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Fishery Bulletin 118(4) 
over the time series (Baum et al., 2003; Cortés et al., 2007). 
In contrast, pelagic longline fisheries observer data indi- 
cate that relative abundance had either stabilized at low 
levels (Baum and Blanchard, 2010) or increased (Cortés 
et al., 2007) during the 1990s into the early 2000s. Results 
of recent analyses of pelagic longline logbook and observer 
data for common thresher sharks from the period 1992— 
2013 also indicate a stabilized trend in relative abundance 
at low levels (Young et al., 2016) to an overall increasing 
trend when temperature profiles are incorporated into the 
analyses (Lynch et al., 2018). Relative abundance indices 
derived from data from 5 long-term recreational sportfish- 
ing tournaments based out of New York and New Jersey 
also indicate an overall increasing trend but with high 
annual variability (Young et al., 2016). 
Although recent analyses provide an optimistic out- 
look, the true extent to which these trends are represen- 
tative of the population of common thresher sharks in the 
WNA remains unknown. For example, although observer 
data are often considered more reliable than fishery log- 
book data, especially for bycatch species, the sample size 
for observer data is much smaller and interactions of 
individuals with the pelagic longline fishery are likely 
limited by the coastal and temperate distribution of this 
species (Young et al., 2016). Nonetheless, this species 
is currently listed as vulnerable by the International 
Union for Conservation of Nature (IUCN) in the IUCN 
Red List of Threatened Species, with a decreasing popu- 
lation trend assumed throughout its global range (Rigby 
et al., 2019). There are also no formal stock assessments, 
international management measures, or species-specific 
quotas for common thresher sharks in the WNA, and 
there are only limited biological and ecological data for 
the development of domestic and international manage- 
ment policies. 
Detailed information on seasonal distribution, habitat 
use, migration patterns, and population structure of com- 
mon thresher sharks in the WNA is limited. Although 
records from fishery-dependent catch data support a 
north-south movement pattern along the U.S. east coast 
(e.g., Castro, 2011; Natanson and Gervelis, 2013), pub- 
lished movement data are limited to fishery-dependent 
data from the recapture of 2 individuals from the 230 
common thresher sharks tagged by the National Marine 
Fisheries Service (NMFS) Cooperative Shark Tagging 
Program from 1962 through 2018 (tag return rate: 
~0.9%). Both recaptured sharks had a north-south move- 
ment pattern along the northeastern coast of the United 
States (Kohler and Turner, 2019). However, the records 
of these recaptures provide minimal insight into the spa- 
tial and temporal extent of the annual movements of this 
species, given the extended times at liberty (1533 and 
2934 d) and the fact that both recapture events occurred 
in June. Migratory routes are also poorly known (Castro, 
2011), and only general descriptions of distribution and 
habitat use by sex or life stage have been presented from 
historical catch data. 
Given the uncertainty over the status of the population 
of common thresher sharks in the WNA, there is a need 
to improve our understanding of this species’ life history 
and general susceptibility to capture in commercial and 
recreational fisheries. Improved knowledge is needed con- 
sidering that their life history parameters (i.e., relatively 
slow growth, late age to maturity, and low fecundity; 
Gervelis and Natanson, 2013; Natanson and Gervelis, 
2013) increase their susceptibility to population decline 
(Smith et al., 1998) and that ecological data on their tem- 
perature and habitat preferences are few. Accordingly, 
we compiled and summarized fishery-dependent data on 
catch of common thresher sharks throughout the WNA 
1) to document their seasonal distribution and habitat 
use by sex and life stage, 2) to better understand fishery 
interactions, and 3) to provide information that facili- 
tates the identification of essential fish habitat (EFH) by 
life stage in U.S. waters. 
Materials and methods 
At-sea observer and other catch data on common thresher 
sharks originated mainly from the NMFS, specifically 
from the Northeast Fisheries Observer Program, Pelagic 
Longline Observer Program of the Southeast Fisheries 
Science Center, Shark Bottom Longline Observer Pro- 
gram, Cooperative Shark Tagging Program of the Apex 
Predators Program, Cooperative Tagging Center of the 
Southeast Fisheries Science Center, and Large Pelag- 
ics Survey. Additional records were obtained from the 
Industry Surveys Database of Fisheries and Oceans 
Canada (H. Bowlby, personal commun.), which is used 
to archive at-sea observer information from commer- 
cial fisheries. Data from each source were subjected to 
editing and quality control a priori by the provider and 
represent all records of common thresher sharks avail- 
able in their respective databases through 2019. Data 
requested from each source include the date of capture, 
location of capture (i.e., latitude and longitude), shark 
sex, shark length (i.e., fork length [FL] and total length 
[TL]) and nature (i.e., measured or estimated) of the 
length, shark weight (i.e., whole [or round] and dressed) 
and nature (i.e., measured or estimated) of the weight, 
gear type used for capture, and the observed sea-surface 
temperature (SST, in degrees Celsius) at the time and 
location of capture. 
Prior to analysis, records with no geographical informa- 
tion were removed, and a series of steps were taken to stan- 
dardize and validate length and weight data among the 
various sources. All capture records for which a measured 
FL was provided were considered reliable and retained. 
Records with a measured or estimated TL or whole weight 
were converted to an estimated FL by using the appropri- 
ate conversion equation (TL to FL: coefficient of multiple 
determination [R?]=0.83; whole weight [in kilograms] to 
FL: R?=0.93; NMFS’). Reported weights were also used to 
! NMFS (National Marine Fisheries Service). 2020. Unpubl. data. 
Apex Predators Program, Northeast Fish. Sci. Cent., Natl. Mar. 
Fish. Serv., NOAA, 28 Tarzwell Dr., Narragansett, RI 02882. 
