The gut of newly-hatched turbot larvae contained very few bacteria but was rapidly colonised once feeding commenced and was generally dominated by Vibrio species. The source of the larval gut flora was the rotifers used as food, rather than the ambient water. In extensive-rearing systems the larval turbot gut was colonised more slowly than in intensive-rearing systems and this was correlated with better growth and survival rates. A wide variety of bacteria were represented in the gut flora of turbot larvae but recognised pathogens were rarely found and their presence was not correlated with low survival rates. Scanning electron microscopy revealed that the bacterial flora of rotifers was almost entirely associated with the surface of the rotifers; approximately 65% of bacteria were removed by rinsing rotifers in sterile seawater and only 5% remained after treatment with benzalkonium chloride. Attempts to reduce the viable bacterial load of rotifers with detergents and other antibacterial compounds were unsuccessful, as all the substances tested were either toxic to rotifers, or were ineffective at reducing the number of rotifer-associated bacteria. Exposure to lysozyme at 0.2% salinity resulted in >70% reduction in the bacterial load of rotifers and lysozyme-treated rotifers were palatable to turbot larvae. However, the osmotic shock due to the low salinity caused the rotifers to stop swimming and sink to the bottom of the larval-rearing tanks. Although the rotifers recovered overnight, their guts were empty, thus, they were probably of little nutritional value to the turbot larvae. Ultra-violet irradiation of rotifers was more successful. Large numbers of rotifers were treated by passing the culture through a water jacket surrounding a U.V. tube lamp. A >99% reduction of the bacterial load of unenriched rotifers was obtained at a flow rate of 1.5 litres per min and a concentration of 200 rotifers per ml. This decreased to a >96% reduction if the rotifers were enriched with algae prior to treatment. U.V.-treated rotifers were palatable to turbot larvae and in field trials at Golden Sea Produce, Hunterston, the gut of larvae fed with U.V.-treated rotifers was colonised more slowly than in controls fed untreated rotifers. In two field trials with extensively-reared turbot larvae, the survival rates of the larvae fed with U.V.-treated rotifers were higher than those of larvae fed untreated rotifers. The gut microflora of intensively-reared turbot larvae fed with U.V.-treated rotifers was diverse and resembled that of unfed larvae, whereas the gut microflora of larvae fed untreated rotifers was dominated by a few Vibrio species. The use of rotifers as carriers was considered a potentially effective means of introducing a probiotic bacterial flora into the gut of turbot larvae. Axenically cultured rotifers were rapidly colonised when the culture water was inoculated with defined bacteria. Within two hours, approximately 250 bacteria adhered per rotifer, or were ingested by rotifers, with each of the five bacterial species tested. Attempts were made to colonise rotifers subsequently fed to larval turbot, with a strain of Vibrio alginolyticus isolated from healthy, intensively-reared larvae. However, the bacteria either lost viability or did not stably colonise the gut of turbot larvae.
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