Les gobies amphidromes et, en particulier, ceux de la sous famille des Sicydiinae représentent une part importante de la diversité et de l'abondance des peuplements de poissons d'eau douce des îles tropicales. Ces espèces se reproduisent dans les rivières et ont une phase larvaire marine pendant plusieurs mois avant de retourner croitre et maturer en eau douce. L'objectif de cette thèse est d'acquérir des connaissances concernant la phénologie des dynamiques de migration en eau douce de deux espèces de Sicydiinae de La Réunion, Sicyopterus lagocephalus et Cotylopus acutipinnis et les mécanismes qui peuvent en être à l'origine. Ces études montrent que la dévalaison des larves vers la mer immédiatement après leur éclosion avait principalement lieu pendant l'été austral et en début de nuit dans les zones aval. Le débit des cours d'eau et ses fluctuations saisonnières et journalières jouent un rôle prépondérant dans cette dynamique de dévalaison en termes d'abondances de larves dérivant, de temps de dévalaison jusqu'à la mer et de survie des larves. Après leur arrivée en eau douce, les juvéniles vont coloniser l'ensemble des zones des bassins versants. Les plus fortes abondances de juvéniles en migration vers les zones amont sont observées en fin d'étiage et pendant l'après-midi. Enfin, S. lagocephalus présente des performances locomotrices supérieures à celles de C. acutipinnis soutenues par des morphologies plus diverses. Ces meilleures performances, soutenues par des morphologies plus diverses, sont l'un des facteurs qui peuvent expliquer la large aire de répartition de S. lagocephalus, présent dans les océans Indien et Pacifique, par rapport à C. acutipinnis qui est endémique de l’archipel des Mascareignes. Des recommandations de gestion, permettant principalement de restaurer la continuité biologique au niveau des barrages, sont faites au regard des résultats obtenus durant cette thèse. / Amphidromous gobies, especially those of the Sicydiinae subfamily, represent most of the diversity and abundance of the freshwater fish assemblages in tropical islands. These species spawn in rivers, spend months in the ocean as pelagic larvae and return to freshwater to grow and reproduce. This doctoral thesis aims at describing the phenology and some mechanisms of the dynamics of migration in freshwater of two Sicydiinae species in Réunion Island: Sicyopterus lagocephalus and Cotylopus acutipinnis. These studies highlighted that larval downstream migration to the sea, immediately after hatching, occurs mainly during austral summer and a few hours after sunset in downstream reaches. Seasonal and daily fluctuations of the flow regime also greatly influence the abundances, the transport duration from spawning site to the sea and the survival of larvae during their downstream migration. After their return to the freshwater, juveniles settle in the watersheds from the estuary to the most upstream reaches. The highest abundances of juveniles migrating to upstream reaches are observed during low flow conditions at the end of the afternoon. Finally, locomotor performances are better for S. lagocephalus compared to C. acutipinnis. These better locomotor performances, supported by more diverse morphologies, are among the factors explaining the presence of S. lagocephalus from Eastern Pacific Ocean to Western Indian Ocean when C. acutipinnis is endemic to the Mascarenes Archipelago. Based on the results obtained in this doctoral thesis, management and conservation recommendations are suggested especially for the restoration of fish passage at several migration barriers such as dams and weirs.

<p> A large gravel pit at East Paris exposed 14m of Pleistocene braided outwash. Large scale longitudinal bars (30 - 400m long) were developed utilising irregular topographic highs (4 - 6m) of the basal gravels as Bar Cores. These bars prograded downstream by means of larce-scale cross-strata, periodically interupted by reactivation surfaces. Contemparaneous upstream addition of material took place by the migr ation of mixed sand and gravel bed-forms on the stoss-side of the bars. The gravel supply to the avalanche face of the major bars took the form of imbricated gravels which passed from the stoss-side, through the Bar Top, onto the Bar Front. Adjacent, sandy side-channels (4 -6m deep) exhibit a fining upward fill of trough cross beds, planar lamination and ripples; and interdigitated with the gravel bars alongside. </p> <p> Aggradation of the flood plain led to the extension of the Bar Top facies over the length of the bars, and then to the development of a Shallow-braided stream facies over the whole system. The latter is recognised by its smaller (less than 1m) bar forms composed of crossbedded gravel supplied by an imbricate gravel, and numerous small, sandy channel forms. </p> <p> At West Paris another large gravel pit exposed 6m of sandier, but stratigraphically equivalent deposits. Large scale side-bar accumulations (200-300m across) of gravel developed from river bank lateral accretion deposits. The downstream progradation was by lobes of cross-stratified and imbricated gravels, alternating with sand drapes of low flow stages. Sandy Side-channels (4m deep) developed between the gravel lobes and the accreting river bank. </p> <p> The concept of braid bar growth from a pre-existing form (bar core or river bank) is in contrast to the classic theory of development from a gravel lag. The differences may be due to depth relationships of the systems, which in this case may be the effect of position on the sandur surface. </p> / Thesis / Master of Science (MSc)