Kneebone et al.: Seasonal distribution and habitat use of the A/opias vulpinus in the western North Atlantic Ocean 409 
female sharks in late summer (senior 
author and D. Bernal, unpubl. data). 
Sea-surface temperatures recorded 
at the time of capture indicate that the 
common thresher shark occurs over a 
wide range of temperatures but is most 
Life stage 
_] yoy 
[|_| Juvenile 
[| Adult 
BB Unknown 
Percentage of records 
) 
xe) 
— 
fe) 
13) 
® 
—_ 
— 
° 
® 
jo») 
© 
~~ 
c 
£0) 
) 
— 
fo) 
jal 
Depth (m) 
Figure 8 
Percentage of the total number of capture records from fishery-dependent data 
of common thresher sharks (Alopias vulpinus) in the western North Atlantic 
Ocean between 1964 and 2019 by (A) sea-surface temperature (SST) and 
(B) depth for each life stage: young of the year (YOY), juvenile, adult, and 
unknown. 
24.0% of the captured sharks and the low number of 
adults (i.e., 423 sharks) hindered our ability to describe 
the distribution and habitat use of sharks in these life 
stages, particularly in Canada. However, the clear associ- 
ation of YOY with shallow waters (depths <50 m) indi- 
cates that records from deep, offshore waters of Canada 
were of juveniles and adults. Difficulties measuring or 
estimating the total length of common thresher sharks 
and variability in size at maturity (Natanson and Gervelis, 
2013) may have also led to the misclassification of life 
stage for some individuals and confounded our descrip- 
tions of distribution for each life stage. 
Contrary to research in other regions (Moreno et al., 
1989; Smith et al., 2008; Kinney et al., 2020), our results 
indicate that there is no strong evidence of sex-specific spa- 
tial or depth segregation of the population in the WNA. Seg- 
regation by sex tends to be most apparent in the adult life 
stage (Speed et al., 2010), and it is possible that we did not 
have sufficient information on this life stage for trends to be 
apparent. There is some evidence of sexual segregation of 
common thresher sharks in southern New England based 
on observations of large aggregations of predominantly 
commonly associated with temperatures 
from 10°C to 22°C. Previously, this spe- 
cies has been observed in SSTs ranging 
from 16.5°C to 19.8°C off the east coast of 
Florida (Castro, 2011), from 8°C to 28°C 
near the Republic of the Marshall Islands 
(Cao et al., 2011), and from 9°C to 21°C 
off the U.S. west coast (Cartamil et al., 
2016). Preferred temperatures have been 
reported as 18—20°C (Cao et al., 2011) and 
14-17°C (Cartamil et al., 2016), which 
are relatively similar to the temperature 
range over which the greatest number of 
capture events were recorded during this 
study (i.e., 12-18°C; Fig. 8). The overall 
range and mean SST at capture was also 
similar for all life stages (Table 2); how- 
ever, YOY were observed over the narrow- 
est temperature range. This result may 
be due to the lower capacity for metabolic 
heat retention in small individuals, which 
have a higher surface-to-volume ratio 
and therefore a decreased capacity to 
retain heat through regional endothermy 
(Bernal and Sepulveda, 2005). 
The monthly and seasonal patterns in 
SST at capture indicate that tempera- 
ture may be one of the key determinants 
of distribution and migration of common 
thresher sharks in the WNA. In general, 
captured sharks rarely occurred north of 
~37°N in the winter and south of ~37°N in the summer (Fig. 
4), the seasons during which the lowest and highest mean 
SSTs were observed, respectively (Fig. 9). This pattern is 
typical of migratory species that inhabit coastal waters of 
the WNA. By contrast, temperature was not one of the main 
factors driving movements and distribution off the U.S. 
west coast, with more broad-scale extrinsic seasonal factors 
(e.g., North Pacific Gyre Oscillation) being more influential 
(Kinney et al., 2020). Of note, the trends reported herein 
solely represent SST at the time and location of capture and 
do not necessary represent the temperatures that sharks 
experience at depth, which are likely the main drivers of 
movement and distribution. This is supported by Lynch 
et al. (2018), who demonstrated that water temperature 
profiles influenced catch rates for common thresher sharks 
in the pelagic longline fishery that operates off the Atlantic 
coast of the United States. 
Fishery interactions and management implications 
Capture records were available for common thresher 
sharks of all life stages; however, YOY was the most 
