350 
Fishery Bulletin 109(4) 
Temperature (°C) 
1 1 Apr 1 Mav 5 10 15 20 25 30 
15:00 19:00 23:00 03:00 07:00 11:00 30% 20% i0% 0% 10% 20% 30% 
Time of day (HST) Percent of time (%) 
Figure 2 
Blue shark (Prionace glauca ) depth and thermal ranges as identified by pop-up satellite archival tags (PSATs). 
(A) Representative vertical movements. Dashed lines indicate expanded area shown in C. (B) Temperature- 
depth profiles obtained from the aggregated data from all sharks. (C) Expanded section from panel A with 
horizontal black bars representing nighttime. (D) Percentage of time spent in individual temperature strata 
for daytime and nighttime diving activities (all tags). (E) Average hourly depth (±standard deviation, SD) 
readings calculated for all samples, illustrating variability at crepuscular times (all tags). (F) Percentage of 
time spent in individual depth strata for daytime and nighttime diving activities (all tags). (Note: to prevent 
excessive clutter, SDs were not shown in some panel figures). 
whitetip sharks; shortfin makos did not display strik- 
ing changes in behavior during crepuscular transitions 
(Fig. 7E). Traveling west to east from deployment to 
pop-up location can alter times of sunrise and sunset 
by as much as -4 and —1.5 h, respectively, but one 
shortfin mako (ID 38572) made no obvious depth cor- 
rections to account for spatial changes in the times 
of local sunrise and sunset as did bigeye threshers 
(cf. Fig. 4E). Shortfin makos made regular excursions 
beneath the uniform temperature surface layer, and 
vertical movement patterns were more variable dur- 
ing daytime than at nighttime (Figs. 3 and 7). Around 
27 January 2003, a shortfin mako (ID 38572) crossed 
the ~18°C SST isotherm, the southern boundary of the 
North Pacific Transition Zone (Polovina et al., 2001), 
at 31.34°N, 135.18°W (Fig. IE) and moved into cool- 
er water. Comparisons of pooled daytime and night- 
time data showed that this individual remained sig- 
nificantly deeper (median=113 m, mean=125 m ±2 SE, 
IQR=91- 161 m) in warmer water than after it crossed 
the boundary and entered cooler water (median=87 
m, mean=90 m ±2 SE, IQR = 39-118 m) (MWBC, 2 = 
-16.45, Monte Carlo P<0.0001). Temperature data in- 
dicated that significantly warmer temperatures were 
encountered before 27 January 2003 (median=21.02°C, 
mean=20.5°C ±0.01 SE, IQR = 19-22°C) than after this 
date (median=14.9°C, mean=14.8°C ±0.05 SE, IQR=14- 
16°C) (MWBC, z— -35.04, Monte Carlo PcO.0001). The 
switching between water masses is clearly seen in the 
temperature-depth profile (Fig. 7B). 
