Influence of algal exudates and bacteria on growth and survival of white shrimp larvae fed entirely on microencapsulated diets

Intact microencapsulated protozoeal diets (MED), added to 5 mu m filtered hatchery water at the recommended concentration of 4 mg L-1 gave no significant difference in bacterial numbers after 6 h, over a 48 h period compared to control groups in autoclaved hatchery water. Disruption of the MED resulted in a significant increase in bacterial numbers up to 12 h compared to controls with intact microcapsules. Protein leach rates were similar in sterile water and 5 mu m filtered water, indicating that leaching was independent of microbial activity. MED leached significantly less dietary solubles compared to a microparticulate diet (MPD). There was no evidence to suggest significant attachment of bacteria to diet particles for at least 48 h. In low-quality water after algal blooms, survival of white shrimp (Penaeus indicus) larvae was poor when reared in both sterile and 5 mu m filtered water. The addition of bacteria filtered from xenic algal cultures together with the algal exudates to such water gave significant improvements in survival to larvae fed MED in 5 mu m filtered water but not in autoclaved culture water. Live algal diets promote high larval survival and growth irrespective of the inclusion of bacteria. It is suggested that algal exudates act as selective bacteriocides and total replacement of algae with MED requires a balanced bacterial community which can be attained at times from natural sea water, or when the natural bacterial community balance is disturbed, corrected by the use of algal exudates.