Thesis Abstract of AGS Students

Evaluation of prawn harvesting management on yields and economic return of rice-prawn (Macrobrachium rosenbergil) system in the Mekong Delta of Viet Nam

Tuyen Quang Nguyen (1994)

Aquaculture plays an important role in the Mekong Delta in which prawn culture in rice-based system is a new integrated farming system to generate more income for rice farmers. However, its potential has yet to be met with the improved practices.

The present study was carried out to assess the alternatives of prawn harvesting practices for improving the yield and size of prawn in the rice based system. A field experiment was conducted to evaluate the effect of cull-harvesting methods on the productivity and size of prawn; to determine effect of harvesting methods on environment of rice-prawn system; and to measure effect of prawn harvesting management on rice yield and economic return of rice-prawn system. Rice-monoculture (T₀) was compared with rice-fresh-water prawn culture. The prawns were raised beginning in April rice-based system arranged with three harvesting alternatives varying from the batch-harvesting of prawn one time at the end of growing season in November (T₁); replacing of size 1 and 2 prawn number by the young prawn in July (T₂); and harvesting all size of prawn then resupplying the same amount of young one in the wet rice season (T₃). The last three treatments were batch-harvested at the end of prawn growing season in November. Rice production, prawn production, prawn numbers in term of size, and economic return were estimated.

The experiment was carried out on a farmer field at a key site of Phung Hiep District, Cantho Province. Ten farmers were selected to record prawn and rice yields for comparison their yields with the experimental result in the same study area.

The treatment T₃ resulted the highest prawn production of 197 kg ha-1 and the highest size 2 prawn number of 4420 prawns ha-1. It was found that the better prawn growing season was wet season rice (WSR) from August to November than early wet season rice (EWSR) from April to July. The average weight growth rate of prawn was also higher in WSR (0.22 g day-1) than that in EWSR (0.14 g day-1). The prawn growing duration was not long enough to provide the significant effect of harvesting methods on environment of rice-prawn system.

Rice production of the rice-prawn pattern was not significantly different from that of rice-monoculture. Total rice and prawn production averaged 8.0 t ha-1yr-1 and 161 kg ha-1yr-1 in the experiment results whereas those yielded 4.5 t ha-1yr-1 and 98 kg ha-1yr-1 in farmer fields in 1992, respectively.

The highest net return of US $ 663 ha-1 was found if prawn were harvested all sizes in mid-season, then resupplied by the same amount of young prawn, and batch harvested in November. Rice-prawn system provided more net return of US $ 524 than that of US $ 308 of rice-monoculture.

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