Methods of microbial control in marine fish larval rearing: clay‐based turbidity and passive larval transfer

This study focused on methods to reduce bacterial loads in the larval culture tanks of California yellowtail ( Seriola lalandi ). We conducted two trials to evaluate methods to minimize bacterial loads in the larval rearing water. The first trial examined the use of bentonite clay as a turbidity agent to replace algae in a green water‐type environment. This trial consisted of three treatments: (1) clay with continuous feeding ( CCO ), (2) clay with batch feedings ( CBA ) and (3) algae paste with batch feedings ( ALG ). The results showed that both clay treatments had significantly fewer Vibrio colonies in the water column ( CBA – 180 ± 78; CCO – 377 ± 120  CFU  mL −1 ) than the ALG treatment (5692 ± 2396  CFU  mL −1 ) after 14 days of culture. Survival was significantly higher in the CCO treatment (14.1 ± 2.6%) than either the CBA (2.3 ± 0.5%) or ALG treatments (2.8 ± 1.5%). The second trial attempted to limit bacterial loading in the larval culture tank by passively transferring the larvae into an adjacent, clean tank at 1, 5 and 9 days post hatch during the first 2 weeks of culture. The results from this trial showed that after 12 days of culture, water in the transfertank had fewer Vibrio colonies (1025 ± 541  CFU  mL −1 ) than the water in the control tanks (1962 ± 1415 CFU  mL −1 ). Also, survival was significantly higher among larvae that were transferred (43.9 ± 13.5%) than in the control tanks (23.1 ± 6.3%).