Analysis of bacterial populations in the gut of developing cod larvae and identification of Vibrio logei, Vibrio anguillarum and Vibrio splendidus as pathogens of cod larvae

Poor larval survival rates currently limit production in aquaculture and a better understanding of the bacterial flora in the larval digestive tract is essential to improving survival in hatchery reared larval fish. Identification of bacteria alone is insufficient, as the concentrations of key bacterial species may be important in determining survival. Here, we analysed bacteria, principally vibrios, in developing cod larvae from a Scottish hatchery. Six tanks of larvae were sampled weekly in parallel to monitor the microbial populations in larvae successively fed on rotifers and Artemia before weaning onto dry feed. When cod larvae were fed on rotifers the digestive tract microbiota was dominated by Ruegeria/Roseobacter, Pseudoalteromonas, and Microbacterium but when the diet was changed to Artemia over 90% of the microbiota consisted of vibrios. Polymerase chain reaction denaturing gradient gel electrophoresis (PCR-DGGE) analysis was used to identify the vibrios present and this showed that the microbiota was affected by changes in larval diet. Whilst feeding on Artemia, the larval digestive tract microbiota was dominated by Vibrio alginolyticus, but the population rapidly switched to one dominated by V. splendidus when larvae were weaned onto dry feed. Other Vibrio species were detected at lower levels at various times. During the survey, one tank experienced heavy mortalities associated with an elevated level of Vibrio anguillarum. The V. anguillarum isolates differed from those typically associated with mortalities in gadoid fish. Apparently healthy larvae from the tank experiencing heavy mortalities had a culturable microbiota almost equally represented by V. anguillarum and V. logei. Both species, as well as a V. splendidus strain isolated from turbot larvae, were pathogenic to cod larvae in laboratory experiments, and these vibrios could be targets for intervention to control the microbial flora of the larval digestive tract.