Tosa Bay

The area is located on the southern coast of Kochi Prefecture, north of the line connecting Cape Muroto and Cape Ashizuri. The mouth of the bay is about 130 km, the depth is about 50 km, and the coastline is about 240 km long. About 70% of the bay is continental shelf with a depth of less than 200 meters, and the depth increases toward the southeast to the Tosa Terrace. The formation of a valley is clearly visible on the shelf slope offshore of the Shimanto River and Nahari River. The area north of the line connecting Aki City and Kuroshio Town is prone to subsidence, and branch bays such as Urado Bay, Uranouchi Bay, and Susaki Bay are formed. The southern side is prone to uplift, and coastal terraces have developed at Cape Muroto, Cape Gyoto, and Cape Ashizuri. A sandy coastline is also formed from the mouth of the Niyodo River to the mouth of the Nahari River. The branch of the Kuroshio Current, which flows westward through the bay toward Cape Muroto and Cape Okitsu, has played a role in the rise and fall of coastal fishing since ancient times. In the inner part of the bay with its muddy bottom, mechanical bottom trawls were used, in the eastern part of the bay and around Cape Ashizuri, fixed nets such as Oshiki nets were used, and beach seine nets were used on the sandy coast, but these are now in decline. The line-and-line fishing of skipjack tuna, which is representative of Tosa Bay, expanded its fishing grounds outside the bay around the Taisho period, and small skipjack tuna are currently caught off the coast of Cape Ashizuri. Representative skipjack tuna fishing ports include Shimizu (Tosashimizu City), Saga, Kure (Nakatosa Town), and Usa (Tosa City), and Cape Muroto Port and Murotsu Port (Muroto City) have expanded into ocean-going tuna bases. In addition, yellowtail and sea bream are farmed mainly in Uranouchi Bay and Nomi Bay. In recent years, whale watching has also been conducted, focusing on whales and dolphins in the bay.

[Masaki Hisahito]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

高知県の南岸、室戸(むろと)岬と足摺(あしずり)岬を結ぶ線以北の海域。湾口約130キロメートル、奥行約50キロメートル、湾岸延長約240キロメートルに及ぶ。水深200メートル以浅の大陸棚が約70％を占め、南東に向かって深度を増し土佐海段に至り、四万十(しまんと)川や奈半利(なはり)川沖合いの陸棚斜面には海谷の形成が明瞭(めいりょう)である。安芸(あき)市と黒潮町を結ぶ線以北は沈降傾向にあり、浦戸湾、浦ノ内湾、須崎(すさき)湾などの支湾が形成される。南側は隆起傾向にあり、室戸岬、行当(ぎょうとう)岬、足摺岬には海岸段丘が発達する。また、仁淀(によど)川河口から奈半利川河口にかけて砂浜海岸が形成されている。室戸岬と興津崎に向かって突入し湾内を西流する黒潮の分流は、古来沿岸漁業の盛衰にかかわった。泥質底の湾奥では機船底引網、湾東部や足摺岬付近では大敷(おおしき)網などの定置網、砂浜海岸では地引網が行われていたが、現在は低迷している。土佐湾を代表するカツオ一本釣りは、大正期ごろから湾外へ漁場を拡大し、現在は足摺岬沖で小型のソウダガツオを漁獲する。代表的カツオ漁港として清水(しみず)（土佐清水市）、佐賀、久礼(くれ)（中土佐町）、宇佐（土佐市）などがあり、室戸岬港、室津港（室戸市）は遠洋マグロ基地へと伸展した。また、浦ノ内湾、野見湾を中心にハマチ、タイなどの養殖も行われる。近年では湾にいるクジラやイルカを対象としたホエールウォッチングも行われている。

［正木久仁］

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