Appendix S1. Detailed description of methods used to estimate diet and fasting behavior.
We examined the contribution of different prey species to the diet of adult and subadult
independent polar bears captured in the CS in 2008-2011 based on quantitative fatty acid
signature analysis (QFASA; Iverson et al. 2004). The fatty acid compositions of prey were
determined from polar bear-killed bearded and ringed seal samples collected during CS capture
operations in 2008-2011; samples from bearded, ringed, ribbon, and spotted seals harvested
around the community of Point Hope and on Diomede Island (n = 83 bearded seals, 32 ribbon
seals, 23 ringed seals, and 24 spotted seals); subsistence-harvested beluga whales sampled in the
southern Beaufort Sea and Amundsen Gulf (n = 29; Thiemann et al. 2008b); subsistenceharvested walrus sampled on St. Lawrence Island (n = 102 adult females; US Geological Survey,
unpubl. data); and bowhead whales from spring and fall subsistence hunts in 1997-2002 at
Barrow and Kaktovik, Alaska (n = 64; Budge et al. 2008). All species were sampled within the
range of the CS polar bear population except beluga and bowhead whales which were sampled
within areas known to be part of the migration patterns of beluga and bowhead whales that
occupy the CS (O’Corry-Crowe et al. 1997; Quakenbush et al. 2009). Several studies have also
shown that temporal and spatial variation can often be detected in fatty acid profiles of marine
mammals, but between-species differences are far greater than spatial and temporal variation
within species (Iverson et al. 2006; Thiemann et al. 2007, 2008b)
Lipid was extracted from each polar bear fat biopsy and prey fat sample, and fatty acid
methyl esters (FAME) were prepared following Budge et al. (2006). Fatty acid composition of
biopsies has been determined to be uniform with depth (Thiemann et al. 2006). FAME were
analyzed in duplicate using temperature-programmed gas chromatography on a Perkin Elmer
Autosystem II capillary gas chromatograph (GC) with a flame ionization detector (FID), using a
1
polar column (Agilent Technologies, DB-23; 30 m x 0.25 mm ID; Budge et al. 2006). Diet
composition was estimated for each polar bear sample using the QFASA model (Iverson et al.
2004; Thiemann et al. 2008a) with diet estimates reflecting the relative biomass of each item.
Generalized linear models were used to determine if the contribution of a food item to dietary
biomass differed between males and females and subadults and adults or exhibited a trend with
body mass.
Levels of blood urea nitrogen (BUN) and creatinine were measured in serum blood
samples for CS bears captured 2008-2011 using an Abaxis VS2 analyzer (Abaxis, Union City,
CA, USA) to identify fasting behavior in individual bears. Serum urea-to-creatinine ratios (U/C)
were calculated as urea nitrogen (mg/dl) to 0.466/creatinine (mg/dl) (Nelson et al. 1984) to allow
comparison with the frequency of fasting behavior in the SB in 2005-2006 (Cherry et al. 2009).
Bears with U/C values ≤ 10.0 were considered to be in a physiological fasting state (Nelson et al.
1984; Derocher et al. 1990). Because bears that have not fed for extended periods of time and
have exhausted fat reserves can have high serum U/C (>10.0) ratios as a result of urea recycling
and elevated protein catabolism, we also recorded a subjective condition index on a scale of 1-5
(Stirling et al. 2008). A bear that had depleted fat reserves would exhibit a body condition score
of 1.
LITERATURE CITED
Budge SM, Iverson SJ, Koopman HN (2006) Studying trophic ecology in marine ecosystems
using fatty acids: a primer on analysis and interpretation. Marine Mammal Science, 22,
759-801.
2
Budge SM, Springer AM, Iverson SJ, et al. (2008) Blubber fatty acid composition of bowhead
whales, Balaena mysticetus: Implications for diet assessment and ecosystem monitoring.
Journal of Experimental Marine Biology and Ecology, 359, 40-46.
Cherry SG, Derocher AE, Stirling I, Richardson ES (2009) Fasting physiology of polar bears in
relation to environmental change and breeding behavior in the Beaufort Sea. Polar
Biology, 32, 383-391.
Derocher AE, Nelson RA, Stirling I, Ramsay MA (1990) Effects of fasting and feeding on serum
urea and serum creatinine levels in polar bears. Marine Mammal Science, 6, 196-203.
