Search for beneficial bacterial strains for turbot (Scophthalmus maximus L.) larviculture

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 t he survival percentage of turbot larvae. There was no correlation between t he number of bacteria present in the gut of turbot larvae and the larval su rvival rate. During both experiments, all replicates followed nearly the sa me rate of bacterial development in the gut of turbot larvae going from cir ca 10(2) CFU larva(-1) just before first feeding at day 3 post hatch to 10( 5) CFU larva(-1) at day 9 post hatch. In total, 127 bacterial isolates from 12 rearing tanks were sampled for fur ther investigation. Based on their fatty acid profile obtained by FAME-anal ysis, 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 thes e clusters were screened on their ability to enhance the survival rate as w ell as the poor reproducibility in larval survival in a small-scale turbot confrontation test. Also, a Vibrio mediterranei Q40 strain, isolated from s ea bream larvae, was included in these small-scale confrontation tests. Onl y cluster A and the V. mediterranei Q40 strain had a distinct positive and reproducible effect on larval survival. In conclusion, cluster A and V. med iterranei 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 bact eria. (C) 2001 Elsevier Science B.V. All rights reserved.