This study analyses the combined effects of organic and inorganic fertilization and of feeding with pellets on fish performance and water quality in a polyculture of common carp. Cyprinus Carpio L., silver carp. Hypophthalmichthys molitrix (Val.), grass carp. Ctenopharyngodon idella (Val.) and hybrid tilapia, Oreochromis niloticus L. x O. aureus Stetndachner. Experiments included six treatments, which are combinations of inorganic fertilisation (daily or fortnightly), manuring (supplied or not), feed pellets (supplied or not) and density of phytoplanktivorous fish species (high-density treatments with 50% more hybrid tilapia and 300% more silver carp than low-density treatments). The effects of the management procedures on fish weight, growth, survival and yield. plankton abundance, water quality, and their complex interactions at the different levels of the natural food web were analysed. The hypothesis that daily fertilization should increase algal production, which in turn should sustain a higher density of phytoplanktivorous fish, proved to be correct within certain limits. Algal reproduction rate was higher under daily fertilization, pointing to better supply of food for herbivorous fish, while algal biomass was similar under both fertilization regimes, pointing to the utilization of that algal food supply. The increased algal food supply sustained a 50% density increase of the omnivorous tilapia, but not the 300% density increase of the grazer silver carp.

The present study was designed to quantify the effect of manure application on fish performance and on the ecology of the ponds. Ponds stocked with common carp, tilapia hybrids, silver carp and grass carp received dry chicken manure as the only nutritional input, at three different rates: the standard level used at Dor (50 kg dry matter ha-1 day-1, increasing by 25 kg ha-1 day-1 every 2 weeks up to 175 kg ha-1 day-1, half of it, and twice the standard. Ecological data included water and sediment parameters, and were analysed through factor analysis and ANOVA.Organic loading increased in two directions, time and treatment. The main sources of variability in the system were related to time. The low effect of treatment was shown in both fish performance and ecological parameters. Of the fish parameters tested, only total yield and common carp growth rate and yield increased with manuring rate. Water quality variability was mainly related to photosynthesis-decomposition balance and algal and bacterial action to metabolize nitrogen, processes which were affected by time and not by manuring rate. A third source of variability in the water column, the respiration of the system, was lower in the treatment with the lower manuring rate. Manuring rate and time strongly affected ecological processes in the sediments: organic loading and anaerobic conditions increased and heterotrophic activity decreased with time and manuring rate, while microbial metabolization of nitrogen decreased with time and increased with manuring rate. The lack of treatment effect on growth and yield of all fish species, except common carp, seems to be related to this difference in the effect of manure level on water and sediment: while common carp feeds on the bottom and is more dependent on feed supplied than the other fish present in the pond, silver carp feeds on phytoplankton in the water body, and tilapia hybrids also favour natural foods of photosynthetic origin.

Daily organic manuring plus chemical fertilization of three earthen, 400-m2 ponds produced fish yields which were not statistically different from the yields produced in three similarly stocked ponds to which only nitrogen and phosphorus chemical fertilizers were added daily. Averaged over the entire 98-day grow-out period, daily yields were 29.5 kg/ha in the organically manured ponds and 27.2 kg/ha in the chemically fertilized ponds. The fish polyculture was composed of common carp (CC), silver carp (SC), tilapia hybrids (TH) and grass carp (GC), with a total stocking density of 20 000 fish/ha. Chemical fertilization provided 0.4 g N and 0.2 g P/m2, 6 days/week. Organic manuring provided an average of 3 g carbon and 0.3 g nitrogen and approximately 0.3 g phosphorus/m2 per day. Twenty-four-hour net primary production added an average of 4.0 algal carbon/m2. Fish yield data, based on comparisons of stable carbon isotopes in the fish muscle and in the natural foods of the ponds and observations of stomach and intestinal contents showed that more than 90% of the fish yield in the organically manured ponds was based on food webs originating with algal carbon. This was harvested either as live algae (by the SC and possibly by the TH) or as microbially processed detritus (by the CC, TH and GC). Organic matter of the manure contributed only marginally to fish growth. These findings indicate a need to re-evaluate, with regard to the specific species of fish and type of organic manure, the relative contributions to fish growth of the mineral and organic fractions of organic manures and composts used in fish ponds.