The effect of repeated cropping of half the number of young and large adults on productivity and dynamics of Oreochromis niloticus was investigated during polyculture with Heterobranchus longifilis. Mixed-sex, large-sized and young adult tilapia of 841±0.3 g and 841±0.23 g were cultured with 40 catfish fingerlings in triplicate 36 m2 earthen ponds in 1 and 2 treatments respectively. At the 4th, 6th, 12th, 26th and 32nd week, cropping was done in treatment 2 and the weights of the crop were used to enrich supplementary feed, by displacing an equal weight of ingredients in treatment 1. Realized supplementary feed crude protein level was 12.9 and 18.3±5% in treatments 1 and 2 respectively. Feeding was at 1% body weight thrice daily. Weight data were analyzed with ANOVA and t-test. Surplus production of O. niloticus from treatment 2, calculated as sum of periodic harvest + final harvest minus final harvest in treatment 1, was 249±25 g per week. Treatment 2 had significantly higher H. longifilis harvest mean weight and growth rate (percentage per day) (p<0.01), with densities (p<0.05). It also has higher numbers of zooplankton and phytoplankton species, cyanophyceae and diatoms, as well as O. niloticus size susceptible to catfish predation. Chlorophyceae dominated the phytoplankton in both treatments. The implication of tilapia surplus production for catfish-tilapia polyculture is thus discussed.
Key words: Oreochromis niloticus, partial harvest, population dynamics, compensatory growth, standing crop, surplus production, food availability, earthen pond.
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