Kneebone et al.: Seasonal distribution and habitat use of the A/opias vulpinus in the western North Atlantic Ocean 407 
75°W 70°W 65°W 60°W 55°W 50°W 
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Sa —— x 
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Ie ————— 
iS 
E 
i Nee 
a 
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Figure 6 
Distribution of juvenile and adult common thresher sharks (Alopias vulpinus) by season in the western North Atlantic Ocean 
between 1964 and 2019. The number of capture records (Rec) in each grid cell (0.5° x 0.5°) is natural log transformed. The lack 
of records in the maps for waters of Canada is a result of missing data on sex of sharks and the subsequent inability to assign 
a life stage to sharks captured in Canada. Records from the Gulf of Mexico are presented in the inset. The 200-m depth contour 
delineates the boundary of the continental shelf. 
(Beerkircher et al., 2004; Hanke et al.”) indicates that this 
species does not occur as frequently in deep, offshore waters. 
Capture of common thresher sharks has been recorded spo- 
radically throughout tropical offshore waters, including in 
the Gulf of Mexico. However, species identification could not 
be verified, and it is possible that some capture records rep- 
resent misidentifications of bigeye thresher, the only other 
alopiid that occurs in the WNA (Compagno, 2001). Of note, 
misidentification would be most likely in the pelagic long- 
line records from offshore areas in the Gulf of Mexico, where 
the distribution of bigeye threshers is well-documented 
? Hanke, A. R., I. Andrushchenko, and G. Croft. 2012. Observer 
coverage of the Atlantic Canadian swordfish and other tuna 
longline fishery: an assessment of current practices and alter- 
native methods. DFO Can. Sci. Advis. Secr. Res. Doc. 2012/049, 
84 p. [Available from website.] 
(Fernandez-Carvalho et al., 2015) and the species tends to 
be encountered 4 times more frequently than the common 
thresher shark (Young et al., 2016). 
Trends in distribution are consistent with a seasonal, 
north-south movement pattern in the WNA, with distri- 
bution focused shoreward of the continental shelf edge in 
depths <200 m along the east coasts of the United States 
and Canada. This distribution and movement pattern 
is similar to those of many other highly migratory species 
that inhabit the temperate WNA, such as the shortfin 
mako ([surus oxyrinchus) (Vaudo et al., 2017) and bluefin 
tuna (Thunnus thynnus) (Galuardi et al., 2010), except 
with no evidence of frequent movements to the tropical 
Atlantic Ocean. It also aligns with the seasonal distribu- 
tion and movements of common thresher sharks in the 
eastern Pacific Ocean, where northward (spring and sum- 
mer) and southward (fall) movements occur between Baja 
