S.J. Kaushik

Energy requirements, utilization and dietary supply to salmonids

Abstract Energy needs of fasting salmonids are about 30–80 kJ/kg body weight/day, depending mainly on water temperature. Maintenance energy requirements are estimated as 75–100 kJ/kg/day, about 10- to 20-fold lower than those of terrestrial vertebrates. Under negative N-balance or maintenance conditions, body protein oxidation accounts for a greater part of the energy supply in fish than in higher animals. Energy requirements for body growth and tissue deposition have been studied to some extent and current estimates vary between 15 and 18 MJ of DE/kg of live weight gain, being affected by water temperature, genotype, body size and growth rate. While the apparent digestibility coefficients of protein and fat from most commonly used feed ingredients are high, those of carbohydrates vary widely. The additive nature of DE values appears to make the use of DE values reliable in feed formulation and feeding and the use of metabolizable energy (ME) values, especially those drawn from poultry nutrition, bears little practical significance. Due to the relatively high contribution of protein and amino acids as preferential energy substrates in fish, much attention is paid to the digestible protein to digestible energy ( DP DE ) ratios, but precise, optimal values are not available for all species. Heat increment of feeding appears to be mainly related to protein intake. Few attempts have been made in the recent past to draw feeding tables based on DE requirements for maintenance and growth, the validity of which needs confirmation under practical farming conditions.

Nutritional regulation of hepatic glucose metabolism in fish

Glucose plays a key role as energy source in the majority of mammals, but its importance in fish appears limited. Until now, the physiological basis for such apparent glucose intolerance in fish has not been fully understood. A distinct regulation of hepatic glucose utilization (glycolysis) and production (gluconeogenesis) may be advanced to explain the relative inability of fish to efficiently utilize dietary glucose. We summarize here information regarding the nutritional regulation of key enzymes involved in glycolysis (hexokinases, 6-phosphofructo-1-kinase and pyruvate kinase) and gluconeogenesis (phosphoenolpyruvate carboxykinase, fructose-1,6-bisphosphatase and glucose-6-phosphatase) pathways as well as that of the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. The effect of dietary carbohydrate level and source on the activities and gene expression of the mentioned key enzymes is also discussed. Overall, data strongly suggest that the liver of most fish species is apparently capable of regulating glucose storage. The persistent high level of endogenous glucose production independent of carbohydrate intake level may lead to a putative competition between exogenous (dietary) glucose and endogenous glucose as the source of energy, which may explain the poor dietary carbohydrate utilization in fish.

Contribution of digestible energy from carbohydrates and estimation of protein/energy requirements for growth of rainbow trout (Oncorhynchus mykiss)

Abstract Four diets having different digestible protein (DP) to digestible energy (DE) ratios were formulated to study the protein-sparing effect of DE from carbohydrates and to estimate the DE requirements for growth of rainbow trout. A growth trial was conducted for 8 weeks. Digestibility measurements were made over 2 weeks. Trout (initial weights 37 and 100 g for growth and digestibility trials, respectively) were fed to satiation twice a day at a constant water temperature of 17.5 °C. Apparent digestibility coefficients (ADC) for dry matter, starch and energy decreased with an increase in dietary raw starch level. The metabolic fecal nitrogen loss was estimted to be 150 mg N/ 100 g of dry diet. Fish fed a high protein diet with 48% DP, 9% digestible fat (DL) and 20% digestible carbohydrates (DC) showed the best performance for weight gain, feed conversion ratio and daily growth index. However, the highest protein efficiency ratio (2.3) and protein retention efficiency (41.4%) were observed in fish fed a diet containing 38, 9 and 30% of DP, DL and DC, respectively. The whole body composition of the fish was not affected by dietary treatments. Liver glycogen and hepatosomatic index were positively related to dietary digestible carbohydrate levels. The digestible protein requirement per unit weight gain (g DP required per kg production) varied depending upon the DP/DE ratios. On the other hand, DE requirement per unit weight increment was relatively constant (about 17.5 MJ for the production of 1 kg of rainbow trout), irrespective of the dietary treatment.

Effect of digestible carbohydrates on protein/energy utilization and on glucose metabolism in rainbow trout (Salmo gairdneri R.)

