FAUNA AND FLORA O F MANGROVE SWAMPS
239
necessarily exclude their larvae. With the addition of water, bacteria,
blue-green algae and green algae succeed one another on the surface of
the soil and in the plankton. When the caretaker of the pond considers
that conditions are just right a number of fry are introduced from the
adjacent nursery pond, the number being known from experience of
the carrying capacity of the pond. The fish feed on the algae, grow,
and when they have reached a weight of upwards of 250 g ($ lb) they
may be harvested. I n a normal pond this weight will be reached in
about 8 months. The fish can reach weights of 2-4 kg (5-10 lb).
Harvesting of the crop is based on knowledge of the schooling habits of
the fish, and then decoying them into a place convenient for capture ;
for example, they may be decoyed towards the gate by admitting a
strong flow of sea water, and there they are relatively easy to net.
The annual production of protein per unit of area is high, amongst
the highest for any form of cultivation. I n certain of the tambaks of
eastern Java, which are situated on juvenile volcanic soils, the production of Chanos may be as high as 178-338 kg per hectare (200-380 lb
per acre). The average in eastern Java is around 160 kg per hectare
(180 Ib per acre), while on the marl soils of Madura the yield averages
only 71 kg per hectare (80 lb per acre).
I n some areas the yield is improved by green manuring. Mangrove
leaves or other chopped vegetation is scattered over the pond to sink
and decay, and add to the available nutritive material.
Of other species of fish some kinds of mullet, Mugilidae, which feed
on algae in much the same way as does Chanos, give similar yields of
protein. These will breed in the tambak areas and the larvae go in with
the tide a t the time of filling. Another fish of varying importance and
popularity is Tilapia mossambica. This fish, a stray from the eastern
coa3ts of Africa, prefers the less saline waters of the ponds farthest
from the sea. Yields may reach 89 kg per hect,are (100 Ib per acre) in
relatively unproductive regions. At all stages they feed most economically on the green algae while chanos, at least in youth, seems to prefer
the softer blue-greens. Hence if tilapia is present growth of the green
algae is easier to control and rarely, if ever, gets out of hand. Tilapia
seems to be limited to water where the depth is more than three times
the depth of the fish. Where tilapia is abundant malaria is controlled,
for there seems to be some sort of an association between the anopheline
larvae and certain of the green algae (cf. p. 159). Tilapia also breeds in
the ponds. This may be a nuisance, for these fish become sexually
mature at a small size and tend to expend their energies on sexual
display and reproduction rather than on growing to a marketable size.
But hybrid strains are available which give up to 98% of males and
239
necessarily exclude their larvae. With the addition of water, bacteria,
blue-green algae and green algae succeed one another on the surface of
the soil and in the plankton. When the caretaker of the pond considers
that conditions are just right a number of fry are introduced from the
adjacent nursery pond, the number being known from experience of
the carrying capacity of the pond. The fish feed on the algae, grow,
and when they have reached a weight of upwards of 250 g ($ lb) they
may be harvested. I n a normal pond this weight will be reached in
about 8 months. The fish can reach weights of 2-4 kg (5-10 lb).
Harvesting of the crop is based on knowledge of the schooling habits of
the fish, and then decoying them into a place convenient for capture ;
for example, they may be decoyed towards the gate by admitting a
strong flow of sea water, and there they are relatively easy to net.
The annual production of protein per unit of area is high, amongst
the highest for any form of cultivation. I n certain of the tambaks of
eastern Java, which are situated on juvenile volcanic soils, the production of Chanos may be as high as 178-338 kg per hectare (200-380 lb
per acre). The average in eastern Java is around 160 kg per hectare
(180 Ib per acre), while on the marl soils of Madura the yield averages
only 71 kg per hectare (80 lb per acre).
I n some areas the yield is improved by green manuring. Mangrove
leaves or other chopped vegetation is scattered over the pond to sink
and decay, and add to the available nutritive material.
Of other species of fish some kinds of mullet, Mugilidae, which feed
on algae in much the same way as does Chanos, give similar yields of
protein. These will breed in the tambak areas and the larvae go in with
the tide a t the time of filling. Another fish of varying importance and
popularity is Tilapia mossambica. This fish, a stray from the eastern
coa3ts of Africa, prefers the less saline waters of the ponds farthest
from the sea. Yields may reach 89 kg per hect,are (100 Ib per acre) in
relatively unproductive regions. At all stages they feed most economically on the green algae while chanos, at least in youth, seems to prefer
the softer blue-greens. Hence if tilapia is present growth of the green
algae is easier to control and rarely, if ever, gets out of hand. Tilapia
seems to be limited to water where the depth is more than three times
the depth of the fish. Where tilapia is abundant malaria is controlled,
for there seems to be some sort of an association between the anopheline
larvae and certain of the green algae (cf. p. 159). Tilapia also breeds in
the ponds. This may be a nuisance, for these fish become sexually
mature at a small size and tend to expend their energies on sexual
display and reproduction rather than on growing to a marketable size.
But hybrid strains are available which give up to 98% of males and