128
C . I?. KICKLINQ
costly. Entire pond systems may be jeopardized by siltation, and in
Java it was necessary to introduce legislation to enforce the upkeep
of the canal system and other waterways. As a result, says Schuster,
great changes for the better were seen.
Rabanal (1962) writing of conditions in the Philippines, advises
sites which have an elevation between - 1 and +4.5 ft above mean low
water as being most suitable for brackish-water fishponds. At greater
elevations, between +4.5 and + 7 ft, ponds can only be made suitable
by excavation, which is costly, because otherwise they will only be
filled with sea water at extreme high tides and therefore at rare intervals. Conversely, sites on low levels between -1 and -2 ft would
become suitable only if filled in, while sites below -2 ft are unsuitable
because they could never be completely drained by gravity.
Lin (1968), writing of the brackish-water complexes in the south
and west of Taiwan, thinks that the ideal site is one where an extensive
tidal flat is covered at high tide to a depth of at least 60 cm, and is
wholly exposed at low tide. Even after the building of embankments
and feeder canals and sluices, this will still allow the optimum depth
of 30-40 cm to be maintained in the ponds.
So far, the situation of ponds has been considered in relation to
irrigation by gravity. However, by the use of pumps, an artificial head
of water can be created which would make the siting of ponds independant of tide levels and possible in tideless estuaries. Many fresh-water
fish farms make use of pumps, and where the ponds are large and highyielding, pumping is economical or even cheap (Hickling, 1962).
4. Pumping
Generally speaking, the yield of brackish-water fishponds is one
half or less that of fresh-water ponds, so that in their case the economy
of pumping is more doubtful. Lin (1968) considers that pumping would
be uneconomic in Taiwan, even though the rate of fish production is
the highest in the world’s brackish-water pond industries. Schuurman
(1964) is also doubtful of the technical and economic aspects, since the
equipment must be technically reliable for salt water, and must also
be inexpensive to buy and operate. The pump would have to fulfil two
requirements, namely, to fill the pond at the start of the exploitation
period, which would call for a rapid rate of pumping, and thereafter to
keep the pond topped up t o make good seepage and evaporation.
Schuurman considers, as an example, a &hectare pond, to be filled to
a depth of 40cm and kept at that depth with a pump which must
work against a head of two metres. He finds that a 6 hp pump would
satisfy both requirements, since it would fill the pond with 48 hours
C . I?. KICKLINQ
costly. Entire pond systems may be jeopardized by siltation, and in
Java it was necessary to introduce legislation to enforce the upkeep
of the canal system and other waterways. As a result, says Schuster,
great changes for the better were seen.
Rabanal (1962) writing of conditions in the Philippines, advises
sites which have an elevation between - 1 and +4.5 ft above mean low
water as being most suitable for brackish-water fishponds. At greater
elevations, between +4.5 and + 7 ft, ponds can only be made suitable
by excavation, which is costly, because otherwise they will only be
filled with sea water at extreme high tides and therefore at rare intervals. Conversely, sites on low levels between -1 and -2 ft would
become suitable only if filled in, while sites below -2 ft are unsuitable
because they could never be completely drained by gravity.
Lin (1968), writing of the brackish-water complexes in the south
and west of Taiwan, thinks that the ideal site is one where an extensive
tidal flat is covered at high tide to a depth of at least 60 cm, and is
wholly exposed at low tide. Even after the building of embankments
and feeder canals and sluices, this will still allow the optimum depth
of 30-40 cm to be maintained in the ponds.
So far, the situation of ponds has been considered in relation to
irrigation by gravity. However, by the use of pumps, an artificial head
of water can be created which would make the siting of ponds independant of tide levels and possible in tideless estuaries. Many fresh-water
fish farms make use of pumps, and where the ponds are large and highyielding, pumping is economical or even cheap (Hickling, 1962).
4. Pumping
Generally speaking, the yield of brackish-water fishponds is one
half or less that of fresh-water ponds, so that in their case the economy
of pumping is more doubtful. Lin (1968) considers that pumping would
be uneconomic in Taiwan, even though the rate of fish production is
the highest in the world’s brackish-water pond industries. Schuurman
(1964) is also doubtful of the technical and economic aspects, since the
equipment must be technically reliable for salt water, and must also
be inexpensive to buy and operate. The pump would have to fulfil two
requirements, namely, to fill the pond at the start of the exploitation
period, which would call for a rapid rate of pumping, and thereafter to
keep the pond topped up t o make good seepage and evaporation.
Schuurman considers, as an example, a &hectare pond, to be filled to
a depth of 40cm and kept at that depth with a pump which must
work against a head of two metres. He finds that a 6 hp pump would
satisfy both requirements, since it would fill the pond with 48 hours