カキ養殖における抑制種苗の使用とその生産的意義

This paper presents the results of the study which was undertaken as a part of the countermeasure against the mass mortality of the cultured oyster, the disaster which recurred in western Japan since 1945 and killed 20 to 100% of the oysters in many oyster-farms located in the area. The damage was most severe in Hiroshima Bay, but was also reported from other localities in western Japan, including Okayama, Tottori and Shimane Prefectures. Prior to this incident, a similar case of mass mortality of the cultured oyster had been known in the Miura Peninsula region, Kanagawa Pref., besides many other cases of more localized mortality which occurred in the oyster-farms in different parts of Japan. In the Miura Peninsula region, mass mortality of the cultured oyster began in 1927 or soon after the raft-culture method was introduced into this region, and recurred almost annually for nearly 10 years, causing the death of 50 to 90% of the oysters in the oyster-farms located all along the coast of the peninsula. The cause of this misfortune was not clarified, though it was generally said that impeded spawning due to high water temperatures and high salinities during the spawning season was likely to be the major cause. There was striking resemblance between the mass mortality of the cultured oyster which occurred in Hiroshima and other localities of western Japan since 1945 and the one which began in the Miura Peninsula in 1927. The following points were common to both cases. 1. Massive death occurred in oysters in their spawning season in most of the cases, and in September or October in some localities. 2. Adult oysters one year or more in age suffered mass mortality. 3. Percent motality was higher in those oysters which had grown fast and therefore was larger in size. 4. Hydrographic conditions in the oyster-farms did not differ significantly in the years of mass mortality and in normal years, at least in Hiroshima Bay. 5. Death of oysters began when water temperature rose to 21 or 22°C, and increased somewhat in parallel with rising water temperatures or salinities for a certain period of time. However, the period of maximal mortality did not coincide with the period of the temperature or salinity maximum. 6. Percent mortality was not always related to the distance from the shore or to the depth at which the oysters were suspended. 7. During a spawning season the death of oysters occurred neither in an explosive manner nor intermittently. The cases of massive death of cultured oysters have been known in other countries as well as in Japan, and the following factors, singly or in combination, have been suspected of being the causative agent. 1. Abnormally low water temperature. 2. Low salinity. 3. Physiological impediment due to high water temperature and/or high salinity. 4. Insufficient insolation and consequent shortage of food supply, due to cloudy or rainy weather. 5. Parasite. 6. Unfavorable water quality produced by artificial or natural cause. 7. Simple disease. 8. Pathogenic micro-organism. The present authors deduced that, if the causative agent, whether it be a sort of chemical substance or a certain physical or biological factor or factors, is such that is inseparably combined with the environment of the oyster-farm, prevention of the mass mortality will be very difficult to achieve, because it necessarily involes an improvement of the environment. They deduced also that there would be little chance of success if preventative measures were sought among those simple artificial measures. which could be easily thought of, because many investigators had failed, after years' of research, to find even the slightest clue to the prevention of these disasters. However, the authors discovered the following noticeable facts after analyzing closely the aforementioned cases of mass mortality which occurred in Hiroshima and in the Miura Peninsula region. 1. Death was not suffered by those oysters whose spawning period was delayed as compared with the oysters which were killed. 2. Natural oysters scarecely suffered death. 3. Death was not suffered by those oyster spat which were born and set during the period of massive death of adult oysters. 4. In the experiment in which the seed oysters from different localities of Japan were cultured under similar environmental conditions in Hiroshima Bay for the purpose of finding the seed oysters resistive to mass mortality, percent mortality was low in the stunted or "hardened" seed oysters. which were specially produced for shipment abroad. These hardened seed oysters were produced by restricting the subsequent growth of settled spat by exposing them to the air for comparatively long periods of time. On the basis of these findings. research was carried out along two lines. One line was concerned with the culture of the "one-year oyster", which is the culturists' term for those oysters which are raised from the spat collected around July and reach the marketable size as early as in the following February or March. In this type of culture, spat are collected as early as possible, raised as rapidly as possible, and harvested as the food oysters before they ever attain sexual maturity. The other line of research, which is the subject of this paper, was concerned with the elucidation of the reason why natural oysters, as well as the cultured oysters which are either young in age or slow in growth, do no suffer mass mortality. In this line of study, hardened seed oysters were produced on an experimental scale by exposing the spat to increased lengths of time and thereby restricting their growth to an extreme extent, and their physiological and ecological characters were studied in comparisor: with those of ordinary seed oysters. In other words, attempts were not made to search for the cause of mass mortality so much as to produce oysters which are resistive to mass mortality. The study was further extended and dealt with the method of growing hardened seed oysters to the food oysters of marketable .size. The results obtained in these studies are summarized below. 