ESTUARINE FISH FARMING
127
(1952) considers that the most suitable level for the floor of a fishpond
should be about sixteen inches above the level of mean high-water
springs. It then becomes possible to fill the ponds during the hours in
which sea level is higher than the floor level of the pond. Conversely,
the ponds can be completely drained during the hours when sea level
is below that of the floor of the pond.
Schuster gives a table which shows that, in the years 1937-1941,
the smallest number of hours in any month during which such ponds
could be filled was two in September, 1937 (none in February, 1937),
and the greatest number was 60 hours in May, 1938. He also shows
that, where the level of the pond is too high (110-130 cm above mean
sea level) the filling of the ponds can be done for only 11; hours per
month, on average ; whereas where the level of the pond bottom is 80100 cm above mean sea level, the ponds can be filled for 32 hours per
month, on average.
Schuurman (1964) considers that the water of a brackish-water pond
should be renewed at least once every ten days. To maintain the ponds
at the optimum depth of 40 cm, to change the water as required and
to replace water lost by seepage and evaporation, the pond must be
sited so that the tide regularly exceeds a height of 40 cm, above the
level of the pond bottom. To get this result, it may be necessary to
excavate the pond bottom so as to adjust its level in relation to the
tide. However, to lower an area of, say, 5 hectares by 10 cm involves
moving some 5 000 cubic metres of soil. As this may have to be done
by hand labour, where the soil is too soft to bear earth-moving equipment, the cost will play a major part in determining whether the
exploitation is practicable.
I n fact, a height difference greater than 40 cm, would have to be
allowed for. Except where the pond directly abuts on the sea, the sea
water must be brought along a canal, which may also have to supply
other ponds. Also, each pond will draw its water from the canal
through one or more gates whose total area will be small in comparison
with that of the pond. The loss of ( ( head ” on both these counts could
be substantial, and would have to be allowed for in considering levels.
A notable difficulty in the efficient working of large areas of brackishwater fishponds has been to keep the sea water supply canals clear, and
their depth in correct relation with the tidal movements and so with
the level of the pond bottoms. The inflowing estuarine water may
carry with it a large load of silt, which may be deposited in the canal
at times of slow flow, The bheris, or estuarine fishponds of West
Bengal, are in danger of disuse through the siltation of the feeder
canals (Pillay, 1954 ; Pakrasi et al. 1964) and clearance would be very
127
(1952) considers that the most suitable level for the floor of a fishpond
should be about sixteen inches above the level of mean high-water
springs. It then becomes possible to fill the ponds during the hours in
which sea level is higher than the floor level of the pond. Conversely,
the ponds can be completely drained during the hours when sea level
is below that of the floor of the pond.
Schuster gives a table which shows that, in the years 1937-1941,
the smallest number of hours in any month during which such ponds
could be filled was two in September, 1937 (none in February, 1937),
and the greatest number was 60 hours in May, 1938. He also shows
that, where the level of the pond is too high (110-130 cm above mean
sea level) the filling of the ponds can be done for only 11; hours per
month, on average ; whereas where the level of the pond bottom is 80100 cm above mean sea level, the ponds can be filled for 32 hours per
month, on average.
Schuurman (1964) considers that the water of a brackish-water pond
should be renewed at least once every ten days. To maintain the ponds
at the optimum depth of 40 cm, to change the water as required and
to replace water lost by seepage and evaporation, the pond must be
sited so that the tide regularly exceeds a height of 40 cm, above the
level of the pond bottom. To get this result, it may be necessary to
excavate the pond bottom so as to adjust its level in relation to the
tide. However, to lower an area of, say, 5 hectares by 10 cm involves
moving some 5 000 cubic metres of soil. As this may have to be done
by hand labour, where the soil is too soft to bear earth-moving equipment, the cost will play a major part in determining whether the
exploitation is practicable.
I n fact, a height difference greater than 40 cm, would have to be
allowed for. Except where the pond directly abuts on the sea, the sea
water must be brought along a canal, which may also have to supply
other ponds. Also, each pond will draw its water from the canal
through one or more gates whose total area will be small in comparison
with that of the pond. The loss of ( ( head ” on both these counts could
be substantial, and would have to be allowed for in considering levels.
A notable difficulty in the efficient working of large areas of brackishwater fishponds has been to keep the sea water supply canals clear, and
their depth in correct relation with the tidal movements and so with
the level of the pond bottoms. The inflowing estuarine water may
carry with it a large load of silt, which may be deposited in the canal
at times of slow flow, The bheris, or estuarine fishponds of West
Bengal, are in danger of disuse through the siltation of the feeder
canals (Pillay, 1954 ; Pakrasi et al. 1964) and clearance would be very