3. Results
3
.
1
. Bacterial distribution
:
general trends The number of fixed bacteria was relatively constant at all three sampling sites
pipes, rearing tank and biological filter during the experimental period Fig. 2. The average number of CFU in the pipes was 1.1 × 10
7
9 2.88 × 10
6
cm
− 2
, which is not significantly different from the average of 1.5 × 10
7
9 1.09 × 10
7
cm
− 2
found in the rearing tank F
5.3
= 0.179, P-value = 0.68. The average number of CFU in
the biological filter was 7.3 × 10
6
9 7.25 × 10
6
g
− 1
of packing. Fig. 3 shows that three levels of heterotrophic free bacteria could be distinguished
in the different components of the recirculating water system over a period of 8 months Newman – Keuls test, P B 0.05, after the ANOVA F
2.62
= 16.39, P-value =
4.7 × 10
− 7
: the lowest level was observed at the UV disinfection units outlet B 10
3
CFU ml
− 1
, an intermediate level at the biological filter inlet \ 10
3
CFU ml
− 1
, situated at a distance of 20 m PVC pipe from the disinfection unit, and the highest level was observed between the biological filter outlet and the UV disinfec-
tion units inlet Fig. 1, Table 1. In this section, there was a general tendency for the CFU to increase between the biological filter outlet and the UV sterilizer inlet.
The increase of viable bacteria in the rearing tank, expressed as the difference between the inlet and the outlet Fig. 4, was not statistically significant, but the
tendency was independent of fish density in the tank R = − 0.435 and P-value = 0.242 for a fixed dilution rate 0.38 kgm
3
of water. Fig. 5 shows the number of bacteria in each component, expressed as the
difference between the inlet and the outlet CFU. The values shown were the means
Fig. 2. Concentration of fixed heterotrophic bacteria.
Fig. 3. Concentration of heterotrophic bacteria in the different components of the recirculating system.
Fig. 4. Relation between the numbers of heterotrophic bacteria in the rearing tank and the fish biomass.
obtained for each sampling spot for a fixed ratio of 0.38 kg of ingested feed per m
− 3
of replacement water. The biological filter was the main source of bacteria, although the average concentration was lower than in the tank, and the UV
disinfection unit reduced the number of viable heterotrophic bacteria to below 10
3
CFU ml
− 1
.
3
.
2
. Influence of replacement water flow rate on bacterial distribution During a 7-month period the experimental aquaculture system functioned with
three replacement water flow rates 24, 50 and 7 m
3
day. The concentration of bacteria in the biofilter effluent increased from 2.4 9 1.2 × 10
4
to 7.0 9 2.3 × 10
4
CFU ml
− 1
Fig. 6, ANOVA, F
7.7
= 8.17, P-value = 0.045, and the concentration
of bacteria on the biofilter media increased from 5.1 9 3.43 × 10
6
to 1.1 × 10
8
9 3.41 × 10
7
CFU g
− 1
of packing Fig. 7, ANOVA, F
7.7
= 30.23, P-value = 0.005
when the ratio of the average amount of ingested feed replacement water flow was increased from 0.38 to 1.1 kg feed m
− 3
of fresh water.
3
.
3
. Identification and generation time Various biotypes of gram negative rod bacteria were found, some of which
differed by only one characteristic. A first group contained strict aerobic or microaerophilic bacteria from Bergey’s Group IV: Pseudomonas, Oceanospirillum,
Marinobacter, Paracoccus and Erythrobacter. A second group, containing aerobes facultative anaerobes, belonged to the Vibrionaceae family: one strain was iden-
tified as Vibrio and another, isolated only once, as Aeromonas. The fixed bacteria contained all the genera isolated, but the Vibrionaceae constituted up to 20 of the
total population. On the other hand, the free bacteria contained only Group IV genera Pseudomonas, Oceanospirillum, Marinobacter, Paracoccus and Erythrobac-
Fig. 5. Bacterial growth in the different components of the rearing system.
Fig. 6. Concentration of heterotrophic bacteria in the biofilter effluent and the ratio of ingested feed per replacement water.
Fig. 7. Concentration of heterotrophic bacteria on the biofilter media and the ratio of ingested feed per replacement water.
ter. This situation was consistently observed, except one time in June, when the biological filter was clogged when the Vibrio represented 50 of the total free
population at the biological filter outlet Fig. 8. The average growth rate at 25°C was 0.25 9 0.01 h
− 1
for all the free bacteria, which corresponds to a generation time of 2 h 50 min.
4. Discussion
Kategori