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.