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Fishery Bulletin 115(1) 
I I I I I M I I I I I I I I I I I I I I I I I M I I I I I I I I I I I I I I" ! I ri t I I I I I I I I I I I I 
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3/17/2015 3/18/2015 3/19/2015 
Time 
Figure 9 
Time and depth profile for a 2-d period in March 2015 for a longfin mako 
(Isurus paucus] LFM2) tracked with a satellite-linked tag in the northwest- 
ern Atlantic Ocean, indicating nighttime forays by this shark from warm, 
near-surface waters to cold waters at depths around 300 m. The shaded 
areas approximate nighttime. Depth data points are color-coded by tempera- 
ture where available. 
Lamnid shark species have the ability to conserve 
metabolic heat by means of vascular countercurrent 
heat exchangers (a complex called the retia mirahilia) 
and can maintain their tissues at temperatures signifi- 
cantly above ambient temperatures, likely as a means 
for broadening their habitat (Block and Carey, 1985; 
Goldman, 1997; Goldman et al., 2004). In a study of 
body temperature measurements and anatomical fea- 
tures related to heat production and conservation for 
lamnid species, the longfin mako was found to have a 
relatively small amount of red muscle and poorly de- 
veloped retia mirahilia and consequently was rated 
as the least endothermic of the lamnid shark species 
(Carey et al., 1985). However, these authors suggest 
that the retia mirahilia of the longfin mako could in- 
crease its thermal inertia and prolong cooling time so 
that this shark species can maintain a body tempera- 
ture higher than the ambient temperature in cold, deep 
waters after spending periods of time in warm surface 
waters. Our findings that daytime vertical activity and 
interforay duration both are related to temperature at 
depth (Figs. 7 and 8) are consistent with this hypoth- 
esis. Tracked shortfin makos have shown a similar pat- 
tern in the southeastern Pacific Ocean in that reduced 
thermal structure of the water column coincided with 
a decrease in the vertical activity of these fish (Abascal 
et al., 2011). 
We hypothesize that LFM2 conducted regular for- 
ays during daytime from cold depths to surface waters 
to gain heat lost at depth. A similar daytime move- 
ment pattern from depth to surface waters has been 
described for the blue shark (Prionace glauca-, Carey et 
al., 1990), and this movement pattern is a thermoregu- 
latory strategy that is aided by the property of muscle 
to warm up more quickly than the time it takes to cool 
down (Carey and Gibson, 1987). In contrast to the blue 
shark, the endothermic longfin mako may be able to 
remain warmer than the ambient water temperature 
for longer periods of time at depth and then re-warm 
rapidly during relatively brief forays to the surface. 
This form of behavioral thermoregulation may enable 
longfin makos to remain active predators in cold water 
and to exploit agile, vertically migrating prey, such as 
pelagic squid species that would largely become un- 
available (during the day) to competing ectothermic 
predators. 
We also noted nighttime vertical oscillations of 
LFM2 from near surface waters to depths >300 m 
(Fig. 9). The subsequent ascents brought this shark 
within 3 m of the surface, perhaps to regain heat after 
a period (20-40 min) at cold depths. For the shortfin 
mako, directed descents into deeper water, followed by 
rapid ascents (also known as bounce dives), have been 
associated primarily with daytime hours (Abascal 
et al., 2011) and have been correlated with success- 
ful feeding events at depth (Sepulveda et al., 2004). 
Although a DVM pattern has been reported for sat- 
ellite-tracked jumbo squid, a highly variable amount 
of nighttime diving to depths in excess of 300 m has 
been observed for this species (Gilly et al., 2006). It 
is likely that the nighttime forays we observed with 
LFM2 are related to foraging and may be a response 
to variable nighttime movement patterns of its pe- 
lagic prey. The large size of the longfin mako (second 
largest of all lamnid species after the white shark, by 
length) may also be thermally advantageous by fur- 
ther minimizing conductive heat loss to surrounding 
sea water. Larger shortfin makos have been found to 
