Experiments to evaluate the production performance in multiple cropping systems with two crops in a year over single cropping, and single stocking and multiple harvesting system in carps polyculture were conducted in six ponds of 0.08 ha each during March 1996 to March 1997. The ponds were stocked with six carp species, viz., catla, rohu, mrigal, silver carp, grass carp and common carp in the ratio of 2:2:2:2:0.5:1.5, at a combined density of 10,000 fingerlings/ha with average weight of 17.5 g. While in treatment with single cropping (Treatment I), the fishes were harvested after rearing period of one year; in the case of treatment with single stocking and multiple harvesting (Treatment II), the fishes of larger size were harvested at monthly intervals from the seventh month onwards. However, in treatment with multiple cropping system (Treatment III), the fishes were harvested only after a rearing period of 6 months as first crop, which was restocked again after due pond preparation for second crop of 5.5 months. The gross and net production levels recorded in Treatments I, II and III were 6011.3 and 5843.8 kg/ha/year, 6487.8 and 6320.3 kg/ha/year and 7194.7 and 6828.4 kg/ha/year, respectively, showing significantly higher production levels in Treatment III (P < 0.05) over Treatments I and II. The pooled growth rates of different species as obtained from two crops of 6 months each in Treatment III (858 64 g) showed significantly higher values (P < 0.05) than that of Treatment 1 (680 13 g). However, the mean survival levels between the treatments did not differ significantly (P>0.05). Further, with 40% and 60% higher feed provision in Treatments I and II, respectively, over Treatment III, the recorded feed conversion ratio (FCR) values in Treatments I, II and III were 3.16, 2.53 and 1.67, respectively, showing significant differences between the treatments (P<0.01). The studies found that two crop-system over an annual cycle was superior to single stocking-multiple harvesting, and single cropping, in terms of feed conversion ratios as well as field rates. (C) 2002 Elsevier Science B.V. All rights reserved.
