Eighty
pieces each. of prawns belonging to two (2) stages of growth A = Stage I (61
day-old), B = Stage II (100 day-old), obtained
from Nasugbu, Batangas were transported to the experimental site. Upon
arrival, the transported prawns were placed in 100-liter capacity tanks
containing 40 liters of pond water with a salinity of 26 ppt. The set up
was provided with moderate aeration. The experimental animals were conditioned
to its new environment for 24 hours. They were then acclimated to a water
salinity of 18 ppt prior to the conduct of the experiment.
Eight pieces each of prawns representing the two stages of growth
indicated above were stocked in 18 units of 100-liter capacity tanks
containing 30 liters of water at 18 ppt or an effective stocking density of
33.6/sq.m. Three dosage levels of NS Series SPO
were inoculated to each tank as follows:
| T
R E A T M E N T |
D
O S AG E |
| g/cu.m. |
g/30
Ii |
| I |
0.5 |
0.015 |
| II |
1.0 |
0.030 |
| III |
1.5 |
0.045 |
The
required amount of NS Series SPO was applied by preparing a stock solution of
1 gram of culture in 1 liter of water. Aliquot amounts were added in
each tank i.e. 15 ml, 30 ml, and 45 ml each for treatments I, II and III
respectively. Prawns were fed with commercially prepared
diet (SMI-President Feeds) used in earlier experiments given at 12% BW. Feed
ration for the day was divided into six equal parts and fed to the prawns at
every 4 hours interval.
The physio-chemical parameters of the rearing water were closely monitored as
in the first two experiments. These consisted of pH, D.O., nitrite, ammonia
and total solids (unconsumed feeds and fecal matter). Samples were taken
initially and every two days thereafter using standard methods. No
water change was done for the entire duration of the experiment which lasted
for 14 days. Initial and final length and weight of the test animals
were taken. The survival of the test animals in each of the treatments were
taken at the termination of the experiment.
Effect on the Nitrite level
Varying
dosages of NS Series SPO (0.5, 1.0, 1.5 g/cu.m.) inoculated into the growing
medium of prawns did not produce differences in the nitrite level. In all the
treatments, the values were maintained well within optimum limits which were
set at 0.1 ppm - 0.5 ppm for nitrite.
Among
the three inoculum levels applied, no marked differences in the nitrite levels
were observed during the conduct of the experiment which lasted up to 14 days.
This means that 0.5 g/cu.m. which is the lowest rate of application of NS
Series SPO is sufficient to reduce the nitrite content of the water to a safe
level. This may be explained by the fact that bacteria multiply and grow as
long as there is available substrate in the environment.
In both the 61 day-old prawns and 100 day-old prawns, the three dosages of NS
Series SPO were equally effective in controlling the nitrite levels within
optimum limits. This demonstrates the efficiency of the bacteria inoculated in
the growing medium which act on higher amounts of uneaten feeds and fecal-
matter as the prawns grew older. Statistical analysis showed no
significant: differences among the three dosages of NS Series SPO for two
growth stages of prawns.
Effect
on the Ammonia level
The
ammonia level in all the treatments remained very low and within tolerable
limits set for prawn. The same trend was observed, that is, there were no
differences in the ammonia level of the water inoculated with varying dosages
of bacteria. The rearing water in both Stage I (61 day-old) and Stage II (100
day-old) prawns showed comparable values throughout the experimental period.
Effect
on Dissolved Oxygen
The varying levels of bacterial inoculation produced comparable values of
dissolved oxygen. The D.O. levels in levels in the various treatments were all
greater than 3.5 ppm, the optimum limit set for prawn farming.
Based
on the above results, bacterial inoculation may be reduced to 0.5 g/cu.m.
without adversely affecting the D.O. level in the water. Moreover,
bioaugmentation of NS Series SPO at the start of the culture period prevented
the accumulation of organic matter in the medium. This being the case, there
were no drastic reduction in the D.O. level since decomposition occurred
gradually with time.
Total
Solids
The
total solids content of the rearing water inoculated with three levels of
bacteria were comparable for specific stages of growth of prawns. Note that
the total solids content of the 100 day-old prawn ii is 2 to 3 times more than
in the younger (61 day-old) prawn. This demonstrates further the efficiency of
NS Series SPO given at a low dosage of 0.5 g/cu.m.
The
bacterial population did not differ among the different levels of inoculation.
Since bacterial growth is dependent on the amount of substrate in the medium,
the amount of substrate (waste materials) available can only support bacterial
growth to this level. Between the two growth
stages of the prawns, similar bacterial counts were also observed. Apparently,
the minimum amount of bacteria applied (0.5 g/cu.m.) has the capacity to
efficiently decompose even the higher substrate level produced by Stage II
(100 day-old) prawns. Statistical analysis did not show
significant differences among the various treatment dosages.
Statistical
analysis showed no significant differences among the three dosages of NS
Series SPO inoculated in the rearing medium for both 61 day-old and 100
day-old prawns.
Effect on Prawn Growth and Survival
High survival of prawn (2 growth stages) in a closed semi-controlled culture
system without water change was observed in all the treatments. Survival
values ranged from 87.5% to 95.8%. This may be attributed to the maintenance
of good water
quality throughout the duration of the experiment.
Duncan's Multiple Range Test did not show significant differences among the
three varied dosage applications.
