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
D.W. Verner-Jeffreys, R.J. Shields, I.R. Bricknell,
T.H. Birkbeck-2004
Aquaculture, 232(1-4): 129-143
Abstract:
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<0.01) than group F (28.8%). Both
of the recycled water groups performed relatively well (42.3% survival for
group RN and 47.8% for group RS), and physical environmental conditions were
most stable in the two recycled water groups. The average incidence of
abnormal developmental morphologies was low with an average of >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
(<10 CFU/larva for group FA compared to 9.6×102 CFU/larva for
group F). Higher levels of bacteria were recovered from both groups of
larvae reared in recycled water (5.8×103 CFU/larva for group RN
and 7.79×103 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.
(Division of Infection and Immunity, Institute of
Biomedical and Life Sciences, University of Glasgow, Joseph Black Building,
Glasgow, Scotland G12 8QQ, UK, e-mail of T.H. Birkbeck: h.birkbeck@bio.gla.ac.uk)