Bioflocs-assisted mass culture of Brachionus plicatilis under prolonged zero-water exchange and its application to Pagrus major larviculture

Abstract

The sustainable mass production of rotifers is crucial to sufficiently supply marine fish hatcheries. However, maintaining a stable rotifer culture is still a challenge mainly due to unionized ammonia (NH₃) toxicity and biological contaminants. This study investigated the effectiveness of single initial inoculation of bioflocs (BF) in addressing the abovementioned problems on rotifer mass culture using <i>Brachionus plicatilis</i> subjected to three treatments for 29 days, namely: 1) water exchange (WE), 2) zero water exchange (ZE), and 3) bioflocs × zero water exchange (BF × ZE). The effects of feeding BF-cultured <i>B. plicatilis</i> and supplementing BF in the rearing water on the survival and growth of red sea bream <i>Pagrus major</i> larvae for 14 days were evaluated. Bacterial composition of BF was determined. BF inoculation resulted in significantly higher rotifer densities from day 14 to day 29 (BF × ZE with 226–434 individuals/mL vs WE and ZE with 69–131 individuals/mL). BF inoculation significantly controlled ciliate contamination (BF × ZE with 7–25 ciliates/mL vs ZE with 421–531 ciliates/mL) from day 21 to day 25. TAN and NH₃ concentrations were significantly reduced in the BF × ZE than in ZE and WE treatments on day 29. <i>P. major</i> larvae fed BF-cultured rotifers and reared in BF-supplemented water did not show reduced survival and growth. BF inoculation significantly improves <i>B. plicatilis</i> productivity through the effective removal of NH₃ via the ammonia-oxidizing activity of <i>Nitrosomonas nitrosa</i> and bacteria within the phylum Chloroflexota under prolonged zero-water exchange conditions. Furthermore, the study demonstrates the viability of BF in <i>P. major</i> larviculture.