Iverson SJ, Field C, Bowen WD, Blanchard W (2004) Quantitative fatty acid signature analysis:
a new method of estimating polar bear diets. Ecological Monographs, 74, 211-235.
Iverson SJ, Stirling I, Lang SLC (2006) Spatial and temporal variation in the diets of polar bears
across the Canadian arctic: indicators of changes in prey populations and environment.
pp. 98-117 in Boyd, I.L., Wanless, S. and Camphuysen (eds.) Top Predators in Marine
Ecosystems. Cambridge University Press.
Nelson RA, Beck TDI, Steigler DL (1984) Ratio of serum urea to serum creatinine in wild black
bears Science, 226, 841-842.
O’Corry-Crowe, GM, Suydam RS, Rosenberg A, Frost KJ, Dizon AE (1997) Phylogeography,
population structure, and dispersal patterns of beluga whale Delphinapterus leucas in the
western Nearctic revealed by mitochondrial DNA. Molecular Ecology 6,955-970.
Quakenbush LT, Citta JJ, George JC, Small RJ, Heide-Jørgensen MP (2010) Fall and winter
movement of bowhead whales (Balaena mysticetus) in the Chukchi Sea and within a
potential petroleum development area. Arctic 63, 289-307.
3
Stirling I, Thiemann GW, Richardson E (2008) Quantitative support for a subjective fatness
index for immobilized polar bears. Journal of Wildlife Management, 72, 568-574.
Thiemann GW, Iverson SJ, Stirling I (2006) Seasonal, sexual and anatomical variability in the
adipose tissue of polar bears (Ursus maritimus). Journal of Zoology, 269, 65-76.
Thiemann GW, Iverson SJ, Stirling I (2007) Variability in the blubber fatty acid
composition of ringed seals (Phoca hispida) across the Canadian Arctic. Marine
Mammal Science 23: 241-261.
Thiemann GW, Iverson SJ, Stirling I (2008a) Polar bear diets and arctic marine food webs:
insights from fatty acid analysis. Ecological Monographs, 78, 591-613.
Thiemann GW, Iverson SJ, Stirling I (2008b) Variation in blubber fatty acid composition among
marine mammals in the Canadian Arctic. Marine Mammal Science, 24, 91-111.
4
Appendix S2. Detailed description of statistical methods used to compare body condition and
reproduction between the two polar bear populations and time periods.
Females accompanied by COY weigh less than lone females or females accompanied by
yearlings or two-year-olds (Rode et al. 2010), so we excluded COY and females with COY from
our growth curve analysis because they were largely absent from the CS 2008-2011 sample. We
controlled for the following covariates: capture date (cdate, because bears were expected to be
gaining body condition during the capture season), age (age, which is nonlinearly related to
size), and cub age (cubage) and litter size (litsize, which can affect offspring size; Rode et al.
2010). Because environmental variation can affect sex and age classes differently (Rode et al.
2010, 2012), we compared body mass and skull width for separate sex and age classes as well as
for all ages combined. Energy density and body length were compared for adults only because
in younger bears all morphometric measures (body length, mass and energy density) can be
reflective of annual variation in environmental conditions. Therefore, controlling for structural
size is less important and additional measures of energy density and body length would be
redundant with skull width and body mass measures. Sample sizes were insufficient to compare
body size and condition metrics for CS males between periods. We also examined relationships
between sea ice metrics and condition and reproduction by including each of our ice metrics,
independently, in candidate models.
LITERATURE CITED
Rode KD, Amstrup SC, Regehr EV (2010) Reduced body size and cub recruitment in polar bears
associated with sea ice decline. Ecological Applications, 20, 768-782.
Rode KD, Peacock E, Taylor M, et al. (2012) A tale of two polar bear populations: ice habitat,
harvest, and body condition. Population Ecology, 54, 3-18.
5
Table S1. The number of polar bears in each sex and age class category captured in the Chukchi
Sea (CS) 1985-1994 and 2008-2011 and in the southern Beaufort Sea 2008-2011. Two years of
the sampling effort in the CS 1985-1994 occurred in the Russian Chukchi and targeted females
with cubs-of-the-year (COY) at den sites whereas the 2008-2011 capture did not include the
Russian Chukchi resulting in differences in the occurrence of females with COY in these
samples. Adult males were also not targeted in the CS 1986-1994. All measures were not taken
for all bears captured. Samples sizes for each measure for each sex/age class are provided in
Table 1.
