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Demographic Response of Black Bears at Cold Lake,
Alberta, to the Removal of Adult Males

Results


Our criteria for mark-recapture modeling (age ≥2, captured during May-Sep) were met by 692 captures of 276 individual bears. However, only the first capture within each annual capture period was relevant to our analysis. After removing duplicate captures within periods, we were left with 65 females captured 123 times as adults, 40 females captured 46 times as subadults, 93 males captured 136 times as adults, and 125 males captured 146 times as subadults. Twenty-three adult males and 3 large subadult males captured during 1971-72 were euthanized.

We used Jolly-Seber models with time-specific survival and capture probabilities to estimate numbers of females (Model A of Pollock et al. 1990) because they fit better than models with constant survival probabilities (Model B: all females χ²5 = 12.3, P = 0.03; adults only χ²3 = 10.9, P = 0.01; truncated capture histories χ²3 = 8.6, P = 0.03) and models with constant survival and recapture probabilities (Model D: all females χ²13 = 27.3, P = 0.01; adults only χ²13 = 25.84, P = 0.02; truncated capture histories χ²8 = 14.4, P = 0.07). Further, goodness-of-fit was satisfactory only for Model A (all females χ²9 = 4.07, P = 0.91; adults only χ²5 = 3.52, P = 0.62; truncated capture histories χ²4 = 3.28, P = 0.51). Estimated numbers of females (Table 1) were relatively stable from 1968 to 1970, increased abruptly during 1971-72, remained high through 1974, then declined abruptly (Fig. 2).

Table 1.  Estimated numbers of black bears using a 218-km² area near Cold Lake Alberta, 1968-77. Estimates of total numbers (all bears) are from Young and Ruff (1982) unless otherwise noted. Estimates for females are from this study and are based on Jolly-Seber models with time-specific survival and recapture probabilities.
Year All bears All females Adult females
N-hat N-hat SE N-hat SE
1968 84 a a a a
1969 71 15.9 2.88 16.1 3.11
1970 92 21.4 4.36 17.1 3.51
1971 75 31.6 9.49 18.9 5.80
1972 117 45.8 11.38 40.4 15.63
1973 175 46.7 15.60 31.5 19.42
1974 136 59.1 24.61 41.4 27.02
1975 137 25.5 5.85 22.9 7.24
1976 123 b 34.2 5.26 25.0 c
1977 99 b c c c c
a Not estimable by Jolly-Seber method
b R.L. Ruff, unpublished data
c Limited data precluded estimation


Figure 2: Comparative line graph showing standardized deviates for 4 variables: male and female bears age 2+ years (filled square), females age 4+ years (open circle), females age 2+ years (filled circle), and female captures age 2+ years (open square) during 1968-77.
Fig. 2.  Trends in estimated numbers of bears using the CLSA during 1968-77, standardized for comparison. Estimates from Young and Ruff (1982) are based on male and female bears aged Greater than or Equal to sign2 years (filled square). Others are Jolly-Seber estimates for females (age Greater than or Equal to sign4 years [open circle]; age Greater than or Equal to sign2 years [filled circle]; captures prior to 1975, age Greater than or Equal to sign2 years [open square]).

The abrupt nature of the increase in estimated numbers of female bears suggests a change in spatial distribution, rather than a change in reproduction or survival. Survival records of radiomarked bears suggest that mortality may have been partially responsible for the abrupt decline in 1975 (Table 2). K-M estimates of mortality caused by humans were similar for radiomarked male and female bears (M:F risk ratio = 1.05, χ²1 = 0.014, P = 0.91) and were not significantly lower for subadults than for adults (ad:sa risk ratio = 0.80, χ²1 = 0.20, P = 0.66).

Table 2.  Kaplan-Meier estimates of mortality rates from deaths due to all agents and natural agents only for radiocollared black bears at Cold Lake, Alberta, 1974-77.
Year Natural mortality All mortality Bears Deaths
Rate SE Rate SE
1974 0   0.14 0.122 11 1
1975 0   0.51 0.086 36 16
1976 0.08 0.079 0.28 0.087 30 7
1977 0.03 0.077 0.24 0.080 26 6

Human activity was the proximate cause of at least 25, and probably 28, of 30 deaths that befell radiomarked bears during 1974-77. Principal causes of mortality were depredation control (11 deaths) and a combination of legal hunting by native people and poaching (11 deaths), which could not be distinguished. Legal sport hunting, a drug overdose, a collision with a train, another bear, and other natural causes each caused 1 death. Fates of 3 bears could not be confirmed by recovery of a carcass or radiocollar, but each was rumored to have been shot.

The number of subadult males captured on the CLSA was larger during each year of 1972-75 (range = 21-25) than during any year of 1968-71 or 1976-77 (range = 2-17; Table 3). We believe this result indicates a difference in the number of subadults that were at least temporarily present on the study area because estimated capture probabilities of subadults were not significantly higher (χ²1 = 1.13, P = 0.29) during the post-removal period (P-hatpost, sa = 0.55, SE = 0.11) than during the pre-removal period (P-hatpre, sa = 0.72, SE = 0.13).

Table 3.  Numbers of individual black bears captured annually at Cold Lake, Alberta, May-Sep, 1968-77.
Year Males Females Total
Subadults Adults Subadults Adults
1968 2 14 3 14 33
1969 5 9 0 14 28
1970 14 15 4 12 45
1971 7 14 5 6 32
1972 21 17 6 14 58
1973 25 8 8 5 46
1974 24 7 6 7 44
1975 22 12 3 12 49
1976 17 31 6 25 79
1977 13 13 7 18 51

Moreover, estimated settling rates did not differ enough between periods (Phi-hatpre, sa = 0.49, SE = 0.10; Phi-hatpost, sa = 0.49, SE = 0.07) to offset the increase in captures. Thus, numbers of subadult males using the study area were probably larger during 1972-75 than during other years.


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