WEKO3
アイテム
マサバ卵・稚仔輸送の場としての海洋構造の研究
https://fra.repo.nii.ac.jp/records/2005322
https://fra.repo.nii.ac.jp/records/2005322e65df997-e07f-404a-bbc0-c403bd43d37c
| Item type | 紀要論文 / Departmental Bulletin Paper(1) | |||||
|---|---|---|---|---|---|---|
| 公開日 | 2024-05-22 | |||||
| タイトル | ||||||
| タイトル | マサバ卵・稚仔輸送の場としての海洋構造の研究 | |||||
| 言語 | ja | |||||
| タイトル | ||||||
| タイトル | Studies on the Kuroshio Related to the Transportation of Mackerel Eggs and Larvae | |||||
| 言語 | en | |||||
| 言語 | ||||||
| 言語 | jpn | |||||
| 資源タイプ | ||||||
| 資源タイプ識別子 | http://purl.org/coar/resource_type/c_6501 | |||||
| 資源タイプ | departmental bulletin paper | |||||
| アクセス権 | ||||||
| アクセス権 | metadata only access | |||||
| アクセス権URI | http://purl.org/coar/access_right/c_14cb | |||||
| 著者 |
友定, 彰
× 友定, 彰 |
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| 抄録 | ||||||
| 内容記述タイプ | Abstract | |||||
| 内容記述 | The spawning grounds of the Japanese common mackerel, Scomber japonicus HOUTTUYN are generally located in the surroundings of islands or submarine banks in the Izu-Shoto sea area such as Zenis and Omuro-Dashi. The current speed of sea water is weak in the spawning grounds because of its location inshore of the Kuroshio. The following phenomena have been observed by the survey of the spawning grounds which was conducted by the Tokai Regional Fisheries Research Laboratory since 1949: namely, the mackerel eggs and larvae are absorbed into the strong current of the Kuroshio stream, drift along the cyclonic shear zone in the Kuroshio off Boso Peninsula and some of them are emitted from the Kuroshio to the coastal sea area of Kashima-Nada(WATANABE, 1970). However, the oceanographic conditions in and inshore of the Kuroshio in relation to the transport of mackerel eggs and larvae have not yet been made clear. It has been considered that the survival rate in the early stages of life has a great effect on the variation of the abundance of population of mackerel as well as the other important commercial stocks of pelagic-neritic fishes. Therefore, it is important to make clear the subject of the mechanism of transport of mackerel eggs and larvae. The purpose of this study is to make clear the oceanographic conditions with relation to the transport of mackerel eggs and larvae in and inshore of the Kuroshio in Izu-Shoto, Boso and Kashima-Nada sea areas. 1. Data sources The oceanographic data was acquired in the Izu-Shoto, Boso and Kashima-Nadasea areas by the R/V Soyo Maru (494 tonages) during the summer seasons of 1975, '77, '79, '80 and '82. Thermal pictures were taken by MSS (multi-spectro-scanner) from an airplane in the Izu-Shoto sea area in the summer of 1978 and '79. Sea surface temperatures have been continuously measured by a navigational thermometer between Tokyo and Hachijyo-Shima Island from the commercial ship 'SUTORETIA Maru' (Tokai Kisen Ltd) daily since the autumn of 1978. Prompt reports of oceanographic conditions have been published weekly from Ibaraki and Chiba Prefectural Fisheries Experimental Stations by gathering the sea surface temperature data measured by the fishing boats. Regular observations at the coastal area have been carried out monthly by Ibaraki Prefectural Fisheries Experimental Station as a part of Fisheries and Oceanographic Forecasting Project. Temperatures are measured at the coast of Izu -Shoto islands every morning. These data are used to analyze the oceanographic conditions with relation to the transport of mackerel eggs and larvae. 2. The Kuroshio and its perturbation in the Izu-Shoto sea area. 2-1. Perturbations of the Kuroshio observed in Izu-Shoto sea area. At the sea area north of Miyake-Shima Island, the bottom topography appears to have rugged features with many islands and submarine banks. In this area, Zenis Ridge lies from Oshima Island to south west as an additional ridge of Izu-Ogasawara Ridge. Near Miyake-Shima Island and Mikura-Shima Island, the bottom is deeper than in the northern or southern sea areas. The bottom with the depth of more than 1000 meters is located south of Mikura-Shima Island with the width of about 10 nautical miles and joins west and east of Izu Ridge. At Hachijyo-Shima Island and its neighbourhood, the bottom is shallower than 1,000 meters and appears in the shape of a plateau. As the bottom topography has these characteristic features in Izu-Shoto sea area, Izu-Shoto sea area is conveniently separated into the three following areas; The northern sea area for north of Miyake-Shima Island, the mididle sea area for the neghbourhood of Miyake-Shima Island and Mikura-Shima Island and the southern sea area for the neighbourhood of Hachijyo-Shima Island. The index temperature of the Kuroshio is 8 - 10 °C at a depth of 500 meters wherever the Kuroshio streams in these three areas. The temperature at a shallower depth than 500 meters can not be taken occasionally as the index of the Kuroshio axis. When the Kuroshio passes the Izu Ridge in the northern Izu-Shoto sea area, the space among 8 and 10 °C at the depth of 500 meters becomes wide because some parts of the Kuroshio streams north of the Zenis Ridge and other parts of it streams south of the Zenis Ridge. But, the space of these temperature zone becomes narrow again just east of the Izu Ridge. When the Kuroshio streams