COY
Yearlings
Two year olds
Subadult females
Subadult males
Adult females with
COY
Adult females with
yearlings
Adult females with
two year olds
Lone adult females
Adult males
Total sample
Total sample
excluding COY and
females with COY
Chukchi- Bering Seas
1986-1991
153
33
24
7
9
84
Chukchi-Bering Seas
2008-2011
2
31
20
13
27
2
Southern Beaufort Sea
2008-2011
65
33
7
20
13
34
16
21
45
11
12
8
17
16
370
133
19
65
212
208
82
101
408
309
6
Table S2. Parameters of modified von Bertalanffy growth equations1 for length (y = L(1 – e-k(ta)
) fit to total body length (cm) and zygomatic skull width (cm) measures and for body mass (y =
W(1 – e-k(t-a))3 of polar bears captured in in the Chukchi Sea between 2008 and 2011. Data sets
for males and females exclude first year cubs since they were not available within the capture
sample. All model fits were significant at P < 0.0001.
L or W
k
a
F
BODY LENGTH
Chukchi Sea 2008-11
Females (90)
199.5 ± 2.6
0.37 ± 0.1
-3.2 ± 1.2 F2,79 =45.9
Males (113)
232.6 ± 2.0
0.34 ± 0.04 -2.6 ± 0.4 F2,109 = 269.9
Chukchi Sea 1986-94
Females (108)
191.9 ± 1.3
0.56 ± 0.1
-1.0 ± 0.3 F3,105 = 218.0
Males
Data insufficient
Southern Beaufort Sea 2008-11
Females (201)
199.9 ± 1.1
0.47 ± 0.05 -2.0 ± 0.3 F2,181 = 431.7
Males (129)
235.5 ± 1.9
0.30 ± 0.03 -2.4 ± 0.4 F2,113 = 447.9
SKULL WIDTH
Chukchi Sea 2008-11
Females (90)
Males(126)
Chukchi Sea 1986-94
Females (128)
Males
Southern Beaufort Sea 2008-11
Females (214)
Males (134)
SCALE BODY MASS
Chukchi Sea 2008-11
Females (90)
Males (121)
Chukchi Sea 1986-94
Females (99)
Males
Southern Beaufort Sea 2008-11
Females (191)
Males (130)
21.5 ± 0.25
28.5 ± 0.5
0.44 ± 0.09
0.17 ± 0.02
-1.8 ± 0.6
-4.5 ± 0.6
F2,87 = 102.2
F2,123 = 396.0
21.2 ± 0.20
Data insufficient
0.32 ± 0.04
-2.3 ± 0.5
F2,125 =285.1
21.0 ± 0.2
26.6 ± 0.3
0.3 ± 0.0
0.2 ± 0.0
-3.8 ± 0.4
-3.1 ± 0.4
F2,194 =588.9
F2,116 =499.4
242.5 ± 7.1
556.8 ± 16.5
0.32 ± 0.1
0.16 ± 0.01
-3.4 ± 0.9
-5.2 ± 0.6
F2,82 = 105.1
F2,117 = 553.5
210.3 ± 5.1
Data insufficient
0.5 ± 0.1
-1.6 ± 0.8
F2,98 =76.9
199.6 ± 3.9
432.4 ± 11.2
0.38 ± 0.05
0.26 ± 0.03
-2.8 ± 0.5
-2.4 ± 0.4
F3,177 = 223.1
F2,113 = 375.0
1
t is a fitting constant (years), k is the growth rate constant (per year), and L or W is the
asymptotic skull width, body length, or body mass.
7
Table S3 Results from comparisons of body size and condition for polar bears captured in the Chukchi Sea (CS) between 1986-1994
and 2008-2011 using general linear models. Differences between time periods (time) and ice effects were included in separate
models. Test statistics (χ2), coefficients (β) and P-values are provided for time and ice. Two ice metrics were examined in separate
models. Ice = the number of reduced ice days defined when ice extent (≥50% ice concentration) over the continental shelf was less
than 6,250 km2 and mndist = the mean minimum distance between the continental shelf and ice of 50% concentration in September.
Time is a categorical variable (1 = 1986-1994 and 2 = 2008-2011) such that a positive β indicates an increase between time periods.
Covariates that could affect measures were included initially in models and retained when P >0.10, including capture date (cdate),
litter size (litsize), and the age of cubs accompanying a female (cubage). Adult males were not targeted in CS captures 1986-1994;
thus sample sizes for most measures were insufficient for comparison.
