162 



Fishery Bulletin 100(2) 



30 



20 



10 10 



Time at deptti (%) 



20 10 10 20 30 40 

 Time at temperature (%) 

 Figure 7 

 Vertical distribution of five juvenile bluefin tuna ex- 

 pressed as percent time (mean ±SEMi spent at specific 

 deptbs I Ai and at specific temperatures iBi. Shaded bars 

 indicate nighttime and open bars indicate daytime. 



of juvenile bluefin tuna in the western Pacific recently ob- 

 tained with archival (i.e. electronic data recording) tags 

 have shown that the vertical movements of juvenile blue- 

 fin tuna can have strong seasonal and geographic com- 

 ponents (Kitagawa et al., 2000). In areas and at times 

 (e.g. winter) when there was a strong thermocline, bluefin 

 tuna remained in the uniform-temperature surface layer 

 and demonstrated vertical movement behaviors similar 

 to those observed during the short-term ultrasonic tele- 

 metry studies. In the summer, when the themocline was 

 less pronounced, the fish showed very distinct diel peri- 

 odicity in their vertical movement patterns. They would 

 remain at the stirface at night and make rapid vertical 

 movements (from surface to =120 m and from =21''C to 

 14°C) during the day. Kitagawa et al. (2000) concluded 

 that the differences in behavior patterns were related to 

 foraging. It is also still an open question as to what ex- 

 tent bluefin tuna's ability to conserve metabolic heat and 

 maintain elevated muscle temperatures (Carey and Teal, 

 1966) enhances vertical mobility. 



Roffer (1987) was apparently the first to propose that 

 movements and abundance of juvenile Atlantic bluefin tuna 

 are controlled by the depth and thickness of the 18.5-20.5°C 

 "preferred habitat" temperature layer Likewise, Inagake 

 et al. (2001), using archival tags implanted into juvenile 



37,8 



37.6 - 



75 8 75.6 75 4 75.2 75.0 74 8 74 6 74 4 74 2 74.0 



Figure 8 



Composite satellite sea surface temperature image (17 June-10 

 July 1998) and movements of the five juvenile bluefin tuna (Brill 

 and Lutcavage, 2001 ). Figure reprinted with permission of Ameri- 

 can Fisheries Society. 



bluefin tuna in the western Pacific, found evidence that 

 this temperature range is indeed always "preferred." Dur- 

 ing the periods of our observations, juvenile bluefin tuna 

 spent the majority of their time (=809c) in water greater 

 then 22°C. A plausible explanation is that under the condi- 

 tions of our observation period, juvenile bluefin tuna simply 

 occupy the warmest water available, although a relatively 

 uniform temperature surface layer was evident only dur- 

 ing tracks of fish 3, 4, and 5. We also did not find any con- 

 clusive indication that juvenile bluefin tuna avoided sur- 

 face water temperatures above 26°C. Although fish spent 

 less than 'ZO'^i of time at these temperatures (Fig. 7), less 

 than 20% of the recorded sea surface temperatures (i.e. the 

 warmest water available) were above 26°C. 



We also found no relationship between sea surface tem- 

 perature and horizontal movements (Figs. 6 and 8), al- 

 though this relationship has been demonstrated for other 

 tuna species in other areas (e.g. Laurs et al., 1977; Fiedler 

 and Bernard, 1987; Uda, 1973). We argue that our results 

 are due to the differences in the vertical and horizontal 

 temperature gradients occurring along the Virginia coast. 

 Juvenile bluefin tuna routinely traveled through the ther- 

 mocline, moving from the relatively warm surface layer 

 into the mid-Atlantic cold-pool water (Houghton et al., 

 1982; Houghton and Marra, 1983) underlying it. The fish 

 thus experienced temperature gradients of up to =0.6°C/m 

 (Figs. 5 and 6). In contrast, the steepest horizontal tem- 

 perature gradient in the area where the fish were tracked 

 was approximately three orders of magnitude smaller 

 (=0.5°C/km). In other words, the frequent vertical move- 

 ments of juvenile bluefin tuna probably prevent them from 

 detecting and responding to SST gradients. 



