Effect of covering water surface with azolla (Azolla filiculoides Lam.) on water quality, growth and production of nile tilapia fed practical azolla-diets in earthen ponds

Abstract Although common in Africa's rural areas, covering fish pond with Azolla is an extensive aquaculture practice that generates lower fish growth performances. To improve fish yield, a 90-days experiment was implemented to evaluate the effects of several Azolla cover (AC) levels on water nutrient, plankton dynamic, growth and production of Nile tilapia supplementary fed in ponds. Six triplicate groups of ponds were covered with one of the following AC extensions: 0%, 15%, 30%, 45%, 60% and 75% of the surface. Fingerlings were fed with a practical diet containing 20% of Azolla meal (AM). Except for temperature, all the other physicochemical parameters, and Chlorophyll a concentration and zooplankton abundance were affected by AC (P < 0.05). The SGR strongly correlate with the AC level [SGR (%/day) = - 0.0003 (AC)² + 0.0135 (AC) + 2.2045; R² = 0.96; P < 0.001, N = 18)], and values ranged from 1.50 %/day to 2.24%/day (P < 0.05). Yield and fish production ranged from 710 kg/ha to 1940 kg/ha for yield, and from 2900 kg/ha/year to 7870 kg/ha/year for fish production. The best values were obtained with 30% AC (P < 0.05), whereas performances with 0, 15 and 45% AC was similar. It appears that fish growth is a result of an interaction between the negative effects of AC on phytoplankton biomass and the positive effects of the food supplied. The study recommends applying a maximum AC level of 30% in ponds for Nile tilapia rearing in tropical areas.