1) Hardened seed oysters were produced from the spat which settled on the strings of cultch in late July. In late September or early October, these spat, as attached to the strings of cultch, were placed on one of the horizontal shelves installed on the beach at four different intertidal levels respectively corresponding to the average daily exposure of about 18, 15 3/4, 13 and 10 1/3 s hours, and cultured (i.e., hardened) there until next August ("hardening period"). As a control, a group of spat of the same origin were cultured during the same period by the simplified hanging method at a low level which was scarcely exposed to the air ("ordinary seed oyster"). 2) At the beginning of the herdening period, spat measured 11.5 mm in average shell height. When the hardening period was over in next August, shell height measured about 25 mm in the hardened group and about 67 mm in the ordinary group. This shows that the growth was very slow in the hardened group during the hardening period. 3) The hardening period was survived by 65.0% of the spat in the hardened group as against 36.4 % in the ordinary group. Most of the mortalities occurred in the earliest part of the hardening period and in the spawning season. In the former period injuries due to handling were the cause of the loss. In the latter period, the ordinary group suffered mortalities ascribable to the physiological processes associated with spawning, and the hardened seed oysters which had been given average daily exposure of 18 hours suffered higher motalities than other hardened seed oysters, owing to the excessively long exposure. 4) In the ordinary group, water content of the meat was 78.2% in June or just before spawning, and increased to 84.4% in August or after spawning. The corresponding values were respectively 80.5% and 80.4% in the hardened group, indicating that before spawning the water content of meat was higher in the hardened group than in the ordinary group and that the reverse was the case after spawning. In addition, glycogen content of the meat (wet basis) of the ordinary group dropped from 1.6% to 0.4% during the spawning period. These facts indicate that hardened seed oysters waste less energy in connection with spawning activity than ordinary seed oysters. 5) The rate of ciliary movement of the gill was almost always greater in the hardened group than in the ordinary group, when there was a change in water temperature or in salinity. This fact suggests that hardened seed oysters have greater adaptability to varying environmental conditions than ordinary seed oysters. The rate reached the maxima at water temperatures around 32°C and at salinities of about 15 to 17‰- 6) In ordinary seed oysters, the gonad began to develop around March and its maturing proceeded gradually, until large amounts of genital products were released from June through August. This was followed by the state commonly known as the "watery oyster". In contrast, in stunted seed oysters, development of gonad was not distinct until about May. Thereafter the gonad developed rapidly, and then small amounts of genital products were liberated. This was immediately followed by the absorption of the gonad and the infiltration of phagocytes, both of which accelerated the disappearance of the gonad. Consequently, hardened seed oysters were scarcely found to be in the state of the "watery oyster". 7) As to the sex composition, the proportions of males and monoecious individulas were greater in the hardened group than in the ordinary group. 8) In June and July, i. e., in the early part of the spawning season, measurements were made on the cross section of the middle part of the soft body of the seed oyster in order to determine the area occupied by the gonad. In average, gonad occupied 35% and 69% of the area of the cross section respectively in the hardened and the ordinary seed oysters, indicating that the proportion of the gonad to the soft part, as well as the amount of ovarian eggs, was much smaller in the hardened seed oysters than in the ordinary seed oysters. 9) Individuals with abnormal multinuclear eggs or abnormal mass of male genital products occurred. in the ordinary group, but not in the hardened group. However, it is not likely that this phenomenon is related to mass mortality. 10) Hardened seed oysters of average shell height of 28.81mm and ordinary seed oysters of 80.90 mm were transferred to an environment favorable for their growth, and cultured there. In three months, the hardened seed oysters grew to about 80 mm in shell height, or nearly the same size as attained by the ordinary seed oysters, the growth of which was very poor during the same period. 