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A14.15; International audience

International audience; The effect of stocking density on feed intake, feed utilization and feeding behaviour was studied in groups of rainbow trout submitted to different levels of food accessibility. In the first experiment, 18 groups of 100, 300 or 500 fish (initial individual weight 26 +/- 0.5 g) were held in 1 m(3) tanks where they were fed: in excess using belt feeders; or by means of self-feeders wit a fixed reward level (4 g per trigger actuation) or a reward level proportional to fish density (2 g/100 fish/trigger actuation). Final biomass after 125 days was c. 25, 70 and 100 kg/m(3) in groups of 100, 300 and 500 fish respectively. Growth and feed intake was affected by fish density, feeding technique and reward level. However, feed efficiency, body composition, as well as nutrient retention and loss were unaffected by the treatment. In the self-fed groups, daily pattern of feeding activity was affected by fish density when the reward level was fixed but not when the reward level was proportional to fish density. In a second experiment, groups of 100 or 500 fish (same initial individual weight) had access to one or three self-feeders. Access to multiple self-feeders had no positive effect on any of the studied parameters. The fish used only one of the three feeders at a time, and the three feeders where not used equally. We conclude that feed intake but not feed utilization was impaired by increased density, indicating that density it itself is not stressful to the fish. Food accessibility is the main factor responsible for the observed density - related decrease in growth.

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International audience; Background: One of the characteristic features of the two types (α and β) of “mitochondria-rich” (chloride) cells in the gill epithelium of freshwater fishes is the presence in their apical region of tubulovesicular structures. A further analysis of the ultrastructural features of these apical elements as well as that of their modifications under various living conditions should help to understand better the respective rǒle of both α and β cells in these conditions.Methods: Atlantic salmon (Salmo salar) maintained in fresh water as well as tilapia (Oreochromis niloticus) maintained either in fresh water or in deionized water or in 20% saltwater were examined. Measurements of surface areas of apical structures in the various living conditions were also performed.Results: In the α cells of freshwater fishes, the apical structures consisted of isolated vesicles containing a filamentous material resembling that coating the apical surface. They were closely related to the apical plasma membrane and did not penetrate the region containing the tubular system. When fishes were transferred to deionized water, the number of the apical membrane folds increased significantly, as did the number and size of apical structures which became elongated. In saltwater-adapted fishes, the apical structures showed a tendency to collapse and took the appearance of flattened and slightly curved elements. These observations tended to indicate that in α cells the apical structures were extensions of the apical plasma membrane and thereby might be implicated in sodium uptake when fishes are placed in fresh or deionized water and in chloride excretion when they are transferred to salt water. In β cells, the apical structures were usually separated from the apical plasma membrane by a zone rich in cytoskeleton elements. They penetrated deeply into the supranuclear region, where they intermingled with the elements of the tubular system. They consisted mainly of tubular elements that contained a material resembling that present in the trans tubular Golgi network from which they might originate. The apical structures remained unaltered in β cells whatever the medium (fresh or deionized water) in which the fish was placed.Conclusions: The α cells which are usually thought to be mainly involved in chloride excretion when fishes are transferred into seawater might also be implicated in sodium uptake in freshwater living conditions. The rǒle of β cells, in contrast, still remains to be established.

International audience; In tilapia, there are two forms of prolactin (PRL) whose effects on sodium and chloride movements differ and depend on the living environment of the fish. To see whether different receptors or the same receptor mediates these different effects, we have characterized the specific binding of both forms of tilapia (ti)PRL in two osmoregulatory organs, the gill and kidney. Two recombinant tiPRLs were used for this analysis. The recombinant hormones had the same properties as the native hormones in a tilapia gill radioreceptor assay. Specific binding to gill and kidney membranes was increased by optimizing the quality of the tissue preparations (physiological state of fish, membrane preparation) and the incubation conditions (pH, salt concentrations, temperature, time). Under these optimized conditions, we detected only one class of high affinity PRL receptor in gill and kidney. Its binding affinity was higher for tiPRL(I) than for tiPRL(II) in both gill and kidney (for tiPRL(I) the respective affinity values were 2.9 and 2.3 x 10(10) per M, for tiPRL(II) they were 1.9 and 0.5 x 10(10) per M). In competition studies, tiPRL(I) was more potent, followed by tiPRL(II) and ovine (o)PRL. tiGH and oGH did not significantly displace either tiPRL. The receptor we have characterized thus recognizes quite specifically both tiPRLs.

International audience