Rabbit droppings as an organic fertilizer in earthen ponds, to improve growth and production of Nile tilapia, Oreochromis niloticus L., in Rwanda

Abstract

Nine earthen ponds of 400 m2 were stocked with mixed sexes of Nile tilapia Oreochromis niloticus (LINNE, 1757) fingerlings of 14g mean weight at a density of 2 fish/m2 and fertilized with fresh dung dropped directly from rabbits stocked at three different densities: T1: 1 rabbit/are; T2: 2 rabbits/are and T4: 4 rabbits/are (1are = 100m2 surface area) of pond in hutches suspended above the ponds. After a culture period of 140 days, an increasing pattern in all nutrients was observed in
the three treatments with only nitrates decreasing with time in all treatments. However, lower concentrations of total nitrogen (TN) were recorded within ponds receiving droppings from the highest stocking rate of rabbit (T4) but the difference between treatments was not significant (P<0.05). In addition, similar trends in decreases over time were observed for water transparency, dissolved oxygen (1.41 – 10.48 mg/l) and water daily primary productivity (178.01 – 426.82 mg C.m-2.d-1).
Temperature was higher during the rainy season (20.6 – 27.8°C) than in the dry season (19.6 – 24.4°C) with significantly higher afternoon pH compared to the morning. Fish masses at harvest and fish net-production were higher in ponds
fertilized by the largest number of rabbits (T4: 42.32 g & 18.38 kg.are-1.yr-1). This study showed that the rearing of rabbits over fish ponds establishes a better environment for fish (O. niloticus) and offers superior hygiene conditions for rabbit
production. Although the largest number of rabbits resulted in the best water quality for O.niloticus production, it is urged that the rabbit load-rate be optimized in order to obtain a sustainable system of rabbit and fish production.