This study examined how using the fresh water prawn (Macrobrachium rosenbergui) at two different stocking densities impacts production of the Chinese Cabbage, Pok Choi (Brassica rapa) compared to Deep-Flow Culture Hydroponic Raceway (DFCHR) production of Pok Choi without fresh water prawns. The experiment was done at a commercial aquaponic project near Cairo, Egypt. The aquaponic system was composed of four, 8 m3 fiberglass fish tanks, a solids removal device, net tank, sump, pump and nine DFCHR (30 m x 1 m x 0.75 m) with a water surface area of 30 m2 per raceway. The aquaponic system was covered by raft made of Expanded Polystyrene (EPS). Pok Choi seedlings were inserted into a light weight mesh net pots within the rafts at a density 36 plant/m2. The prawn post-larvae (PL30) weight typically ranged from 0.01 to 0.09 g and length ranged from 0.8 to 1 cm. The DFCHR were stocked at densities of 5 and 10 PL30/m3 respectively for a rearing period of 55 days versus DFCHR without prawn. Each treatment had three replicates. The prawns were fed three times a day using a 32% crude protein and 5% crude fat commercial extruded sinking tilapia formulated diet. The water quality parameters were monitored daily (salinity; dissolved oxygen – DO; pH and temperature( and water chemistry parameters were monitored weekly (total ammonia nitrogen – TAN; nitrite; nitrate; potassium; and soluble reactive phosphorus - SRP). In addition, the other main culture parameters that were monitored included: plant/ freshwater prawn biomass growth, feed consumption, survival, final Pok Choi and prawn production at harvest).
Water quality parameters in both experiments were within the acceptable range for prawns and all increased as prawn stocking density increased (0.25-0.32 ppt salinity; 3.1–5.3 mg/L DO; 7.2-8.4 for pH; 0.012- 0.029 mg/L for TAN; 0.030–0.034 mg/L for nitrite; 0.001-0.002 mg/L for nitrate; 1.2-3.1 mg/L, for potassium 5.2-5.6 mg/L and SRP for 2.4-4.9 mg/L). The final prawn weight was 15.3 ± 4.1 g and 13.5 ± 3.6 g for 5 and 10 PL30/m3, respectively. Specific growth rate (SGR) and survival rate was reduced with increasing stocking density, whereas food conversion ratio (FCR) increased at the higher stocking density. Aquaponic systems with the lower prawn stocking densities (5 PL30/m3) performed better than 10 PL30/m3. The growth performance of prawns in terms of SGR, survival rate and FCR were significantly influenced by the stocking density. The SGR was estimated at 2.7% and 2.6% at 5 PL/m3 and 10 PL30/m3, respectively. The survival rate was 75% and 69% at 5 PL30/m3 and 10 PL30/m3, respectively. The FCR was reported to 1:1.9 and 1:2.1 at 5 PL30/m3 and 10 PL30/m3, respectively. The prawn biomass at harvest was 57.4 and 93.2 gm/m3 at 5 PL30/m3 and 10 PL30/m3, respectively. The water chemistry with prawn in both experiments was significantly different for nitrate, potassium, and SRP in the high prawn density treatment, which was impacted positively on the mean of plant weight at harvest, 750 and 545 gm/Pok Choi, at 5 PL30/m3 and 10 PL30/m3, respectively because the prawns damaged the plant roots. In the raceways without prawns, the Pok Choi has reached 790 gm/plant. This presentation will report on the actions needed to achieve efficient production and economic returns.