets for rainbow trout, Oncorhynchus mykiss (Walbaum)

A 65-day study was undertaken to evaluate the utilization of lysine-supplemented wheat gluten meal as a protein source for rainbow trout, Oncorhynchus mykiss (Walbaum). The performance of the trout fed wheat gluten was compared to that of trout where protein was principally supplied using fish meal and full-fat soyabean meal. Six isonitrogenous diets (approximately 45% crude protein) were formulated. Thus, the reference diet (FS) was based on fish meal and full-fat soya bean meal while a further five diets were formulated with a mixture of wheat gluten and wheat middlings used to isonitrogenously replace approximately 75% and 57% of the full-fat soya and fish meal respectively. Crystalline l -lysine representing 0.00%, 0.29%, 0.58%, 0.87% and 1.16% of the complete diet was then added to the wheat-gluten-based diets. Optimal performance in terms of weight gain and apparent net protein utilization was achieved by the fish fed a wheat-gluten-based diet supplemented with lysine (0.58%) yielding a digestible lysine level of 1.9% of the complete feed. Within the range of supplements provided, lysine digestibility was high. However, at the highest levels of supplemental lysine the relationship between uptake and supplement level was not linear. Additionally, arginine digestibility was not affected by the level of lysine supplementation. However, amongst the wheat-gluten-based diets, optimal performance was associated with a lysine: arginine ratio of 1.1: 1. Despite a lysine: arginine ratio of approximately 1.1: 1, the overall performance of the fish fed the fish-meal-soya-based reference diet was poorer than expected. The results are discussed with respect to optimal patterns of gross and digestible amino acids, lysine-arginine antagonism and the possible relationship between antinutritional factors and the poor performance of the fish fed the fish-meal-soya-based reference diet.     

Influence of rearing conditions on Flavobacterium columnare infection of rainbow trout, Oncorhynchus mykiss (Walbaum)

The influence of rearing conditions on Flavobacterium columnare infection of rainbow trout, Oncorhynchus mykiss (Walbaum), was studied experimentally in the laboratory and at a fish farm. In experiment I, the effect of parasitic infection on columnaris disease was studied using F. columnare carrier fish. The fish were exposed to Diplostomum spathaceum cercariae and a set of other stressors in order to induce clinical columnaris infection. Parasitic infection and other stressors failed to induce the disease. Disease occurred when the fish were challenged with F. columnare, but D. spathaceum infection did not enhance the severity of the infection. In experiment II, the influence of rearing density and water temperature was studied. Overall mortality was highest in fish at normal rearing density with high temperature (+23 degrees C). At low temperature (+18 degrees C) mortality was not affected by rearing density, but the transmission of columnaris disease was faster at normal rearing density at both temperatures. This supports the view that reduction of fish density could be used in prevention of columnaris disease especially if water temperature is high. Because the lower rearing density can also decrease the transmission of ectoparasites and penetrating endoparasites, it could be an efficient tool in ecological disease management.