The experiment was carried out from 23rd Junuary to 24th March 2011 at Sealab. Atlantic cod larvae were reared with water from three different systems: flow-though, maturated water and recirculation water. The water, larvae and feed specimens will be sampled from those three systems for microbial analysis by PCR-DGGE (denaturing gradient gel electrophoresis) and sequencing of bands. DNA will be extracted from water, feed and larvae, and used as template in PCR to amplify 16S rRNA encoding genes of bacteria. Subsequently, the PCR products will be separated by DGGE. The phylogenetic affiliation of the predominant community members, as represented in the DGGE band pattern, will be excised, re-amplified and sequences, and used for taxonomic assignment of the bacteria. Analyses of similarity (ANOSIM) with Bray-Curtis dissimilarity will be used to compare microbial community composition in three systems as well as microbiota of gut larvae.