water temperatures in the study area are close to the optimum during the hot
summer months, they are not satisfactory for fish growth in the dry, winter season
(Diaz et al. 1998), especially in shallow ponds that do not have a source of fresh
water during those periods (Dan and Little 2000). In addition to growth reduction,
immune suppression characteristics have been revealed at the low water
temperatures (Yang and Zuo 1997) occurring within the research area, and so the
relatively cold winters in this area can limit fish production and even kill tropical
species such as tilapia.
As water temperature increases, the dissolved oxygen (DO) content of water
decreases due to the lower oxygen solubility, while at the same time the DO
requirements of the fish increase. Oxygen represents the key limiting variable
within the aquatic environment for fish growth, as the food intake of fish may be
suppressed if their oxygen supply is limited (Ross 2000; Black 1998). In the study
area, low DO levels are regularly observed at dawn, a time when fish are seen
gulping at the pond surface, reflecting emergency respiration behavior.
Inflowing water carries DO into the system, but at the same time also carries
sediments from bare and eroded upland fields. Especially during the hot, wet
season, these sediments frequently color pond water red-brown, and during the
study, the transparency of the water, as measured by the Secchi Disk Depth, was
frequently found to be around 20 cm or even lower. Water is considered turbid
when it has a Secchi Disk Depth lower than 30 cm (Sevilleja et al. 2001), and
turbidity limits photosynthesis due to diminished sunlight penetration; thereby
reducing the internal production of DO.
Fish in the study area are frequently stressed by low DO levels, and it is quite
likely that the potential entry of pesticides and detergents, as well as sediments from
the uplands, which may carry heavy metals (e.g., as a result of weathering of rocks
and soils) into the pond system, may stress fish additionally. All these factors
probably contribute to the relatively low fish yields in Yen Chau ponds.
Available water resources can become scarce, especially during the dry season.
Usually, when paddy fields are irrigated and water is distributed among fields and
ponds, the local irrigation authorities give priority to paddy fields, which in some
cases leads to a complete halt in the supply of water to ponds. In combination with
low rainfall, which was the case during the study phase, this leads to an enormous
decrease in the water level, which negatively impacts on the fish. In severe cases
farmers are forced to sell fish ahead of time.
In general, farmers practice multiple stocking. The main source of juvenile fish
in the research area is a hatchery in Son La town, which was established by the
government in the 1960s and was privatized in 2004. The most important fish
species produced by the hatchery and reared by local farmers are grass carp, mud
carp, mrigal, rohu, common carp, silver barb, the cichlid Nile tilapia and the filter
feeders silver carp and bighead carp. During the study, farmers stocked between
four and eight different fish species at an average fish stocking density of 1 fish
per m
2 . The continuous supply of young fish is often not guaranteed, thus farmers’
stocking system often depends on the availability of fish rather than on long-term
planning.
8 Improved Sustainable Aquaculture Systems for Small-Scale Farmers . . .
285
summer months, they are not satisfactory for fish growth in the dry, winter season
(Diaz et al. 1998), especially in shallow ponds that do not have a source of fresh
water during those periods (Dan and Little 2000). In addition to growth reduction,
immune suppression characteristics have been revealed at the low water
temperatures (Yang and Zuo 1997) occurring within the research area, and so the
relatively cold winters in this area can limit fish production and even kill tropical
species such as tilapia.
As water temperature increases, the dissolved oxygen (DO) content of water
decreases due to the lower oxygen solubility, while at the same time the DO
requirements of the fish increase. Oxygen represents the key limiting variable
within the aquatic environment for fish growth, as the food intake of fish may be
suppressed if their oxygen supply is limited (Ross 2000; Black 1998). In the study
area, low DO levels are regularly observed at dawn, a time when fish are seen
gulping at the pond surface, reflecting emergency respiration behavior.
Inflowing water carries DO into the system, but at the same time also carries
sediments from bare and eroded upland fields. Especially during the hot, wet
season, these sediments frequently color pond water red-brown, and during the
study, the transparency of the water, as measured by the Secchi Disk Depth, was
frequently found to be around 20 cm or even lower. Water is considered turbid
when it has a Secchi Disk Depth lower than 30 cm (Sevilleja et al. 2001), and
turbidity limits photosynthesis due to diminished sunlight penetration; thereby
reducing the internal production of DO.
Fish in the study area are frequently stressed by low DO levels, and it is quite
likely that the potential entry of pesticides and detergents, as well as sediments from
the uplands, which may carry heavy metals (e.g., as a result of weathering of rocks
and soils) into the pond system, may stress fish additionally. All these factors
probably contribute to the relatively low fish yields in Yen Chau ponds.
Available water resources can become scarce, especially during the dry season.
Usually, when paddy fields are irrigated and water is distributed among fields and
ponds, the local irrigation authorities give priority to paddy fields, which in some
cases leads to a complete halt in the supply of water to ponds. In combination with
low rainfall, which was the case during the study phase, this leads to an enormous
decrease in the water level, which negatively impacts on the fish. In severe cases
farmers are forced to sell fish ahead of time.
In general, farmers practice multiple stocking. The main source of juvenile fish
in the research area is a hatchery in Son La town, which was established by the
government in the 1960s and was privatized in 2004. The most important fish
species produced by the hatchery and reared by local farmers are grass carp, mud
carp, mrigal, rohu, common carp, silver barb, the cichlid Nile tilapia and the filter
feeders silver carp and bighead carp. During the study, farmers stocked between
four and eight different fish species at an average fish stocking density of 1 fish
per m
2 . The continuous supply of young fish is often not guaranteed, thus farmers’
stocking system often depends on the availability of fish rather than on long-term
planning.
8 Improved Sustainable Aquaculture Systems for Small-Scale Farmers . . .
285