Sex/age class
Measure
Time effect
Ice effect
βtime
χ2
Ptime
Model
βice
χ2
Pice
Model
Yearling Females Mass
19.2
5.3
0.02
Time
>0.10
None
Skull width 1.5
17.6
<0.0001
Time + litsize
0.02
4.1
0.043
Ice
0.003
7.9
0.005
Icedist
Yearling Males
Mass
31.4
17.2
<0.0001
Time + litsize
0.05
5.7
0.017
Icedist + litsize
Skull width 1.5
22.9
<0.0001
Time + litsize
None
Subadult Females Mass
Age
0.33
3.6
0.058
Ice + age + cdate
Skull width
None
0.013
5.7
0.017
Ice + age
0.001
3.1
0.08
Icedist
Subadult Males
Mass
Age
Age
Skull width 0.92
5.6
0.018
Time + age + cdate 0.002
6.2
0.013
Icedist + age + cdate
Adult Females
Mass
29.8
14.0
<0.0001
Time + cdate
0.55
18.4 <0.0001 Ice + cdate + cubage
without cubs-ofSkull width 1.0
11.9
0.001
Time + cdate
None
the-year
Length
10.0
46.1
<0.0001
Time + age
Adult Males
Yrlgs Per Female
Yrlg Lit Size
Energy D
Skull width
Cdate
None
Age
None
0.014
0.003
-0.006
4.4
5.5
3.7
0.035
0.019
0.055
Cdate
Ice
Icedist
Ice + age + cdate
None
8
Table S4 Results from comparisons of body size and condition for polar bears captured in the Chukchi Sea (CS) and the Southern
Beaufort Seas (SB) 2008 and 2011 using general linear models. Population effects (pop) were included in separate models from ice
metrics. Test statistics (χ2), coefficients (β) and P-values are provided for pop and ice. Two ice metrics were examined in separate
models. Ice = the number of reduced ice days defined when ice extent (≥50% ice concentration) over the continental shelf was less
than 6,250 km2 and icedist = the mean minimum distance between the continental shelf and ice of 50% concentration in September.
Pop is a categorical variable (1 = CS and 2 = SB) such that a positive β represents CS metrics > SB. Covariates that could affect
measures were included initially in models and retained when P >0.10, including capture date (cdate), litter size (litsize), and the age
of cubs accompanying a female (cubage).
Sex/age class
Measure
Pop effect
Ice effect
Yearling
Females
Yearling Males
Subadult
Females
Subadult Males
Adult females
without cubs-ofthe-year
Adult Males
βpop
18.2
χ2
5.4
Ppop
0.02
Model
Pop
Skull width 0.77
4.8
0.028
Pop
Mass
39.5
30.1
<0.0001
Pop + litsize
SW
1.3
22.8
<0.0001
Pop + litsize
Mass
SW
Mass
SW
Mass
Skull width
0.99
44.3
0.90
30.5
0.90
7.4
9.8
4.6
23.7
14.5
0.24
0.007
0.002
0.03
<0.0001
<0.0001
None
Pop
Pop + age
Pop + age
Pop
Pop + cdate
Length
Energy D
Mass
4.0
52.6
26.9
33.5
<0.0001
<0.0001
None
Pop
Pop + age
25.7
<0.0001
Mass
Skull width 1.2
Length
Pop
Age
Age
βice
-0.24
8.3
-0.01
-0.004
-0.54
-0.11
-0.02
-0.004
0.20
0.01
0.10
χ2
5.4
7.1
4.1
11.8
30.3
10.7
27.1
13.7
13.7
19.1
3.8
Pice
0.02
0.008
0.043
0.001
<0.0001
0.001
<0.0001
<0.0001
<0.0001
<0.0001
0.05
-0.28
-0.014
-0.002
8.3
17.0
5.3
0.004
<0.0001
0.022
-0.036
-0.53
-0.10
-0.012
8.1
13.3
5.0
11.1
0.004
<0.0001
0.025
0.001
Model
Ice
Icedist
Ice
Icedist + litsize
Ice + litsize
Icedist + litsize
Ice + litsize
Icedist + litsize
Icedist + age
Icedist + age
Icedist + age + cdate
None
Ice
Ice + cdate
Icedist + cdate
NA
Ice + cubage
Ice
Icedist
Ice
NA
9
Energy D
Yrlg Per Fem
Yrlg Lit Size
1.8
29.6
<0.0001
Pop + age
0.4
8.7
0.003
Pop + age +
cdate
None
-0.02
-0.004
-0.01
18.5
6.8
25.2
<0.0001
0.009
<0.0001
Ice + age
Icedist + age
Ice + age
-0.001
-0.001
3.0
4.9
0.08
0.028
Icedist + age + cdate
Icedist
10
Table S5. Estimated energy density (MJ/kg, based on models by Molnar et al. 2009) of male and
female polar bears from 8 populations across the Arctic based on asymptotic body length and
body mass from a modified von Bertalanffy growth curve. Values using calculated and scale
body mass are provided for bears captured in the Chukchi (CS) and southern Beaufort Seas (SB).
