The effect of the type of non-protein energy (NPE) on energy utilisation in Nile tilapia was studied, focusing on digestible energy utilisation for growth (k gDE ). Furthermore, literature data on k gDE across fish species were analysed in order to evaluate the effect of dietary macronutrient composition. A total of twelve groups of fish were assigned in a 2 £ 2 factorial design: two diets ('fat' v. 'starch') and two feeding levels ('low' v. 'high'). In the 'fat'-diet, 125 g fish oil and in the 'starch'-diet 300 g maize starch were added to 875 g of an identical basal mixture. Fish were fed restrictively one of two ration levels ('low' or 'high') for estimating k gDE . Nutrient digestibility, N and energy balances were measured. For estimating k gDE , data of the present study were combined with previous data of Nile tilapia fed similar diets to satiation. The type of NPE affected k gDE (0·561 and 0·663 with the 'starch' and 'fat'-diets, respectively; P,0·001). Across fish species, literature values of k gDE range from 0·31 to 0·82. Variability in k gDE was related to dietary macronutrient composition, the trophic level of the fish species and the composition of growth (fat:protein gain ratio). The across-species comparison suggested that the relationships of k gDE with trophic level and with growth composition were predominantly induced by dietary macronutrient composition. Reported k gDE values increased linearly with increasing dietary fat content and decreasing dietary carbohydrate content. In contrast, k gDE related curvilinearly to dietary crude protein content. In conclusion, energy utilisation for growth is influenced by dietary macronutrient composition.Key words: Nile tilapia: Digestible energy: Dietary nutrient composition: Energy metabolism Fish feeds will have to further diversify ingredient composition due to the limited availability of wild fishery-derived fishmeal and fish oil as dietary protein and lipid sources (1) . This diversification is already reflected by the substantial amount of ingredients from oilseeds, pulses and cereals in fish diets (2 -5) , which increases the variability in dietary nutrient composition. Inclusion of plant ingredients as a protein source inevitably increases the dietary carbohydrate content. Digestible carbohydrates (i.e. starch) constitute a partial alternative for the digestible energy (DE) supplied by fish oil or vegetable oils in diets, especially for species such as tilapia.An adequate formulation of animal feed in terms of energy supply requires information on (1) nutrient digestibility of ingredients, (2) energy requirements for maintenance and (3) utilisation efficiency of DE or metabolisable energy (ME) for growth (respectively, k gDE and k gME ). Most energy evaluation systems for farm animals, which have a net energy approach, take into account dietary macronutrient composition-induced differences in energy utilisation efficiency (k gDE or k gME ) as seen in pigs (6,7) . For instance, in the net energy evaluation system for the Dutch pig, the u...
The hypothesis was tested that fish fed to satiation with iso-energetic diets differing in macronutrient composition will have different digestible energy intakes (DEI) but similar total heat production. Four iso-energetic diets (2×2 factorial design) were formulated having a contrast in i) the ratio of protein to energy (P/E): high (HP/E) vs. low (LP/E) and ii) the type of non-protein energy (NPE) source: fat vs. carbohydrate which were iso-energetically exchanged. Triplicate groups (35 fish/tank) of rainbow trout were hand-fed each diet twice daily to satiation for 6 weeks under non-limiting water oxygen conditions. Feed intake (FI), DEI (kJ kg−0.8 d−1) and growth (g kg−0.8 d−1) of trout were affected by the interaction between P/E ratio and NPE source of the diet (P<0.05). Regardless of dietary P/E ratio, the inclusion of carbohydrate compared to fat as main NPE source reduced DEI and growth of trout by ∼20%. The diet-induced differences in FI and DEI show that trout did not compensate for the dietary differences in digestible energy or digestible protein contents. Further, changes in body fat store and plasma glucose did not seem to exert a homeostatic feedback control on DEI. Independent of the diet composition, heat production of trout did not differ (P>0.05). Our data suggest that the control of DEI in trout might be a function of heat production, which in turn might reflect a physiological limit related with oxidative metabolism.
