Carppmducon
297.
fish food organisms present along the food chain. The control of water quali—
ty through fish population also has high priority in these organically loaded
ecosystems.
Filter
feeder
fish
species
like
silver carp, catla and tilapia
utilize the filtrable and digestible particles. of the seston and maintain the
plancton
in
a
rather
balanced
state
without
harmful
fluctuations.
Over—
stocking and undigestible algal species like Microcystis may create severe
unbalance in plankton life. Bottom stirrers or bioturbator fish species like
common
carp
and
mrigal utilize benthic food organism and to keep the water
sediment interface oxidized preventing the harmful accumulation of reduced
substances
in the
sediment.
A permanently disturbed sediment surface is unfa—
vorable
to
the
mass
development of Microcystis species which are so abundant
but undesirable
in tropical fish ponds.
Applying a proper manuring rate Schroeder (1978) has discovered a clear
linear
relationship
between
stocking density and maximum fish yield in the
range
of
1500—9000
fingerlings/ha
(Fig.
1).
In
the
same
stocking
range
Schroeder
(1974),
Sharma
and
Olah
(1976) an Buck et al.
(1978) have got much
decreased
maximum
fish
yield,
but
in
these
experiments the ponds were under—
manured.
Even
in slightly undermanured experiments maximum fish yield decrea—
sed
but
less
(Noriega—Curtis,
1979).
If
stocking
density was
increased
to
about
18000/ha the
maximum
fish yield remained the same as with an optimal
fish
density of 8 to 9000/ha (Moav et al. 1977; Olah, 1985). In a more over—
stocked pond (23000/ha) there is a severe grazing pressure on both the plankton
and
benthic
organism.
This
destroys
the
reproductive potential of the food
populations and decrease in the*maximum-fish yield is significant (Tang,1970).
Atpresent,
a'level
8‘—9000
fingerlings or
yearlings per hectare is
advisable
as
a
stocking parameter when the man‘uring rate
is
optimal. With
intermittent
harvesting
these
numbers
may
increase
up
to
18
and
10000
respectively.
'
‘
’
_
_
35
.
=
,
-
o
each from
Schroeder,1978
,
Moav et al.,1977
/
’
.overstocked
,
o
/
-
30
…,
…
“
‘
‘
0lâh,1985
.
/
ovèrstocked
,,ê‘=’, 25
,
/
'..
f'
»
'
.
,
2…
,
_/o
Noriega-Cuis,1979 ,
_
'
/
o
.
‘
Buch et al., 1978
% 20
/°/
'
undermanured
'
o—>23
“;
Sha…“ “"“ °'àhïm55harma
O‘|ah,‘1‘980
‘
Tang,1970
,
_.
'undermanured
_
,
.
_:
,
,
‘
£
15
/
'
Burns and
Stickney,1980
‘
_
,.
,o
,
,
…
_'
.
*:;g
o
.
monoculture
,»
-
:,
,
/
_.
«
u
,
«
,
;
,
,
,
.
Schroeder,1974
.
'
'
'_
,
.
‘
‘
—
°“
10
,
…
_
,
/
,
,‘.
.
'
'
/
,
.
5
;!
'
/
/
/
.
.
'1234567891011121314l5161718192021
,
_
stocking
density 103
ha'1
Fig- 1 : Relation between stocking density and fish production in manured ponds