カキ養殖場における生物源堆積現象 (Biodeposition) の研究 (I) : 養殖適正密度について
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It has been pointed out that the biodeposition of marine organisms influences the physical, chemical and ecological conditions of the surrounding seas (ITO & IMAI, 1955 ; HAVEN & MORALES-ALAMO, 1966 etc.). As a matter of facts, in many of the oyster beds in Hiroshima Bay, the oyster crops decrease with years by their repeated use which causes bulky accumulations of biodeposits of oysters and their competitors under the bottom of the beds. Biodeposits thus accumulated gradually change the physical and chemical nature of the bottom sediments of the beds and not only interfere with the lives of the oysters but also often cause a high mortality during the summer season when temperatures are high and waters are stratified. From these points of view, the present investigations were initiated to determine the economic density for oyster culture of raft hanging method (CAHN, 1950 ; Fis. 20 & 28) which prevails in this district. The first part of our work, which is now being continued, is summarized below : -1) Daily studies relating the defecation cycle of oysters (Crassostrea gigas THUNBERG) and their competitors (Mytilus, Ciona, Styela etc.) indicated that they are producing larger quantities of faecal matter at night than those in the daytime (Tables 1-1 & 1-2, Text-fig. 4). Further studies on the daily cycle of the rate of water transport of these species in relation to the period of light and darkness seem to support the above findings (Table 2 & Text-fig. 5). But in either case, such a rhythm tends gradually to show more irregular patterns due to repeated use of the same materials for the experiments. 2) The results obtained from seasonal studies of defecation cycle of oysters and their competitors are similar to those of HAVEN & MORALES-ALAMO (1966) and show that within a single oyster culture cycle (from August to April ; 9 months) the rate of defecation largely increases during Autumn, passes through a maximum in October, and gradually falls in Winter (Table 3 & Text-fig. 7). 3) From the above data, a raft of oysters (a single raft holds 350, 000∿630, 000 oysters, averaging 420, 000 ones) in Hiroshima may be calculated to produce 20 metric tons or more of faeces and pseudofaeces on a dry basis within one culture cycle. Of these, 20∿30% may be settled onto the bottom under the oyster raft (21.8×9.1 m in size) as biodeposits (Table 4). 4) Organic nitrogen contained in the faeces and pseudofaeces of oysters and their competitors varies with season from 0.5 to 2.1% (the proportion of organic nitrogen to the total faecal material by dry weight), averaging 1.0% or so (Text-fig. 8). 5) Total physiological requirements of water by a raft of oysters and their competitors for maintaining normal rate of water transport are estimated to be approximately 233×10^4l/day. In order to meet the requirements, it may be necessary to draw its supply of water from tidal current of the surrounding seas with 1.5∿1.7 cm/sec or more in constant velocity.
- 日本貝類学会の論文
- 1971-11-30
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