11) 57.4% of the ordinary seed oysters and 78.4% of the hardened seed oysters survived the abovementioned growing period, indicating the higher survival rate of the hardened seed oysters. 12) During the abovementioned growing period, degeneration of gonads was in progress in both hardened and ordinary seed oysters. The majority of the ordinary seed oysters, however, contained genital products and spawning took place until as late as September. In contrast, hardened seed oysters contained those genital products which had been carried over from the spawning season, but these were not liberated in the subsequent period owing to absorption and the infiltration of phagocytes. As a result, the connective tissue was restored quickly and fattening of the meat began early in the hardened seed oysters. 13) In another experiment, hardened seed oysters were transferred to a growing environment at three different times (i. e., April, June and August) and their growths during the subsequent period were compared. The results indicated a definite tendency that the earlier the transplantation, the slower the growth during the subsequent spawning season : the group transferred to the growing environment in April and that transferred in June attainted nearly the same size (about 90 mm in shell height) in next January; the groups transferred repectively in June and in August also reached similar sizes to each. other (about 75 mm) in November. There was also a tendency that the earlier the transplantation, the slower the growth in winter. 14) It was found that the rate of shell growth, the rate of increase in meat weight and the time and duration of spawning period vary with the localities and, within a single locality, with the depths. at which the oysters are suspended. It was also found that the manner in which the spawning proceeds. is closedly related to the water content of the meat. It was not always the case that fattening began earlier in the oysters which spawned earlier. 15) When transferred to a growing environment early, hardened seed oysters tended to spawn in a similar manner to ordinary seed oysters. 16) It was found that, if oysters are transferred from one locality to another even for a short period. time during the raft-culture period, the rate of shell growth and the rate of increase in meat weight are much different before and after the transfer. When the oysters which had been grown in an environment favorable for their growth and fattening were transferred to a less favorable environment, their rates of growth and fattening dropped to extremely low levels. On the contrary, when the oysters. grown in an unfavorable environment were transferred to an favorable environment, their rates of growth and fattening were remarkably improved, and better crops were harvested than from those oysters which were raised in the favorable environment throughout the period. 17) Different localities have different characters as oyster culture grounds : some localities favor shell shell growth, some favor fattening, some accelerate spawning, etc. By taking advantage of this fact intelligently, i. e., by transferring oysters from one locality to another according to a reasonable plan, one can accelerate either the shell growth or the fattening at his will. 18) The adverse effect of crowding upon the growth and fattening of the oysters depends to a marked degree on the fertility of the water area. In those waters where the micro-suspensoid, which is considered as the food of oysters, is present in an amount more than about 1.5 mg/l, growth and fattening proceed generally at high levels almost independently of the number of oysters per cultch, though there is tendency that slight differences in growth and fattening occur between the central and the peripheral portion of a single oyster raft, which covers an area of about 1.3 to 2.0 are. Effect of crowding is obscure also in the waters where the amount of micro-suspensoid is less than 0.8 mg/l. In the waters where the amount of micro-suspensoid is intermediate between the two values mentioned above, the growth and fattening of the oysters are much affected by the population density of the oysters. Fouling organisms attach to the oysters during the culture period. The adverse effect of these organisms to the growth and fattening of oysters also depends on the fertility of the water area. The foregoing results indicate that the use of hardened seed oysters in the culture of the food oyster will lead not only to the economization of labor and other expenses but also to reduced mortalities and improved meat quality. The finding that different localities have different effects upon the growth, spawning and fattening of the oyster suggests that one can accelerate growth and/or fattening by transferring the oysters to the suitable culture grounds which are selected on the basis of this finding and according to the growth stages of the oyster. It is anticipated that, if this principle is put into practice, the food oyster can be produced and marketed on the schedule, and that the culture of the food oyster will therefore become a more stabilized industry than what it is now.

