(1.4 % of DM) of the buffalo manure used in the case study ponds corresponded to
those reported by Edwards et al. (1994b, 1994a, 1996). Due to the low N-content of
buffalo manure, it was necessary to apply high quantities to ponds in order to reach
the recommended N-rate of 4 kg N ha
À1 day
À1 (Edwards et al. 1994b, 1996). The
daily loading rates in the case study were lower than 200 g N ha
À1 , and thus far
below these recommendations. In addition, the relatively high water exchange in
the ponds limited the effect of fertilization.
Usually, higher fertilization is associated with an increase in plankton production (Nandeesha et al. 1984; Boyd 1982); however, the amount of plankton biomass
in the case study ponds was generally low, which can be attributed to an inappropriate water management system with high flow rates, as there tends to be an
inverse relationship between water flow rates and the abundance of plankton in
the study area. On one side, the high flow rates in the study area caused a flushingout of the phototrophic top water layer, whilst on the other side, they increased the
turbidity and inflow of eroded particles, which limited the penetration of sunlight.
The current aquaculture system focuses on grass carp, but grass carp production
has become a risky venture due to the occurrence of diseases that cause high fish
mortality rates (see Sect. 8.2). As long as the diseases identified cannot be prevented
or treated, it seems unwise to focus so much labor and other resources on this fish
species. Further, according to the local fish traders, the local production of common
carp, mud carp, mrigal and rohu is not sufficient to supply the demand of the Yen
Chau market. Increasing the portion of common carp may therefore be a promising
alternative for the farmers. Common carp generally fetch similarly high prices on
the local market as grass carp, and may be reared by farmers themselves, which may
save money and further help them to become more independent of the unreliable
seed supplies provided by the hatcheries. Therefore, Steinbronn (2009) proposed a
“non-grass carp-dominated system” with a species combination that favors common carp. Despite this, this proposed system still advocates that a certain number
of grass carp should be stocked (e.g., 10 %) in order to utilize the abundant leaf
and grass material and to provide fish feces for fertilizing the pond. Here, small
amounts of green plants of comparatively high nutritional quality should be used to
supplement the feed. Furthermore, grass carp stocked at low densities have shown
extremely high growth rates in polyculture ponds, as investigated by Sinha and
Gupta (1975).
Under the proposed non-grass carp-dominated system, the feed base of the nongrass carp species would be improved through: (a) an increase in the availability of
natural food and use of a proper water management system, and (b) an improved
supplemental feed based on local products such as maize and cassava. Tuan (2010)
tested different supplemental diets for common carp based on locally available
resources such as meal from cassava, maize and soybean, as well as rice bran. In a
feeding trial, common carp were fed either a control diet (with fishmeal being the
major protein-source) or a diet in which 25 %, 50 % or 75 % of fishmeal was
replaced by these plant-derived resources. Even though fish feeding on the control
diet showed the highest growth rates, the feed costs per fish produced were lowest
in the diet in which 75 % of the fishmeal was replaced by local products. This feed,
8 Improved Sustainable Aquaculture Systems for Small-Scale Farmers . . .
293
those reported by Edwards et al. (1994b, 1994a, 1996). Due to the low N-content of
buffalo manure, it was necessary to apply high quantities to ponds in order to reach
the recommended N-rate of 4 kg N ha
À1 day
À1 (Edwards et al. 1994b, 1996). The
daily loading rates in the case study were lower than 200 g N ha
À1 , and thus far
below these recommendations. In addition, the relatively high water exchange in
the ponds limited the effect of fertilization.
Usually, higher fertilization is associated with an increase in plankton production (Nandeesha et al. 1984; Boyd 1982); however, the amount of plankton biomass
in the case study ponds was generally low, which can be attributed to an inappropriate water management system with high flow rates, as there tends to be an
inverse relationship between water flow rates and the abundance of plankton in
the study area. On one side, the high flow rates in the study area caused a flushingout of the phototrophic top water layer, whilst on the other side, they increased the
turbidity and inflow of eroded particles, which limited the penetration of sunlight.
The current aquaculture system focuses on grass carp, but grass carp production
has become a risky venture due to the occurrence of diseases that cause high fish
mortality rates (see Sect. 8.2). As long as the diseases identified cannot be prevented
or treated, it seems unwise to focus so much labor and other resources on this fish
species. Further, according to the local fish traders, the local production of common
carp, mud carp, mrigal and rohu is not sufficient to supply the demand of the Yen
Chau market. Increasing the portion of common carp may therefore be a promising
alternative for the farmers. Common carp generally fetch similarly high prices on
the local market as grass carp, and may be reared by farmers themselves, which may
save money and further help them to become more independent of the unreliable
seed supplies provided by the hatcheries. Therefore, Steinbronn (2009) proposed a
“non-grass carp-dominated system” with a species combination that favors common carp. Despite this, this proposed system still advocates that a certain number
of grass carp should be stocked (e.g., 10 %) in order to utilize the abundant leaf
and grass material and to provide fish feces for fertilizing the pond. Here, small
amounts of green plants of comparatively high nutritional quality should be used to
supplement the feed. Furthermore, grass carp stocked at low densities have shown
extremely high growth rates in polyculture ponds, as investigated by Sinha and
Gupta (1975).
Under the proposed non-grass carp-dominated system, the feed base of the nongrass carp species would be improved through: (a) an increase in the availability of
natural food and use of a proper water management system, and (b) an improved
supplemental feed based on local products such as maize and cassava. Tuan (2010)
tested different supplemental diets for common carp based on locally available
resources such as meal from cassava, maize and soybean, as well as rice bran. In a
feeding trial, common carp were fed either a control diet (with fishmeal being the
major protein-source) or a diet in which 25 %, 50 % or 75 % of fishmeal was
replaced by these plant-derived resources. Even though fish feeding on the control
diet showed the highest growth rates, the feed costs per fish produced were lowest
in the diet in which 75 % of the fishmeal was replaced by local products. This feed,
8 Improved Sustainable Aquaculture Systems for Small-Scale Farmers . . .
293