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In order to reveal feature of cultured yellowtail swimming behavior in the aquaculture pen, we investigated their swimming depth by the data logger in relation to the ambient water temperature between April 2004 and November 2004. The yellowtail exhibited two diel vertical swimming behavior patterns, i.e. day-shallow and night-deep pattern, and day-deep and night-shallow pattern. We defined daytime and nighttime as from 08 : 00 to 16 : 00 and from 20 : 00 to 04 : 00, respectively. Average swimming depth and ambient water temperature were calculated in the daytime and nighttime in each day. The yellowtails changed the swimming depth in accordance with the seasonal temperature fluctuations. The swimming depth shifted from the deep layer to the shallow layer as the water temperature increased in spring. In summer, the yellowtail frequently swam around 2m depth in the nighttime. However, the swimming depth deepened during periods when the thermal stratification developed and diurnal large amplitude temperature fluctuations occurred. The swimming depth was changed from shallow layer to deep layer in relation to the temperature falling in the daytime and nighttime in fall., Organized by Graduate School of Informatics, Kyoto University ; JSPS Bangkok Liaison Office ; Japanese Society of Bio-logging Science ; Informatics Research Center for Development of Knowledge Society Infrastructure, December 13-15, 2004, Imperial Tara Hotel, Bangkok, Thailand