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1)本研究はカキ発死対策の一環として進められたものであるが,本研究を行なった背景には2及び3の項目に述べるようなカキ養殖技術の基礎的研究として行なわねばならない諸問題が含まれていた。2)カキ養殖はこの10年間に驚異的な発展を示し,生産量は実に4~5倍に達した。しかしこの上昇はいわゆる質よりも量の増加であって,カキを作ったと言うよりはむしろ出来たと言うべきであり,必ずしも科学的根拠に立脚した養殖技術の確立によるものではない。したがって現在まで未解決のまま放置された数多くの問題点が提起されている。3)養殖場の老朽化密殖と言う言葉に抽象されて成長肥満の低下は一般的な重要問題となっている。4)·1,2,3の項目と関連して生産の阻害,品質の低下となる諸要因は,直接的,間接的を問わず多数あるが,その最も大きいものは,いわゆる原因不明の大量斃死である。5)1945年から広島を中心として島根,鳥取,岡山の諸県下に大量斃死が起き,その披害率は60~100%に達した。6)過去における大量斃死原因は養殖場の異状高水温と高比重による産卵障害とされていた。また斃死の対策は斃死の起こる養殖場を避けること,KIイオンを含んだ物質を投与することなどであった。一方その間に斃死原因に関する研究も進み海水中の bacteria による伝染性の疾病であると言う説も出ている。7)著者等は天然着棲ガキや,若年ガキ,さらに長時間干出を与えた種苗(抑制種苗)に大量発死の現象が起こらないとと,また一方において斃死を防止するために,海況の人為的な改変は非常に困難であると思われることから,種苗の時期に長時間の干出を与えて成長を抑制したいわゆる抑制種苗を作り,まず生理,生態を含めた一般性状について研究を進めることにした。8)抑制種苗は7月末に着棲した種苗を9月末から10月初めに1日平均約18時間,15時間45分,13時間,10時間20分干出するように高さの異なった棚を作り,この棚で4段階の抑制種苗を作り,対照として簡易垂下式によるほとんど干出時間のないカキを普通種苗として育成した。9)抑制開始当時 spat の殻長は 11.5mm であったが抑制終了時の翌年8月には抑制種苗の殻高が 25mm前後で普通種苗は 67mm で,抑制種苗の成長は非常に緩慢であった。10)抑制期間中の着棲個体の生残率は抑制種苗が65%で普通種苗は36.4%であって,歩減りのはなはだしい時期は試験開始直後と産卵期で,前者は諸作業による損傷の歩減りであり,後者は普通種苗が産卵生理によるもので,抑制種苗の中で特に1日平均18時間の干出を与えた抑制種苗は長時間の干出による斃死であると思われる。11)普通種苗は産卵直前の6月に肉中含水率が78.2%で抑制種苗の80.5%より低いが,産卵後の8月には普通種苗が84.4%,抑制種苗は80.4%で逆に普通種苗が高くなり,生肉中のglycogen 含有率は同じく普通種苗が1.6%から0.4%に減少し,抑制種苗は1.2%が0.7%となって,抑制種苗は産卵行為による体消耗が少ない。12)鰓の繊毛運動速度は抑制種苗が普通種苗よりも水温,塩分の変化に対して常に大となる傾向を示して,抑制種苗には強い適応性のあることがみられたが,両種苗共に水温では32°C附近 Cl では15~17%附近で最大の速度を示した。13)gonad の発達は普通種苗では3月頃から漸次発達し6月から8月にかけて大量の産卵を行ない「水ガキ」状態となるが,抑制種苗は5月頃から急激に発達し,わずかに産卵した後吸収作用と食細胞の浸潤によって gonad が消滅し,「水ガキ」状態になることはほとんどなかった。14)抑制種苗の sex-ratio は雄性個体と雌雄同体が多く現われた。15) 6月~7月の産卵初期に肉の中央部を切断し,切断全面積に対する gonad 部位の面積の割合を比較すると普通種苗の平均が69%,抑制種苗は35%で,抑制種苗の gonad 面積の占める割合は少なく抱卵量は非常に少なかった。16)普通種苗には多核現象の異状卵や異状精塊が発生した個体が出現するが抑制種苗には現われない。この現象は異状斃死と関係はないと思われる。