Agriculture Reference
In-Depth Information
7.8.2 Biology
Records in China indicate that common carp were raised in rice paddies during the Wei
dynasty (
AD
220 - 265).
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Many other Asian countries have recorded histories of culti-
vation of fish with rice. Before 1930, tilapia was cultivated almost exclusively in its
continent of origin, Africa. It was introduced into most countries of the world starting
in the 1940s.
Each species of fish has unique feeding habits and growth stages. Bighead and
silver carp both feed on plankton. Silver carp in the larval stage feed mainly on zoo-
plankton but turn to phytoplankton as adults, while bighead feed on zooplankton all
their lives. This difference is due to anatomical differences in their gill rakers. Gill
rakers are a series of projections located along the front edge of the gill arch. These
act to filter zooplankton from the water. The gill rakers of bighead are short and
sparse and unable to retain the smaller phytoplankton that the more numerous gill
rakers of the silver carp can retain.
Grass carp are herbivorous consuming most types of aquatic and land grasses with
their saw-toothed teeth. This species is often used to control aquatic weeds in water-
ways and canals. Dense populations of grass carp in rice paddies can cause damage
to the rice crop but sparse populations are useful for weed control.
Black carp are carnivorous, feeding on snails and clams. Their teeth and callous
pad act together to crush shells and eat the meat. Under cultivated conditions, they
feed on silkworm pupae, earthworms, and animal entrails.
Common carp feed on rotifers, insect larvae, snails, young clams, shrimp, some
plants, and seeds. Their mouth is shaped to permit them to dig in mud for organic matter.
Tilapia are omnivorous. At the larval stage they prefer zooplankton, but as they
grow larger they will eat a large variety of plants, algae, and animals. The dense gill
rakers of Tilapia nilotica makes it more efficient at feeding on the green algae,
Chlorophyta, and the blue algae, Cyanophyceae. This gives it an advantage over
other species that cannot digest these algae.
Growth rate is often a major consideration for choosing an aquacultured species.
Most of the carp species have high growth rates. The optimum temperature for
growth and food intake is 25 to 328C with death occurring above 408C. The best pH
range of water for growth is between pH 7.5 to 8.5. Water below pH 6.0 or above
pH 10.0 will retard growth. At higher levels of dissolved oxygen (4 to 5 mg/L)
feeding is intense. Levels of dissolved oxygen below 1 mg/L will stop feeding and
cause fish to gasp for air.
7.8.3 Management Systems
Integrated rice fields and pond aquaculture will be discussed here. Both systems are
present in China, the largest producer of aquaculture species. It should be mentioned
that a large part of the commercial production in China is done on large operations
similar to that shown in Figure 7.30.
Integrated rice fields use the flooded conditions of rice paddies to nurture fish for
sale. Some modification of the pure rice planting scheme is needed to accommodate
