The aerobic bacterial flora in the gut of turbot larvae and their influence on larval survival was examined. Two turbot experiments were run with six replicates each time. Large variation, from 0% up to 44%, was observed in the survival percentage of turbot larvae. There was no correlation between the number of bacteria present in the gut of turbot larvae and the larval survival rate. During both experiments, all replicates followed nearly the same rate of bacterial development in the gut of turbot larvae going from circa 102 CFU larva-1 just before first feeding at day 3 post hatch to 105 CFU larva-1 at day 9 post hatch.In total, 127 bacterial isolates from 12 rearing tanks were sampled for further investigation. Based on their fatty acid profile obtained by FAME-analysis, and using principal component analysis, the isolates were subdivided in 12 major gaschromatographic-groups or clusters (GC-groups), 11 isolates remained unclustered. Four specific GC-groups (namely cluster A, B, I and J) were selected as potential beneficial bacteria for turbot larviculture as the majority of the isolates of these clusters derived from rearing tanks with a survival percentage higher than 35%. Representative isolates of these clusters were screened on their ability to enhance the survival rate as well as the poor reproducibility in larval survival in a small-scale turbot confrontation test. Also, a Vibrio mediterranei Q40 strain, isolated from sea bream larvae, was included in these small-scale confrontation tests. Only cluster A and the V. mediterranei Q40 strain had a distinct positive and reproducible effect on larval survival. In conclusion, cluster A and V. mediterranei Q40 seemed to play a role as first coloniser of the gut of turbot larvae and could prevent the colonisation of the gut by opportunistic bacteria.
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