Des systèmes vidéo rotatifs pour étudier l'ichtyofaune : Applications à l’analyse des variations spatiales et temporelles dans le lagon de Nouvelle-Calédonie

Estimating diversity and abundance of fish is fundamental for the understanding of community structure and dynamics of coral reefs. Observations are generally gathered using Underwater Visual Census (UVC). These informations can also be collected by underwater video techniques involving no diver and allowing a high spatial and temporal coverage. The first part of this PhD thesis is a review of publications using underwater video techniques between 1952 and 2012. The second part present two rotating video systems used to study the diversity of ichthyofauna and habitats in the lagoon of New Caledonia: the STAVIRO (for "STAtion VIdeo ROtative") and the MICADO (for "Module d’Imagerie Côtier, Autonome pour le Développement de l’Observation sous-marine"). The third part of this work is a comparison of these techniques with UVCs and an analysis of the “observer effect” for the rotating video techniques. The comparison between rotating video systems and UVC showed that: (i) fish community data collected by UVC and by STAVIRO differed significantly; (ii) species richness and density of large species were not significantly different between techniques; (iii) species richness and density of small species were higher for UVC; (iv) density of fished species was higher for STAVIRO; and (v) only UVC detected significant differences in fish assemblages structure across reef type. The comparison of fish counts obtained from the same videos by different observers showed that for a defined list of species, estimates of species richness and total abundance can be considered as similar between observers with sufficient experiences (> 6 months). The last part of this PhD is an application of rotating video techniques to the study of daily temporal variations of the ichthyofauna. Daily variations were observed depending on the time of day and the tidal state and typical patterns of variations have been described for some species groups. The outcomes of this work bring original insights of new techniques complementary to traditional techniques in order to enhance our understanding of the functioning and dynamic of coral reef.