in the northern and middle Izu-Shoto sea area, many eddies with the scales of islands or submarine banks are generated by the perturbation induced by islands or submarine banks. The enclosed temperature contour lines are drafted with the same scales as the islands at the depth shallower than 200 meters. When the Kuroshio streamed in the southern Izu-Shoto sea area, southerly strong current separated from the main Kuroshio which flowed to the north east was observed just east of Izu Ridge. Both the perturbations and the spacious shear zone of the Kuroshio due to the islands or submarine banks can not be recognized in the short distance from the Izu Ridge. The perturbation of the Kuroshio due to the islands or submarine banks seems to give an effect to the south-south-east of Cape Nojima-Saki at the farthest. 2-2. Perturbations of the Kuroshio causing to the convergence of mackerel eggs and larvae. Temperatures and surface velocities were measured by XBT (expendable bathy-thermograph) and GEK, respectively in the sea area of Oshima, Miyake-Shima Islands' to south of Cape Taito-Saki located at the middle of the Boso Peninsula on 31 July 1979 when the Kuroshio streamed in the northern Izu-Shoto sea area. At the same time, thermal pictures were taken 5 consecutive times by an airplane from the height of 4,500 meters in the area of Oshima to Cape Nojima-Saki located south of the Boso Peninsula. As the result, the pattern of the stream line suggested the water convergence just east of Oshima Island to Omuro-Dashi submarine banks where the bottom changes from a shallow to a deeper depth. During the first flight of the picture taking by airplane, the colder water flowed southward immediately east of Oshima Island and warmer water flowed eastward south of Oshima Island, and the warmer and the colder water flowed together and formed a striped eddy east of Oshima Island. The result shows that topographic eddies are generated east of Oshima Island and these eddies have a role to converge inshore water to the marginal portion of the Kuroshio. The temperature of this topographic eddy became lower than the temperature of the surrounding water with the passing of time. The processes of the topographic eddy generation indicate that the eddy has convergency at the birth stage and changes to divergency as the time passed. The navigational thermometer record of the SUTORETIA Maru indicates that the lower temperature area east of Oshima Island is 3 to 5 days in duration with repetion of temperature variations. Such a topographic eddy with stripe taken just east of Oshima Island was also observed east of Mikura-Shima Island on 30 July 1978. These topographic eddies were generated in the sea area adjacent to islands where the bottom deepened for the strong Kuroshio current. It is estimated that the eddy has the character of convergency in the stage of having a striped shape. In the period when the Kuroshio streamed around Hachijyo-Shima Island, a set of CTD (conductivity-temperature-depth system) and a current meter was repeatedly raised and lowered at two hours intervals from 9 to 11 August 1980. This was done at the plateau with 300 meters in bottom depth at the south of Hachijyo-Shima Island in order to investigate the relation between the current and the temperature or salinity variation. According to the time dependent variation of temperature or salinity versus current, high temperature/low salinity was observed in the shallower layer than the salinity maximum and high temperature/high salinity was observed in the deeper layer than the salinity maximum when the current directed from a shallow to a deep bottom depth. This indicates that water columns stretches vertically and convergence is induced when the current directs from shallow to deep bottom depth. It is suggested that the cyclonic vorticity is induced and counterclockwise current is generated by the cyclonicvorticity in the stretched water columns, when the water converges. The observed result south of Hachijyo-Shima Island supports that the topographic eddies east of Oshima Island or Mikura-Shima Island are in counterclockwise motion and convergent. These results from investigation suggest the following process on the absorption of mackerel eggs and larvae to the margin of the Kuroshio; namely, the mackerel eggs and larvae spawned in Izu-Shoto sea area inshore of the Kuroshio are diffusively drifted eastwardly by weak current while they drift the Izu-Shoto sea area. The mackerel eggs and larvae will converge by the stretching of water column or by the topographic eddy with convergence, when they arrive at the east of Izu Ridge where the bottom deepens. Therefore, the fact that the bottom deepens steeply east of Izu Ridge and that the Kuroshio streams adjacent to islands such as Oshima, Miyake-Shima or Mikura-Shima has a great role in the absorption of mackerel eggs and larvae to the margin of the Kuroshio. 