N. Misciattelli, J.W. Latchford, D.A. Jones, D. Fletcher. Recirculation systems in aquaculture are receiving increasing attention since they conserve water and increase flexibility in site selection. Whilst many studies have focused on the water chemistry in recirculation systems and the effect of ozone and UV-light on bacterial pathogens, less attention has been paid to the dynamics of bacterial populations within such systems.

Larval numbers ranged from 0 to 1700 and nitrate, nitrite and ammonia levels remained within acceptable limits. A significant correlation was noted between larval numbers and pH of the rearing water. Total culturable bacteria counts (TCC) and the proportion of presumptive Vibrio species varied from 0 to 100 % respectively during 13 weeks trial. The UV treatment was more efficient in initially reducing the numbers of presumptive Vibrios spp compared to the bacterial population as a whole, but the proportion of presumptive Vibrios spp increased steadily as the water circulated through the system. No significant correlation was found between the total numbers of larvae in the system and either the TCC or presumptive Vibrio counts in any part of the recirculation system.

A total culture collapse was observed during the experiment, at a time when TCC and numbers of presumptive Vibrios spp in the effluent from all tanks and system reservoir were 3.62 x 10^5 and 1.79 x 10^4 respectively. These values are lower that the maxima recorded earlier in the trails when the larvae were apparently healthy but the proportion of presumptive Vibrios spp in the tank effluent and system reservoir were at their maximum. These results may indicate that the balance of the bacterial populations rather than the total numbers of bacteria present in the rearing water has a major impact on larval survival.

(School of Ocean Sciences, University Wales Bangor, Menai Bridge, Anglesey, N. Wales, LL59 5EY, UK) back