Food organism availability and resource partitioning in organically or inorganically fertilized Tilapia rendalli ponds

Ž . Ž . Inputs of either napier grass NG or diammonium phosphate plus urea NP containing similar Ž y1 y1. Ž y1 y1. amounts of nitrogen 17 kg N ha week and phosphorus 1.2 kg P ha week were made to Tilapia rendalli ponds from which offspring were either partially removed or left to grow. After Ž . 364 days, average weight of stocked fish in NG ponds 74.3 g was significantly higher Ž . Ž . P-0.05 than in NP ponds 57.4 g . Partial removal of offspring reduced net yield and standing stock at harvest from 1515 to 1131 kg ha and 1679 to 1099 kg ha, respectively; but it improved final average weight of stocked fish from 59.2 to 72.3. Reduction of offspring density Ž . had no affect on size of fingerlings juveniles )5 g , but it significantly reduced the size of fry Ž . Ž . juveniles -5 g . Phytoplankton chlorophyll a and rotifer populations were larger in NP ponds Ž y1 y1 . Ž y1 106.1 mg l and 3042 organisms l , respectively than in NG ponds 23.5 mg l and 1075 y1 . organisms l , respectively . Microcrustacean densities were highly variable, but statistically similar between input and offspring removal treatments, averaging 385 organisms l overall. Offspring removal had no effect on rotifer populations. Sediments in NG ponds contained a higher percentage of organic matter throughout the study, achieving equilibrium between decomposition and loading of around 8%, compared to 5% in NP ponds. Offspring number did not affect phytoplankton density or organic matter loading in the sediment. Fish growth and stomach content data confirmed that T. rendalli of less than 25 g average weight are basically detritivors while larger T. rendalli target larger plant materials. q 2000 Elsevier Science B.V. All rights reserved.