TERRAPUB Journal of Oceanography

Journal of Oceanography, Vol. 63 (No. 5), pp. 761-773, 2007

Jellyfish Patch Formation Investigated by Aerial Photography and Drifter Experiment

Shinya Magome1*, Tomohiro Yamashita2, Takeshi Kohama3, Atsushi Kaneda4, Yuichi Hayami5, Satoru Takahashi1 and Hidetaka Takeoka2,4

1Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology, Hirosuehiro, Kure, Hiroshima 737-0197, Japan
2Faculty of Engineering, Ehime University, Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan
3Faculty of Risk and Crisis Management, Chiba Institute of Science, Shiomi-cho, Choshi, Chiba 288-0025, Japan
4Center for Marine Environmental Studies, Ehime University, Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan
5Ariake Sea Research Project, Saga University, Honjyo-cho, Saga, Saga 840-8502, Japan

(Received 18 December 2006; in revised form 8 February 2007; accepted 30 March 2007)

Abstract: Jellyfish patch formation is investigated by conducting a drifter experiment combined with aerial photography of a sustained patch of the moon jellyfish in Hokezu Bay, Japan. Jellyfish patches are aggregations of individuals that are caused by a combination of swimming (active influence) and advection by currents (passive influence). The drifter experiment involved the injection of 49 drifters around a distinct surface patch of jellyfish within an area of approximately 300 m × 300 m. The drifters' motion, caused only by the passive influence, was recorded in a series of 38 aerial photographs taken over approximately 1 h. The ambient uniform current field larger than the patch scale was estimated from the movement of the centroid position of drifters, while the distribution of horizontal divergence and relative vorticity around the patch was estimated from the time-derivative in areas of triangles formed by the drifters. The centroid positions of both drifters and patches moved stably toward the bay head at different speeds. The difference vector between the patch and drifter centroids was directed to the sun, and was opposite to the ambient current. The distributions of vorticity and divergence around patches exhibited inhomogeneity within the patch scale, and the drifters in this nonuniform current field aggregated near the convergence area within 1 h. The results suggest that horizontal patch formation is predominantly influenced by passive factors at the surface of Hokezu Bay. Furthermore, the upward swimming against downwelling may make sustained patch in surface layer.

*Corresponding author E-mail: s-magome@aist.go.jp

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