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In the year, during the period from spring to early summer remarkable rise of salinity in the estuary water of the River Tone and the corresponding change of the aquatic livings were observed. The results of studies conducted by the present author are as follows: The trace of the oceanic water was detected at the station upstream about 45 km distant from the mouth of River Tone even at the time of low tide. At the station upstream about 27 km distant from the river mouth, the ratio of oceanic water and fresh water indicates about 50% to 50%, where marine organisms (such as mackerel, octopus) were found against normal fresh water organisms (eel, silver-carp, corbicular) which died or escaped away. Such anomalous condition in this district has been aroused by the extremely dry weather which continued from spring to early summer in that year. It resulted in the decrease of fresh river water and as its consequence the inflow of oceanic water into the river (especially in the lower layer) increased. In addition to it, some other factors to strengthen the upstream flushing of sea water into the interior of the river might be considered, i.e. the deepening of the river floor by the artificial dredge on river bottom under the flood prevention works or by the rise of estuary water level effected by the occasional storms passing by the river. I. 緒 言 昭和33年 春から初夏にかけ,東 日本は非常な 干天続きで,農 作物に大被害を生 じたばか りでな く,干 天に伴 う種 々の異変現象をみた。河川特に 利根川では水量が減少し,外洋水の逆流があつて, かなり上流 までその影響を うけ,淡 水魚貝の弊死 や逸散を生 じ,海 産生物の異常遡河がみられた。 筆者は被害のもつ ともひどかつた昭和33年6月 上旬,利 根川下流域で異常現象の聞き取 り調査を 行い,ま た数地点で河水中の塩素量を測定した。 ここにその調査の概要を記 し,原 因などについて 検討を試みたので報告する。 II. 聞き取 り調査 調査はこの水域で漁撈に従事 している人を対象 とした。第1図 は利根川下流域の地形図である。 A. 銚子市笹本町地先(河 口より12km),西 谷千代松氏(中 利根漁協組合長) a) カタクチイワシの遡河をみ る。b)枝 コンブの 漂流多し,c)種 ウナギ(2~3匁 大)漁 撈皆無 と なる。d)鎌 ウナギ(食 用)ゴ カイ飽食,e)塩 分増 加につれ ゴカイ増加す,f)蓄 養中の鎌 ウナギが シジ ミの死貝を吐 く,g)夜 光虫多く,ボ ラ刺網 発光 して不漁 〔以上昭和33年6月 中旬以降〕, h)30cm大 のサバ水田中で捕獲 〔昭和33年6月 上旬〕,i)塩 分は河底淡蝶 工事の終了と共 に増加 傾向にあ り 〔昭和30年 以降〕。 B. 銚子市宮原町地先(河 口より14km),西 * 1960年11月2日 受 理(1958年10月 日本 水 産 学 会秋季大会(新 潟)講 演) * * Tokyo University of Fisheries 東京水産大学

On the Anomalous Phenomenon Caused by the Dry Spell in the Early Summer of 1958 in the Tidal Estuary of the River Tone