アコヤガイの飼育餌料としての米粉末について

Abstract

A series of experiments on the rice powder as the diet of the pearl oyster were made with special reference to the sedimenting velocity and the frequency distribution of size of the particles of rice powder, their distribution and sedimenting rate in culture vessels, and the feeding rate and growth of the oyster. 1. In the rice powder which was made by soaking grains of rice for one day in distilled water and pulverizing them with a homogenizer, about 85% (in weight) of the raw material was available as the diet of oyster. The 15% loss may be a total of the embryos which are not easily broken, the moisture lost during powdering and drying and a part of the soluble components (Table 1 and 2). 2. From the velocity of sedimentation, the size of most (about 90% in weight) of the particles prepared as stated above was calculated to be under 10μ in diameter. These particles took one hour or more to sediment 10cm in sea water (σ15 1.025). Particles less than 5-6μ (about 4 hours to settle) account for 60% of the total (Table 2, 3 and Fig. 1). 3. Particles suspended from the surface to a depth of 10cm were removed by filteration and dried. The actual size of these particles measured 2-10μ in diameter, averaging 5.3μ immediately after preparation of suspension and 4.5μ after 5 hours (Fig. 2 and Table 4). These observations show that the rice powder may be suitable in respect of size as a diet for the pearl oyster. 4. The distribution of rice powder particles in two vessels, CV and FOV in a re-circulating rearing system shown in Fig. 3, became approximately similar (51.5:46.0% in weight of the total particles added) 20 minutes after the beginning of circulation and almost the same (49.0:48.5%) after 40 minutes. The diminution of the amount of suspended particles due to sedimentation in this rearing system amounted to only 11.5% after 4 hours (Table 5 and Fig. 4). These results suggest the possibility that almost all of the rice powder particles supplied may be utilized by pearl oysters in this type of culture equipment. 5. Additional experiments using 7 culture equipments of the same type and 56 full 1-year-old pearl oysters (7 groups of 8 animals each) which had been reared on diets of various quantities of rice powder for 56 days before experiments were conducted as follows: Each group of oysters was supplied with rice powder of the same quantities as in the rearing period (20-200mg/time; twice a day, a. m. and p. m.) and the amount of the rice powder suspended in the water of the culture vessels were determined at an interval of 1 hour for 5 hours. Starch content was taken as a measure of the amount of rice powder. The diminution rate of rice powder varied depending on the quantity of diet given. When 120mg was supplied, the rice powder was diminished more rapidly than in case of other quantities (Table 6, Fig. 6 and Table 7, Fig. 7). 6. The starch content of the substances sedimented on the bottom of the culture vessels was measured after 4 hours. By deducting the amounts of rice powder remaining suspended in the water and sedimented one from the quantity supplied at the beginning, the feeding rate of the oyster was calculated, on the assumption that the rice powder fed by the oyster was digested and no starch was paesent in the faeces. The results showed that the amount of rice powder remaining suspended in the water 4 hours after supplying was negligible as compared with that fed by the oysters, while the starch content of the sediments was in direct proportion to the amount of rice powder supplied (Table 8 and Fig. 8). The calculated feeding quantities were in direct proportion to and the feeding rates in inverse proportion to the amount supplied. 7. A relationship between the amount of rice powder ingested and the growth (increase in under-water weight) in the last week of the rearing period (Fig. 5) was shown to be a linear regression (Fig. 9).