ESTUARINE FISH FARMING
169
differing accounts of the food of this fish, but they indicate an omnivorous habit with the exception of larger animals, since this fish does not
have the short gut and carnivorous teeth of a carnivore. The gut
contents are generally green in colour, which means that the food
most usually available is of vegetable origin. Schuster (1952) describes
how the introduction of T . mossambica into the Javanese fishponds
led to control of the filamentous green algae which had always been a
health hazard, and the same has proved true of Indian bheris (Pakrasi
et al., 1964). The feeding habits of this fish resulted in a good level of
fish production in the Javanese fishponds at the end of the last war,
when Chanos was not available.
I n Japan and Taiwan eels are raised very intensively in ponds (Fig.
14). Onodera (1963) states that yields unsurpassed by any other fish
culture can be obtained here. However, these are fish which are
wholly fed from outside-the analogy is with the broiler house and
not the farm. Giving as an example a group of five ponds, Onodera
calculates that an initial biomass of 16 110 kg grew in 230 days to one
of 32 980. This is equivalent to a stocking density of 0.51-0.71 kg/m2,
and an increment in weight of 1.36-2-63 kg/m2 per annum, or 13 600
to 26 300 kg/hectare/year. This certainly is high, but intensive trout
ponds regularly give a crop a t the rate of 10 000 kg/hectare/year, and
intensive carp rearing in running water can give a crop of 161 kg/m2
or a notional fish crop of 1.5 million kg/hectare/year (Hickling, 1962).
The unit rearing pond for eels is 1 000 m2, and the most productive
depth, 4 ft. The fodder which is so intensively fed is cheap trash fish
such as sardine, skipjack heads, akta mackerel, and silkworm pupae.
The food conversion rate varies from 7.6-19.9 per pound of eel (Fig. 14).
In Taiwan, where the culture of eels flourishes, the conversion rate
is roughly 13 to 1. Clearly, large amounts of fresh food must be always
available, and this could be a serious problem. However, Onodera says
that, in spite of the great quantities of fodder necessary, the culture pays.
Certainly, in Taiwan, eels are very well-priced, and can stand high
production costs.
I n Japan and Taiwan, the eel which is cultured is Anguillajaponica,
but in the Italian valli it is Anguilla anguilla. The eel comprises 80%
of the fish caught in the valli of Commachio, but there is no intensive
culture. The elvers or young eels enter in the spring, spend some years
in the valli, and are caught when, a t approaching maturity, they seek
to return to the sea. They are given no supplementary food, but grow
well on the rich food available to them. This again is the result of the
natural fertility of the estuarine environment in which the valli are
constructed. Brunelli (1 937) points out that decomposing organic
169
differing accounts of the food of this fish, but they indicate an omnivorous habit with the exception of larger animals, since this fish does not
have the short gut and carnivorous teeth of a carnivore. The gut
contents are generally green in colour, which means that the food
most usually available is of vegetable origin. Schuster (1952) describes
how the introduction of T . mossambica into the Javanese fishponds
led to control of the filamentous green algae which had always been a
health hazard, and the same has proved true of Indian bheris (Pakrasi
et al., 1964). The feeding habits of this fish resulted in a good level of
fish production in the Javanese fishponds at the end of the last war,
when Chanos was not available.
I n Japan and Taiwan eels are raised very intensively in ponds (Fig.
14). Onodera (1963) states that yields unsurpassed by any other fish
culture can be obtained here. However, these are fish which are
wholly fed from outside-the analogy is with the broiler house and
not the farm. Giving as an example a group of five ponds, Onodera
calculates that an initial biomass of 16 110 kg grew in 230 days to one
of 32 980. This is equivalent to a stocking density of 0.51-0.71 kg/m2,
and an increment in weight of 1.36-2-63 kg/m2 per annum, or 13 600
to 26 300 kg/hectare/year. This certainly is high, but intensive trout
ponds regularly give a crop a t the rate of 10 000 kg/hectare/year, and
intensive carp rearing in running water can give a crop of 161 kg/m2
or a notional fish crop of 1.5 million kg/hectare/year (Hickling, 1962).
The unit rearing pond for eels is 1 000 m2, and the most productive
depth, 4 ft. The fodder which is so intensively fed is cheap trash fish
such as sardine, skipjack heads, akta mackerel, and silkworm pupae.
The food conversion rate varies from 7.6-19.9 per pound of eel (Fig. 14).
In Taiwan, where the culture of eels flourishes, the conversion rate
is roughly 13 to 1. Clearly, large amounts of fresh food must be always
available, and this could be a serious problem. However, Onodera says
that, in spite of the great quantities of fodder necessary, the culture pays.
Certainly, in Taiwan, eels are very well-priced, and can stand high
production costs.
I n Japan and Taiwan, the eel which is cultured is Anguillajaponica,
but in the Italian valli it is Anguilla anguilla. The eel comprises 80%
of the fish caught in the valli of Commachio, but there is no intensive
culture. The elvers or young eels enter in the spring, spend some years
in the valli, and are caught when, a t approaching maturity, they seek
to return to the sea. They are given no supplementary food, but grow
well on the rich food available to them. This again is the result of the
natural fertility of the estuarine environment in which the valli are
constructed. Brunelli (1 937) points out that decomposing organic