Bacteria-food or food competitors of silver carp, Hypophthalmichthys molitrix Val.?

The role of bacteria in the nutrition of silver carp was studied by in‐vitro tests, which confirmed earlier results that the potentially important source of nitrogen in bacteria is not utilized by the fish. Cultures of aerobic and anaerobic bacteria, recovered from a fish pond, were incubated with digestive fluids from the gut contents and from tissues surrounding the alimentary tract (Organ of Leydig) respectively. Growth of aerobic isolates was not affected by the digestive fluids whereas growth of all anaerobes was inhibited. A susceptibility test revealed a bacteriostatic rather than bactericidal effect of the digestive fluid on the bacteria. During 42 h incubation at approximately 20°C of gut fluid, trypsin activity decreased between 14% (in fore‐gut fluid) and 62% (in mid‐gut fluid). The addition of aerobic bacteria (to the mid‐gut fluid) or anaerobic bacteria (to the fore‐gut fluid) did not accelerate the degradation of trypsin.