ESTUARINE FISH FARMING
135
6. The harsh estuarine physical environment
The estuarine environment has been described as “ harsh ”,
especially in respect of rapid changesin physical factors such as temperature, salinity, and oxygen content. Only rather a narrow group of fish
and other organisms can flourish there, and, of the fish, only a few
species are also acceptable food fish, and so profitable for fish farming.
Rapid changes of salinity are self-evident where sea water and
fresh water are mixing, and where evaporation on the one hand and
flooding on the other may cause extreme fluctuations. Rapid changes
of temperature may be caused by the tide rising over flats heated by
the sun or chilled by severe frosts or cold winds. Rapid changes of
oxygen may be caused by water flushed out of reed beds or polluted
with organic matter, or which has passed over beds of green vegetation
actively photosynthesising in bright sunshine. When such waters are
impounded in ponds, there will still be diurnal changes in temperature
and oxygen content. If the impounded water is too deep and there is
no wind, stratification may set in, leading to a deoxygenated layer of
bottom water and the death of fish and other organisms. In the
Italian valli, which are rich in nutrients, this stagnation and oxygen
deficiency commonly accompany high productivity (d’Ancona, 1954).
The control of salinity is achieved by having a system of freshwater canals, with branches to each pond, as well as sea water canals.
Schuster advises that, in the Javanese ponds, salinity should be maintained between the limits of 10 to 35%, by regular replenishment with
new water. Too high a salinity diminishes the quality of the food
grown in the ponds as well as harming the fish. Salinity tolerances
will be discussed later, but here it can be said that the most important
fish able t o cope with these harsh conditions are the Chanos (Fig. 8),
a silvery-white herring-like fish very popular as food throughout its
range which is the tropical Indo-Pacific; the Grey Mullets (Fig. 8)
(species of Mugil and Liza, but chiefly M . cephalus), fish of temperate
as well as tropical and sub-tropical waters ; and eels, Anguilla japonica
(Temminck and Schlegel) in the East, and A . anguilla L. in Europe.
The penaeid prawns (Fig. 8) are also notably euryhaline and are a
very important product of brackish-water fishponds in the tropics.
7. Intensive fish culture in cages
The sheltered conditions offered by estuaries have made possible
the intensive culture of fish and prawns in open nets and cages. For
example, the Hitsuishi fish farm in the Inland Sea of Japan makes use
of a channel between two islands, one end of which is closed by a con-
135
6. The harsh estuarine physical environment
The estuarine environment has been described as “ harsh ”,
especially in respect of rapid changesin physical factors such as temperature, salinity, and oxygen content. Only rather a narrow group of fish
and other organisms can flourish there, and, of the fish, only a few
species are also acceptable food fish, and so profitable for fish farming.
Rapid changes of salinity are self-evident where sea water and
fresh water are mixing, and where evaporation on the one hand and
flooding on the other may cause extreme fluctuations. Rapid changes
of temperature may be caused by the tide rising over flats heated by
the sun or chilled by severe frosts or cold winds. Rapid changes of
oxygen may be caused by water flushed out of reed beds or polluted
with organic matter, or which has passed over beds of green vegetation
actively photosynthesising in bright sunshine. When such waters are
impounded in ponds, there will still be diurnal changes in temperature
and oxygen content. If the impounded water is too deep and there is
no wind, stratification may set in, leading to a deoxygenated layer of
bottom water and the death of fish and other organisms. In the
Italian valli, which are rich in nutrients, this stagnation and oxygen
deficiency commonly accompany high productivity (d’Ancona, 1954).
The control of salinity is achieved by having a system of freshwater canals, with branches to each pond, as well as sea water canals.
Schuster advises that, in the Javanese ponds, salinity should be maintained between the limits of 10 to 35%, by regular replenishment with
new water. Too high a salinity diminishes the quality of the food
grown in the ponds as well as harming the fish. Salinity tolerances
will be discussed later, but here it can be said that the most important
fish able t o cope with these harsh conditions are the Chanos (Fig. 8),
a silvery-white herring-like fish very popular as food throughout its
range which is the tropical Indo-Pacific; the Grey Mullets (Fig. 8)
(species of Mugil and Liza, but chiefly M . cephalus), fish of temperate
as well as tropical and sub-tropical waters ; and eels, Anguilla japonica
(Temminck and Schlegel) in the East, and A . anguilla L. in Europe.
The penaeid prawns (Fig. 8) are also notably euryhaline and are a
very important product of brackish-water fishponds in the tropics.
7. Intensive fish culture in cages
The sheltered conditions offered by estuaries have made possible
the intensive culture of fish and prawns in open nets and cages. For
example, the Hitsuishi fish farm in the Inland Sea of Japan makes use
of a channel between two islands, one end of which is closed by a con-