All others use calculated body mass based on the equation of Kolenosky et al. (1989) with the
exception of Svalbard where a model developed specifically for the population was used
incorporating girth and length (Derocher and Wiig 2002). Data are presented from highest to
lowest energy density. Bears in Western Hudson Bay and Foxe Basin were primarily captured in
the autumn whereas bears in all other populations were captured in the spring (Derocher 1991).
Data from the Central Arctic (including McClintock Channel and Gulf of Boothia populations),
Foxe Basin, High Arctic (including the Lancaster Sound population), Western Hudson Bay,
Davis Strait and Beaufort Sea (including the northern and southern BS populations) were
collected between 1966 and 1991 (Derocher 1991). Data from Svalbard were collected between
1990 and 2000 (Derocher and Wiig 2002).
Females
Males
Central Arctic
22.1
CS 2008-2011
22.9, 23.01
Foxe Basin
21.7
Foxe Basin
22.8
1
CS 2008-11
19.6
Central Arctic
21.8
CS 1986-94
18.91
Davis Strait
21.5
High Arctic
18.6
High Arctic
20.0
Western Hudson Bay
16.9
Beaufort Sea2
20.0
CS 2008-11
16.6
Svalbard
19.5
CS 1986-94
16.5
SB 2008-11
18.4, 19.11
Davis Strait
16.4
Western Hudson Bay
18.7
Beaufort Sea2
15.6
Svalbard
14.4
SB 2008-11
14.01, 14.2
1
2
Scale body mass
Bears sampled in the northern and southern Beaufort Sea
11
Table S6. Contribution of prey items to diets of polar bears captured in the Chukchi and Bering
seas during the spring based on fatty acid composition of fat biopsies. Data represent the %
contribution to polar bear fatty acid profiles and thus reflect the relative contribution to polar
bear diets on a biomass basis. Subadults include bears age 2-4 years.
Adult Females (55)
Adult Males (61)
Subadult Females (13)
Subadult Males (25)
MEAN
Bearded Seal
6.5 ± 9.2
20.7 ± 17.3
2.9 ± 3.5
6.8 ± 7.5
11.9 ± 14.5
Beluga Whale
0.5 ± 3.5
1.4 ± 4.5
0.3 ± 0.9
1.8 ± 6.9
1 ± 4.5
Bowhead Whale
5.1 ± 6.0
7.7 ± 7.4
5.8 ± 7.9
4.8 ± 5.7
6.1 ± 6.8
Ringed Seal
87.4 ± 11.2
65.7 ± 17.3
86 ± 26.1
86.4 ± 10.8
78.5 ± 18.5
Walrus
0.6 ± 1.4
4.4 ± 6.0
5 ± 18
0.2 ± 0.5
2.4 ± 6.7
12
Minimum distance to ice of 15% concentration or more
Fig. S1. Annual variation in the average minimum distance between sea ice of 15% (a) and 50%
(b) concentration and the continental shelf break (300 m isobath) between 1979 and 2010 in the
Chukchi and Southern Beaufort Seas.
(a)
600
Chukchi Sea
Southern Beaufort Sea
500
400
300
200
100
0
1980
1985
1990
1995
2000
2005
2010
Minimum distance to ice of 50% concentration or more
Year
(b)
700
600
Chukchi Sea
Southern Beaufort Sea
500
400
300
200
100
0
1980
1985
1990
1995
2000
2005
2010
13
Fig. S2. Annual variation in the number of reduced ice days defined as days in which there was
less than 6,250, 10,000, or 25,000 km2 of either 15% (a) or 50% (b) ice concentration over the
continental shelf (<300 m ocean depth) in the Southern Beaufort and Chukchi Seas between
1979 and 2010.
(a)
(b)
14