Feeding with microbial flocs by tilapia in minimal discharge bio-flocs technology ponds

The uptake of microbial floes by tilapia was evaluated. Fish (tilapia Mozambique, 107 g) were stocked in 1 m(3) tanks filled with water from a limited exchange intensive tilapia producing pond (bio-floc technology, BFT system). Tagged ammonium nitrogen ((NH4)-N-15(SO4)(2)) and starch to ensure incorporation of the N-15 into the bio-flocs, were added. Fish were held in the tanks for ca 2 weeks, not fed during a week period, when the only source of feed was microbial flocs. Floc volume, total suspended solids, as well as total carbon and nitrogen in suspension were monitored. The floc plug in settling cones contained 1.4% as dry matter. Feed uptake, evaluated through the decrease with time in respect to 4 independently determined parameters, namely floc volume, TSS, C and N in suspension, was found to be 10.3 +/- 1.0 g kg fish(-1) day(-1), averaged for the computed values for these parameters However, this preliminary evaluation was based on the assumption that fish harvesting is the only mechanism affecting bio-flocs mass, neglecting biodegradation and production of flocs. Gross daily uptake of nitrogen as determined using 15 N uptake data was 0.25 gN kg(-1) (1.6 g protein), equivalent to the daily uptake of 6.2 g kg(-1) of dry bio-flocs, 60% of that computed by the simplified mass balance approach. This difference may be attributed to microbial degradation of the bio-flocs. Even the lower flux as evaluated through N-15 uptake, constituted, in the specific case studied, almost 50% of conventional feed ration. (c) 2006 Elsevier B.V. All rights reserved.