302 



Fishery Bulletin 103(2) 



Oct-Nov 



441 range 

 ▲ deployment point 

 X recapture point 



Figure 8 



Grayscale contours of seasonal spatial use and movement 

 pattern for fish 159 and 233 combined, displaying "core areas" 

 of use represented by volumes of 10-50% . The smaller total 

 range of fish 441 is illustrated by the polygon. 



temperature is less. Temperature and depth tolerances 

 and preferences indicated in our study are similar to 

 those of bluefin tuna studied in other parts of the world 

 (Carey and Teal, 1969; Carey and Lawson, 1973; Block 

 et al., 1997; Kitagawa et al., 2000, 2004; Block et al„ 

 2001; Brill et al., 2002; Itoh et al., 2003b). 



The migration of a fish with an archival tag from the 

 western Pacific to the eastern Pacific (Itoh et al., 2003a) 

 provides an interesting comparison to our data. This 

 individual, tagged off Japan, made the trans-Pacific 

 migration in about two months and then resided in 

 the eastern Pacific for about eight months before being 

 recaptured by a recreational angler. The fish arrived off 

 the coast of northern California in the month of Janu- 

 ary — a time when fish from our study were found to be 

 at the southern extreme of their eastern Pacific range. 

 By the month of March, the western Pacific migrant 

 had traveled to the winter-spring grounds where it 

 then seemed to behave in a pattern similar to that of 

 fish tagged for our study. Whether or not the Itoh et al. 

 (2003a) tagged fish illustrated a typical transition from 

 trans-Pacific migrant to eastern Pacific resident will 

 require more tag recoveries. It will be equally interest- 

 ing to see future descriptions, from archival-tag data, 

 of maturing Pacific bluefin tuna making the trip back 

 to the western Pacific. 



Two of the Pacific bluefin tuna with archival tags 

 were captured and recaptured in very close proximity 

 in both space and time of year. The computed tracks 

 for these two fish, both relatively large for the eastern 

 Pacific, also showed that they kept close to each other 

 for most of the year. A smaller fish, tagged a month 

 later, underwent a similar north to south movement, 

 but did not range as far north, particularly, or south. 

 Given our extremely low sample sizes, very little can be 

 concluded, but the question is raised as to whether or 

 not Pacific bluefin tuna of different year classes have 

 distinct schools and migratory behaviors. It is also im- 

 portant to point out that the two larger fish were tagged 

 in the dorsal musculature, whereas the smaller fish was 

 tagged in the peritoneal cavity. The orientation of the 

 light stalk is different for these two methods, one point- 

 ing towards the surface and the other in the shadow of 

 the fish and pointing down. How this tag orientation 

 may influence the detection of light and subsequent 

 position estimates is unknown. 



Two of our fish with archival made rapid northward 

 migrations into much colder water in the early spring. 

 This northward migration is similar to that made by 

 Itoh's fish in the early spring of 1998. Because these 

 movements occurred at a time when the light-based 

 latitude estimates prove unreliable, it would not have 



