Depending on the source of water, the surrounding land use and soil types, ponds
in the research area showed a high variability in terms of water quality, natural food
availability and hence in the level of fish production and financial benefits to be
gained from the aquaculture activities. Unsuitable water quality elements such as
water temperature (Sifa et al. 2002; Cagauan 2001; Black 1998; Alcaraz et al. 1993;
Chervinski 1982), water oxygen (Black 1998; Ross 2000) and water pH (Svobodova
et al. 1993) levels were shown to have a negative effect on fish growth. Especially
within extensive and semi-intensive aquaculture systems, fish growth is correlated
with the availability and abundance of natural food resources (Rahman et al. 2008b;
Muendo et al. 2006; Schroeder et al. 1990; Spataru et al. 1983; Kolar and Rahel
1993; Pucher et al. 2011b), and both water quality and the availability of natural food
are primarily or secondarily affected by the suspended particle load. During the
research in Yen Chau district, ponds with maize and cassava fields on yellow soils in
the watershed above and fed by rain water, showed a distinctly higher suspended
solids load than ponds located on the rice paddy plateaus and fed by reservoir water.
Under the modified pond management system, a reduction in turbidity was obtained,
leading to higher primary production activity levels and with higher oxygen production peaks during the day (Pucher et al. 2010a). Oxygen limitations occurred in the
morning, similarly to those observed under the traditional pond management system.
This suggested better primary production activities, resulting in higher oxygen
production during the day but also higher oxygen depletion at night. Nevertheless,
higher primary production levels stimulated by fertilization and reduced water flowthrough resulted in a greater abundance of natural food resources and provided
more support for growth, especially of the filter-feeder fish species such as silver
carp, bighead carp and tilapia. Also, grass carp under the modified pond management system showed a higher growth rate than under the traditional system,
indicating a better feed base of natural food due to lower stocking densities (Sinha
and Gupta 1975) and consumption of pelleted supplemental feed which was
intended to feed common carp. All stocked species under the semi-intensive pond
management system, such as the common carp, grass carp, silver carp, bighead carp
and tilapia, showed significantly higher specific growth rates (in %), of 2.6 Æ 0.2,
2.4 Æ 0.3, 2.5 Æ 0.1, 2.9 Æ 0.2, and 3.4 Æ 0.6 respectively, and when compared to
those under the traditional management system, which were 1.8 Æ 0.1, 1.9 Æ 0.7,
1.7 Æ 0.0, 2.0 Æ 0.0, and 2.5 Æ 0.4 respectively (Pucher et al. 2010b). Net production (in kg/1,000 m
2
) over the 7 months of the trial under the traditional pond
management system was 87.4 Æ 43.6, with 36.8 Æ 18.8 for grass carp, 4.3 Æ 5.5
for common carp, 16.7 Æ 3.8 for silver carp, 8.3 Æ 4.4 for bighead carp and
16.1 Æ 15.4 for tilapia (Pucher et al. 2010a, b). Net production (in kg/1,000m
2
)
under the semi-intensive management system was 227.5 Æ 41.6, with 45.5 Æ 11.8
for grass carp, 35.8 Æ 12.7 for common carp, 60.8 Æ 2.8 for silver carp,
39.6 Æ 13.0 for bighead carp and 35.7 Æ 21.9 for tilapia. All fish species, except
grass carp and tilapia, had significantly higher production levels under the semiintensive than the traditional pond management system. The level of production
for the common carp was relatively low under the semi-intensive management
system, which could be explained by a low recovery rate for the fish of around
8 Improved Sustainable Aquaculture Systems for Small-Scale Farmers . . .
301
in the research area showed a high variability in terms of water quality, natural food
availability and hence in the level of fish production and financial benefits to be
gained from the aquaculture activities. Unsuitable water quality elements such as
water temperature (Sifa et al. 2002; Cagauan 2001; Black 1998; Alcaraz et al. 1993;
Chervinski 1982), water oxygen (Black 1998; Ross 2000) and water pH (Svobodova
et al. 1993) levels were shown to have a negative effect on fish growth. Especially
within extensive and semi-intensive aquaculture systems, fish growth is correlated
with the availability and abundance of natural food resources (Rahman et al. 2008b;
Muendo et al. 2006; Schroeder et al. 1990; Spataru et al. 1983; Kolar and Rahel
1993; Pucher et al. 2011b), and both water quality and the availability of natural food
are primarily or secondarily affected by the suspended particle load. During the
research in Yen Chau district, ponds with maize and cassava fields on yellow soils in
the watershed above and fed by rain water, showed a distinctly higher suspended
solids load than ponds located on the rice paddy plateaus and fed by reservoir water.
Under the modified pond management system, a reduction in turbidity was obtained,
leading to higher primary production activity levels and with higher oxygen production peaks during the day (Pucher et al. 2010a). Oxygen limitations occurred in the
morning, similarly to those observed under the traditional pond management system.
This suggested better primary production activities, resulting in higher oxygen
production during the day but also higher oxygen depletion at night. Nevertheless,
higher primary production levels stimulated by fertilization and reduced water flowthrough resulted in a greater abundance of natural food resources and provided
more support for growth, especially of the filter-feeder fish species such as silver
carp, bighead carp and tilapia. Also, grass carp under the modified pond management system showed a higher growth rate than under the traditional system,
indicating a better feed base of natural food due to lower stocking densities (Sinha
and Gupta 1975) and consumption of pelleted supplemental feed which was
intended to feed common carp. All stocked species under the semi-intensive pond
management system, such as the common carp, grass carp, silver carp, bighead carp
and tilapia, showed significantly higher specific growth rates (in %), of 2.6 Æ 0.2,
2.4 Æ 0.3, 2.5 Æ 0.1, 2.9 Æ 0.2, and 3.4 Æ 0.6 respectively, and when compared to
those under the traditional management system, which were 1.8 Æ 0.1, 1.9 Æ 0.7,
1.7 Æ 0.0, 2.0 Æ 0.0, and 2.5 Æ 0.4 respectively (Pucher et al. 2010b). Net production (in kg/1,000 m
2
) over the 7 months of the trial under the traditional pond
management system was 87.4 Æ 43.6, with 36.8 Æ 18.8 for grass carp, 4.3 Æ 5.5
for common carp, 16.7 Æ 3.8 for silver carp, 8.3 Æ 4.4 for bighead carp and
16.1 Æ 15.4 for tilapia (Pucher et al. 2010a, b). Net production (in kg/1,000m
2
)
under the semi-intensive management system was 227.5 Æ 41.6, with 45.5 Æ 11.8
for grass carp, 35.8 Æ 12.7 for common carp, 60.8 Æ 2.8 for silver carp,
39.6 Æ 13.0 for bighead carp and 35.7 Æ 21.9 for tilapia. All fish species, except
grass carp and tilapia, had significantly higher production levels under the semiintensive than the traditional pond management system. The level of production
for the common carp was relatively low under the semi-intensive management
system, which could be explained by a low recovery rate for the fish of around
8 Improved Sustainable Aquaculture Systems for Small-Scale Farmers . . .
301