3. The Kuroshio and mackerel eggs and larvae in Boso sea area. The velocity of the Kuroshio greatly increases from its coastal margin to the strong current zone and gradually decreases offshore from the strong current zone. According to KAWAI (1955), a secondary flow would be drived perpendicular to the main flow due to the frictional force by the velocity variation of the main flow. Applying this hypothesis to the Kuroshio current off Boso Peninsula, it is estimated that almost the cyclonic shear zone between the coastal side margin of the Kuroshio and the strongest current zone in the Kuroshio is the convergent area and the anticyclonic shear zone on the offshore side from the strongest current zone is the divergent area. Furthermore, the northern boundary of the Kuroshio is the land along Boso sea area and the Kuroshio streams along its coast. This suggests that great turbulences with the scale of more than 10 nautical miles are difficult to form, although small scale turbulences with the scale of a few miles are frequently formed. As it is proved that the diffusion coefficient is in proportion to the 4/3 th power of the eddy scale, the eddy diffusion is small in Boso sea area due to the small scale turbulence. The mackerel eggs and larvae are densely distributed along the cyclonic shear zone of the Kuroshio and drift with little diffusion perpendicular to the main Kuroshio current in the Boso sea area. The convergence of mackerel eggs and larvae is caused by the physical properties that the effect of convergence induced by the secondary flow across the main current is greater than the effect of divergence due to the eddy diffusion in the Boso sea area. 4. Emission of mackerel eggs and larvae in Kashima-Nada. 4-1. Correlation between the oceanographic condition and transport of mackerel eggs and larvae. Warm water tongues exist in about half periods of winter and spring when the mackerel eggs and larvae are distributed in the Kashima-Nada. The warm water tongue is generated by peeling off the coastal margin of the Kuroshio and overhanging to Kashima -Nada. The western boundary of the warm water tongue is trapped at the shallow bottom of the continental shelf. Stability of velocity, vector mean and current direction are statistically calculated from the currents measured by GEK in the area with the bottom depth of more than 100 meters in Kashima-Nada. As the results, the Kuroshio streams in a north-east or east- north-east direction with high stability off Cape Inubo-Saki (northern edge of the Boso Peninsula) and off Kashima Port in all seasons. In the coastal area of Kashima-Nada at the north of the Kuroshio, the currents have the southward component and the water in the coastal area of Kashima-Nada is absorbed into the Kuroshio in all seasons. The stability of current in the coastal area of Kashima-Nada is great in the summer and autumn, and little in the winter and spring. According to the individual observation of current, the northward currents are measured in a few cases along the western boundary of a warm water tongue. This is caused by the east-west fluctuation of the western boundary of a warm water tongue. Northward currents are observed when the western boundary of the warm water tongue is located at the current measuring stations. In opposition, southward currents are observed when the western boundary of the warm water tongue is adjacent to the coast and the current measuring stations are situated at offshore side from the sharp temperature gradient zone of the western boundary of the warm water tongue. Therefore, the mackerel eggs and larvae are transported by the northward current along the western boundary of a warm water tongue and are distributed adjacent to the coast of Kashima-Nada, when the warm water tongue exists. 4-2. Problem on the relation between the oceanographic condition and the transport of mackerel eggs and larvae. Calculating the mean flow in Kashima-Nada, it has a southward component even in winter and spring when the mackerel eggs and larvae are drifting in Kashima-Nada. Estimating from the flow pattern, it is difficult to speculate that the mackerel eggs and larvae which are emitted in the coastal area of Kashima-Nada are enough to support the abundance of population of the Japanese common mackerel. According to the rough estimation, the quantities of mackerel larvae emitted from the Kuroshio to the coastal area of Kashima-Nada and the quantities transported to the east of 142 °E, are generally the same. This indicates that the mackerel eggs and larvae which are transported to the east of 142 °E by the Kuroshio are not so small in magnitude as to be neglected, but are considerable enough to support the abundance of population of the Japanese common mackerel. | |||||
| 言語 | en | |||||
| 書誌情報 |
ja : 東海区水産研究所研究報告 en : Bulletin of Tokai Regional Fisheries Research Laboratory 巻 117, p. 47-119, ページ数 73, 発行日 1985-10 |
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| 出版者 | ||||||
| 出版者 | 東海区水産研究所 | |||||
| 言語 | ja | |||||
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| 出版者 | Tokai Regional Fisheries Research Laboratory | |||||
| 言語 | en | |||||
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| 収録物識別子タイプ | PISSN | |||||
| 収録物識別子 | 0040-8859 | |||||
| 書誌レコードID | ||||||
| 収録物識別子タイプ | NCID | |||||
| 収録物識別子 | AN00156834 | |||||
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| 識別子タイプ | Local | |||||
| 関連識別子 | tokai_k_117_47 | |||||
| 関連サイト | ||||||
| 識別子タイプ | URI | |||||
| 関連識別子 | https://agriknowledge.affrc.go.jp/RN/2010330406 | |||||
| 言語 | ja | |||||
| 関連名称 | 日本農学文献記事索引(agriknowledge) | |||||
