五島西沖アジ・サバ漁場の海洋学的特性に関する研究

In order to study the oceanographic characters of the mackerel and jack macke-rel fishing ground off the western region of the Goto Islands, the data mainly on temperature, salinity, fish finder records, the positions of fishing grounds and other items which were collected in the fishing grounds of this sea area from January 1971 to June 1972, were analyzed and the following results were obtained. 1. In the western region off Goto Islands which is located on the right hand side against the flow axis of the Tsushima Current, it was confirmed by dynamic calculation that stationary vortices exist in summer and a countercurrent against the Tsushima Current develops in winter. 2. Since this area was covered by the Tsushima Current from June to December, there were not found the Yellow Sea Cold Water, the South Korean Coastal Water and the coastal water of mainland China. In winter, however, being nearly of the same density from surface to bottom, a unique homogeneous water mass (105σ240-300) was formed in this area. 3. The fishing grounds formed in this area could be classified into following three; 1) the fishing ground formed in the area of the stationary vortex, 2) the fishing ground formed on the oceanic front made up in the mixing area between the South Korean Coastal Water and the Tsushima Current, 3) the fishing ground formed in the area of separation of currents. Especially, the first type was predominant. 4. Jack mackerels were often detected within the thermocline, while mackerels were found in the layer with hardly any difference of vertical thermal gradient just above or beneath thermocline. 5. During the observation period from November 1971 to June 1972, the main fish shoals, in the night, were frequently detected at a depth of 80 to 130 meters, the temperature and salinity ranges being 15° to 17℃ and 34.5 to 34.7‰, respectively. 6. In June 1972, the upper layer less than 100 meters of depth began to increase in temperature accompanied with the depletion of salinty, but the lower layer did not seem to be affected by such changes and the detection of fish shoals at night was limited within the lower layer.

1971年1月から1972年6月までの水温,塩分,魚群発見水深(夜間),漁場位置などを中心に資料を整理分析した結果,五島西沖アジ・サバ漁場の海洋学的特性は次の通りである。1) 対馬暖流主流の東側にあたる五島西沖には,渦流および対馬暖流に対し反流である南下流の存在が認められる。特に渦流は夏季に又反流は冬季によく発達している。2) 6月から12月までは,同海域は対馬暖流域にあたり,9月を除けば110m以深には西部北太平洋中央水,それ以浅には対馬暖流表層水がそれぞれ存在しているので,黄海冷水,韓国南岸沿岸水,中国大陸沿岸水などは出現しない。又1月から4月までの冬季は表層から底層までほとんど同一水塊で,この海域独特の水塊を形成する(105σ240～300)。3) 同海域で形成される漁場は,主にイ)渦流と関係がある漁場,ロ)潮境漁場,ハ)海流の分岐域と関係がある漁場などに分類することが出来る。このうちイ)の型が一番多かった。4) アジは水温躍層でも発見出来た例が多かったのに対し,サバはこの躍層を避けてその真下あるいは真上の鉛直水温傾度が小さい層でよく発見出来た。又冬季表層から底層までの水温差がほとんどない時によく漁獲された。5) 同海域の夜間魚群発見水深は,11月から6月まではおおむね80m～130mの水深で,その層の水温・塩分は主としてそれぞれ15～17℃, 34.5～34.7‰であるが,最高23.0℃,最低12.5℃の水温で漁獲された例もあるから,かなり広い水温範囲で漁獲されるものと思われる。6) 6月になって,水温・塩分の変化が進行する時にも,100m以深層はその影響を直接受けないように思われ,アジ・サバの夜間発見水深この層にかぎられている。

長崎大学水産学部研究報告, v.35, pp.67-84; 1973