Effects of manuring regime and stocking rate on primary production and yield of tilapia using liquid swine manure

Abstract

The influence of manuring regime was investigated to determine its effect on primary production and yield of Oreochromis niloticus stocked at high (1.5/m 2), medium (1.0/m 2), and low (0.5/m 2) rates in concrete tanks (20.2 m 2). Over a period of 14 weeks, liquid swine manure was added to tanks in three regimes: decreasing rate, constant rate and increasing rate. By the end of the season all tanks had received equal quantities of manure. Interaction between stocking density and manuring regime did not significantly ( P>0.05) influence fish production. Addition of manure at a constant or decreasing rate resulted in higher net fish yield (17.5 and 18.7 kg/ha day −1) than addition of manure at an increasing rate (13.7 kg/ha day −1). Maximum primary production was achieved most quickly in the decreasing regime. Both the decreasing and increasing regimes resulted in periods of low dissolved oxygen which necessitated emergency aeration. The increasing regime resulted in lowest net fish yield, probably because slow growth early in the season prohibited the fish from taking full advantage of high quantities of manure added later in the season. An optimum manuring regime would probably consist of a decreasing rate whereby a maximum quantity of manure would be applied initially to stimulate primary production and early fish growth. Subsequent fertilization rates would be moderated, and adjusted primarily according to dissolved oxygen concentrations. Final mean weight of fish decreased with increasing stocking rate: 243 g, 214 g and 174 g at 0.5, 1.0 and 1.5 fish/m 2, respectively. However, net fish yields for the medium and high stocking rates were similar (18.1 and 19.3 kg/ha day −1) and were greater than net yield at the low stocking rate (10.8 kg/ha day −1). The low stocking rate resulted in lower primary productivity and greater fish mortality than the medium and high stocking rates.