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International Journal of Marine Science 2013, Vol.3, No.40, 319-332
http://ijms.sophiapublisher.com
324
1.5 Average weight, feed conversion ratio and
survival
Cultured shrimps attained an average weight ranging
from 0.05 ± 0.01 g on 0-doc to 32.0 ± 4.0 g on
135-doc with a total production of 650 ± 50 kg per
pond. Average survival was about 78% and the feed
conversion ratio (FCR) was 1.5 ± 0.1.
2 Discussion
Monitoring of bacterial abundance and assessment of
the taxonomic diversity of potential pathogens is
essential to avoid viral outbreaks in aquaculture ponds.
Very few studies have been carried out to
systematically monitor the bacterial composition with
special reference to vibrios in shrimp ponds. In the
present study, we examined the abundance of THB
along with potential pathogens like TVLO and LB.
The abundance of THB in pond water recorded in this
study were lower by 2-orders of magnitude than those
previously reported from shrimp culture ponds (Putro
et al., 1990; Ruangpan et al., 1995; Prem Anand et al.,
1996; Bright Singh et al., 1998; Otta et al., 1999) and
by an order of magnitude higher than those reported
by Fonseka (1990), Nayyarahamed et al (1995) and
Sharmila et al (1996). Such differences may be
attributed to the species cultured, intensity of the
culture system, feed used and media used to
enumerate bacteria. The bacterial load of source water
is also known to influence that of rearing water as
reported from semi-intensive shrimp aquaculture
farms by Nayyarahamed et al (1995) and Sharmila et
al (1996). In the present study, no significant
difference between THB in source/rearing water was
observed. This could be due to the modified extensive
system followed wherein the feed inputs and stocking
rates are relatively lower than semi-intensive/intensive
culture systems.
Vibrios constitute a major portion of the micro biota in
brackish water pond ecosystems. In shrimp farms
from India, vibrios account for 1 - 10% of the total
bacterial CFU mL
-1
(Otta et al., 1999). Compared to
the total heterotrophs enumerated in the present study,
the retrievability of TVLO in pond water and pond
sediment were high varying between 1 - 100% and 1 -
10% respectively. Considering a total bacterial load of
10
7
cells mL
-1
(unpublished data) in the ponds during
the culture period, the TVLO form only 0.01% of the
total bacterial population in the pond water whereas in
pond sediments, they constituted ~ 0.1%. The
abundance of TVLO and LB during the culture were
below the threshold levels of 10
4
CFU mL
-1
and 10
2
CFU mL
-1
respectively (Leano et al., 1998;
Vaseeharan and Ramasamy, 2003). Hence, it can be
implied that in low-input modified extensive shrimp
culture systems and well managed shrimp ponds, the
usage of antibiotics could be avoided.
Bacterial densities of 1-3 orders of magnitude higher
in pond sediment as compared to rearing water have
been reported (Moriarty, 1986; Allan et al., 1995;
Ruangpan et al., 1997; Janakiram et al., 2000).
However, in this study, the average bacterial counts in
rearing water were significantly higher than in pond
sediments (
t
-test,
P
< 0.05). This observation is
unexpected as one could anticipate higher bacterial
counts in pond sediments which contain nutrients
in higher concentration resulting from the
accumulation of leached and/or unconsumed feed,
phytoplankton/zooplankton detritus and chitin
deposition as a result of moulting. Gopal et al (2005)
have reported a higher relative abundance of vibrios
by up to 2-orders of magnitude in rearing water than
in aquaculture pond sediments located along the west
coast of India. Such differences have not been
observed in shrimp ponds on the east coast where
farmers stock relatively higher PL per m
2
. Shrimps are
known to be deposit feeders. It is possible that they
ingest bacteria adhered to settled organic particles.
Hence, a lower bacterial density in the sediments
could be expected.
No clear-cut trend in TVLO abundance was observed
in rearing waters. Absence of any particular trend may
be attributed to the ‘artificial’ nature of culture system
where the manipulation of the pond environment by
the way of water quality management and pond
bottom management (water exchange, liming) leads to
frequent changes in biotic and abiotic factors in the
pond. The only major change in TVLO abundance
observed in the rearing water, was an increase of these
bacteria by about two orders of magnitude until
30-doc. At the end of 60-doc, their numbers were
restored to those recorded at the commencement of the
culture. This change during the first 30-doc could
be attributed to the lack of water exchange
accompanied by input of feed and accumulation of
feacal matter in the ponds. Once the water exchange