Effects of different water treatment methods and antibiotic addition on larval survival and gut microflora development in Atlantic halibut (Hippoglossus hippoglossus L.) yolk-sac larvae

The survival of Atlantic halibut ( Hippoglossus hippoglossus ) yolk-sac larvae and the development of their gut bacterial flora were compared in pilot-commercial scale rearing trials under different conditions. Four different treatments were tested; water recirculation using non-disinfected embryos (RN); water recirculation using disinfected embryos (RS); flow-to-waste water with disinfected embryos (F); flow-to-waste water with disinfected embryos and addition of antibiotics, oxytetracycline, oxolinic acid and amoxycillin at 20 mg l −1 (FA). Mean survival was greatest in the FA group (53%), which was significantly higher ( p 71% morphologically normal larvae in all treatments. Addition of antibiotics appeared to prevent the colonisation of yolk-sac larvae by Marine Agar (MA)-culturable bacteria ( 2 CFU/larva for group F). Higher levels of bacteria were recovered from both groups of larvae reared in recycled water (5.8×10 3 CFU/larva for group RN and 7.79×10 3 for group RS), indicating that bacterial abundance is not, in itself, a determinant of survival. Differences in the types of bacteria that colonised the halibut yolk-sac larvae under the different rearing regimes were described by characterization of the isolates. There appeared to be a relationship between larval survival rate and the presence, or absence, of certain phenotypes. In particular, the gut microflora in the poorest performing group (group F) was dominated by isolates biochemically and morphologically distinct from the predominant Pseudoalteromonas species isolated from the better performing recycled water groups. This lends weight to the hypothesis that certain types of bacteria may act as opportunistic pathogens in yolk-sac incubators while others are benign.