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In fish, the voluntary feed intake is influenced by dietary, environmental and/or physiological factors. It is well known that under hypoxia the concentration of oxygen in the water (DO) determines the feed intake of fish. However at non-limiting water DO levels (normoxia), several other mechanisms might play a role in feed intake regulation. Under hypoxia feed intake and oxygen consumption are interrelated. In this thesis we proposed the ‘oxystatic’ concept of feed intake regulation, which states that even at normoxia and in the absence of other constraints, the long term (weeks) voluntary feed intake of fish can be constrained by a set-point value of oxygen consumption. Dietary macronutrient composition affects the ‘dietary oxygen demand’ (i.e., amount of O2 consumed per unit of feed). This oxystatic concept implies that fish fed to satiation with diets differing in ‘dietary oxygen demand’ (mg O2/ g or kJ feed) will have a different digestible energy intake but a similar oxygen consumption. The validity of the oxystatic concept was assessed in two species, Nile tilapia and rainbow trout. These fish were fed diets which had large contrasts in nutrient composition (i.e., protein to energy ratio; type of the non-protein energy source (starch vs. fat); amino acid composition) in order to create contrasts in dietary oxygen demand. In all conducted studies with both species, the digestible energy intake was affected by the diet composition. However, in some studies oxygen consumption was similar and in others it differed between the diets, which respectively supports and contradicts the oxystatic concept. In all studies with both species, the digestible energy intake of tilapia and trout was negatively related to dietary oxygen demand and positively related to efficiency of oxygen utilization for energy retention. Furthermore it was observed in tilapia that the within-day variation in feed intake was affected by dietary macronutrient composition. The variation in within

Compromisation of food intake when confronted with diets deficient in essential amino acids is a common response of fish and other animals, but the underlying physiological factors are poorly understood. We hypothesize that oxygen consumption of fish is a possible physiological factor constraining food intake. To verify, we assessed the food intake and oxygen consumption of rainbow trout fed to satiation with diets which differed in essential amino acid (methionine and lysine) compositions: a balanced vs. an imbalanced amino acid diet. Both diets were tested at two water oxygen levels: hypoxia vs. normoxia. Trout consumed 29% less food under hypoxia compared to normoxia (p0.05). This difference in food intake between diets under normoxia together with the identical oxygen consumption supports the hypothesis that food intake in fish can be constrained by a set-point value of oxygen consumption, as seen here on a six-week time scale.

Sustainable aquaculture, which entails proportional replacement of fish-based feed sources by plant-based ingredients, is impeded by the poor growth response frequently seen in fish fed high levels of plant ingredients. This study explores the potential to improve, by means of early nutritional exposure, the growth of fish fed plant-based feed. Rainbow trout swimup fry were fed for 3 weeks either a plant-based diet (diet V, V-fish) or a diet containing fishmeal and fish oil as protein and fat source (diet M, M-fish). After this 3-wk nutritional history period, all V- or M-fish received diet M for a 7-month intermediate growth phase. Both groups were then challenged by feeding diet V for 25 days during which voluntary feed intake, growth, and nutrient utilisation were monitored (V-challenge). Three isogenic rainbow trout lines were used for evaluating possible family effects. The results of the V-challenge showed a 42% higher growth rate (P = 0.002) and 30% higher feed intake (P = 0.005) in fish of nutritional history V compared to M (averaged over the three families). Besides the effects on feed intake, V-fish utilized diet V more efficiently than M-fish, as reflected by the on average 18% higher feed efficiency (P = 0.003). We noted a significant family effect for the above parameters (P,0.001), but the nutritional history effect was consistent for all three families (no interaction effect, P.0.05). In summary, our study shows that an early shortterm exposure of rainbow trout fry to a plant-based diet improves acceptance and utilization of the same diet when given at later life stages. This positive response is encouraging as a potential strategy to improve the use of plant-based feed in fish, of interest in the field of fish farming and animal nutrition in general. Future work needs to determine the persistency of this positive early feeding effect and the underlying mechanisms.

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/kg0·8 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.

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 (P0.05). 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.

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 (kgDE). Furthermore, literature data on kgDE 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 kgDE. Nutrient digestibility, N and energy balances were measured. For estimating kgDE, data of the present study were combined with previous data of Nile tilapia fed similar diets to satiation. The type of NPE affected kgDE (0·561 and 0·663 with the ‘starch’ and ‘fat’-diets, respectively; P <0·001). Across fish species, literature values of kgDE range from 0·31 to 0·82. Variability in kgDE 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 kgDE with trophic level and with growth composition were predominantly induced by dietary macronutrient composition. Reported kgDE values increased linearly with increasing dietary fat content and decreasing dietary carbohydrate content. In contrast, kgDE related curvilinearly to dietary crude protein content. In conclusion, energy utilisation for growth is influenced by dietary macronutrient composition.

The quantity of amonia excreted by Artemia reared under axenic conditions depends on the quality and quantity of purine nucleotides used to satisfy the essential purine requirement of this crustacea. Results obtained under the rigorous conditions of culture in synthetic media, rise the problem of the metabolic significance of the adenylate deaminase activity., La quantité d'ammoniaque excrétée par Artemia élevé dans des conditions axéniques, dépend de la nature qualitative et quantitative des nucléotides puriques utilisés pour satisfaire le besoin essentiel de ce crustacé en dérivés puriques. Les résultats obtenus dans les conditions expérimentales rigoureuses de culture sur milieu synthétique permettent de poser le problème de la signification métabolique de l'activité de l'adénylate désaminase.