Domeier et al.: Tracking Thunnus thynnus onentalis with the aid of an automated algorithm 



51)1 



northern latitude estimates calculated 

 by PSAT Tracker placed fish 441 slightly 

 north of Point Conception, California, 

 and fish 159 near the California-Oregon 

 border before it returned to wintering 

 grounds off Baja California (Fig. 10). 

 This movement is corroborated by a 

 westerly trend in longitudes and a dra- 

 matic drop in SSTs. For fish 159, SST 

 dropped from 19. TC on 4 April 2003 to 

 12.4°C on 18 April 2003. Similarly, fish 

 441 experienced an SST drop from 19.5° 

 to 13.5°C between 1 and 18 April. 



Data from the archival tags provided 

 near daily positions for each fish. The 

 longest time between successive fixes 

 was four days. The calculated swim- 

 ming speeds between successive posi- 

 tion fixes ranged from to 14.7 knots 

 for all three fish combined. The mean 

 swimming speed for all three fish was 

 1.3 knots (±1.3 km. 



Depth and temperature ranges Vertical 

 movement was similar to that reported 

 for other bluefin tuna (Block et al., 1997; Block, 2001; 

 Kitagawa et al., 2004). Detailed analyses of vertical 

 movement and temperature preferences and tolerances 

 are beyond the scope of this article and will be pre- 

 sented in a future publication. In general, dives were 

 most common during the day; maximum dive depths 

 ranged from 341 to 382 m. Fish with archival tags 

 spent nearly 70. 1% of the time near the surface (<20 m 

 deep). Ambient water temperatures ranged from 5.7° 

 to 25.0"C (mean=17.4°C). The internal temperature 

 offish tagged with archival tags ranged from 14.1° to 

 29.5 C (mean=21.8°C); average internal temperatures 

 of the fish were 4.4°C warmer than ambient waters 

 and at times were up to 19.2°C warmer. 



Discussion 



Although we used SST matching as the sole means 

 of estimating latitude for the fish tracks and spatial 

 analyses presented in our study, the extent of the 

 northward fall migration of juvenile Pacific bluefin 

 tuna in the eastern Pacific has been corroborated by 

 occasional commercial landings of Pacific bluefin tuna 

 in Oregon (McCrae 3 ). Because Pacific bluefin tuna 

 are apparently capable of existing in the northern 

 part of the eastern Pacific range, even during the 

 colder months of the year, it is not clear what dictates 

 the movement pattern of these fish. It is reasonable 

 to speculate that the tuna are taking advantage of 

 seasonal ocean warming to exploit distant prey when 

 the physiological expense to maintain optimum body 



3 McCrae, J. 2004. Personal commun. 

 Fish & Wildlife, Newport, OR 97365. 



Oregon Dept. 



Figure 7 



Fixed kernel home range analysis illustrating relative 

 importance of the range of juvenile Pacific bluefin tuna 

 {Thunnus thynnus orientalist in the eastern Pacific; dis- 

 played are all points for fish 159, 233, and 441 and volume 

 contours of 95% (outer line) and 50% (inner line) for all 

 three fish combined. Isolated circle to the north is a 95% 

 contour. 



