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Fishery Bulletin 101 (3) 



1 Oct 1997 20 Nov 1997 9 Jan 1998 28 Feb 1998 19 Apr 1998 8 Jun 1998 28 Jul 1998 



Figure 11 



Changes in ambient water temperature recorded by an archival tag in a young Pacific bluefin 

 tuna. Data are for the fish that traversed the Pacific Ocean. The symbols from Ml to M5 designate 

 from the first to fifth traveling phase of the fish. Each dot is an ambient water temperature at m 

 depth recorded in the summary file of the archival tag each day. Large black and brighter circles 

 are temperatures at the onset and end of traveling, respectively. Arrows indicate the observed 

 ambient water temperature changes several days before the onset of the travel phase. 



China Sea, off the east coast of Choshi, off the east coast 

 of Hokkaido in the western Pacific Ocean, off Southern 

 California and Baja CaHfornia, off northern California in 

 the eastern Pacific Ocean, and off the west coast between 

 Akita and Hokkaido in the northern Sea of Japan. The first 

 four areas correspond to the major known fishing grounds 

 of young Pacific bluefin tuna. 



The last two areas do not correspond to previously known 

 fishing areas for young Pacific bluefin tuna. In the eastern 

 Pacific, young bluefin tuna are usually caught in an area 

 from 23 to 34°N, off California to Baja California, from May 

 to October by purse seine (Calkins, 1982). Catch records 

 in the northern area around 40°N were scarce, and all of 

 them were for catches from summer to autumn (Radovich, 

 1961; BaylifT, 1994). It was not expected that young Pacific 

 bluefin tuna were to be found around 40°N in winter, but 

 the archival tag records showed fish staying in an area of 



33-40°N, off northern California, from winter to spring. In 

 the northern Sea of Japan, young Pacific bluefin tuna are 

 usually caught by set nets in coastal areas, and are not 

 caught in the offshore area. The archival tag records again 

 showed fish staying in this area in summer and moving 

 southward without being captured. These cases clearly in- 

 dicate the ability and advantage of archival tags to provide 

 information on fish distribution and migration when and 

 where fishing has not been conducted. 



An archival tag demonstrated that an individual young 

 Pacific bluefin tuna was able to travel more than 7000 km 

 without pause and to travel for more than two months. The 

 daily moving distance during the traveling phase ranged 

 from 59 to 182 km, and averaged 104 km. Assuming a con- 

 stant swimming speed, the daily average swimming speed 

 was estimated as a range from 1.3 to 4.1 knots (average of 

 2.3 knots). 



