Studies on the ecological effects of varying the size of fish ponds loaded with manures and feeds

Abstract

The major species of Chinese carp: grass carp ( Ctenopharyngodon idella), bighead carp ( Aristichthys nobilis), silver carp ( Hypophthalmichthys molitrix), common carp ( Cyprinus carpio), crucian carp ( Carassius auratus), and Wuchang fish ( Megalobrama terminalis) were cultured using the traditional Chinese polyculture system in different-sized ponds loaded with pig manures. The grass carp were given aquatic grass as supplementary feeds. Two experiments were conducted in 1982 and 1983 using nine fish ponds of sizes varying from 1 mu (667 m 2) to 11 mu (7333 m 2), each for a culture period of 200 days. The average yield attained 6090 kg/ha. The conversion rate of pig manure and aquatic grass to fish biomass was found to average 17:1 and 85:1, respectively. Results indicated higher yield obtained in pond sizes between 3 and 5.7 mu (1999 and 3800 m 2) using standard pond management. Bigger ponds with greater surface area were more difficult to manage and often resulted in lower fish yields. The study also indicated that the fish yield in various experimental ponds was affected by the growth and survival rate of the grass carp, the main species cultured. Dissolved oxygen was found to be lower in smaller ponds and increasing frequency of ‘fish surfacing’ was noted, while other nutrient levels such as that of total nitrogen, ammonia, nitrite, and phosphate did not appear to have a direct effect on the fish yield. There was no direct relationship between primary productivity, organic detritus, and fish yield in different-sized ponds. However, production of filter feeders (silver and bighead carp) was higher than previously recorded indicating the optimal stocking rate in the polyculture system had been adopted.