ON THE “NIGASHIO” WATER IN THE BAY OF OMURA

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  • 大村湾の苦潮について
  • オオムラワン ノ ニガシオ ニ ツイテ

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Abstract

In summer, the water-mass lacking in dissolved oxygen often covers the bottom in the central part of the Bay of Omura. This water-mass has been called “Nigashio” water by the fisher-men, and there has been an influence upon fishing conditions of the drag-net fishery in the Bay.<br> In this work, the author has made a study on the appearance of the “Nigashio” water in this Bay, and investigated the vertical distributions and seasonal variations of oxygen as one of the methods for analysis of its factors.<br> The “Nigashio” water often appears in July at first, then develops well in August to September, and it disappears in October, soon after the vertical mixing of sea water begins.<br> Stagnation of the water-mass of central part of the Bay, owing to the anti-clockwise eddy system, causes a well developed stratification of sea water and consequently the occurrence of the “Nigashio” water.<br> As one method to know a degree of development of the “Nigashio” water, the vertical gradient of water temperature is useful, and at the same time it is said that the great abundance of plankton plays an important rÔle for onset of the “Nigashio” water.<br> Though the “Nigashio” water occurs most frequently at the central part of the Bay, west of the Mishima Is., the area is not always fixed, and the thickness of the “Nigasho” water (oxygen lacking layer) changes by area.<br> The “Nigashio” water front which has an effect upon the benthic fauna acts as a cause of a temporal increase of catches of drag-nets around the “Nigashio” water area, but ultimately the productivity of fishing grounds in that area decreases very much.

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