Brill et aL: Horizontal and vertical movements of juvenile Thunnus thynnus 



161 



26 



24-- 



C/5 

 C/5 



22 



4 c 



-- 3 



-- 2 in 



1400 



1800 



2200 



0200 



0600 



1000 



1400 



1800 



8 12 16 20 24 28 

 Temperature ( 'C) 

 Figure 6 



Swimming speed (solid line, upper panel) and vertical movements (lower panel) of fish 4. The change in tempera- 

 ture in the horizontal direction (expressed as sea surface temperature, SST) is shown by the broken line in the 

 upper panel (Brill and Lutcavage, 2001). As in Figure 5. the change in temperature in the vertical direction (mean 

 ±SEM) is shown to the right of the vertical movement plot. Note that changes in swimming speed are not cor- 

 related with changes in SST, and that the steepest temperature change the fish could experience moving horizon- 

 tally (generally less then 0.5°C/km) is several orders of magnitude less then that experienced moving vertically 

 (=0.6°C/m). 



occur throughout the water column during daylight, are 

 abundant in the areas where we tracked the fish, and 

 dominate the diet of tunas in this area (Mason, 1976; Egg- 

 leston and Bochenek, 1989). The nature offish 4's descents 

 up to =160 m while off the continental shelf (Fig. 6) re- 

 main unclear, although they too may be related to foraging 

 (Dagorn et al., 2000a, 2000b). Their brevity is most likely 

 due to the inability of Atlantic bluefin tuna to withstand 

 temperatures below 10°C for long periods of time, rather 

 than to an intolerance of low ambient oxygen conditions. 

 Although no depth-oxygen profiles were obtained during 

 our study, available data- suggest that juvenile tuna did 

 not encounter ambient oxygen levels that were likely to be 

 stressful (Bushnell and Brill, 1991, 1992). 



The behavior pattern we observed of short oscillatory 

 dives near the surface is similar to that of both juvenile 

 bluefin tuna in the eastern Pacific (Marcinek et al., 2001) 

 and adult bluefin tuna tracked in the Gulf of Maine (Lut- 

 cavage et al., 2000). In all cases, fish spent the majority 

 of their time in the surface layer, although in the Gulf of 

 Maine and eastern Pacific, the temperature of the warm- 

 est water available was lower (=13-22°C) and more vari- 

 able. As shown in Figure 11, when expressed as the rel- 

 ative change in temperature with depth (i.e. in relation 

 to the surface water temperature occurring during each 

 track), time-at-temperature distributions of juvenile and 



adult Atlantic bluefin tuna become essentially identical. 

 Moreover, the limiting effects of temperature change on 

 vertical movements are independent of body size. Simi- 

 larly, yellowfin tuna tracked near the main Hawaiian Is- 

 lands and off the coast of California occupy the warmest 

 water available, regardless of body size, even though sur- 

 face water temperature in the two areas differs by more 

 than 5°C (Holland et al., 1990; Block et al., 1997; Brill et 

 al., 1999). Atlantic bluefin tuna, however, are more eury- 

 thermal than yellowfin tuna. The latter will rarely expose 

 themselves to more than an 8°C change in temperature, 

 whereas the former regularly subject themselves to a tem- 

 perature change of up to 13°C (Fig. 11). Surprisingly, the 

 behavior of juvenile bluefin tuna observed by Marcinek 

 et al., (2001) was more like that of yellowfin tuna in that 

 these juvniles would not expose themselves to more then 

 an 8°C temperature change. 



It still remains to be conclusively demonstrated, how- 

 ever, whether the vertical movement patterns of tunas and 

 other large pelagic fishes are (as suggested by Brill et al., 

 1993, 1999) limited by the effects of ambient temperature 

 on cardiac function. Or whether, as suggested by Mar- 

 cinek et al. (2001), that depth distributions "... may have 

 more to do with the location of prey, and the physiological 

 limitations of the prey, than physiological limitations of 

 the bluefin Itunaj." Moreover, months-long observations 



