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Fishery Bulletin 115(3) 
Spike dives 
Oceanic whitetip sharks tagged in the North Atlantic 
Ocean (Bahamas) have been reported to make sporadic 
deep dives to the mesopelagic zone, down to a depth 
of 1190 m (Howey-Jordan et al., 2013; Howey et al., 
2016). Similar to the deep dives observed in our work, 
descent rates of dives of oceanic whitetip sharks in the 
study off the Bahamas were significantly faster than 
ascent rates. However, unlike most sharks in our study, 
the sharks in the Bahamas performed deep dives pri- 
marily at night and dusk. The difference between the 
2 studies in the period during which oceanic whitetip 
sharks made deep dives could be related to local envi- 
ronmental conditions; however, 1 shark (shark AOCS7) 
in our study also made spike dives primarily during 
the night. These contrasting periods and the rarity of 
these deep dives make it difficult to identify their driv- 
ing forces. In any case, similarly shaped deep dives are 
relatively common among pelagic sharks and often are 
believed to be associated with prey searching (Sepul- 
veda et al., 2004; Hoffmayer et al., 2013; Howey-Jordan 
et al., 2013; Tyminski et al., 2015). Gleiss et al. (2011) 
suggested that v-shaped dives might help sharks to 
efficiently scan the water column for patches of food 
while expending minimal energy. They also suggested 
that slower ascent rates might improve the chances of 
pelagic predators detecting prey because backlighting 
is improved during ascents. 
If searching for prey is the driver behind the occa- 
sional spike dives observed for oceanic whitetip sharks 
in our study, they could have been triggered after 
sharks remained in less productive surface waters for 
prolonged periods. It has been hypothesized that deep 
dives by whale sharks ( Rhincodon typus) in the Indian 
Ocean were triggered when individuals were crossing 
less productive areas (Brunnschweiler and Sims, 2012). 
Another hypothesis for the occurrence of spike dives is 
that sharks make these deep dives to search for navi- 
gational cues through magnetic gradients (Willis et 
al., 2009). Seafloor magnetic anomalies associated with 
bathymetric features form a predictable gridded pattern 
that is believed to aid navigation (Walker et al., 2002). 
Sharks can detect magnetic fields (Kalmijn, 1982), and 
deep dives could represent a mechanism to acquire these 
magnetic cues (Gleiss et al., 2011; Tyminski et al., 2015). 
Howey et al. (2016) concluded that foraging or naviga- 
tion are the only viable hypotheses to explain the rea- 
sons for the mesopelagic excursions of oceanic whitetip 
sharks, but they suggested that foraging is the most 
likely hypothesis. Nevertheless, the 2 proposed hypothe- 
ses are plausible and not mutually exclusive. It is, there- 
fore, possible that isolated spike dives taken by oceanic 
whitetip sharks can be triggered by both foraging and 
navigational needs, depending on the circumstances. 
Vertical movements and the environment 
Water temperature is a limiting factor for ectothermic 
species whose body temperature is dependent on the 
external environment, because body temperature is a 
central factor in the control of their physiological pro- 
cesses (Sims, 2003). For such species, like the oceanic 
whitetip shark, which occupies the tropical epipelagic ! 
niche (Musyl et al., 2011; Howey-Jordan et al., 2013; \ 
Tolotti et al., 2015b), variations in the extent and heat I 
content of the warmer mixed layer are expected to play I 
a major role in their vertical movements. Our study | 
confirmed this strong relationship by modeling the | 
SD of daily depth records as a proxy for the ampli- t 
tude of vertical movements. The GAM results indicate J 
that tagged oceanic whitetip sharks tended to increase | 
their vertical range in the water column as the depth | 
of the mixed layer increased and, consequently, their I 
optimal habitat expanded. A similar relationship was I 
recently observed for another tropical epipelagic spe- ' 
cies; dolphinfish tagged in the Pacific Ocean extended I 
their vertical depth ranges as the depth of the thermo- 
cline increased (Furukawa et al., 2014). 
Besides the MLD, other factors were found to influ- 
ence the vertical movements of tagged oceanic whitetip jj 
sharks in our study. Results from the GAM that used ; 
data for multiple sharks indicate an effect of shark 
size, in which larger individuals tended to have wider 
use of the water column. Values of average depth per 
hour, displayed in Figure 1, also indicate that vertical \ 
behavior might vary with shark size. Such an effect • 
might be linked to the increased thermal inertia that 
results from a larger body mass, enabling larger indi- 
viduals to extend their thermal habitat (Neill et al., 
1974, 1976; Wilson et al., 2006). A possible effect of size i 
on the vertical movements of oceanic whitetip sharks I 
and silky sharks also has been reported from a tag- jj 
ging study conducted off Hawaii (Musyl et al., 2011). j 
The results of a cluster analysis by Musyl et al. (2011) r 
revealed that the vertical behavior of large individuals I 
(>200 cm in total length) of these 2 closely related spe- * 
cies appear to be well separated from that of juveniles. ; 
For oceanic whitetip sharks tagged in the Bahamas, 
a correlation between average daily depth and SST 
was observed (Howey-Jordan et al., 2013). The authors \ 
reported that the average daily depth increased when 
individuals experienced warmer SSTs. For the oceanic i 
whitetip sharks that we studied, the results from most f 
GAMs also indicate a positive relationship between 1 
vertical movement and SST. Interestingly, for 2 individ- 1 
uals (AOCS3 and AOCS5), this relationship occurred i 
simultaneously with an inverse relationship between 
MLD and the vertical activity of the sharks. In short, j 
when SST was above average, these 2 sharks increased 
the amplitude of their vertical movement despite the ■ 
reduced depth of the mixed layer. This pattern may 
indicate behavioral thermoregulation. Accordingly, oce- < 
anic whitetip sharks could be using the warmer SST to { 
accumulate heat and subsequently explore cooler deep 
waters or they could be diving below the thermocline jj 
to cool down. 
Howey-Jordan et al. (2013) also suggested that the 
correlation between SST and average depth observed 
for the oceanic whitetips sharks in the Bahamas could 