17)抑制種苗と普通種苗を同一成長環境に移殖すると,移殖した時普通種苗の殻高は 80.90mm で抑制種苗殻高は 28.81mm であった。移殖後3ヶ月で普通種苗はその間成長が非常に緩慢で抑制種苗は 80mm の前後まで成長して両種苗間にはほとんど差がみられなくなった。18)成長環境に移殖した後の生残率は普通種苗が57.4%で抑制種苗は78.4%となり,抑制種苗の生残率が高い。19)成長環境に移殖された後両種苗の gonad の変化は共に消減の過程であるが,普通種苗は9月になっても大多数の個体が生殖素をもち産卵を行なっている。抑制種苗も未放出の生殖素は残っているが吸収作用と食細胞による浸潤で産卵は行なわれず結締組織の回復が早くから始っている。20)抑制種苗を4月,6月,8月の3回にわけて成長環境へ移殖すると,早期に移殖した種苗ほど産卵期における成長の停滞が大きく,4月に移殖した種苗と6月に移殖した種苗は翌年1月には共に殼高が 90mm前後で同じ大きさとなり,6月に移殖した種苗と8月に移殖した種苗は11月には共に殻高が 75mm 前後で同じ大きさになった。早期に移殖した種苗ほど冬期の成長が緩かである。21)漁場の場所的な,また同一漁場でも垂直的な違いによって成長,肥満の度合が異なり産卵速度にも遅速がある。産卵の状態と産卵期における肉中含水量は密接な関係があり,早く産卵した個体が必ずしも早く肥満するとは限らない。22)抑制種苗でも早期に成長環境に移殖すれば,産卵状態が普通種苗と似通った傾向を示すようになる。23)養殖期間中に短期間でもカキを移動させると,以後の成長,肥満が非常に変化するものであって,成長,肥満の好適漁場で養殖していたカキを好適でない漁場に移動すれば極度に成長,肥満が悪くなり,逆に好適でない漁場から好適漁場に移せば,最初から好適漁場に養殖されていたカキよりも更によい結果となる。24)各養殖場は殼の成長,肉質の肥満,産卵を促進させる環境特性を異にしていて,これらの特性を合理的に利用することによって,成長,肥満の促進を行なうことが出来る。25)着棲個体数が,成長,肥満に与える密度効果は漁場の豊度によって非常に異なり,カキの餌料と思われる micro-suspension が 1.5mg/1L前後以上あるような養殖場では着棲個体数にほとんど関係なく全般にカキの成長,肥満がよく行なわれるが,養殖筏を単位とした場合に中央部位と周辺部位には多少ながら差のあらわれる傾向がある。また 0.8mg/1L 以下のような場合には密度効果は現われず suspension 量が上記の中間的な値を示す漁場に密度効果が大きく現われる。また養殖中に着棲するカキ以外のfouling 生物の着棲量とカキの成長,肥満との関係もまた漁場豊度に左右される。以上のような諸研究結果から抑制種苗を使用することによって養殖期間中の斃死率を低下させ肉の品質を向上させることが出来,さらにカキの成長段階に応じて各漁場の特性を合理的に利用すれば成長肥満を促進させることが出来,カキ養殖をより安定した計画産業に導けると思われる。

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bibliographic_information

ja : 内海区水産研究所研究報告
en : Bulletin of Naikai Regional Fisheries Research Laboratory

巻 19, p. 1-153, ページ数 153, 発行日 1962-03

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内海区水産研究所

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Naikai Regional Fisheries Research Laboratory

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0497-5022

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収録物識別子

AN00176718

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