「マガキ」異状斃死の防止に関する研究「Kイオン」特に「KI」の注射の効果について

Abstract

Various authors have worked on the cause or causes of unusual mortality observed during summer in the Japanese oyster, Ostrea gigas, with the following results:- (1) Proof against such an unusual event are those culture-beds which enjoy an appropriate water condition ensuing from either (a) the temperature being below 24°C coupled with the salinity ranging 33-34‰, or (b) the temperature somewhat higher (26°-36°C) but the salinity slightly lower (30-32‰) than the former. (2) The summer conditions of water consequent upon higher temperature (26°-32°C) and salinity (33-35‰) were found in Moroiso Bay decidedly fatal to the profusely egg-laden oysters which however rather gradually succumed to death. (3) Spawning was followed by sudden fate at Kanazawa and Akamizu during a period from late August to early September in those oysters which had been previously exposed in summer months to a temperature above 26°C and at the same time to a salinity of 32-34‰, although the water conditions were taking a favorable turn for them while dying. (These three items were verified by Seno, Fujimori, Kusakabe, and Yokota). (4) Transplanted anywhere else from Shimizu Bay as it was, the Shimizu stock of oysters still maintained its regular spawning season of mid-autumn irrespective of conditions of life, of its new habitat, briskly continuing its existence during the afore-mentioned critical period amidst all other dying native ones. Hence the mortality of the latter dependent on coincidence of spawning season with the time of upturned temperature and salinity (Seno, Fujimori, and Kusakabe). (5) Nothing suggestive of contagion could be detected in the perishing oysters (Seno, Kusakabe, and Miyazaki); nor harmful bacteria in them (Orton). (6) Autolysis products of ovary of an oyster induced “gaping” condition in another otherwise healthy one when injected in its visceral mass (Yokota, ). (7) The higher temperature and salinity of water markedly lessened reproductirvity of diatoms (Yokota) which were found indispensable food for rearing Olympia oyster lavae (Hori). (8) Gaping oysters were found to have their digestive diverticula rediced in size and faded in color. It may be mentioned a measure was taken by Yokota, Inoue, and Sawto with a view to reducing the unusual mortality of the oyster in summer. Of injections of Vitamin B2, KI, KCl, KIO ??, diatom extract crone of them, a certain dose of KI appeared to answer the purpose (Preliminary Report of Experiment to Reduce Mortality of Oyster in Summer 1950. BULL. JAP. SCO. SCI. FISH. VOL. 15 No. 9.) The premnt paper deals with an account of further studies conducted in summer of 1948 and 1950 on the same subject, and the attention of the reader is called to the accompanying Tables I and II which tabulate the results of experiments, Tables III and IV which give a summary of environmental conditions of Akamizu oyster farm under consideration, and Table V which sums up mortality of oysters. Remarkably enough, shots of KI solutions were likely to reduce the mortality in summer even under adverse conditions resulting from unsuitable temperarure and salinity and also shortage of food. Such effects of KI and other already mentioned related facts when taken together appear to point to the conclusion that unusual deathrate ensues from accumulation of harmful autolysis products due to disturbed metabolism which was given rise to by persistence of higher temperature and salinity of water during the spawning season. A large dose of KI is therefore recommended to be administered to the oysters when they are spawning in summer.