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Block, 2001). Our data also indicate that vertical mobil- 
ity patterns are species-specific. Moreover, the vertical 
movement patterns of bigeye thresher and blue sharks 
and shortfin makos appear to allow them to remain in 
the vicinity of prey organisms in the deep sound scatter- 
ing layer (SSL), as is the case for swordfish and bigeye 
tuna (Childress and Nygaard, 1974; Carey, 1990; Josse et 
ah, 1998; Musyl et al., 2003, Musyl et al. 2 ), during their 
extensive daytime vertical migrations, with additional 
adjustment of nighttime vertical movement behaviors to 
lunar illumination (e.g., Musyl et al., 2003). By contrast, 
the epipelagic silky and oceanic whitetip sharks remain 
in the upper mixed layer (-120 m) both night and day. 
Diet studies (Tricas, 1979; Harvey, 1989; Preti et al., 
2008) and observations from submersibles (Davies and 
Bradley, 1972) indicate overlap among pelagic shark 
species which are in concordance with the overlap in 
diel vertical movement patterns, especially at nighttime 
when species remain near the surface. 
With the exception of shortfin makos, the pelagic 
sharks in our study displayed distinct changes in verti- 
cal movement patterns during crepuscular transitions. 
Pronounced or regular activity at crepuscular periods 
has been hypothesized to aid in orientation and naviga- 
tion (e.g., by detecting sun angles and geomagnetic or 
electric fields; Carey and Scharold, 1990; Musyl et al., 
2001, 2003; Klimley et al., 2002; Willis et al., 2009). 
Other authors have suggested this strategy reflects 
movements of the organisms of the SSL (Josse et al., 
1998; Musyl et al., 2003). Klimley et al. (2002) pos- 
tulated that shortfin makos occasionally dive deep to 
sample magnetic gradients, but also need to sample 
the earth’s main dipole field at the surface where it is 
strongest. The absence of pronounced vertical move- 
ments during crepuscular transitions indicates sun 
elevations or changes in light-intensity may not be criti- 
cal for navigation. 
Examining data from 22 blue sharks carrying ultra- 
sonic transmitters, Carey and Scharold (1990) noted 
the largest vertical oscillations during the day (de- 
scents to 620 m and 7°C) and smaller excursions at 
night. Blue sharks appear to have no unique anatomi- 
cal or physiological adaptations (e.g., thermoconserving 
mechanisms necessary for regional endothermy) and 
Carey and Scharold (1990) suggested this “up and 
down movement” pattern might be a hunting tactic, 
behavioral thermoregulation, or an efficient way to 
sample odor plumes that tend to spread horizontally 
throughout the water column. Lastly, divergent verti- 
cal movement behaviors could be specific search be- 
haviors tailored to finding the availability of specific 
resources (Sims et al., 2008; Humphries et al., 2010). 
For example, when resources are scarce and patchily 
distributed, pelagic sharks adopt a Levy flight be- 
havior, but at thermal fronts, where there are abun- 
dant resources, they switch to Brownian movement 
(Humphries et al., 2010). 
PSAT data from blue sharks in eastern Australia 
have shown diel vertical movement patterns (i.e., deeper 
in daytime and near the surface at nighttime) with 
the majority of the time spent between 17° and 20°C 
and approximately 80% of vertical movements above 
-200 m, but maximum depths reached may have been 
constrained by bathymetry (Stevens et al., 2010). In 
contrast, blue sharks in our study experienced a larger 
range in temperatures (e.g., 80% of temperatures oc- 
cupied were from 13-26°C) as a result of their greater 
vertical mobility. In the tropical Indian Ocean, catch 
data indicated the abundance of blue sharks was great- 
est at depths of 80-220 m and at temperatures from 12° 
to 25°C (Compagno, 1984) — data that correlate with our 
results. Nakano et al. (1985) offered that 14-21°C was 
the preferred temperature of blue sharks in the North 
Pacific, whereas Strasburg (1958) claimed that 99% of 
the blue shark catch in the Pacific was taken by long- 
line hooks in waters between 7° and 20.5°C — hooks that 
were in or immediately below the thermocline. 
The horizontal movements of blue sharks that we ob- 
served generally followed the seasonal and ontogenetic 
north-south migratory patterns reported by Strasburg 
(1958) and Nakano and Stevens (2008). Weng et al. 
(2005) reported movements of blue sharks from the 
eastern Pacific into the central Pacific, but it is unclear 
if populations of blue sharks in the central Pacific are 
regularly supplemented by recruits from the eastern 
Pacific. Moreover, to our knowledge, movements of blue 
sharks from the central to eastern Pacific have not been 
documented. Understanding these movement patterns 
would be helpful for stock assessments. 
Apart from anecdotal and taxonomic information 
(Compagno, 1984; Bonfil et al., 2008) very little data 
exist about the life history and ecological requirements 
of oceanic whitetip sharks. The movement data reported 
herein are in agreement with published summaries on 
the biology of this species (Bonfil et al., 2008), which 
generally indicate their habitat to be primarily in the 
uniform temperature surface layer. We found that 
oceanic whitetip sharks spend >95% of their time at 
temperatures within 2°C of SST. Strasburg (1958) con- 
cluded that the whitetip “was surface dwelling north 
of the equator and bathypelagic to the south,” whereas 
Compagno (1984) suggested that this species can toler- 
ate temperatures from 18° to 28°C but normally prefers 
water above 20°C. Bonfil et al. (2008) suggested that 
blue and oceanic whitetip sharks — the most abundant 
oceanic sharks — have evolved an efficient partitioning 
of the oceanic environment,” and our data clearly sup- 
port this conclusion. 
Silky sharks have been reported to be limited to wa- 
ter temperature >23°C (Last and Stevens, 2009) which 
agrees with our data. Compagno (1984), however, sug- 
gested that silky sharks could inhabit depths below 500 
m, something we did not observe. Watson et al. (2009) 
reported finding smaller, immature silky sharks cap- 
tured by purse seine north of the equator in the eastern 
tropical Pacific. In our cluster analysis, the most unique 
cluster was composed of immature silky sharks south of 
the NEC, and silky sharks segregated by body size and 
also by latitude. Presumably this topology is temporary 
and changes through ontogeny. 
