Eco-physiological studies of Chinese integrated fish culture-I Decomposing processes of grass carp feces under aerobic laboratory conditions

Abstract

In a Chinese integrated fish culture conducting around Dianshan Lake in Shanghai, Grass carp (Ctenopharyngodon 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 feces were found in the intestinal contents of Carassius auratus and Aristichthys nobilis. In the present study, grass carp feces (fragments of Vallisneria spiralis) collected from the pond were decomposed under aerobic laboratory condition, in order to discover the role of grass carp feces in the food-webs of this pond ecosystem. During 16 days of decomposition, carbon contents in the feces 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 feces increased up to the maximum level on the 8th day of the experiment, but changed little during further decomposition. 15NH4+ and 3H-thymidine added the incubation medium were actively incorporated into various sized fecal fragments. It is suggested that decomposing grass carp feces (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. © 1992, The Japanese Society of Limnology. All rights reserved.