Kneebone et al.: Seasonal distribution and habitat use of the A/opias vulpinus in the western North Atlantic Ocean 405 
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Figure 4 
Distribution of common thresher sharks (Alopias vulpinus) in the western North Atlantic Ocean by season during 1964-2019 
and in relation to average sea-surface temperature (SST) climatologies for the period 2009-2016. The number of capture records 
(Rec) in each grid cell (0.5° x 0.5°) is natural log transformed. Records from the Gulf of Mexico are presented in the inset. The 
200-m depth contour delineates the boundary of the continental shelf. 
evaluating time or area closures to assist in the recovery 
of overfished populations. 
Analyses of presence-only data are a useful and 
cost-effective way to understand the distribution and 
habitat use of wide-ranging species for which limited 
species-specific survey data exist (Elith and Leathwick, 
2009; Curtis et al., 2014). Their main limitation is the 
potential for bias due to spatial and temporal variabil- 
ity in fishing effort or in catchability by both gear type 
and life stage (Pearce and Boyce, 2006; Curtis et al., 
2014). For example, common thresher sharks are the 
target of a relatively large, seasonal recreational shark 
fishery that operates from Virginia to Maine (Gervelis 
and Natanson, 2013), and >99% of the observed rod- 
and-reel capture events occurred over this geographic 
area. However, commercial fisheries data would be 
expected to be less biased, given that common thresher 
sharks are caught almost exclusively as bycatch in the 
WNA (Castro, 2011; Young et al., 2016) and that effort 
in trawl, gill-net, and pelagic longline fisheries is rela- 
tively consistent, widespread, and year-round along the 
U.S. east coast (Beerkircher et al., 2004; Guiet et al., 
2019). Because 94.9% of records originated from a vari- 
ety of commercial fisheries that operate throughout the 
range of this species, the overall and seasonal trends 
we report herein may largely reflect animal distribu- 
tion. Nonetheless, spatial and temporal concentration 
in fishing effort may have influenced the number of 
captured sharks by season; for example, the large num- 
ber of YOY observed off North Carolina during the win- 
ter and spring. 
Seasonal distribution and habitat use 
Our results are consistent with those of previous reports 
that indicate that the common thresher shark is primarily 
