Hueter et al.: Horizontal and vertical movements of Isurus paucus in the northwestern Atlantic Ocean 
111 
the pattern of movements of longfin makos observed 
in our study and indicate that the diel pattern of the 
longfin mako may be related to a habit of following 
cephalopod prey. 
The importance of the convergence of the tracks of 
LFMl and LFM2 in the MAB in summer is unclear, but 
their movements likely are influenced directly or indi- 
rectly by water temperature, given their observed ten- 
dency to remain near or within the Gulf Stream (Fig. 
2). The water column in this region becomes strongly 
stratified by mid-summer and is characterized by the 
formation of a warm, mixed surface layer of about 30 
m in depth (Castelao et al., 2008). We noted a shallow- 
er mean depth at nighttime (-30 m) for LFM2 during 
June and July that indicates a nighttime preference for 
this mixed layer. Similarly, satellite tagging of bluefin 
tunas (T. thynnus) in the Atlantic Ocean has indicat- 
ed a shallower summertime depth distribution when 
bluefin tunas occupy well-stratified water in this region 
(Galuardi and Lutcavage, 2012). The 60-d track of a 
shortfin mako tagged in Gulf Stream waters off South 
Carolina also had a decrease in nightly depth range 
after the summer solstice, indicating that the shark or 
its prey was tracking a thermal regime at night (Loefer 
et al., 2005). In a study of the diet of large, pelagic, 
predatory fish species in this same region of the North 
Atlantic Ocean, the most common component of stom- 
ach contents was cephalopods, and the highest biomass 
was represented by ommastrephid squid species (Logan 
et al., 2013). 
Taken together, the data from these studies indicate 
that longfin makos use this area off the MAB as a sum- 
mer feeding area and adjust their vertical movement 
patterns in response to seasonal changes in distribu- 
tions of cephalopod or teleost prey to maximize foraging 
efficiency. Because both LFMl and LFM2 were sexually 
mature males, we cannot discount the possibility that 
this area may also be a mating ground for this species. 
Using NMFS POP data from the MAB (Keene'^), we es- 
timated that 58% of female and 46% of male longfin 
makos caught in this area are of a sexually mature 
size. There is evidence that other lamnids, such as the 
white shark, use the MAB as both feeding and mating 
grounds (Gilmore, 1993). However, additional informa- 
tion, such as the presence of female longfin makos with 
fresh mating scars, will be needed to verify that the 
MAB is a mating ground for this species. 
Use of deep, cold habitat and endothermy 
Profiles of time at depth and time at temperature for 
the 2 longfin makos (Fig. 6) indicated that this species 
is capable of using cold, deepwater habitat for extended 
periods of time (42-54% of time at depths >200 m; 14- 
20% of time at temperatures <12° C), particularly dur- 
ing daytime hours. This pattern contrasts with that of 
its congener, the shortfin mako, which appears to make 
only brief excursions below the thermocline (Abascal 
et al., 2011) and reportedly spends only 4% of its time 
at depths greater than 300 m in the southwest Pacific 
(Stevens et al., 2010) and only about 6-10% of its time 
at depths exceeding 200 m in the northwestern Atlan- 
tic Ocean (Vaudo®). 
Keene, K., Jr. 2016. Personal commun. Southeast Fish. 
Sci. Cent., Natl. Mar. Fish. Serv., 75 Virginia Beach Dr., Bldg. 
2, Miami, FL 33149-1003. 
® Vaudo, J. 2013. Personal commun. The Guy Harvey Res. 
Inst., Nova Southeastern Univ., 8000 N Ocean Dr., Dania 
Beach, FL 33004. 
