Impact of the Kuroshio on dispersal of coral spawn and larvae around Ryukyu Islands in the East China Sea

琉球諸島周辺海域におけるサンゴ浮遊幼生分散に対する黒潮の影響

小谷 瑳千花*; 内山 雄介*; 鹿島 基彦*; 上平 雄基   ; 御手洗 哲司*

Odani, Sachika*; Uchiyama, Yusuke*; Kashima, Motohiko*; Kamidaira, Yuki; Mitarai, Satoshi*

2012年から2015年の春季を対象として、領域海洋循環モデルROMSをベースとした高解像度の琉球諸島周辺海域流動モデリングとその結果を用いた3次元Lagrange中立粒子追跡を実施し、黒潮がサンゴ幼稚仔輸送過程に及ばす影響を評価した。その結果、石西礁湖から放出された中立粒子は、サンゴ幼稚仔浮遊期間である3週間後も大部分がその場に留まるが、約1割が黒潮や黒潮反流によって沖縄本島まで輸送されていた。また、石西礁湖南で発生する東向きの流れがサンゴ幼稚仔輸送に重要な役割を果たしている可能性を示唆した。

We conduct a double nested high-resolution synoptic ocean modeling using ROMS with grid spacing down to 1 km coupled with an offline Lagrangian particle tracking model. The modeled dispersal of coral spawn and larvae released from about 20 major islands and lagoons are examined to quantify connectivity using Lagrangian probability density functions (PDFs) of the Lagrangian particles among Ryukyu Islands. We introduce 145 patches with a diameter of 1km around Sekisei Lagoon between Yaeyama and Ishigaki Islands close to Taiwan to compare with an in-situ surface drifter measurement. Lagragian particles are released from the patches in spring for 2012-2015 and are tracked for the advection time of 3 weeks suitable to representing active spawning and lifespan of coral spawn and larvae. The PDF analyses suggest that the particles mostly remain near the released areas, while about 10% of the particles are entrained by the Kuroshio to travel long distance, leading to inter-island connectivity. Some of these particles are further transported northeastward, and then partially trapped by the Kuroshio Counter Current formed between the Kuroshio and Main Island to approach Okinawa Main Island. We reveal that eastward current in the south the lagoon occurred shortly after the releases suppresses the entrainment and the resultant long-distance transport.