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International Journal of Marine Science 2013, Vol.3, No.40, 319-332
http://ijms.sophiapublisher.com
320
pathogenic vibrios, findings by Costa et al. (1998)
suggest that some diseases occurring in penaeid
shrimps have been caused by
Vibrio
spp., which
behave more like true pathogens rather than
opportunists. Pathogenic vibrios usually invade the
host through the hepatopancreas, the common target
organ of most bacterial pathogens of shrimps (Chen et
al., 1992; Frelier et al., 1992). They then proliferate
and colonize the host digestive tract becoming
pathogenic.
Studies on vibrios in shrimp ponds have focused on
their isolation, characterization and identification
(Gomez-Gil et al.,
1998; Lavillo-Pitogo et al.,
1998;
Leaño et al., 1998; Sung et al., 1999; Abraham and
Palaniappan, 2004). A comparison of vibrios in water,
sediment and shrimp samples from multiple shrimp
ponds from east and west coast of India has been
made by Gopal et al (2005). However, the changes in
the abundance and composition of
Vibrio
spp. in
shrimp pond components during the entire crop cycle
from post larvae (PL) till the harvestable size has not
been studied, particularly from modified extensive
ponds. Elevated organic loading from shrimp farming
activities can result in an increase in the abundance of
heterotrophic bacteria (Zaccone et al., 2005) as they
are active degraders of organic matter. Generic
composition of heterotrophic isolates in an aquaculture
pond has shown the predominance of
Vibrio
spp.,
(Ganesh et al., 2010). A comprehensive knowledge of
the changes in the abundance and species composition
of vibrios would therefore be inevitable for a chosen
set of environmental conditions in aquaculture ponds.
This approach would not only help understanding the
dynamics but also plan strategies for disease
containment and increasing shrimp yield. Therefore,
during a 135 day production cycle of
Penaeus
monodon
we aimed to (i) monitor changes in
physico-chemical parameters along with the abundance
of total heterotrophic bacteria (THB), total
Vibrio
like
organisms (TVLO; potential pathogens) and luminescent
bacteria
(LB; confirmed pathogens) in different pond
components (rearing water and sediment, pond water
and sediment and shrimp hepatopancreas) (ii) examine
the relationship between environmental variables and
abundance of vibrios and (iii) assess changes in the
composition of the
Vibrio
community in different
components of shrimp pond. Results obtained during
the study revealed prevalence of a healthy
environment within the culture ponds with no
outbreak of pathogenic vibrios during the culture
period. The
Vibrio
diversity within the shrimp
hepatopancreas was similar to the aquatic system with
no detection of pathogenic LB towards the end of the
culture. Thus, this study enabled us to establish a
framework within which the modified extensive
shrimp ponds operated optimally leading to a
sustainable yield.
1 Results
1.1 Physico-chemical characteristics
Results of physico-chemical characteristics analyzed
in the pond water and sediment during the culture of
P.
monodon
have been tabulated in Table 1a. Between
ponds, no significant differences in physico-chemical
parameters were observed. Hence, the values from
two ponds were pooled (n = 6) to obtain an average (±
SD). A gradual increase in the temperature and salinity
of the pond water through the culture period was
noted. Temperature of pond water increased from 22.2
± 0.4
℃
at the start of the cycle reaching to a
maximum of 32.5 ± 0.4
℃
at the end of 135 days.
Similarly, salinity increased from 8.7 ± 0.2 at 0-doc to
34.0 ± 0.2 at the end of the cycle. Near neutral pH (7.3
± 0.2 to 8.0 ± 0.1) was recorded in the pond water
whereas the pH of sediment in the ponds was more
acidic with little variation through the culture period
(6.2 ± 0.2 to 6.7 ± 0.2). Eh values in the pond water
ranged between 27.90 ± 0.19 and 142.77 ± 3.25
whereas in the pond sediments the range was
relatively lower varying from 26.78 ± 2.57 to 127.73 ±
1.60. Dissolved oxygen generally showed a
decreasing trend as the culture period progressed. The
lowest dissolved oxygen content of 4.4 ± 0.3 mg L
-1
was recorded at 120-doc. POC in the pond (Table 1a)
and creek water (Table 1b) followed a similar trend
with a steady increase in concentration until 60-doc to
reach about 25.2 ± 8.4 g C m
-3
. Thereafter, POC in the
pond and creek varied with doc. The organic matter
content in the pond sediments increased with the
progress of the culture period. A maximum of 58.1 ±
10.6 mg g
-1
was recorded on 120
th
day of observation.
In the water/sediment sourced from the nearby creek,
the trend in the variation of temperature, salinity, pH,
Eh, dissolved oxygen and sediment organic matter
were similar to those observed in the culture ponds
(Table 1b).