It is the aim of this project to investigate the bacterial flora of rotifers and the effect of environmental manipulations on it. Manipulation of the larval health by bacteria. This project is designed to develop methods of hygiene management and disease control for marine fish larviculture, at the industrial level. The culture and enrichment of rotifers under different environmental conditions and the performance of different hygiene measures will investigate the possibility of influencing the microflora of these rotifers. The effect of these parameters will also be considered in relationship to the survival and growth of the rotifers; Subsequently the effect of these rotifers on the intestinal flora of fish larvae will be investigated. Bacterial strains and species can serve as probionts either by facilitating food digestion or by preventing the colonization of opportunistic pathogenic bacteria. The isolation of bacteria from the digestive tract of adult wild fish and healthy larvae will enable suitable bacterial strains to be selected as probionts. These strains will be determined as potential probionts by in vitro and in vivo challenge tests and will be administered through the rotifers. The effect of these adjusted rotifers on the health status and survival of the larvae will be followed. In addition the host specificity of pathogenic strains towards different fish species will be determined as well as investigating the infection pathways and the most critical periods for infection in the ontogeny of the involved fish species. The experiments will initially be carried out on the laboratory scale and will overlap and continue on the semi-industrial scale on the farms.