La pisciculture africaine : enjeux et problèmes de recherche

Cet article présente les enjeux et contraintes majeurs du développement de la pisciculture sur le continent africain, traduits en problématique de recherche scientifique et technique. Les principaux poissons concernés appartiennent aux groupes des tilapias et des siluriformes, dont l'utilisation en aquaculture dépasse largement le cadre du continent africain, conférant aux travaux menés une portée internationale. Les principaux axes de recherche porteurs concernent l'optimisation du contrôle de la reproduction chez les tilapias et de la maîtrise du cycle biologique chez les siluriformes, l'étude de la biodiversité de la faune piscicole africaine appliquée à la mise en évidence de "nouvelles" espèces d'intérêt aquacole (en parallèle avec le même type d'approche menée sur d'autres grands bassins hydrographiques sud-américains ou du sud-est asiatique) et la connaissance des réseaux trophiques au sein des écosystèmes aquacoles. En outre, une démarche scientifique en sciences humaines dans le domaine de l'aquaculture tropicale apparaît aujourd'hui comme une nécessité. (Résumé d'auteur

Sperm characteristics and motility in Pangasianodon hypophthalmus (Sauvage, 1878) and Pangasius djambal Bleeker, 1846 (Pangasiidae, Siluriformes)

A drop in osmotic pressure appeared as the main trigger for initiating spermatozoa motility in the two catfish species studied. Motility duration was particularly short in both species (20 to 30 s). Sperm preserved at 4 degrees C in a saline medium (NaCl 155-175 mM, pH 7.5-8.5) Survived longer than in any of the other media tested

Inertial coalescence of droplets on a partially wetting substrate

We consider the growth rate of the height of the connecting bridge in rapid surface-tension-driven coalescence of two identical droplets attached on a partially wetting substrate. For a wide range of contact angle values, the height of the bridge grows with time following a power law with a universal exponent of 2/3, up to a threshold time, beyond which a 1/2 exponent results, that is known for coalescence of freely-suspended droplets. In a narrow range of contact angle values close to 90°, this threshold time rapidly vanishes and a 1/2 exponent results for a 90° contact angle. The argument is confirmed by three-dimensional numerical simulations based on a diffuse interface method with adaptive mesh refinement and a volume-of-fluid method

Transitional random matrix theory nearest-neighbor spacing distributions

This paper presents a study of the properties of a matrix model that was introduced to describe transitions between all Wigner surmises of Random Matrix theory. New results include closed-form exact analytical expressions for the transitional probability density functions, as well as suitable analytical approximations for cases not amenable to explicit representation

Tidal currents, winds and the morphology of phytoplankton spatial structures

Chlorophyll a, nutrients and salinity distributions were studied at two spatial scales (10 cm and 0.25 to 2.5 km) in the St. Lawrence Estuary (Quebec, Canada), in order to investigate the role of tidal currents and winds in the formation and maintenance of spatial structures. Data were collected according to a synoptic sampling pattern using three sampling platforms simultaneously, and they were analyzed using analysis of variance. The sampling pattern was repeated on four occasions during July 1980.Analyses of variance indicated significant spatial heterogeneities of about the same magnitude at the two scales studied for chlorophyll and nutrients, whereas salinity showed only large–scale variability. At the kilometer scale, the frequency distribution spectra of patch length for chlorophyll showed the existence of patches of various dimensions between 0.2 and 6.0 km with a dominance of small patches (≤0.5 km). Frequency maxima were usually observed at the smaller (≤0.5 km) and larger (≥2.0 km) scales for the nutrients and only at larger (≥2.0 km) scale for salinity. The distribution spectra of patch dimensions were characteristic for each sampling experiment, depending on tidal currents and prevailing wind conditions. Estimated patch dimensions were larger parallel to the current direction than perpendicular to current direction, implying that spatial structures are elongated in the sense of the current direction. Higher winds have, first, a tendency to increase the small–scale structure of the environment by breaking up larger patches into smaller patches, before structures are completely eliminated. The implication of these findings is that different results could be obtained depending on the sampling strategy used (sampling either at anchor stations or at random, independent of current direction), which could lead to different conclusions

Effects of spatially displaced feedback on remote manipulation tasks

Several studies have been performed to determine the effects on computer and direct manipulation task performance when viewing conditions are spatially displaced. Whether results from these studies can be directly applied to remote manipulation tasks is quenstionable. The objective of this evaluation was to determine the effects of reversed, inverted, and inverted/reversed views on remote manipulation task performance using two 3-Degree of Freedom (DOF) hand controllers and a replica position hand controller. Results showed that trials using the inverted viewing condition showed the worst performance, followed by the inverted/reversed view and the reversed view when using the 2x3 DOF. However, these differences were not significant. The inverted and inverted/reversed viewing conditions were significantly worse than the normal and reversed viewing conditions when using the Kraft Replica. A second evaluation was conducted in which additional trials were performed with each viewing condition to determine the long term effects of spatially displaced views on task performance for the hand controllers. Results of the second evaluation indicated that there was more of a difference in performance between the perturbed viewing conditions and the normal viewing condition with the Kraft Replica than with the 2x3 DOF