Milkfish Production and Processing ... - WorldFish Center

e) Environmental factors affecting growth
and survival of milkfish in the pond
are as follows:
• Dissolved oxygen – optimum level is
-10 ppm, below this level is harmful
to the fish.
• Weather conditions – sudden change
in weather condition causes oxygen
depletion in ponds.
• Salinity – milkfish growth is retarded
at 45 ppt salinity; frequent water
exchange should be done during the
summer months.
• Water temperature – affects metabolism
of fish and consequently
growth rates; optimum temperature
is 5- °C; growth is reduced
at °C.
• pH – indicates the presence of
metabolites, photosynthetic activity
and fertility of pond water; high pH
value indicates that the pond water
is too fertile, therefore, there is a
possibility of a plankton bloom; low
pH means the water is infertile and
plankton growth is slow; optimum
range is 6.5-8.5; above or below this
range, the water should be changed
immediately.
• Hydrogen sulfide – the by-product
of decomposition, produced by
the chemical reduction of organic
residues which accumulate at the
pond bottom; the bottom soil turns
black and smells like a rotten egg.
It decreases dissolved oxygen and
pH of water and fish will die at
ppm concentration; hydrogen
sulfide concentration could be
reduced by water exchange and
draining the pond.
f) During rainy days, drain the uppermost
freshwater layer in the water column
to prevent sudden drop in salinity.
g) If fish is observed to be gasping or
swimming in circles, replenish with new
water from the river source and if not
possible draw water from adjoining
Milkfish Production and Processing Technologies in the Philippines
ponds. Pumps can be used in such
emergency.
h) Supplemental feeds such as commercial
fish feed, rice brand or bread crumbs
should be provided if lab-lab is prematurely
depleted due to overgrazing,
poor water condition, or persistent
inclement weather.
i) Harvest and transfer the fingerings
from the nursery pond to the transition
pond after 4 to 6 weeks ( -7 cm
with average weight of 1-6 g).
16.4 Harvest and Transfer of
Fingerlings
Harvest or transfer should be preferably done in
the morning just after sunrise or late afternoon
to avoid mass mortality and stress. Oxygen
content of pond water is at its lowest before
sunrise while water temperature increases
considerably after 8:00 o’clock in the morning.
There are several methods of harvesting or
transfer of milkfish fingerlings depending upon
the circumstances. The most ideal is to take
advantage of the behavior of the milkfish which
always try to swim against the current. But this
is not always possible. So different methods
may be employed as detailed below:
a) “Pasulang” or Freshening Method
• Initially decrease the pond water
level in the morning or at noon time
and admit new water to the nursery
the following day. This induces the
fingerlings to go against the water
current and congregate to catching
pond/canal or confinement net
where they are held prior to transfer
or packing.
• Return the gate screen when the
required fingerlings have been confined
in the catching pond or canal
or confinement net if the fingerlings
are to be transported for stocking
in grow-out pond, pens, or cages.
• Allow water to flow continuously
in the catching pond or canal or
confining net while the fish are still
being held.
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