Kneebone et al.: Seasonal distribution and habitat use of the A/opias vulpinus in the western North Atlantic Ocean 401 
reconcile any lengths that appeared to have been inaccu- 
rately labeled (e.g., labeled as TL when the weight was 
more consistent with a fish near the same FL). Length 
data for any record that had a measured or estimated FL 
below the published size of near-term embryos (50 cm FL; 
Gervelis and Natanson, 2013) or markedly above the esti- 
mated maximum length (350 cm FL; Cailliet et al., 1983) 
were considered unreliable and excluded from ontoge- 
netic analyses. Lastly, all common thresher sharks with 
a measured or estimated FL were assigned to 1 of 3 life 
stages, on the basis of published estimates of length at age 
(Gervelis and Natanson, 2013) and length at 50% matu- 
rity (Natanson and Gervelis, 2013) for individuals from 
the WNA: young of the year (YOY), if <100 cm FL; juve- 
nile, for males 101—187 cm FL and for females 101—215 cm 
FL; and adult, for males >188 cm FL and for females and 
sharks of unknown sex >216 cm FL. 
Distribution analysis 
Spatial and temporal patterns in presence and fish- 
ery interactions of common thresher sharks by sex, life 
stage, and season were qualitatively analyzed following 
the general approach of Curtis et al. (2014). All analyses 
were performed in R (vers. 4.0.3; R Core Team, 2020). 
Maps and figures were created by using the tidyverse 
collection of packages (vers. 1.3.0; Wickham et al., 
2019) and the sf package (vers. 0.9.6; Pebesma, 2018) in 
R. Seasons were classified as winter (January—March), 
spring (April-June), summer (July-September), or fall 
(October-December). 
Because of the limitations of using presence-only data 
when detectability is unknown, location data over the full 
time series (1964-2019) were plotted by season, both in 
aggregate and by sex and life stage, to assess distribution 
patterns. To meet confidentiality requirements for com- 
mercially derived data, latitudes and longitudes (under 
the World Geodetic System 1984) were aggregated in a 
raster grid (0.5° x 0.5°) spanning the WNA, including the 
Gulf of Mexico. Grid cell counts (number of captures) were 
natural log transformed for plotting to increase contrast 
among small values. 
Habitat use 
Fishery-dependent records with a recorded SST were 
aggregated by month and summarized by using box 
plots to examine monthly and seasonal trends. To visu- 
ally examine distribution of common thresher sharks in 
relation to typical seasonal SST conditions in the WNA, 
grid cells were plotted over NMFS Southwest Fisheries 
Science Center monthly composite climatologies (data 
set ID: erdAGsstamday_LonPM180; available from the 
ERDDAP server at website) averaged over the most 
recent years for which data were available (2009-2016); 
62% of all capture events occurred over this period. The 
SST data were downloaded by using the rerddap package 
(vers. 0.6.5; Chamberlain, 2019) in R. To explore any asso- 
ciation with bathymetry, the local depth was assigned to 
each record by using data from the NOAA ETOPO1 1 
Arc-Minute Ocean Relief Model (NOAA National Geo- 
physical Data Center, model available from website, 
accessed June 2019; Amante and Eakins, 2009) in the R 
package marmap (vers. 1.0.5; Pante and Simon-Bouhet, 
2013). Histograms were used to examine SST and depth 
occupancy by life stage. 
Results 
A total of 3478 fishery-dependent records of the capture of 
common thresher sharks were compiled between 1964 and 
2019, representing 1035 males, 1039 females, and 1404 
sharks of unknown sex (Tables 1 and 2). The number of 
Table 1 
Source, number, and time period for fishery-dependent capture records com- 
piled to examine spatial distribution and habitat use of common thresher 
sharks (Alopias vulpinus) in the western North Atlantic Ocean between 1964 
and 2019. DFO=Department of Fisheries and Oceans; NMFS=National Marine 
Fisheries Service; SEFSC=Southeast Fisheries Science Center. 
Data source 
DFO Canada: Industry Surveys Database 
NMES Cooperative Shark Tagging Program 
NMFS Large Pelagics Survey 
NMES Shark Bottom Longline Observer Program 
NOAA Northeast Fisheries Observer Program 
NOAA Southeast Pelagic Observer Program 
NOAA SEFSC Cooperative Tagging Program 
Total 
Number of Time 
records period 
299 1982-2019 
1964-2019 
2002-2018 
1996-2019 
1990-2019 
1992-2017 
1978-1996 
1964-2019 