It has been hypothesised that, at non-limiting water oxygen conditions, voluntary feed intake (FI) in fish is limited by the maximal physiological capacity of oxygen use (i.e. an 'oxystatic control of FI in fish'). This implies that fish will adjust FI when fed diets differing in oxygen demand, resulting in identical oxygen consumption. Therefore, FI, digestible energy (DE) intake, energy balance and oxygen consumption were monitored at non-limiting water oxygen conditions in Nile tilapia fed diets with contrasting macronutrient composition. Diets were formulated in a 2 £ 2 factorial design in order to create contrasts in oxygen demand: two ratios of digestible protein (DP):DE ('high' v. 'low'); and a contrast in the type of non-protein energy source ('starch' v. 'fat'). Triplicate groups of tilapia were fed each diet twice daily to satiation for 48 d. FI (g DM/kg 0·8 per d) was significantly lower (9·5 %) in tilapia fed the starch diets relative to the fat diets. The DP:DE ratio affected DE intakes (P,0·05), being 11 % lower with 'high' than with 'low' DP:DE ratio diets, which was in line with the 11·9 % higher oxygen demand of these diets. Indeed, DE intakes of fish showed an inverse linear relationship with dietary oxygen demand (DOD; R 2 0·81, P,0·001). As hypothesised ('oxystatic' theory), oxygen consumption of fish was identical among three out of the four diets. Altogether, these results demonstrate the involvement of metabolic oxygen use and DOD in the control of FI in tilapia.Key words: Feed intake: Oxygen consumption: Non-protein energy sources: Energy balance: FishVoluntary feed intake (FI) in fish, as in other animals, is controlled by a complex combination of nutritional, physiological and environmental factors (1) . It has frequently been suggested that FI is controlled to maintain a relatively constant digestible energy (DE) intake, i.e. to meet the DE requirements (2 -6) .However, a few studies in fish have suggested the involvement of other nutritional factors in the control of FI (7,8) . Studies that verify the importance of other well-known regulatory mechanisms of FI in mammals, such as glucostatic (9) or lipostatic control (10) , either lead to an ambiguous conclusion or show a lesser impact in fish (7,11) than in terrestrial animals. The effect of non-protein energy (NPE) source (fat and starch) on FI in fish is unclear (12) and has not been systematically assessed at least at a similar digestible protein (DP):DE ratio and DE content of diets. Among the abiotic factors, dissolved oxygen (DO), pH and NH 3 are recognised to affect FI in fish (13) . The effect of the availability of oxygen on FI has been relatively well documented. Several studies have demonstrated that FI in fish decreases linearly with declining water DO content (14 -17) .The minimum DO level at which metabolic oxygen demand in fish limits FI is termed as incipient DO (iDO). Thus, * Corresponding author: J. W. Schrama, fax þ31 317 483937, email johan.schrama@wur.nlAbbreviations: ADC, apparent digestibility coefficie...
Insects are emerging as a sustainable alternative to fishmeal and fish oil in aquafeeds. This study assessed the effect of graded incorporation levels of defatted yellow mealworm (Tenebrio molitor) protein meal on juvenile rainbow trout (Oncorhynchus mykiss) growth performance, body composition, and apparent nutrient digestibility. The trial comprised five dietary treatments: control diet with 25% fishmeal, and four experimental diets with yellow mealworm protein meal at 5%, 7.5%, 15%, or 25%, which corresponded to a fishmeal replacement of 20%, 30%, 60%, or 100%, respectively. After 90 days, the graded incorporation of insect protein meal led to a significant stepwise increase in final body weight, and a significant improvement of specific growth rate, feed conversion ratio, and protein efficiency ratio compared to the control treatment. Regardless of the incorporation level, the insect protein meal had no effects on fish whole-body composition and apparent digestibility coefficients of dry matter, protein, fat, phosphorus, and energy. Protein, phosphorus, and energy retention significantly increased in fish fed the diets with an insect protein meal. In conclusion, the yellow mealworm protein meal could effectively replace 100% of fishmeal in the diet of juvenile rainbow trout with positive effects on its overall zootechnical performance.
