2. Materials and methods
2.1 Field
sampling
Hudson Bay is an inland sea that is seasonally ice covered (autumn to
spring) and ice free in summer (Hoccheim et al. 2010; Fig. 1). When sea
ice retreats in summer, WH polar bears come ashore along the western
coast of the Bay in northeastern Manitoba, Canada and remain on land
until sea ice freeze-up (Stirling et al. 1999; Lunn et al. 2016). Polar
bears were captured in the core summering area of the WH population
(Fig. 1) in late August to early October from 1985 to 2018 following
standard methods (Stirling et al. 1989). Bears were measured
(straight-line body length and axillary girth), marked with uniquely
numbered ear-tags and tattoos, and released. Age was determined from an
extracted vestigial premolar (Calvert & Ramsay 1988). Bears were
categorized into seven age, sex, and reproductive classes: adult male (≥
5 years), solitary adult female (≥ 5 years), adult female with offspring
(≥ 5 years and accompanied by offspring), subadult male (2-4 years),
subadult female (2-4 years), yearling (ca. 20-22 months), and
cub-of-the-year (COY, ca. 8-10 months). All capture and handling
techniques were in accordance with the Canadian Council on Animal Care
(www.ccac.ca) guidelines and approved by the University of Alberta
BioSciences Animal Care and Use Committee and Environment and Climate
Change Canada’s Western and Northern Animal Care Committee. Research was
conducted under wildlife research permits issued by the Government of
Manitoba and the Parks Canada Agency.
2.2 Environmental
data
Annual dates of sea ice breakup and freeze-up for the WH management zone
were extracted from 323 grid cells with 25 x 25 km resolution passive
microwave satellite raster imagery from the National Snow and Ice Data
Center (Cavalieri et al. 1996). The first ordinal date in spring when
sea ice concentration was ≤ 50% for three consecutive days was used as
the date of sea ice breakup, while the first ordinal date in autumn when
sea ice was ≥ 10% for three consecutive days was used as the date of
freeze-up (Etkin 1991; Stirling et al. 1999; Lunn et al. 2016). The
length of the open water period (i.e., when bears are on land) was
calculated as the date of freeze-up minus the date of breakup, then
further subtracting 25 days due to the bears arriving onshore
approximately 21 to 28 days after breakup (Stirling et al. 1999; Castro
de la Guardia et al. 2017; Johnson et al. 2019). In addition, the Arctic
Oscillation winter index (AOw) and the North Atlantic Oscillation winter
index (NAOw) were extracted for each year to examine broad climate
variability. The AO affects sea ice distribution (Stroeve et al. 2011)
and is related to polar bear reproduction rates and diet (Derocher 2005;
McKinney et al. 2017), while NAO influences sea ice extent and is
related to polar bear stress hormones (Bechshøft et al. 2013). AOw was
calculated as the mean of January to March AO in each year (National
Ocean and Atmospheric Administration;
https://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao.shtml).
NAOw was calculated as the winter index (December to March) from the
National Centre for Atmospheric Research (Hurrell 2012). To account for
the influence of environmental conditions of the previous year, we also
calculated lagged environmental variables in each year.