Basic HTML Version
International Journal of Marine Science 2013, Vol.3, No.40, 319-332
http://ijms.sophiapublisher.com
326
(Thompson et al., 2010). Compared to many other
opportunistic pathogenic
Vibrio
species, the
occurrence of
V. gazogenes
in the shrimp culture
system should not be a cause of concern to farmers
and diagnosticians as this species has not been found
to cause or occur in association with a disease
outbreak when compared to other
Vibrio
species
(Egidius, 1987).
The important consideration for aquaculture is to
understand the influences of physiological stress and
environmental factors on gut floral composition in
cultured animals (Oxley et al., 2002). Ingestion of the
free-living bacterial community in environments other
than natural stable conditions may lead to
establishment of abnormal gut floral species (Lynch
and Hobbie, 1988), pre-disposing the host to
pathogens (Munro et al., 1994). Moreover, the
digestive system of unhealthy or diseased prawns may
be found to support a gut flora where one or two
species pre-dominate. Researchers have shown that
stressed environment result in a decrease in the
diversity of the microbial community and an increase
in the numbers of functionally specific microbial
groups (Atlas et al., 1991; Geiselbrecht et al., 1996).
Outbreaks of disease in shrimp are poorly related to
the increased number of vibrios in the culture
environment
per se
but are strongly influenced by the
significant increase in the proportion of potentially
pathogenic
Vibrio
community in the rearing water as
well as in the shrimp hepatopancreas (Sung et al.,
1999; Sung et al., 2001). Furthermore, a close
relationship between the incidence of disease and
pathogen population of the surrounding water has also
been documented (Amaro et al., 1995). Gomez-Gil et
al (1998) reported a diverse population of vibrios
associated with healthy
P. vannamei
. This differs from
the situation where in one or two species pre-dominate
in diseased animals. In the present study, different
species of vibrios were found throughout the
production cycle and there was no sudden increase in
potentially pathogenic species, indicating that these
were a part of the normal flora. Absence of a dramatic
decrease in the diversity of
Vibrio
community either in
rearing water, pond sediment or shrimp
hepatopancreas during the culture suggests that the
environmental conditions were apparently stress-free.
In the present study, proper pond management
practices were followed with lime application and
water exchange during the culture period.
Physico-chemical characteristics did not vary
significantly during the culture period and were within
the optimum range recommended for culture of
P
.
monodon
(Chien, 1992). Though some strong
correlations were expected, statistical analysis did not
provide any insights into the relationship between
bacterial abundance and other environmental variables
in rearing water and sediment. Overall, the
relationships between abiotic and biotic variables were
low. Similar results have been reported by Prem
Anand et al (1996) and Janakiram et al (2000).
Previous studies have attributed differences in density
of vibrios to changes in environmental variables like
temperature and salinity (Johnson et al., 2010). Bright
Singh (1998) suggested that increase in the salinity of
ambient water could enhance the proliferation of
vibrios in
Peneaus indicus
culture system. Felix (2000)
hypothesized that higher salinity and nutrient content
could result in a ‘blooms’ of
Vibrio
spp. In the present
study,
Vibrio
counts increased with an increase in
salinity at certain period of the culture. However, this
relationship was found to be inconsistent and
statistically insignificant.
In conclusion, regular water exchange and liming
maintained the water quality of the shrimp ponds
during the culture period. Though the abundance of
pathogenic bacteria in the pond waters was
significantly higher compared to the sediments, their
numbers were maintained below the threshold levels
cited in literature. No dependency between abiotic and
biotic components was observed during the culture
period suggesting that the environmental parameters
were within the threshold did not influence the
abundance of bacteria. Culture-dependent approaches
to examine the composition of the
Vibrio
community
through the culture period revealed that many species
recorded from shrimp hepatopancreas were also
observed in other pond components. This observation
indicates that the pathogenic micro-flora is derived
from the ambient aquatic system. More importantly,
the dominance of particular
Vibrio
spp. was not
observed in the hepatopancreas which was indicative
of a stress-free environment for the shrimps. Thus,
timely monitoring of water quality parameters along
with abundance/composition of pathogenic bacteria in
different pond components can help detect disease
outbreaks if any. Farmers could be trained to use such