Food and feeding
The types of feed used in fresh-water prawn farming vary considerably, from locally available feedstuffs to compound chicken feeds and specially formulated prawn feeds (see D’Abramo and New, 2000). Compared to Penaeid shrimps, prawns are reported to have a lower dietary protein requirement of 20–25 per cent. However, some farms use shrimp feeds containing higher protein levels.Also,the results of research seem to indicate that juveniles may
need higher protein levels, and a 40 per cent protein diet has been reported to yield higher weight gains.Asian farmers use rice byproducts, trash fish, molluscs, prawn waste, poultry waste and meal of ipil ipil (Lecucaena sp.) leaves.
Many farms use commercially available compound chicken feed, sometimes reextruded after mixing with trash fish or prawn meal. Tables 25.4–25.8 present the ingredient composition of a number of practical compound feeds used in commercial production. At least some of them can be prepared on the farm, if
commercial feeds are not available. In most cases, feeding is done manually, but some farmers use automatic blower feeders, as in Hawaii. Many Hawaiian and Thai prawn farms maintain rich blooms of phytoplankton in thegrowout ponds to supplement the compound chicken feed or prawn feed.
The feeding rate is adjusted according to assessments of daily consumption and biomass changes. Recommended rates rise from 2.7– 7.4kg/ha per day in the early rearing period to as much as 37.5kg/ha per day towards the end of the rearing, in about six to eight months (New and Singholka, 1985). Feed conversion ratios generally range from 2:1 to 4:1 for com mercial diets, giving growth rates averaging around 1–2cm/month.
Since M. rosenbergii is a benthic omnivore, itis considered a suitable candidate for polyculture. Experiments in polyculture with Tilapia aureus (Brick and Stickney, 1979) did not reveal any significant interaction between the species.
Relatively high combined yields were obtained in polyculture trials with tilapia, channel catfish and Chinese carp in Alabama (Behrends et al., 1985). However, the yields and individual harvest weights of the prawns seemed to indi cate interspecific competition. In initial experi mental polyculture of the prawn with Chinese and common carp in ponds without supple mentary feeding, Malecha et al. (1981) found that weight gains by the prawns compared favourably with those in monoculture, indicat ing that they could utilize heterotrophic productivity in manured ponds. Pond fertilizationunder monoculture is likely to reduce the need for complete compound feeds, making natural food available to the prawns. In Taiwan, where polyculture is practised with grass carp, bighead carp, grey mullet or milkfish, the species combinations are believed to result in more efficient use of pond productivity and the control of the growth of filamentous algae.
Polyculture of the prawn with the crayfish Pro cambarus clarkii and the channel catfish appears to lower the survival and growth rates of the prawns (Huner et al., 1983).