Manipulation of stocking ratios between surface- and bottom-grazing fishes as a strategy to increase the fertilizer value of rockphosphate in a carp polyculture system

Using twice-monthly application of rockphosphate (100 kg ha^-1) and a fixed stocking density (16 000 ha^-1), the influence of the application of rockphosphate on varying ratios (1:0; 1:1; 1:3) between surface feeders (catla, silver carp and rohu) and bottom grazers (mrigal, common carp and puntius) was examined in six carp polyculture ponds. Two control ponds without rockphosphate treatment with the ratio of 1:1 were used. Water and sediment quality parameters were monitored fortnightly. Maximum fish production and primary productivity were observed in the 1:3 system, followed by 1:1 and 1:0 in the rockphosphate treatments, whereas lowest values were in the control. It is suggested that manipulation in the stocking ratio between surface feeders and bottom grazers in the carp polyculture system might be a useful strategy for utilizing rockphosphate as a direct source of P fertilizer.