Abstract Diplicate groups of rainbow trout were fed low-protein (38%) diets containing high levels (38%) of five different carbohydrate sources (raw starch, extruded corn, extruded wheat, extruded corn starch or extruded wheat starch). A modified pair-feeding method was used to allow equal intakes of protein and of digestible energy by each group of fish. Growth parameters were followed over a period of 18 weeks; the digestibility of the diets, nitrogen and energy balances, and the respiratory and ammonia quotients of fish fed the different diets were determined. To study the long-term effects of dietary carbohydrates, plasma glucose profile and its control by bovine insulin was followed in fish fed over 30 weeks with selected test diets. The results show that inclusion of extruded cereals or extruded starch improves availability of dietary energy. High levels of carbohydrates to not adversely affect overall growth or nutrient retention efficiencies. Long-term feeding with carbohydrate-rich d diets does not confer on trout any adaptive capacity to regulate postprandial glycemia levels. Respirometric measurements appear to provide interesting data on the metabolic utilization of body substrates.

Effects of dietary incorporation of a co-extruded plant protein (rapeseed and peas) on growth, nutrient utilization and muscle fatty acid composition of rainbow trout (Oncorhynchus mykiss)

Abstract Four experimental diets (diets CP0, CP5, CP10 and CP15) containing 42% crude protein were formulated to contain graded levels of colzapro, a co-extruded product of rapeseed and peas, by replacement of protein from fishmeal. Another diet (CPE) with a lower protein (34%) and higher fat content was formulated to contain a high level (45%) of colzapro. A 12-week growth trial was conducted with rainbow trout (initial body weight 40 g; 18°C) to evaluate the potential use of this plant protein in trout diets. Growth performance, digestibility of nutrients, and ammonia nitrogen excretion were measured. The effects of dietary colzapro on the fatty acid composition of trout muscle were studied. Compared to the control group, fish fed diets in which 5, 10 and 15% of protein was replaced by protein of colzapro showed equivalent or slightly betten growth performance. Increasing the dietary colzapro level to 45% (diet CPE) led to a significantly lower final weight, specific growth rate and an increase in feed to gain ratio when compared with the other groups. No significant differences were observed in the digestibility of protein and fat, regardless of dietary treatment, while those of dry matter and energy were affected by colzapro incorporation. Non-faecal nitrogen loss showed a decreasing trend with increasing levels of dietary colzapro. In the muscle, n−6 fatty acids increased with increasing levels of dietary colzapro, mainly due to a rise in linoleic acid levels, while the levels of 20: 4 n−6 did not vary much between treatments. Although the levels of 22: 6 n−3 fatty acids remained stable, a decreasing trend for the total percentage of n−3 fatty acids was observed, thus affecting the n−3 n−6 ratios. Histological observations did not reveal any significant difference in thyroid cells between dietary treatments. The results obtained showed that colzapro can be utilized in rainbow trout diets at levels of up to 20% without negative effects on growth, nitrogen or energy utilization and muscle fatty acid composition.

Dietary ascorbic acid needs of common carp (Cyprinus carpio) larvae

Abstract As some controversy seem to exist regarding the dietary essentiality of ascorbic acid (AA) for larval cyprinids, a study was conducted to determine the dietary AA requirements with first-feeding larvae of common carp. Six semi-purified diets were formulated to contain graded levels (0, 10, 30, 90, 270, 810 mg kg −1 ) of AA supplied as ascorbyl polyphosphate. Triplicate groups of 250 larvae each were fed one of the experimental diets using automatic feed dispensers over the light hours of the day (temperature: 24±1°C; photoperiod 16L:8D). A duplicate group was kept unfed. Survival, body length and weight increments were recorded. At the end of the 21 day trial, samples were withdrawn for analyses of whole body vitamin C content. The results indicate that common carp larvae have a dietary requirement for ascorbic acid. The required level for maximum tissue storage is higher than that needed for survival and maximum growth. Based on growth performance, a dietary requirement of about 45 mg AA equivalent kg −1 was established. Maximal body vitamin C concentration (104–115 μ g g −1 wet weight) was observed in larvae fed 270 mg AA equivalent kg −1 or more.

Studies on the nutrition of Siberian sturgeon, Acipenser baeri: I. Utilization of digestible carbohydrates by sturgeon

Abstract A growth trial was conducted to study the effects of incorporation of four different carbohydrate sources (crude, gelatinized or extruded starch, extruded whole corn) in practical diets (crude protein level: 36%) for Siberian sturgeon, Acipenser baeri. Another diet incorporating gelatinized starch was formulated at a higher protein level of 42%. Based on growth performance, body composition and nutrient retention efficiencies, it was found that growing juvenile Siberian sturgeon do not utilize well complex carbohydrates like crude starch. Extruded or pregelatinized starches or extruded corn can be used as an energy-yielding ingredient in sturgeon grower diets. The level of feeding has a great impact on feed efficiency, the optimal level being 1.45% of body weight per day for sturgeon weighing between 90 and 400 g. The results indicate that the protein requirement for optimal growth of Siberian sturgeon would be around 36%, but below 42%. In absolute terms, the daily protein requirement for optimum growth would amount to 3.7 g digestible protein/kg BW per day with a protein:energy ratio of 18 mg digestible protein/kJ DE. Histological studies revealed that high levels of digestible carbohydrates adversely affect liver size, morphology and function.

