In Brazil, the expansion of freshwater prawn farming is limited by many factors, among these, the lack of a specific feed. Farmers generally use feed for marine shrimp or tilapia to rear freshwater prawns, causing low productivity. On the other hand, natural forage organisms generally are rich in protein (about 50-60%) with high biologic value and amino acid profiles very close to those required by prawns, becoming interesting to test prawn farming using natural feed as an alternative to conventional systems based on manufactured feed.
The term "pellet free" was firstly used in the production of marine shrimps fed with natural food. In Brazil, a preliminary culture of pellet free Macrobrachium rosenbergii was carried out with stimulating results. The present research aimed to test the pellet free system in a M. amazonicum culture, directed to the live bait market. The test was conducted from November 2018 to April 2019 (139 days) in two 40-m2 earthen ponds located at the Research Unity of Pirassununga (SP), Fisheries Institute, Brazil, stocked with 30-d postlarvae (0.029 ± 0.014 g), at 30 PL/m2. One of these ponds was provided with six lines with eight substrates each (pieces of polyethylene mesh vertically oriented and measuring 1 x 0.5 m each). The other pond did not have substrates. The objective of substrates was to improve natural food production and to provide shelters to the prawns. Both ponds were weekly fertilized with urea (45% N) at 10 g/m2 for four weeks, in order to stimulate the production of phytoplankton and natural food. Moreover, both ponds received once a week, about 100 g/m2 of organic material, primarily rotten fruits (mango and guava). There was no water exchange, but water losses were replaced. Water temperature (25 to 28oC), dissolved oxygen (3 to 5 mg/L) and pH (6.5 to 7.5) were adequate for good freshwater prawn growth.
After 188 days, the prawns were harvested, weighed and counted. Mean weight was 2.9 ± 1.4 and 2.5 ± 1.2 g, survival 77.7 and 75.3 % and mean length was 7.0 ± 1.0 and 6.9 ± 0.8 cm for prawns from ponds with and without substrates respectively. All the prawns attained the minimum length for live baits (5.0 cm). Substrates apparently had very little influence on survival and length, thus, they did not should be recommended. Although there were not statistical analyses, the results obtained were promising, considering the reduction of about 11% of operating costs, correspondent to commercial feed acquisition. An economic analysis showed that pellet free system without substrates is viable only from the price of R$ 400.00 (US$ 100) per thousand, with Internal Return Rate of 17.7%, Payback Period of 4,5 years and Net Present Value of US$ 21,725. Profitability Index was 34.1%. Pond building is the highest investment item (53.6%) and acquisition of PLs corresponded to 59% of Total Operating Costs. We concluded that pellet free system can be an alternative to the conventional one, especially in regions where commercial pellets are not available or do not have adequate nutritional quality for freshwater prawns.