Metabolic mechanisms underlying the divergent response of rainbow trout (Oncorhynchus mykiss) and Nile tilapia (Oreochromis niloticus) to changes in dietary macronutrient composition were assessed. Fish were fed one of four isoenergetic diets having a digestible proteinto-digestible energy (DP:DE) ratio above or below the optimal DP:DE ratio for both species. At each DP:DE ratio, fat was substituted by an isoenergetic amount of digestible starch as the non-protein energy source (NPE). Dietary DP:DE ratio did not affect growth and only slightly lowered protein gains in tilapia. In rainbow trout fed diets with low DP:DE ratios, particularly with starch as the major NPE source, growth and protein utilisation were highly reduced, underlining the importance of NPE source in this species. We also observed species-specific responses of enzymes involved in amino acid catabolism, lipogenesis and gluconeogenesis to dietary factors. Amino acid transdeamination enzyme activities were reduced by a low dietary DP:DE ratio in both species and in tilapia also by the substitution of fat by starch as the NPE source. Such decreased amino acid catabolism at high starch intakes, however, did not lead to improved protein retention. Our data further suggest that a combination of increased lipogenic and decreased gluconeogenic enzyme activities accounts for the better use of carbohydrates and to the improved glycaemia control in tilapia compared with rainbow tront fed starch-enriched diets with low DP:DE ratio.Key words: Digestible protein/digestible energy: Non-protein energy: Nutrient utilisation: Hepatic enzyme activity: Glycaemia Recommended dietary macronutrient levels differ among teleosts (1) . Optimal levels of digestible protein (DP) and digestible non-protein energy (NPE; fat or starch) are often related to the species' trophic level or feeding habitat. Dietary protein levels for maximum growth of fish depend also on the digestible energy (DE) level or DP:DE ratio, and hence on the amount of NPE in the diet (2) . Studies on the effect of dietary NPE on growth and nutrient utilisation mostly focused on total NPE levels (3 -8) , despite clear evidence that the protein-sparing effect of digestible fat and carbohydrates is species-dependent, varying according to species' feeding habitat (cold v. warm-water) or feeding habit (carnivorous v. omnivorous) (9 -13) . For example, while rainbow trout or other salmonids efficiently utilise dietary lipid above 10 % without negative effects on growth (13 -15) , other fish species such as turbot, Senegalese sole, Nile tilapia and grass carp seem to display a limited ability to use high dietary fat (16 -22) . On the other hand, most warm-water fish such as Nile tilapia appear to utilise higher levels (up to 40 % of diet DM) of dietary carbohydrates than cold-water fish such as rainbow trout (# 20 % of diet DM) (1,10,23,24) . The limited ability of rainbow trout (carnivorous-like species) compared to, for example, Nile tilapia and common carp (omnivorous-like species) to efficient...
Acid -base disturbances caused by environmental factors and physiological events including feeding have been well documented in several fish species, but little is known about the impact of dietary electrolyte balance (dEB). In the present study, we investigated the effect of feeding diets differing in dEB (2100, 200, 500 or 800 mEq/kg diet) on the growth, nutrient digestibility and energy balance of Nile tilapia. After 5 weeks on the test diet, the growth of the fish was linearly affected by the dEB levels (P, 0·001), with the lowest growth being observed in the fish fed the 800 dEB diet. The apparent digestibility coefficient (ADC) of fat was unaffected by dEB, whereas the ADC of DM and protein were curvilinearly related to the dEB levels, being lowest and highest in the 200 and 800 dEB diets, respectively. Stomach chyme pH at 3 h after feeding was linearly related to the dEB levels (P,0·05). At the same time, blood pH of the heart (P,0·05) and caudal vein (P,0·01) was curvilinearly related to the dEB levels, suggesting the influence of dEB on postprandial metabolic alkalosis. Consequently, maintenance energy expenditure (MEm) was curvilinearly related to the dEB levels (P,0·001), being 54 % higher in the 800 dEB group (88 kJ/kg 0·8 per d) than in the 200 dEB group (57 kJ/kg 0·8 per d). These results suggest that varying dEB levels in a diet have both positive and negative effects on fish. On the one hand, they improve nutrient digestibility; on the other hand, they challenge the acid -base homeostasis (pH) of fish, causing an increase in MEm, and thereby reduce the energy required for growth.
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