representative number of all stocked species without significantly disturbing pond
turbidity; therefore, the primary species, common carp (50 % of all stocked
species), was used as an estimate of the total fish biomass.
In Chieng Khoi commune, farmers were advised to split the daily feed into two
portions; once in the morning and once in the evening (Rahman and Meyer 2009),
but when the farmers were very busy they could only feed the fish once a day.
Feeding in the evening only was thus recommended, because water oxygen levels
in the morning tend to be low.
In our study in Chieng Khoi district, the method of application used for leaves as
feed for grass carp was also modified. It had been observed that the farmers’ normal
practice was to feed cassava, banana and maize leaves complete with stems,
branches and middle ribs, introducing large amounts of indigestible organic matter
into the pond, and thus reducing the oxygen level by decomposition processes.
To reduce the organic load experienced at the bottom of the ponds, farmers were
advised to feed only those parts of the leaves known to be eaten by grass carp.
In order to improve the ability of farmers to calculate the investments and
benefits of aquaculture as a farming activity, they were given a book in which
they recorded all body masses, as well as the amounts and costs of inputs such as
fingerlings, feeds and fertilizer, labor, time and water flows, plus outputs such as the
number and mass of fish caught and other pond products. Dead fish or abnormal
behavior among fish was also recorded. This practice gave farmers an overview of
the input–output data, current stocking densities, species composition, and the
survival and growth rates of different species, and so helped them to appreciate
the financial benefits of aquaculture as a farming activity. Dung and Minh (2010)
recommended that this kind of simple record book should be used in all farming
activities, so that farmers can compare different land uses and farming activities and
use it as an aid to their decision-making.
8.5.3 Evaluation of Implemented Modifications in the Pond
Management and Feeding Regime
Ponds under the traditional management system had a high variability in water
quality (Pucher et al. 2010a; Steinbronn 2009) which was greatly influenced by the
surrounding land use types and also water exchange. Heavy rain storms in particular
rapidly changed the water quality, resulting in higher turbidity, and high ammonia
levels sometimes above 1 mg NH 4 -N l
À1 in the spring and autumn. In addition to
introduced sediments, Steinbronn (2009) noted that there was a tendency for water
inflow rates and plankton densities to be negatively correlated because ponds were
designed with overflow as the only water outflow, meaning water was only ever
removed from the top layer of the pond. However, this top layer is the most
productive water layer of a pond as it contains most of the phototrophic plankton
and is thereby the main source of oxygen and the base of pond productivity.
300
J. Pucher et al.
turbidity; therefore, the primary species, common carp (50 % of all stocked
species), was used as an estimate of the total fish biomass.
In Chieng Khoi commune, farmers were advised to split the daily feed into two
portions; once in the morning and once in the evening (Rahman and Meyer 2009),
but when the farmers were very busy they could only feed the fish once a day.
Feeding in the evening only was thus recommended, because water oxygen levels
in the morning tend to be low.
In our study in Chieng Khoi district, the method of application used for leaves as
feed for grass carp was also modified. It had been observed that the farmers’ normal
practice was to feed cassava, banana and maize leaves complete with stems,
branches and middle ribs, introducing large amounts of indigestible organic matter
into the pond, and thus reducing the oxygen level by decomposition processes.
To reduce the organic load experienced at the bottom of the ponds, farmers were
advised to feed only those parts of the leaves known to be eaten by grass carp.
In order to improve the ability of farmers to calculate the investments and
benefits of aquaculture as a farming activity, they were given a book in which
they recorded all body masses, as well as the amounts and costs of inputs such as
fingerlings, feeds and fertilizer, labor, time and water flows, plus outputs such as the
number and mass of fish caught and other pond products. Dead fish or abnormal
behavior among fish was also recorded. This practice gave farmers an overview of
the input–output data, current stocking densities, species composition, and the
survival and growth rates of different species, and so helped them to appreciate
the financial benefits of aquaculture as a farming activity. Dung and Minh (2010)
recommended that this kind of simple record book should be used in all farming
activities, so that farmers can compare different land uses and farming activities and
use it as an aid to their decision-making.
8.5.3 Evaluation of Implemented Modifications in the Pond
Management and Feeding Regime
Ponds under the traditional management system had a high variability in water
quality (Pucher et al. 2010a; Steinbronn 2009) which was greatly influenced by the
surrounding land use types and also water exchange. Heavy rain storms in particular
rapidly changed the water quality, resulting in higher turbidity, and high ammonia
levels sometimes above 1 mg NH 4 -N l
À1 in the spring and autumn. In addition to
introduced sediments, Steinbronn (2009) noted that there was a tendency for water
inflow rates and plankton densities to be negatively correlated because ponds were
designed with overflow as the only water outflow, meaning water was only ever
removed from the top layer of the pond. However, this top layer is the most
productive water layer of a pond as it contains most of the phototrophic plankton
and is thereby the main source of oxygen and the base of pond productivity.
300
J. Pucher et al.