3.
Results
There were 4346 captures from 1985 to 2018 of 2533 individual bears,
with a mean of 127.8 bears (SE = 11.3) captured per year (Table S3).
There were 1159 adult male, 540 solitary adult female, 807 adult female
with offspring, 296 subadult male, 331 subadult female, 393 yearling,
and 820 COY captures (Table S4).
3.1 Age/sex class energy
patterns
Energy density declined significantly over time for solitary adult
females (linear regression, P = 0.015) and declined non-significantly
over time for the other classes, except adult males and adult females
with offspring (linear regression, P > 0.05; Fig. 2).
Storage energy declined significantly over time for solitary adult
females and yearlings (linear regression, P = 0.001, 0.041 respectively)
and declined non-significantly over time for the other classes, except
adult females with offspring (linear regression, P > 0.05;
Fig. 3).
Energy density was significantly lower at earlier sea ice breakup dates
for adult males and subadult females (multiple linear regression, P =
0.020, 0.034; Fig. S1; Tables S5, S6) and non-significantly lower at
earlier breakup dates for the other classes (multiple linear regression,
P > 0.05; Fig. S1; Table S6). Storage energy was
significantly lower at earlier sea ice breakup dates for adult males,
subadult males, subadult females, and COY (multiple linear regression, P
= 0.004, 0.014, 0.012, 0.029 respectively; Fig S2; Tables S7, S8) and
non-significantly lower at earlier breakup dates for the other classes
(multiple linear regression, P > 0.05; Fig. S2; Table S8).
A longer lagged open water period was associated with significantly
reduced storage energy and non-significantly lower energy density for
solitary adult females (multiple linear regression, P = 0.028 and P =
0.074, respectively; Figs. S3, S4; Tables S6, S8).
Energy density was significantly different among classes
(Kruskal-Wallis, χ2 = 958.35, df = 6, P <
0.001). Solitary adult females had significantly higher energy density
than all other classes (Dunn’s test, P ≤ 0.05; Tables S4, S9). COY and
adult females with offspring had significantly lower energy density than
all other classes, while adult males, subadult males/females, and
yearlings had intermediate energy density. Storage energy was also
significantly different among classes (Kruskal-Wallis,
χ2 = 3398.2, df = 6, P < 0.001). Adult males
had significantly higher storage energy than all other classes, followed
by solitary adult females (Dunn’s test, P ≤ 0.05; Tables S4, S10).
Subadult males/females and adult females with offspring had intermediate
storage energy. Yearlings and COY had significantly lower storage energy
than all other classes.
3.2 Temporal dynamics of population
energy
From 1985 to 2018, the total population energy density declined by 53%
and population storage energy declined by 56% (linear regression, P
< 0.001 and P < 0.001 respectively; Fig. 4). There
was a significant positive correlation between yearly population
abundance estimates and both population energy density and storage
energy (Spearman’s correlation, coefficient = 0.69, P < 0.001
and coefficient = 0.68, P < 0.001 respectively).
3.3 Population energy and the
environment
Sea ice breakup varied from 17 May to 10 July and occurred significantly
earlier from 1985 to 2018, with mean breakup occurring 5.5 days/decade
earlier (linear regression, P ≤ 0.05; Fig. S5). Sea ice freeze-up varied
from 4 Nov to 7 Dec and occurred significantly later over time, with
mean freeze-up occurring 4.3 days per decade later (linear regression, P
≤ 0.001; Fig. S5). The length of the open water period varied from 102
days to 166 days and significantly lengthened over time, with a mean
increase of 9.9 days per decade (linear regression, P ≤ 0.001; Fig. S5).
The top ranked models for population energy density and storage energy
included sea ice breakup and the lagged open water period, while AOw,
NAOw, and their lagged effects were not included in the top models
(Table S11). Total population energy density was significantly lower
when sea ice breakup occurred earlier and the lagged open water period
was longer (multiple linear regression, P < 0.001 and P =
0.001, respectively; Fig. 5, Table 1). The top multiple linear
regression model predicted that at the earliest breakup (ordinal date
137) and 180 day lagged open water
period, total population energy density would be 8303.0 MJ
kg-1 (58% lower than the mean energy density value
that was calculated in our study, 19944.8 MJ kg-1).
Similarly, total population storage energy was significantly lower when
sea ice breakup occurred earlier and the lagged open water period was
longer (multiple linear regression, P < 0.001 and P
< 0.001 respectively; Fig. 5, Table 4). At the earliest
breakup (ordinal date 137) and 180 day lagged open water period,
population storage energy was
predicted to be 838781 MJ (63% lower than our mean estimated storage
energy, 2270218 MJ).