Itoh et al.: Migration patterns of Thunnus onentalis determined with archival tags 



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2003 ) from the second day after release, and their feeding 

 frequency reached a constant level one month after release. 

 Also, the fish maintained weight-at-length similar to that 

 of wild fish. The average growth rate of fish tagged with 

 archival tags observed in our study ( 1.3 cm/month) was less 

 than the growth rate of wild fish observed between ages one 

 and twoin previous studies (1.7-3.3 cm/month, Aikawa and 

 Kato, 1938; Yokota et al., 1961; Yukinawa and Yabuta, 1967; 

 Bayliffet al., 1991; Bayliff, 1993; Foreman, 1996), except for 

 the result of Bell (1963b) (1.3 cm/month). Because similar 

 growth rates were observed for fish recaptured more than 

 a half year after release that spent the summer at large, 

 it appears that the lesser growth rate in the present study 

 is not due to the fact that some fish spent only winter at 

 liberty, when the growth rate is less than that in summer 

 (Yukinawa and Yabuta, 1967; Bayliff, 1993). Judging from 

 these facts, we suggest that the effect of archival tags on 

 fish behavior and physiology seems to be minor, although 

 there is a possibility that carrying an archival tag caused 

 a reduction in growth rate of the fish. 



Horizontal movement 



Archival tags revealed the movement pattern of young 

 Pacific bluefin tuna individuals, which could be divided into 

 the two clearly-separable phases of traveling and residency. 

 These two phases were observed for all individuals that 

 moved out from the East China Sea. 



T3 

 0) 



8 



12 



16 



20 



24 



Time of day (fi) 



Figure 10 



Hourly averages of temperature difference between 

 ambient water and fish viscera in both traveling 

 (circle) and residency phase (triangle) of young 

 Pacific bluefin tuna with archival tags. Data from 

 five individuals are all combined. Bar indicates the 

 standard deviation. 



The residency phase is considered a normal condition for 

 young bluefin tuna, comprising 83-87% of their time. Fish 

 with archival tags tended to stay in the areas of the East 



