Release of phosphorous from igneous phosphate rock through bioturbation: a low-cost approach for phosphorous fertilization in fish farms

The influence of bioturbation caused by common carp fry was treated in 24 transparent polythene jars (5?L each) in the laboratory and in outdoor vats (150?L), increasing the fertilizer value of phosphate rock in eight treatment combinations in triplicate. Input of water soluble reactive phosphate (SRP) was determined to quantify the effects of bioturbation, fish excrements and soil. The level of SRP in water was always lowest in the control series. Introduction of common carp fry resulted in a net increase of 0.009–0.010?mg phosphate g^?1day^?1 of SRP attributable to the effect of fish excrement. Bioturbation caused by common carp resulted in as high as a 64.8 to 90% influx of phosphate from bottom soil in the presence of phosphate rock but only about 6.3 to 7.2% in the absence of phosphate rock. The bioturbation that occurred in this treatment resulted in a significant release of phosphorous into the overlying water from an apatite source. The results confirm the environment friendly application of phosphate rock in fish-farming ponds at low cost.