404 
Fishery Bulletin 118(4) 
Life stage 
| Ov 
[] Juvenile 
[§] Adult 
BB Unknown 
Number of sharks 
GN PLL OT RR HS MWTBLL PS 
Gear type 
Figure 3 
Number of common thresher sharks (Alopias vulpinus) 
caught in the western North Atlantic Ocean, based on 
compiled fishery-dependent data from 1964 through 2019. 
Sharks were captured with the following gear types: gill 
net (GN), pelagic longline (PLL), otter trawl (OT), rod 
and reel (RR), haul (beach) seine (HS), mid-water trawl 
(MWT), bottom longline (BLL), and purse seine (PS). Note 
the difference in scale between the y-axes. YOY=young of 
the year. 
fall, 2 centers of distribution were evident: one from North 
Carolina to New Jersey and another off the Scotian Shelf 
in Canada. 
Young of the year Common thresher sharks classified 
as YOY were distributed almost exclusively (99.1%) in 
continental shelf waters north of 33.5°N (Fig. 5). Only 6 
individuals were captured south of 33.5°N, including a 
single record from the Gulf of Mexico (27.8°N, 88.6°W), 
which may have been a misidentification. Sharks of this 
life stage migrated along the coast in a north-south 
pattern, with overwintering grounds off Cape Hatteras. 
During the summer, sharks were most commonly taken 
off New Jersey and Long Island, New York, but their 
distribution ranged as far north as Cape Cod, Massa- 
chusetts. During the spring and fall, their distribution 
spanned between these summer and overwintering 
grounds. 
Juveniles and adults Because of the inability to assign a 
life stage to all recorded common thresher sharks and 
the limited data from adults (Table 2), composite distri- 
bution maps were made for the juvenile and adult life 
stages (Fig. 6). The lack of sex data for sharks captured in 
Canada also precluded a thorough assessment of juvenile 
and adult distribution in waters of Canada, particularly 
along the Scotian Shelf. The winter distribution of these 
life stages occurred off the coast of North Carolina and 
the center of summer distribution extended from the 
mid-Atlantic states north to the Gulf of Maine. The distri- 
bution patterns of both juvenile and adult sharks are con- 
sistent with a seasonal, north-south distribution pattern 
along the east coasts of the United States and Canada, 
with extensive movements between the winter and sum- 
mer habitat areas. 
Males and females In general, the overall and seasonal 
distributions of male and female common thresher 
sharks were similar and followed the patterns previously 
described for this species and for each life stage (Fig. 7). 
Habitat use 
Sea-surface temperature at the time and location of cap- 
ture was recorded for 2764 common thresher sharks and 
ranged from 4°C to 31°C (mean: 17°C [SD 4]) (Table 2); 
78.2% of all records were for sharks captured in waters 
with temperatures of 10—22°C (Fig. 8). Young-of-the-year 
sharks were associated with the narrowest temperature 
range (6—26°C); however, each life stage and both sexes 
were generally associated with a similar range of SSTs 
(males: 5-30°C [mean: 17°C (SD 4)]; females: 6—30°C 
[mean: 17°C (SD 4)]). Monthly and seasonal fluctuations 
in SST at capture were evident for all life stages (Fig. 9). 
Sharks were taken in areas with depths ranging from 
1 to 5427 m (mean: 433 m [SD 1032]), with 43.7% of all 
records from waters 10-25 m deep (Table 2, Fig. 8). In 
particular, 98.4% of YOY were caught in depths from 1 
to 50 m, and 61.7% of juveniles were caught in depths 
from 10 to 100 m. Males and females were taken in 
areas with depths from 1 to 5427 m (mean: 165 m [SD 
533]) and from 1 to 5401 m (mean: 146 m [SD 497]), 
respectively. 
Discussion 
The results of our analysis of over 50 years of fishery- 
dependent data advance the scientific understanding of 
the distribution and habitat use of common thresher 
sharks in the WNA and will improve our ability to assess 
and sustainably manage the population of this species 
nationally and internationally. Although trends of relative 
abundance for common thresher sharks in the WNA 
appear to have stabilized or slightly increased in recent 
years, current levels are still well below historic values of 
abundance (Young et al., 2016; Lynch et al., 2018) and fish- 
ery interactions are increasing across some gear types 
(Fig. 1) (Gervelis and Natanson, 2013; NMFS, 2019a). New 
insights into habitat use by sex and life stage will also 
promote the identification of EFH in U.S. waters. Accurate 
EFH designations allow potential effects on species in 
areas of proposed development or other activities to be 
considered before approval and provide a basis for 
