Abstract : About sixty species of demersal fishes are living in the coastal waters of the bay of Douarnenez, the road of Brest, the bay of Audierne and the nearby waters of Iroise Sea. They are mainly distributed according to the bio-sedimentary structure of the bottom. The seasonal study of catches shows the influence of the hydro-climatic conditions, particularly in the bay of Douarnenez. Analysis of correspondences on the occurrence of the species allow to separate the winter populations from the summer one. This alternance is also connected with the spawning periods of the characteristic species. On the intertidal nurseries, the study of the physico-chemical factors shows the great influence on the distribution of the fishes, particularly for the Pleuronectiforms juveniles, of the continental fresh-water feeds, and the exposure of the beaches. The monthly evolution of the catches shows the same pattern in the different stations : an increase whenthe sea-water temperatures rises, from May onwards, and the decrease due to an escape of the young fishes into deeper waters in November. The analysis of the demographic structures suggests a double origin for the young plaices in the northern part of the surveyed area. As for Solea lascaris, however the bimodalism of the 0 group could be accounted for by a confusion between two species whose spawning periods could be different. Finally, the study of diets of principal predators shows the large diversity of preys and their variability, particularly as for as the size of fishes are concerned. These facts, completed by ethological analysis, emphasize the main aspects of the predator-prey connections. Several dues show that inter-specific competition between predators is very low.