Dietary Carbohydrate Utilization by European Sea Bass (Dicentrarchus labrax L.) and Gilthead Sea Bream (Sparus aurata L.) Juveniles

European sea bass and gilthead sea bream are two major marine finfish species reared in the Mediterranean region. Feeds for these species have high protein levels, usually supplied by fish meal. Research on alternatives to fish meal is mainly focused on plant protein feedstuffs, which are generally high in carbohydrates. Therefore, understanding the potential use of dietary carbohydrate is crucial for better utilization of such feedstuffs in the diets for these species. Factors that affect carbohydrate digestibility, metabolic utilization, as well as regulation of glucose metabolism are discussed. Data show that to ensure high digestibility, growth, and feed utilization, diets for these species should include around 20% digestible carbohydrate. Digestibility of native starch is high (above 70%) in both species, and it is further improved by processing starches (above 90%). Regarding carbohydrate complexity, it appears that European sea bass perform better with starch than glucose, while the opposite is true for gilthead sea bream. An enhancement in the activity of key enzymes of the glycolytic pathway and an increase of liver glycogen content is generally observed in the two species after feeding carbohydrates. An inverse regulation of the gluconeogenic enzyme activities with dietary carbohydrate intake is not observed in either species.

Studies on the nutrition of Siberian sturgeon, Acipenser baeri. II. Utilization of dietary non-protein energy by sturgeon

Abstract Duplicate groups of juvenile Siberian sturgeons (initial body weight 49 g) were fed two experimental diets (crude protein 51% DM; gross energy 22 kJ/g DM) containing either 21.8% lipid and 9.9% crude starch (diet F) or 12.5% lipid and 20% gelatinized starch (diet M) as non-protein energy sources. Growth parameters were followed over a period of 8 weeks; the digestibility of the diets, nitrogen and energy balances and body composition of sturgeons fed the two diets were determined. The digestibility of lipid decreased when the diet contained a high level of fat, so that the available digestible energy (DE) was higher in the diet containing the lower fat level with digestible carbohydrates. This increase in DE led to an increase in fat deposition, even in the muscle. In spite of the difference in DE, metabolic nitrogen losses were not different between the two dietary treatments, suggesting that, in juvenile Siberian sturgeon, dietary lipids can be more efficient in sparing protein compared with dietary carbohydrates. Lipids appeared to be used as energy sources more efficiently than carbohydrates at the metabolic level.

Skeletal muscle cellularity and expression of myogenic regulatory factors and myosin heavy chains in rainbow trout (Oncorhynchus mykiss): Effects of changes in dietary plant protein sources and amino acid profiles

The nutritional regulation of skeletal muscle growth is very little documented in fish. The aim of the study presented here was to determine how changes in dietary plant protein sources and amino acid profiles affect the muscle growth processes of fish. Juvenile rainbow trout (Oncorhynchys mykiss) were fed two diets containing fish meal and a mixture of plant protein sources either low (control diet) or rich in soybean meal (diet S). Both diets were supplemented with crystalline indispensable amino acids (IAA) to match the rainbow trout muscle IAA profile. Diet S was also supplemented with glutamic acid, an AA present in high quantities in trout muscle. Rainbow trout fed diets C and S were not significantly different in terms of overall somatic growth or daily nitrogen gain, although their parameters of dietary protein utilisation differed. Distribution of skeletal white muscle fibre diameter and expression of certain selected muscle genes were also affected by dietary changes. In the white muscle, diet S led to a significant decrease (x0.9) in the mean and median diameters of muscle fibres, to a significant decrease (x0.6) in the expression of MyoD and to a significant increase (x1.7) in the expression of fast-MHC, with no significant changes in myogenin expression. There was no change in the expression of the genes analysed in lateral red muscle (MyoD, MyoD2, myogenin and slow-MHC). These results demonstrated that changes occurred in skeletal white muscle cellularity and expression of MyoD and fast-MHC, although overall growth and protein accretion were not modified, when a diet rich in soybean meal and glutamic acid was ingested. Present findings also indicated that the white and red muscles of rainbow trout are differently affected by nutritional changes.