Eco-physiological studies on Chinese integrated fish culture

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In a Chinese integrated fish culture conducting around Dianshan Lake in Shanghai, Grass carp (Ctenopharyngon idella) and some other cyprinid fishes with different food habits are coreared with water plants to give extremely high fish yields. In this system, a large amounts of grass carp excreta suspended in the culture pond seem to play an important role in providing food material directly or indirectly to other fishes. In fact, considerable quantities of decomposing grass carp faces were found in the intestinal contents of Carassius auratus and Aristichthys nobilis.In the present study, grass carp face (fragments of Vallisneria spiralis) collected from the pond were decomposed under aerobic laboratory condition, in order to discover the role of grass carp faces in the food-webs of this pond ecosystem. During 16 days of decomposition, carbon contents in the faces decreased at a constant rate to 50% of original contents, while nitrogen contents hardly decreased for the first 8 day periods of the experiment. The nitrogen as well as amino acid contents per unit of dry weight of faces increased up to the maximum level on the 8th day of the experiment, but changed little during further decomposition. ^<15>NH_4^+ and ^3H-thymidine added the incubation medium were actively incorporated into various sized fecal fragments. It is suggested that decomposing grass carp faces (including resultant bacteria and their metabolic products) make a new food resource available to invertebrates as well as to the plankton and benthos cyprinid feeders in this pond ecosystem.