Studies on the Utilization of Petro-yeast as Diet of Freshwater Fish-I

In order to elucidate the reasons for the highly efficient utilization of petro-yeast in freshwater fish, a series of experiments were carried out, laying emphasis on the characteristically high content of chitin in petro-yeast. In this report, the authors examined the distribution of chitin decomposing bacteria in the digestive tracts of ayu (Plecoglossus altivelis), carp (Cyprinus carpo), and rainbow trout (Salmo gairdneri), as well as the biochemical characteristics of the isolated chitin decomposing bacteria. Chitin decomposing bacteria were isolated from the digestive tracts of the above mentioned fishes in relatively high frequencies. The proportion of chitin decomposing bacteria to total bactiria was generally high in every species of fish, particularly in fishes fed with a diet containing petro-yeast. Most of the chitin decomposing bacteria isolated were considered to belong to Ahromonas genus according to their biochemical characteristics. This makes a marked contract with the case of marine fishes whose chitin decomposing bacteria were mainly members of genus Vibrio. In particular, a high incidence of chitin decomposing bacteria in carp was considered to be related to a better utilization of petro-yeast by carp than by the other freshwater fish.